SEARCH

SEARCH BY CITATION

Keywords:

  • Clusia;
  • crassulacean acid metabolism (CAM);
  • ecophysiology;
  • neotropics;
  • plasticity;
  • photosynthesis;
  • phylogeny

Abstract

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

Contents

  •  Summary 7

  • I. 
    The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research 8
  • II. 
    Phylogeny 8
  • III. 
    Photosynthetic physiotypes 10
  • IV. 
    Metabolic flexibility: organic acid variations 12
  • V. 
    The environmental control of photosynthetic flexibility 13
  • VI. 
    Phenotypic plasticity: physiotypes and morphotypes 16
  • VII. 
    Ecological amplitude and habitat impact 16
  • VIII. 
    Conclusions and outlook 21
  •  Acknowledgements 22

  •  References 22

Summary

It is the aim of this review to present a monographic survey of the neotropical genus Clusia on scaling levels from molecular phylogeny, metabolism, photosynthesis and autecological environmental responses to ecological amplitude and synecological habitat impact. Clusia is the only dicotyledonous genus with real trees performing crassulacean acid metabolism (CAM). By way of introduction, a brief historical reminiscence describes the discovery of CAM in Clusia and the consequent increase in interest in studying this particular genus of tropical shrubs and trees. The molecular phylogeny of CAM in the genus is compared with that in Kalanchoë and the Bromeliaceae. At the level of metabolism and photosynthesis, the great plasticity of expression of photosynthetic physiotypes, i.e. (i) C3 photosynthesis, (ii) CAM including CAM idling, (iii) CAM cycling and (iv) C3/CAM-intermediate behaviour, as well as metabolic flexibility in Clusia is illustrated. At the level of autecology, the factors water, irradiance and temperature, which control photosynthetic flexibility, are assessed. The phenotypic plasticity of physiotypes and morphotypes is described. At the level of synecology, the ecological amplitude of Clusia in the tropics and the relations to habitat are surveyed.


I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

As we shall see, species of Clusia are handsome woody plants. Some have small flowers only, but others develop rather large beautiful flowers; for example, the flowers of Clusia grandiflora Engl. have a diameter of 150 mm. Nevertheless, Clusia most likely would have remained just one genus among a vast number of others in the large biodiversity of trees and shrubs in the tropics if it had not turned out that species of Clusia are performing crassulacean acid metabolism (CAM). This mode of photosynthesis is basically characterized by nocturnal uptake of CO2 and dark fixation via phosphoenolpyruvate carboxylase (PEPC), where the resulting fixation product, mainly malic acid, is stored in the central cell sap vacuole. Behind closed stomata and in the absence of any overt gas exchange, the organic acid is remobilized during the light period and decarboxylated, and the CO2 regained is refixed via ribulose-bis-phosphate carboxylase/oxygenase (Rubisco) and assimilated in the Calvin cycle.

Clusia is the only genus of trees with this mode of photosynthesis. True, there are a number of sizeable plants with CAM in the families of Cactaceae, Euphorbiaceae and Didieraceae as well as the monocotyledonous Yucca, all of which have been considered ‘fantastic trees’ (Menninger, 1967; Ellenberg, 1981). Moreover, giant cactus communities in Venezuela have been called ‘cactus forests’ (Vareschi, 1980). However, Clusia is the only genus of trees sensu stricto with typical dicotyledonous secondary growth that performs CAM.

Clusia early raised the curiosity of researchers because of the peculiarities of its photosynthetic physiology. During field work in February 1800 in Venezuela, Alexander von Humboldt discovered that Clusia rosea Jacq. had no obvious overt gas exchange in the light period but built up a high internal gas pressure with an oxygen concentration near 40% (Krätz, 2001; Lüttge, 2002). In 1937, Willy Hartenburg reported observations from glasshouse experiments showing that on bright days Clusia mexicana Vesque did not take up CO2 in the light period but even released some CO2, which was not explicable by respiration. However, on overcast days the CO2 exchange pattern was as expected of photosynthesizing plants (Hartenburg, 1937; Lüttge, 1995b). Neither Alexander von Humboldt nor Willy Hartenburg was able to really explain their observations. Now, as we know that Clusia species are performing CAM, we realize that they saw particular features of CAM, where stomata are closed in the light period when CO2 remobilized from nocturnally stored organic acids is photosynthetically assimilated, which is associated with the build up of high internal partial pressures of oxygen (Spalding et al., 1979). Decarboxylation of nocturnally stored organic acids behind closed stomata also leads to high internal CO2 concentrations, i.e. 2–60 times atmospheric (Lüttge, 2002), which may even cause some diffusive loss of CO2 to the atmosphere in spite of stomatal closure. It remained for the Mexicans Tinoco Ojanguren & Vazquez-Yanez (1983) to demonstrate and clearly explain the performance of CAM in Clusia for the first time, and with an additional seminal study by Ting et al. (1985) this became widely known and created wide interest, moving Clusia into the limelight of international research activities. This now allows me to present Clusia in this review, and we are also preparing a monographic book on Clusia covering all aspects of its biology, i.e. its taxonomy and phylogeny, diversity and phytogeography, ecology and physiology, and circadian clock (Lüttge, 2006c).

II. Phylogeny

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

The question is why there are not more trees with CAM. Similarly, although there are woody plants, shrubs and small trees with C4 photosynthesis, there are no real large trees with this mode of photosynthesis. The two modes, C4 and CAM, share many biochemical features, particularly the primary fixation of CO2 by PEPC and synthesis of organic acids (mainly malate and citrate in CAM and malate and aspartate in C4 photosynthesis) and the subsequent decarboxylation of the organic acids and refixation and assimilation of the CO2 by Rubisco in the Calvin cycle. There are, of course, important differences between C4 photosynthesis and CAM which are well known and need not be reiterated here. However, the observation that trees do not use phosphoenolpyruvate (PEP) carboxylation as widely as CAM and C4 photosynthesis are otherwise distributed among angiosperms shows an intriguing parallel between both modes of photosynthesis. Phylogenetic studies address the question of why this is so, but we must note here, at the outset, that in both cases the question has not been answered to date.

CAM is widespread among the taxa of vascular plants including ferns and fern allies (Isoëtes). There is no doubt that CAM arose polyphyletically many times during the evolution of vascular plants (Lüttge, 1987). This must have been facilitated by the fact that CAM does not require any particular metabolic trait as compared with non-CAM plants. A reorganization and specific management of metabolic modules common in the housekeeping functions of all photosynthesizing plants are sufficient for CAM performance (Lüttge, 2005). The evolution of CAM has been specifically studied in the genus Kalanchoë and the family Bromeliaceae and more recently also in the genus Clusia. There are conspicuous differences among the three taxa. A comparison (Lüttge, 2004, 2005) is useful as it reveals interesting peculiarities of Clusia.

The radiation centre of Kalanchoë is in the moist forests of eastern Madagascar (Kluge et al., 1991; Gehrig et al., 2001; Kluge, 2005). The genus comprises species with obligate C3 photosynthesis and obligate CAM and there are a small number of species that are C3/CAM intermediate and can switch between the two modes of photosynthesis (Brulfert et al., 1973, 1975). The majority of the species in the genus are CAM species. Morphological (Boiteau & Allorge-Boiteau, 1995) as well as molecular (Gehrig et al., 1997, 2000, 2001) evidence suggests that CAM must have evolved monophyletically in the genus from a C3-photosynthesis ancestor during radiation from the moist east to the drier areas up to the arid areas in the west and south-west of Madagascar, where CAM functions as an important adaptation to the limited availability of water.

Conversely, in the neotropical family of the Bromeliaceae with its three subfamilies Pitcairnioideae, Bromelioideae and Tillandsioideae and very many epiphytic species, again as supported by morphological/anatomical, biochemical and molecular genetic evidence (Smith, 1989; Crayn et al., 2004), CAM and the epiphytic habit evolved a minimum of three times. Epiphytism and CAM have evolved independent of each other, although evidently CAM is an adaptation to the water shortage stress of the epiphytic habitat (Zotz & Hietz, 2001). The last common ancestor of the Bromeliaceae was a terrestrial C3 mesophyte. It is important to note that there has also been a reversion from CAM back to C3 photosynthesis in some taxa of the bromelioid line following subsequent radiation into less xeric habitats (Crayn et al., 2004). Diversity is very large in the family in many respects but photosynthetic plasticity is low. There are obligate C3 and CAM species but in the entire large family there appears to be only one clearly C3/CAM-intermediate species, namely Guzmania monostachia (L.) Rusby ex Mez (Maxwell et al., 1994, 1995, 1999; Maxwell, 2002).

The subfamily Clusioideae with the genus Clusia in the family Clusiaceae has a minimum phylogenetic age of 90 × 106 years (Gustafsson et al., 2002). Clusia is a large genus of shrubs and trees with about 300–400 species (Pipoly et al., 1998). The genus is considered monophyletic. However, internal transcribed spacer (ITS) sequencing of nuclear ribosomal DNA has shown that the evolution of CAM in the genus was polyphyletic (Vaasen et al., 2002; Gehrig et al., 2003; Holtum et al., 2004). ITS analyses of some 80 species of Clusia and a derived phylogenetic tree suggest that CAM evolved at least twice in the genus. A reversion from CAM to C3 was noted in eight individual species and in one group of three species (Gustafsson et al., 2006). Thus the situation in Clusia resembles that in the Bromeliaceae and not that in Kalanchoë. However, in contrast to the Bromeliaceae, Clusia is characterized by an enormous plasticity. There are very many C3/CAM-intermediate species. It appears that this even may be the rule in the genus and obligate C3 and especially obligate CAM species are much fewer (Holtum et al., 2004).

This expectation is, however, based on a much smaller sample size. To some extent, the occurrence of CAM can be screened by analysing carbon isotope ratios (δ13C) because, as a result of the much lower 13C discrimination of PEPC (−7 relative to CO2) than of Rubisco (+27 relative to CO2) (Ziegler, 1994), in CAM-performing plants δ13C values are significantly less negative than in C3 plants. However, it has been noted that C3/CAM-intermediate species often make little use of their CAM option in the field, so that δ13C values are C3-like. Although Winter & Holtum (2002) and Winter et al. (2005) have presented a calibration for deducing from δ13C values the extent of primary dark fixation (PEPC) and light fixation (Rubisco) of CO2, respectively, for a definitive identification of a CAM potential in a species additional physiological evidence is required, for example from gas exchange measurements and analyses of diurnal organic acid fluctuations. It is much more tedious to obtain such information for many species than to just analyse δ13C values, but it appears to be very important to do this for as many species of Clusia as possible. This would not only provide insights into the evolution of CAM within the genus. In addition, because of the extraordinary inherent evolutionary plasticity in the genus and its high speciation rate (Gustafsson et al., 2006), the investigation of Clusia may also advance understanding of CAM evolution in general.

In summary, the comparison of the three taxa Kalanchoë, Bromeliaceae and Clusia, in which CAM phylogeny has been studied in the greatest depth, shows that the evolution of CAM not only occurred independently in the three taxa but also went rather different ways, i.e. monophyletically within the genus Kalanchoë, polyphyletically but without generation of much plasticity in the Bromeliaceae and polyphyletically with production of enormous plasticity in the genus Clusia.

The key enzyme of CAM as well as C4 photosynthesis is PEPC. Recently, the molecular characterization of PEPC isoforms has been investigated in detail in C4 photosynthesis (Engelmann et al., 2003; Svensson et al., 2003; Westhoff & Gowik, 2004) and also in CAM plants, with a strong phylogenetic bias. It has been found that Kalanchoë pinnata (Lam.) Pers. has seven PEPC isoforms, only one of which is CAM relevant (Gehrig et al., 2005). Taybi et al. (2004) studied the expression of PEPC and PEPC-kinase (PEPCK) genes in four different species of Clusia. PEPCKs regulate the activity of PEPC which is more active and less sensitive to feedback inhibition by malate in the phosphorylated state. The species compared were C. rosea, an obligate CAM species, Cluisa minor L., a C3/CAM-intermediate species, Clusia aripoensis Britt., a weakly CAM-inducible species, and Clusia multiflora H.B.K., an obligate C3 species. They found that transcriptional control of PEPC abundance is a key factor in determining the genotypic capacity of CAM, while the PEPCK gene is under the control of metabolic feedback, and there are no C3- or CAM-specific isoforms of the PEPCK genes. The obligate CAM species Clusia hilariana Schlecht. produces three isoforms of PEPC, one of which is root-specific and is probably a housekeeping isoform in this organ, while the C3 species C. multiflora and the C3/CAM-intermediate species C. minor only have one isoform (Vaasen, 2005; Vaasen et al., 2006). For C. minor the observation that there is only one isoform each of PEPC and PEPCK is most noteworthy because this implies that just one isoform can support both the housekeeping functions of the C3 state and the particular requirements of the CAM state. This may facilitate C3–CAM switches. PEPC transcript abundance is increased in the CAM state (Taybi et al., 2004; Vaasen, 2005; A. Vaasen, D. Begerov, R. Hampp, Botanisches Institut, Universität Tübingen, Tübingen, Baden-Württemberg, Germany). The observation that a single isoform each of PEPC and PEPCK can support performance of both C3 photosynthesis and CAM also strengthens the speculation that this has facilitated CAM evolution and that C3/CAM flexibility, which is so widespread in the genus Clusia, has played a role in this. However, whether the absence of more isoforms in C. minor is a result of a lack of evolution of more isoforms or the loss of redundant isoforms during evolution remains unresolved at present. More comparative work clearly is highly desirable. However, in the earlier literature it was not C3/CAM-intermediate behaviour but ‘CAM cycling’ that was suggested to have been a starting point for CAM evolution (Guralnick et al., 1986; Guralnick & Jackson, 2001), but this now takes us to the next section discussing photosynthetic plasticity in Clusia.

III. Photosynthetic physiotypes

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

In Fig. 1, I have reduced metabolic complexity to the extent of only using two modules, namely PEPC and organic acids and Rubisco and carbohydrate ([CH2O]n), respectively, and external and internal CO2 for a schematic illustration of four different photosynthetic physiotypes, all of which are found among Clusia species.

image

Figure 1. Minimal scheme of metabolism using two metabolic modules, namely phosphoenolpyruvate carboxylase (PEPC) and nocturnal organic acid accumulation (org.ac.) and ribulosebisphosphate carboxylase/oxygenase (Rubisco) and daytime carbohydrate synthesis ([CH2O]n) and two pools of inorganic carbon, namely external and internal CO2, to illustrate the basic features of four photosynthetic physiotypes and their connections: from left to right, (i) crassulacean acid metabolism (CAM) cycling (CAM cycl), (ii) full CAM with CAM idling (CAM idl), (iii) C3/CAM-intermediate behaviour, and (iv) C3 photosynthesis. Dark arrows, carbon flow in the dark; white arrows, carbon flow in the light.

Download figure to PowerPoint

  • 1
    C3 photosynthesis, where stomata are open during the light period and CO2 taken up is fixed directly via Rubisco in the light.
  • 2
    CAM, where stomata are open in the dark period and CO2 taken up is fixed via PEPC leading to the production of organic acid (both malic and citric in Clusia species), organic acid is decarboxylated during the light period and the CO2 regained is refixed via Rubisco. Some direct C3-type fixation is also possible, i.e. in the later light period, when ecological conditions, particularly water relations, allow stomatal opening after nocturnally accumulated organic acid is consumed. CAM idling, as also shown in Fig. 1, is not considered a separate physiotype but a variant of normal full CAM, where stomata remain closed night and day when water relations are adverse and respiratory CO2 is refixed in the dark period and recycled to carbohydrate during the light period.
  • 3
    C3/CAM-intermediate behaviour with reversible switches between the two modes of photosynthesis in perennial Clusia, where leaves are used for at least two growth periods (Olivares, 1997).
  • 4
    CAM cycling, where CO2 uptake and fixation occur mainly during the light period as in C3 photosynthesis, but respiratory CO2 is recuperated via PEPC behind closed stomata during the dark period and organic acid is produced which is decarboxylated during the light period, supplementing the CO2 taken up from the atmosphere via the open stomata as substrate for Rubisco. This use of PEPC to prevent loss of respiratory CO2 in the dark period was thought to have been an early step towards the evolution of CAM, as alluded to in the previous section (Guralnick et al., 1986; Guralnick & Jackson, 2001).

The scheme illustrated in Fig. 1 symbolizes the modular nature of photosynthetic physiotypes and can be used to elucidate the links that might be important in long-term and medium-term evolutionary and ontogenetic development, respectively, as well as in short-term environmental responses in ecophysiology. For example, (i) CAM can also involve a strong C3 element; (ii) without light-period CO2 uptake CAM cycling would become CAM idling.

Some representative curves of day:night CO2 exchange in Clusia species are shown in Fig. 2. A bona fide obligate C3 species is C. multiflora and obligate CAM species are Clusia alata Pl. et Tr., Clusia major L., C. rosea and C. hilariana. As noted in the previous section, there are very many C3/CAM-intermediate species in the genus Clusia. The most widely studied species in this respect is C. minor. Under different conditions with respect to a combination of different environmental factors, such as irradiance during growth and a variety of day:night temperature regimes, it produced a vast number of different responses including full CAM with CAM idling (Haag-Kerwer et al., 1992). In another experiment, irradiance during growth and nitrogen nutrition were varied. Plants grown at high irradiance without supplemental nitrogen showed no CO2 uptake and a minimal loss of CO2 but accumulated appreciable amounts of citrate plus a small amount of malate in the dark period, while light-period CO2 uptake was substantial, i.e. there were clear indications of CAM cycling (Franco et al., 1991). Most of the experiments cited here were performed with one clone of vegetatively propagated C. minor. The enormous phenotypic plasticity with respect to expression of photosynthetic physiotypes of the genome is evident.

image

Figure 2. CO2 gas exchange curves (inline image) of Clusia minor L. under three different conditions (A–C) and Clusia venosa Jacq. (D), Clusia alata Pl. et Tr. (E) and Clusia major L. (F), showing performance of C3 photosynthesis (A, D), crassulacean acid metabolism (CAM) (B, E, F), with the four phases of (I) nocturnal CO2 uptake, (II) daytime stomatal closure and organic acid remobilization, (III) a transition with CO2 uptake in the early light period, and (IV) CO2 uptake in the later light period (Osmond, 1978), and CO2 uptake almost around the clock and strong expression of all four CAM phases (C). In (A–C) for C. minor, the irradiance and water vapour pressure deficit (VPD) of the atmosphere in mol m−2 s−1/mbar bar−1, respectively, were as follows: (A) 1700/6.6, (B) 400/13.5, (C) 400/3.5, i.e. high irradiance and low VPD favoured C3 photosynthesis, medium irradiance and high VPD favoured CAM, and medium irradiance and very low VPD favoured CO2 uptake around the clock. Dark bars, night-time; white bars, daytime. (After data of Lee et al., 1989, where C. minor is wrongly named C. rosea, and Franco et al., 1990.)

Download figure to PowerPoint

IV. Metabolic flexibility: organic acid variations

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

Clusia also shows flexibility at the level of metabolites in the CAM cycle. Normally the precursor for glycolytic formation of phosphoenolpyruvate (PEP) as a CO2 acceptor for dark fixation is glucan (starch) and the acid produced is malic acid (malate). In Clusia, however, free sugars, i.e. glucose, fructose and sucrose, are also used in addition to starch as PEP precursors (Popp et al., 1987; Ball et al., 1991; Berg et al., 2004). Although noteworthy, this situation is not unique as it also occurs occasionally in other taxa, for example in the bromeliad pineapple Ananas comosus (L.) merr. (Black et al., 1996). More remarkable is the fact that Clusia, in addition or alternatively to malate, can also synthesize citrate. There are other CAM plants that nocturnally form citrate in their CAM cycle but the amounts are far lower than those found in Clusia; for example, day:night oscillations of citrate concentrations in some species of Kalanchoë may be up to 26 mm, and this is considered relatively high (Lüttge, 1988), but C. minor and C. rosea can show values of up to 200 mm (Franco et al., 1992). This really amounts to a qualitative rather than only a quantitative difference. The relative amounts of malate and citrate produced can vary within the same species, clone or plant depending on environmental conditions, where again C. minor is an excellent example (Haag-Kerwer et al., 1992).

I can list three cases where differential responses of nocturnal malate and citrate accumulation in the CAM of Clusia species to environmental factors have been observed.

  • 1
    Under drought conditions in experiments by Franco et al. (1992) and de Mattos et al. (1999) with three Clusia species (Clusia lanceolata Camb., C. rosea and C. minor) the ratio of malate to citrate accumulation declined by a factor of 1.8–3.4 after 10 to 16 d without watering. This was, however, not observed in a study by Borland et al. (1998) with C. minor, C. rosea and C. aripoensis, but in this case the ratios of malate to citrate were already very low in the nondroughted controls.
  • 2
    In C. minor the ratio of malate to citrate accumulated also depended on nitrogen nutrition and irradiance during growth (Franco et al., 1991). At low irradiance there was no malate and only citrate accumulation with and without supplemental nitrogen nutrition. At high irradiance the ratio of malate to citrate was 0.22 without and 3.07 with nitrogen.
  • 3
    In C. minor the ratio of malate to citrate was also affected by day:night (D:N) temperature regimes and irradiance during growth. For example, low irradiance and D:N temperatures of 30 : 15°C produced only malate accumulation; high irradiance and D:N temperatures of 25 : 15°C or 30 : 15°C produced high malate and low citrate accumulation; high irradiance and D:N temperatures of 25 : 20°C produced equal malate and citrate accumulation; low irradiance and D:N temperatures of 30 : 30°C produced only citrate accumulation; high irradiance and D:N temperatures of 20 : 20°C produced high citrate and low malate accumulation (Haag-Kerwer et al., 1992).

Naturally one would expect that a switch in metabolism as important as a change from malate to citrate accumulation should have adaptive benefits in response to the various environmental cues. However, the picture created by the above three examples of environmental responses remains very fuzzy, and we can only try to evaluate comparatively a number of roles of malate and citrate, respectively, which I summarize below in eight points (Table 1; Franco et al., 1992; Lüttge, 1996, 2006a).

Table 1.  Potential advantages and disadvantages, respectively, of night:day change in the concentrations of malate (Δmal) and citrate (Δcitr) during the crassulacean acid metabolism (CAM) cycle (Franco et al., 1992; Lüttge, 1996, 2006a)
FunctionΔmalΔcitr
1. CO2 acquisitionYesNo
2. H2O saving during CO2 acquisitionYesNo
3. Carbon recycling in the dark periodCO2Carbon skeleton
4. Carbon recycling in the light period and increase in internal CO2 concentrationCO2 YesCO2 Yes, more than for malate
5. Energy budget in the dark periodATP consumptionATP consumption and production of redox power
6. Energy budget in the light periodATP consumptionLarger ATP consumption and consumption of redox power
7. Osmotic changesYesYes, less than for malate
8. Buffer capacityLowHigh
  • 1
    With respect to day:night carbon balance, the CAM cycle with citrate is futile. Nocturnal citrate accumulation does not contribute to carbon gain because, starting with a C6-hexose unit as precursor, the result is C6-citrate. Conversely, two molecules of C4-malate are formed by nocturnal fixation of 2 CO2 per C6-hexose unit used (Lüttge, 1988).
  • 2
    As it does not contribute to carbon gain, citrate, of course, also does not contribute to water saving during CO2 acquisition, unlike malate synthesis by nocturnal CO2 uptake when evaporative demand is much lower than during the day.
  • 3
    Like malate, citrate can contribute to carbon recycling during the dark period, for example in CAM idling. With malate accumulation respiratory CO2 is recycled; with citrate accumulation entire carbon skeletons are recycled.
  • 4
    Like malate, citrate can contribute to CO2 recycling in the light period. Potentially more CO2 is generated from citrate breakdown in the light period so that citrate can make a larger contribution to the build-up of high internal CO2 concentrations behind closed stomata than malate.
  • 5
    Energetically, citrate accumulation is marginally cheaper in the dark period. ATP is consumed in similar amounts in both malate and citrate accumulation but reduction equivalents are generated in citrate synthesis which can be used for ATP production in the respiration chain.
  • 6
    Energetically, the return of carbon from citrate to carbohydrate in the light period appears to be more costly than in the case of malate. The decarboxylation of two molecules of malate gives rise to two molecules of pyruvate, which can be used to regenerate two molecules of triose and then one molecule of hexose via gluconeogenesis. The decarboxylation of one molecule of citrate may generate three molecules of CO2 and only one molecule of pyruvate. The refixation of CO2 to generate triose is much more costly energetically than gluconeogenesis starting from pyruvate.
  • 7
    The nocturnal accumulation of malate in the vacuoles of CAM plants has osmotic consequences and can support nocturnal water storage (Eller & Ruess, 1986; Lüttge, 1986; Ruess et al., 1988; Eller et al., 1992; Murphy & Smith, 1998). The CAM cycle with citrate is less effective in this respect; for example, for one hexose unit from glucan, two osmotically active malate molecules and only one citrate molecule are formed. If the precursor is free hexose, citrate formation is osmotically neutral.
  • 8
    Citrate is a more effective pH-buffer compound than malate. The buffer capacity of the vacuole is important for nocturnal organic acid accumulation because it determines the trans-tonoplast proton gradient against which the proton pumps energizing acid accumulation must work (Lüttge et al., 1981). A higher buffer capacity may therefore allow an increased capacity for organic acid accumulation (Franco et al., 1992). Thus, citrate accumulation in Clusia species may be an explanation of the fact that in Clusia species we observe the highest nocturnal accumulation of acids ever found among CAM plants, namely 1410 mm titratable protons in one example of C. minor in the field in Trindiad (Borland et al., 1992).

Hence, there is no benefit from citrate accumulation for carbon gain and improvement of water relations (points 1, 2 and 7 above). Conversely, citrate is useful as it could facilitate a larger total accumulation of acids in the dark period (point 8) which contributes to the build-up of high internal CO2 pressures in the light period, in which citrate is more effective than malate (point 4). This is useful because high internal CO2 concentrations may bring Rubisco closer to substrate saturation, or actually achieve substrate saturation, minimizing photorespiration, photoinhibition and photodestruction at high irradiance during the light period (Lüttge, 2002). The higher energy demand of citrate cycling in the light period (point 6) in this respect may also be considered beneficial as it contributes to energy dissipation, reducing the danger of photoinhibition and photodestruction at high irradiance. In this respect, observations in the field in Trinidad are interesting, where citrate breakdown in C. minor during the day showed a much closer correlation with irradiance as compared with malate (Borland et al., 1996). Thus, one might expect that citrate accumulation might be more favourable than malate accumulation under conditions of drought, when stomata partially or fully close during the dark period and internal carbon recycling and CAM idling become increasingly important, and also because drought is often associated with high-irradiance stress. However, apart from the observation that malate:citrate ratios during nocturnal acid accumulation under drought conditions may decrease, the other results on differential effects of environmental factors on malate and citrate accumulation, respectively, presented above do not show any obvious relations to the theoretically expected benefits from citrate accumulation.

It is astonishing that such a fascinating problem as the putative functional advantage of the metabolic plasticity of performing CAM with malate or citrate is not understood. Even information about the metabolic pathway involved in nocturnal citrate synthesis is scarce (Olivares et al., 1993), and we have no information on light-period citrate metabolism in Clusia, so that we are left with known general biochemistry of intermediate metabolism (see also Holtum et al., 2005), for example in the assessment of energy budgets, as mentioned above. Some classical biochemical studies would be very interesting, and rigorously systematic comparative studies varying environmental parameters may provide important new insights. However, current fashions in plant biology have a very different focus from motivating funding of the study of such an exciting problem in biochemical ecology.

V. The environmental control of photosynthetic flexibility

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

Environmental input is always received by the phenotypes, and in the case of the different modes of photosynthesis these are the photosynthetic physiotypes distinguished above (Fig. 1). There is then a possible feedback to the genotype which may respond with changes of the phenotype expressed (Lüttge, 2005). More straightforwardly, we can ask the questions of which environmental factors determine the reversible switches between C3 photosynthesis and CAM in Clusia species and how this works. As an example, we have already seen above that quite specifically the expression of PEPC may change during a C3 to CAM transition. The major external factors are water, irradiance and temperature (Lüttge, 2004). In the field they never act individually but always interactively. Even in experiments they are often hard to separate. However, individual factors may become limiting and we may try to consider them separately.

1. Water

Although the earliest environmental pressure for the evolution of CAM was probably the problem of CO2 acquisition with the advantage of the 60 times higher CO2 affinity of PEPC than Rubisco and the related function of CAM as a CO2-concentrating mechanism (Lüttge, 2002), it is evident that the driving force for the polyphyletic evolution of CAM in so many taxa of the vascular plants must have been limited availability of water.

In the C3/CAM-intermediate C. minor, CAM can be induced by drought. In an experiment where this was done it was shown subsequently that even the two opposite leaves on one node simultaneously could perform C3 photosynthesis and CAM, respectively. The two leaves were kept in gas exchange cuvettes at a low (6.2 mbar bar−1) and high (13.1 mbar bar−1) water vapour pressure deficit (VPD) of the air, respectively, i.e. subjecting the two leaves to different evaporative demand. When the plant was re-watered after 4 d of drought, the leaf in the dry air continued to perform CAM with dominating CO2 uptake and fixation in the dark, while the leaf in the moist air within a few hours switched to CO2 uptake in the light (Schmitt et al., 1988, where C. minor is wrongly called C. rosea).

While this illustrates the plasticity of a single plant, K. Winter and colleagues have tried to carry out a more general survey of Clusia species, assessing water use efficiency (WUE) by carbon isotope analysis (δ13C). WUE is defined as the ratio of moles of CO2 fixed and assimilated to moles of water lost by transpiration. WUE is high in nocturnal CO2 fixation during CAM because CO2 acquisition in the dark occurs at low evaporative demand. WUE is much lower in C3 photosynthesis in the light and also in CAM plants when they make use of their potential for direct CO2 fixation via Rubisco in the later light period (Fig. 1). As the 13C discrimination of PEPC is so much lower than that of Rubisco, analysis of δ13C values allows screening (see section II) of the extent to which plants use primary CO2 fixation via PEPC and Rubisco, respectively (Borland et al., 1993), and allows this to be related to WUE. Winter & Holtum (2002) and Winter et al. (2005) have shown in general that δ13C values in CAM species strongly correlate with WUE. These authors also screened 38 species of Clusia from Panamá and found that under natural conditions in the field Clusia species only sparingly make use of their PEPC option of CO2 fixation (Holtum et al., 2004) and, thus, the related high WUE. This underlines the importance of other factors in addition to water in the habitats of Clusia species.

2. Irradiance

High irradiance increases VPD and may cause over-energization of the photosynthetic light-harvesting apparatus. Thus, one would expect that it would support the use of the CAM option in plastic Clusia species. However, this is not always so. In well-watered C. minor an increase of the irradiance from 360 to 1200 mol m−2 s−1 suppressed CO2 dark fixation (Schmitt et al., 1988). In studies of the return from CAM to C3 photosynthesis in C. minor, de Mattos & Lüttge (2001) found that, provided water is not limiting and the plant can afford increased transpiration, daily photon use increases when there is unrestricted C3-like CO2 uptake, where the plant may overcome the limitations of the storage capacity of the vacuole for nocturnal organic acid accumulation, improving its daily carbon balance.

A comparative phytotron study of the obligate C3 species C. multiflora and the C3/CAM-intermediate species C. minor (Herzog et al., 1999) is interesting because it also sheds light on the behaviour of the species and their relative use of the CAM option in the field. Plants of the two species were grown at a low daily dose of irradiance (4 mol m−2 d−1) and then transferred to two higher daily doses of 24.5 and 33.5 mol m−2 d−1, respectively. Daily irradiance was applied in a bell-shaped pattern as in nature, i.e. increasing from dawn towards noon and decreasing again towards dusk. Figure 3 shows some results obtained 5 d after the transfer. We can see that the C3 species which had maximum rates of CO2 uptake (inline image) between 2 and 4 mol m−2 s−1 at 4 mol m−2 d−1 could make use of the higher irradiance of 24.5 mol m−2 d−1 for increasing inline image up to 6 mol m−2 s−1 but was strongly inhibited by the still higher irradiance of 33.5 mol m−2 d−1 showing only a inline image of < 1 mol m−2 s−1. Conversely, the C3/CAM-intermediate species which had performed C3 photosynthesis with maximum rates of inline image of c. 2–3 mol m−2 s−1 at 4 mol m−2 d−1 had switched to CAM under the higher daily doses of irradiance with high potential rates of inline image even in the transition phase between nocturnal CO2 uptake and daytime stomatal closure in the morning and during C3-like CO2 fixation in the later part of the day, with no difference between the two high daily doses. At 4 mol m−2 d−1 neither species was photoinhibited. However, under the higher irradiances they were similarly photoinhibited and increasingly so in the second part of the light period after irradiance was increased from 24.5 to 33.5 mol m−2 d−1 as given by the potential quantum yield of photosystem II (Fv/Fm, where Fv is the variable and Fm is the maximum fluorescence of dark-adapted leaves and a ratio below 0.83–0.80 indicates photoinhibition; Björkman & Demmig, 1987). Nonphotochemical quenching (NPQ) of chlorophyll (Chl) a fluorescence of photosystem II was also similar for the two species, as were the levels of the xanthophyll zeaxanthin, which is involved in harmless thermal energy dissipation under irradiance stress (Hager 1980; Demmig-Adams, 1990; Demmig-Adams & Adams, 1992; Horton et al., 1994; Pfündel & Bilger, 1994; Schindler & Lichtenthaler, 1996). With the putative protective function of the high internal CO2 concentrations built up during CAM in the light period (see section IV), one would have expected that Fv/Fm would have been higher and NPQ and zeaxanthin levels lower in the C3/CAM-intermediate species after its switch to CAM than in the C3 species. That this is not so shows that CAM species, like C3 species, are not exempt from photoinhibition and have protective mechanisms, such as thermal energy dissipation, to deal with irradiance and oxidative stress, as was also observed in the CAM species C. hilariana (Franco et al., 1999). Measurements by Winter et al. (1990) showed that in the early light period in CAM-performing C. rosea the level of zeaxanthin greatly increased from much lower levels maintained in the dark period, decreased in the middle of the day when stomata closed and organic acid was remobilized, and subsequently increased again as stomata opened in the later light period. The levels of violaxanthin, the double epoxide of zeaxanthin, showed the opposite pattern during the day. With respect to the pair of Clusia species, C. multiflora and C. minor, compared by Herzog et al. (1999), the long-term comportment after transfer to higher irradiance is important. In the C3 species, under higher daily irradiance leaves became necrotic and died. However, the plant was not principally suffering from the high irradiance. It only suffered from the sudden change from low irradiance during growth to higher irradiances, probably because acclimation of the light-harvesting apparatus takes a long time or is totally insufficient. However, C. multiflora then produced new leaves from dormant buds which were adapted to the new situation. Conversely, the C3/CAM-intermediate species C. minor was not damaged after the transfer to higher irradiance. It was probably the much more rapid switch of the biochemical machinery to CAM combined with protective mechanisms which saved it from destruction of its leaves. In fact, separate experiments showed that the capacity to produce zeaxanthin was even higher in C. minor (c. 130 mol m−2 leaf area) than in C. multiflora (c. 60 mol m−2) but it did not make use of this higher capacity, at least up to 5 d after transfer (Fig. 3).

image

Figure 3. Photosynthetic performance of Clusia multiflora H.B.K. (solid lines) and Clusia minor L. (broken lines) grown at a daily irradiance of 4 mol m−2 d−1 5 d after transfer to high irradiance of 24.5 and 33.5 mol m−2 d−1, respectively, in a phytotron where irradiance was increased up to midday and then decreased again in a bell-shaped pattern (not shown). inline image, CO2 exchange; Fv/Fm, potential quantum yield of chlorophyll a of photosystem II; NPQ, nonphotochemical fluorescence quenching; Z, levels of zeaxanthin, where data are only available for 24.5 mol m−2 d−1. Dark bars, night-time; white bars, daytime. (After data of Herzog et al., 1999.)

Download figure to PowerPoint

These phytotron experiments suggest that the C3/CAM-intermediate species may have a larger niche width, performing well and with high short-term flexibility both under full sun exposure and in more shaded situations, while the C3 species performs well under high irradiance only when it has long-term development under full exposure, for example starting with seedling growth. Indeed, in the field it has been observed that C. multiflora occupies exposed sites in a secondary savanna while C. minor prefers semi-shaded sites in a deciduous dry forest but can also invade the exposed sites of C. multiflora (Franco et al., 1994; Grams et al., 1997).

3. Temperature

It is widely assumed that lower night temperatures and higher day temperatures are favourable for the performance of CAM, based on the observation of effects of temperature on the overall performance of the counteracting enzymes of nocturnal carboxylation (PEPC) and daytime decarboxylation of CAM, where lower temperatures favour the former and higher temperatures the latter (Brandon, 1967; Kluge et al., 1973; Neales, 1973; Medina et al., 1977; Kluge & Ting, 1978; Buchanan-Bollig & Kluge, 1981; Buchanan-Bollig et al., 1984; Nobel, 1988; Fetene & Lüttge, 1991; Carter et al., 1995). However, in the tropics many CAM species perform well under rather high and similar night and day temperatures.

Studies on species of Clusia show that temperature effects are often closely related to effects of irradiance (Haag-Kerwer et al., 1992). We have already seen in section IV that, depending on irradiance during growth, day:night temperature regimes may strongly determine the extent to which either malate or citrate or both are used in the CAM cycle. In C. minor it was found that the amplitude of the day:night (D:N) temperature difference is important in eliciting a C3 to CAM switch and this is modulated by irradiance during growth. At low irradiance, plants performed C3 photosynthesis when D:N temperatures were 30 : 30, 30 : 25 and 30 : 20°C and switched to CAM at 30 : 15°C. A smaller D:N temperature difference was required for the switch in high-irradiance-grown plants which performed C3 photosynthesis at 30 : 30°C but already switched to CAM at 30 : 25°C. At 15 : 15°C gas exchange was reduced under both irradiances, but high-irradiance-grown plants performed CAM and low-irradiance-grown plants C3 photosynthesis. The changes in the mode of photosynthesis were always readily reversible when temperature regimes were changed again.

VI. Phenotypic plasticity: physiotypes and morphotypes

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

Clusia species are woody shrubs and trees with a dichasial cyme. In an earlier review (Lüttge, 1999) I proposed considering all species of Clusia to belong to a single morphotype. Saying this, I referred to the vegetative photosynthetic organs, the leaves, which, although they may have greatly differing sizes in different species, are all somewhat leathery and succulent. Diversity is revealed when we consider the physiotypes with high plasticity of modes of photosynthesis and metabolic pathways during CAM performance, as described in sections III and IV. At the morphotypic level diversity is very high in the generative organs (Engler, 1925; Bittrich & Amaral, 1996; Gustafsson & Bittrich, 2003; Gustafsson et al., 2006). Flower size may range in diameter from 5 to 150 mm. There is substantial diversity in floral morphology with respect to size, degree of fusion and number of floral parts, position of staminodes and stamen and anther morphology. Pollination rewards are nectar, pollen and resin. Floral resin is a rare award. It has evolved polyphyletically in the genus of Clusia (Gustafsson et al., 2006). Diversity at the morphological level is also revealed by the different life forms of Clusia species. They may germinate terrestrially and directly develop into independent free-standing trees. They may also germinate epiphytically, predominantly in humus niches between branches of host trees or of other epiphytes such as epiphyte nests or even the tanks of bromeliads. Then they can establish soil contact via aerial roots, i.e. they are hemi-epiphytes. Other adventitious roots may strangle the bark of the host tree and thus murder the phorophyte, the stem of which may then rot away under a network of strangler roots so that the previous hemi-epiphyte eventually becomes a free-standing tree (Lüttge, 1991). This diversity does not only occur among species, because even individual species can express many of these life forms, so that we have here another example of plasticity in the genus (Fig. 4).

image

Figure 4. Life forms of Clusia. Top row (left–right): terrestrial seedlings of Clusia multiflora; terrestrial seedlings of Clusia minor; seedling of Clusia sp. under the bark of a fallen tree. Centre row (left–right): Clusia rosea in an epiphyte nest; roots of C. rosea in the tank of the bromeliad Aechmea lingulata, where the tank was cut open for photography; C. rosea growing epiphytically in a tree branch. Bottom row (left–right): two pictures of C. rosea as a strangler and C. rosea as a free-standing tree.

Download figure to PowerPoint

In contrast to other hemi-epiphytes, mainly in the genus Ficus, several morphological/anatomical traits, such as leaf thickness, and stomatal density and size, did not differ much between the epiphytic and rooted tree life form of C. minor (Holbrook & Putz, 1996). Often the hydraulic architecture must be adapted in hemi-epiphytes as compared with rooted trees for sufficient water supply. In a comparative study of the hemi-epiphytic C3/CAM-intermediate Clusia uvitana Pittier with other hemi-epiphytes, mainly of the genus Ficus (Zotz & Winter, 1994a,b; Patiño et al., 1995; Zotz et al., 1997), it turned out that C. uvitana had less effective unit leaf area, a rather large leaf area per unit of stem cross-sectional area and a lower specific stem conductance. This suggests that physiotypic flexibility with the water-saving CAM option also relates to a morphotypic trait such as hydraulic architecture.

VII. Ecological amplitude and habitat impact

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

Physiological ecology has always largely been autecology. We have noticed, however, that it is increasingly developing the potential to make contributions to synecology (Lüttge & Scarano, 2004). This is in the cast of mind of Sir Arthur Tansley, whose name is heading the series of reviews to which the current contribution belongs, because quite early ‘He made it clear that synecology and autecology are subsumed in the study of the community by methods firstly descriptive and appreciative, and secondly, analytic and experimental’ (Godwin, 1977). Thus, in this vein, in this section I shall describe the community performance of Clusia. However, one may bear in mind here that laboratory studies of autecology mostly cover the dynamics of short-term acclimations, while field observations represent momentary pictures of long-term adaptations. Long-term laboratory observations are as rare as recordings of gradual responses under gradually varying, for example seasonally, natural conditions. The continuous long-term studies in the field which I allude to below are quite rare.

With the great plasticity found in the genus Clusia we may expect that it will span a large ecological amplitude. Indeed, it is observed that the genus, with its single simple vegetative morphotype, occupies a wide range of habitats in the neotropics, as listed in Table 2 and illustrated in Fig. 5. Not only the genus as a whole but also some individual species, for example the C3 species C. multiflora, the CAM species C. rosea and the C3/CAM-intermediate species C. minor, cover almost the entire range of different habitats listed in Table 2, and some species occur in rather contrasting habitats, such as the coastal restingas of Brazil and inselbergs (the C3 species Clusia parviflora Saldanha et Engl.) and the restingas and dry lowland forest (the CAM species Clusia fluminensis Pl. et Tr.). In the following I shall list a number of sites occupied by Clusia species and try to evaluate the environmental impact of the genus. I shall do this by comparing the ecophysiological performance of Clusia species with those of other co-occurring plants with similar life forms, i.e. shrubs and trees. Much field work is in fact reported in the literature and an ecophysiological comparison can be based on traits such as carbon isotope ratios, rates of CO2 exchange, Chl a fluorescence parameters of photosytem II and day:night changes of organic acid levels in CAM-performing species. Space does not allow me to go into as much detail as elsewhere (Lüttge, 1999, 2006b), but the advantage of a briefer treatment is that it provides a more concise overview.

Table 2.  Ecological amplitude of Clusia with the range of habitats where Clusia is found, and occupation of a large number of these habitats by three selected species with obligate C3 photosynthesis, obligate crassulacean acid metabolism (CAM) and C3/CAM-intermediate behaviour
HabitatC. muliflora (C3)C. rosea (CAM)C. minor (C3/CAM)
Restinga
Coastal rocks  
Savanna/cerrado 
Gallery forest – cerrado ecotone
Open shrubland  
Dry lowland forest 
Secondary shrub forest 
Dry montane karstic limestone forest
Montane rainforest
Upper montane rainforest  
Cloud forest/fog forest/elfin forest  
Inselberg
image

Figure 5. Ecological amplitude of Clusia. Top row (left–right): coastal restinga in Brazil with shrubs of Clusia hilariana in the centre; free-standing tree of C. hilariana in a semidecidous dry forest in Brazil. Second row: Clusia criuva on the rock outcrops of an inselberg in Brazil; C. criuva at the ecotone of a gallery forest with the cerrado savanna in Brazil. Third row: four sympatric Clusia species on karstic limestone mountains in northern Venezuela (Clusia multiflora, Clusia rosea, Clusia alata and Clusia sp.) and roots of Clusia in the crevasses of the limestone rocks. Fourth row: Unidentified Clusia species in a tropical cloud forest, Sierra Maigualida, southern Venezuela; C. multiflora c. 60 cm tall in an elfin forest, Cerro Santa Ana, Paraguana peninsula, Venezuela.

Download figure to PowerPoint

1. The Brazilian restinga complex

The restinga complex consists of a mosaic of plant communities on the sandy coastal plains ranging from open formations to forest ecosystems (Lacerda et al., 1993). The open formation is characterized by shrub islands surrounded by white sand. There are sand dune ridges with dry forest in between and on fixed dunes (Cirne & Scarano, 2001). Species of Clusia are important floristic elements of the restingas, which also have been called ‘Clusia scrub’ (Ule, 1901). According to annual rainfall, height of the ground water table and salinity of the ground water, we may distinguish wet, intermediate and dry restingas. The major Clusia species of the restingas are the C3 species C. parviflora, and the CAM species C. hilariana and C. fluminensis, the former of which is much more dominant than the latter.

In the wet restinga of Jacarepiá (22°47′ to 22°57′ S, 42°20′ to 42°43′ W), C. fluminensis has been compared with the C3 Fabaceae species Andira legalis Vell. Toledo (Geßler et al., 2005a; Scarano et al., 2005). C. fluminensis developed somewhat larger maximum apparent electron transport rates of photosynthesis (ETRmax) and with respect to photoinhibition the two species were very similarly affected at midday and slightly before dawn. In the intermediate restinga of Jurubatiba (22°00′ to 22°23′ S, 41°15′ to 41°45′ W), C. parviflora and C. hilariana were compared with the C3 species A. legalis, Rheedia brasiliensis (Mart.) Pl. et Tr. (another Clusiaceae) and Myrsine parvifolia A. DC. (Myrsinaceae) (Franco et al., 1996; Duarte et al., 2005; Scarano et al., 2005). The dominant C. hilariana (CAM) showed superior photosynthetic capacity (ETRmax) as compared with the other four species, and the C3 species C. parviflora was similar to the other C3 species. Chronic photoinhibition not reversible overnight was absent or low in all species, but an advantage of CAM in avoiding acute photoinhibition at midday was not seen. In the intermediate restinga of Barra de Maricá (22°53′ S, 42°52′ W) C. lanceolata and C. fluminensis were studied (Roberts et al., 1996; Reinert et al., 1997). Both are potential CAM species but δ13C values indicated that C. lanceolata was mostly fixing CO2 by the C3 mode, while C. fluminensis was shifted more towards CAM. Chronic photoinhibition was absent and ETRmax and acute photoinhibition were similar in the two species so that the relative performance of the two modes of photosynthesis did not make much difference. In the dry restinga of Massambaba (22°56′ S, 42°13′ W), C. fluminensis mainly performing CAM and for comparison A. legalis, R. brasiliensis and M. parviflora were investigated (Duarte et al., 2005; Geßler et al., 2005a; Scarano et al., 2005). All species showed similar photosynthetic capacity and acute photoinhibition and there was no obvious advantage of CAM for C. fluminensis. In the dry forest of Buziós (22°49′ S, 41°59′ W), C. fluminensis was compared with three Fabaceae trees, Caesalpinia echinata Lam., Caesalpinia ferrea Mart. ex Tul. and Machaerium obovatum Kuhlm. et Hoehne, and the Euphorbiaceae Croton compressus Lam. (Geßler et al., 2005b; Scarano et al., 2005). C. fluminensis achieved somewhat higher ETRmax than C. compressus and higher rates than the other species. In terms of acute and chronic photoinhibition it was not better off than the two Caesalpinia species or C. compressus but was superior to M. obovatum. C. fluminensis in the very dry forest must be limited by the moisture regime (Scarano et al., 2005) and again there appeared to be no clear advantage of CAM at the dry site. ETRmax measurements (mol m−2 s−1) indicated that C. fluminensis showed a strong dependence on a moisture gradient from wet restinga (290) to dry restinga (160) and dry forest (100).

Overall, the potential of Clusia species for CAM performance may have some subtle benefits but does not appear to provide a very obvious conspicuous advantage as compared with C3 species of similar life form. I have alluded to the fact that Clusia may dominate the restingas in places. Moreover, several studies have shown that Clusia species, especially C. hilariana, can have important pioneer functions in the restingas, starting vegetation islands on the bare sand plain and functioning as nurse trees under the canopy of which other vegetation may become established (Liebig et al., 2001; Dias et al., 2005). In ecology as well as in evolution (Gould, 2002), subtle differences can become decisive. However, functions other than photosynthetic capacity discussed here could be essential, for example reproductive biology. However, in the restingas, sexual reproduction of Clusia species is absent or very rare and we must assume that all growth is clonal.

2. Northern coastal mountain ranges in Venezuela

In the northern coastal range of Venezuela near Caracas (10°24′ N, 66°58′ W, 1500–1740 m asl) the same two Clusia species, C. multiflora and C. minor, investigated in a phytotron study in relation to the irradiance factor (section V.2) and physiotypic plasticity described in section III were measured in the field (Franco et al., 1994; Grams et al., 1997). In comparison to the behaviour discussed in section V.2 the field performance was similar (Fig. 6). The C3 species C. multiflora potentially could achieve higher maximum rates of CO2 uptake, inline image-max, in the light. Notwithstanding the higher inline image-max in C. multiflora at higher irradiance, there was a pronounced midday depression. Chronic photoinhibition at dawn and acute photoinhibition at midday (Fv/Fm values < 0.8) were similar in the two species. Nocturnal CO2 uptake in C. minor when performing CAM increased from low to medium irradiance and then decreased again at the highest irradiance, and this was reflected in the total integrated CO2 uptake for the dark periods and night:day oscillations of organic acids (not shown). Integrated CO2 uptake over 24 h was much higher under high than under low irradiance in C. multiflora, and in C. minor at the highest irradiance day-time CO2 uptake was also somewhat higher and night-time CO2 uptake was reduced as compared with the shaded plants at the lowest irradiance (Franco et al., 1994; Grams et al., 1997). C. minor when performing CAM always had much higher WUE than C. multiflora.

image

Figure 6. Photosynthetic performance of Clusia multiflora H.B.K and Clusia minor L. in the field with different daily irradiances as indicated. inline image-max, maximum rates of CO2 uptake (positive values) and CO2 release (negative values) observed in the light period (open bars) and in the dark period (closed bars); inline image-integr, net CO2 uptake integrated over 24 h; WUE, water use efficiency integrated over 24 h; Fv/Fm, potential quantum yield of chlorophyll a of photosytem II, predawn (dark bars) and at midday (open bars). (After data of Franco et al., 1994; Grams et al., 1997; Lüttge, 1999.)

Download figure to PowerPoint

In the Sierra de San Luis in the state Falcón (11°18′ N, 69°45′ W, 1200 m asl) on a karstic limestone ridge the performance of the C3 species C. multiflora was compared with that of three CAM species, C. rosea, C. alata and an unidentified species (Popp et al., 1987; Franco et al., 1994; Haag-Kerwer et al., 1996). The CAM species made considerable use of their CAM option at this site. Photosynthetic capacity was similar in all four species. However, ETRmax values were higher in the CAM species as a result of the high internal CO2 concentrations built up during acid remobilization in the light period. All four species showed slight chronic and pronounced acute photoinhibition at dawn and midday, respectively.

3. Semi-evergreen moist tropical forest, Barro Colorado Island, Panama

In this forest (9°10′ N, 79°51′ W) the hemi-epiphytic C. uvitana was compared with other hemi-epiphytes by Zotz & Winter (1994a,b). These authors made an important contribution to the understanding of a possible advantage of the CAM option in C3/CAM-intermediate species. The advantage of the CAM option in structuring hydraulic architecture in hemi-epiphytes has been mentioned in section VI. However, the two authors also covered an entire 12-month period with numerous measurements, one of the rare studies meeting the need for continuous long-term studies in the field, and thus revealing an important seasonal effect. In the wet season, with respect to both CO2 uptake integrated over 24 h and maximum rates of inline image, the performance of C. uvitana was close to that of a C3 fern Polypodium crassifolium L. and inferior to the that of the C3 orchid Catasetum viridiflavum Hook. However, in the dry season, when C. uvitana switched to CAM it was superior to the other two species.

4. Tropical forest on the island of Trinidad

A similar advantage of the CAM option in response to fluctuations of habitat conditions in wet and dry seasons was also shown for Clusia species in Trinidad: C. minor, C. tocuchensis, C. aripoensis and C. intertexta. This is another example of rather continuous long-term studies, where especially for C. minor the advantage of C3/CAM switches for performance over the seasons was demonstrated (Borland et al., 1992, 1993, 1994, 1996; Roberts et al., 1998).

5. Inselberg, Pão de Açúcar, Rio de Janeiro

On this inselberg (22°57′ S, 43°59′ W) the C3 species C. parviflora and the two Euphorbiaceae C. compressus and Styllingia dichotoma Muell. Arg. form bushes and shrubbery. The performances of all three species are quite similar (Duarte et al., 2005; Scarano et al., 2005).

VIII. Conclusions and outlook

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References

Clusia, the only dicotyledonous tree genus with CAM, presents all facets of CAM and with its extraordinary plasticity it allows the study of CAM in all its ramifications. It suggests that not so much CAM per se but the high flexibility in carbon acquisition under variable environmental conditions in the tropics inherent in CAM (Fig. 1) is its real ecophysiological advantage. Plasticity with segregation and separation of ecotypes based on the high ecological amplitude might relate to the high speciation rate in the genus (Lüttge, 1995a,b, 2005). The radiative power of the genus is also great. This can be seen in the Atlantic rain forest complex of Brazil, where the Atlantic forest itself on tertiary ground is surrounded by a variety of plant communities (Fig. 7), some of which are also older relicts, such as the dry forest, but some of which are quite young, such as the restingas and swamp forests on tertiary ground. The dioecious species C. hilariana in these communities is a migrant from the Atlantic forest with missing or extremely limited generative propagation in the restingas because its specific pollinators from the Euglossini tribe, for example the bee Trigona spinipes, did not follow it to the new habitat (Scarano et al., 2004).

image

Figure 7. The Atlantic rain forest (AF) of Brazil with its peripheral ecosystems of dry forest (DF), high-altitude rock outcrops (HA), inselbergs (I), restinga (R) and swamp forest (SF), all occurring within a distance of only 60–80 km from the sea (S). (Modified from Scarano, 2002.)

Download figure to PowerPoint

My monographic review on the genus Clusia may have shown that there is a great potential for future study of this single diverse genus spanning all scaling levels from molecular genetics to ecosystems.

  • 1
    While the molecular phylogeny of Clusia has advanced, its scope and data base need to be extended and the molecular approach must also be used in population genetics.
  • 2
    Studies of metabolism should evaluate the organization and management of metabolic reaction modules in the flexibility of photosynthetic physiotypes. Metabolomics will be a useful approach, but some classical biochemistry is also required, for instance regarding the plasticity in carbohydrate metabolism and the citric acid enigma.
  • 3
    More rigorously comparative quantitative laboratory and field studies, such as the comparison of C. multiflora and C. minor in the phytotron and in the field discussed in this review, should be performed to evaluate ecophysiological capacities and potential environmental impact.
  • 4
    In addition to assessment of ecophysiology, reproductive biology should be advanced to foster understanding of the performance of Clusia at the community level.

References

  1. Top of page
  2. Abstract
  3. I. The discovery of crassulacean acid metabolism (CAM) in the trees of Clusia: arrival in the limelight of international research
  4. II. Phylogeny
  5. III. Photosynthetic physiotypes
  6. IV. Metabolic flexibility: organic acid variations
  7. V. The environmental control of photosynthetic flexibility
  8. VI. Phenotypic plasticity: physiotypes and morphotypes
  9. VII. Ecological amplitude and habitat impact
  10. VIII. Conclusions and outlook
  11. Acknowledgements
  12. References
  • Ball E, Hann J, Kluge M, Lee HSJ, Lüttge U, Orthen B, Popp M, Schmitt A, Ting IP. 1991. Ecophysiological comportment of the tropical CAM-tree Clusia in the field. II. Modes of photosynthesis in trees and seedlings. New Phytologist 117: 483491.
  • Berg A, Orthen B, De Mattos EA, Duarte HM, Lüttge U. 2004. Expression of crassulacean acid metabolism in Clusia hilariana Schlechtendal in different stages of development in the field. Trees 18: 553558.
  • Bittrich V, Amaral MCE. 1996. Flower morphology and pollination biology of some Clusia species from the Gran Sabana (Venezuela). Kew Bulletin 51: 681694.
  • Björkman O, Demmig B. 1987. Photon yield of O2 evolution and chlorophyll fluorescence characteristics at 77 K among vascular plants of diverse origins. Planta 170: 489504.
  • Black CC, Chen J-Q, Doong RL, Angelov MN, Sung SJS. 1996. Alternative carbohydrate reserves used in the daily cycle of crassulacean acid metabolism. In: WinterK, SmithJAC, eds. Crassulacean acid metabolism: biochemistry, ecophysiology and evolution. Ecological Studies, Vol. 114. Berlin, Heidelberg, New York: Springer, 3145.
  • Boiteau P, Allorge-Boiteau L. 1995. Kalanchoë (Crassulacées) de Madagascar: systématique, écophysiologie et phytochimie. Paris, France: Editions Karthala.
  • Borland AM, Griffiths H, Broadmeadow MSJ, Fordham MC, Maxwell C. 1993. Short-term changes in carbon-isotope discrimination in the C3-CAM intermediate Clusia minor L. growing in Trinidad. Oecologia 95: 444453.
  • Borland AM, Griffiths H, Broadmeadow MSJ, Fordham MC, Maxwell C. 1994. Carbon-isotope composition of biochemical fractions and the regulation of carbon balance in leaves of the C3-crassulacean acid metabolism intermediate Clusia minor L. growing in Trinidad. Plant Physiology 106: 493501.
  • Borland AM, Griffiths H, Maxwell C, Broadmeadow MSJ, Griffiths MN, Barnes JD. 1992. On the ecophysiology of the Clusiaceae in Trinidad: expression of CAM in Clusia minor L. during the transition from wet to dry season and characterization of three endemic species. New Phytologist 122: 349357.
  • Borland AM, Griffiths H, Maxwell C, Fordham MC, Broadmeadow MSJ. 1996. CAM induction in Clusia minor L. during the transition from wet to dry season in Trinidad: the role of organic acid speciation and decarboxylation. Plant, Cell & Environment 19: 655664.
  • Borland AM, Laszlo IT, Leegood RC, Walker RP. 1998. Inducibility of crassulacean acid metabolism (CAM) in Clusia species; physiological/biochemical characterization and intercellular localization of carboxylation and decarboxylation processes in three species which exhibit different degrees of CAM. Planta 205: 342351.
  • Brandon PC. 1967. Temperature features of enzymes affecting crassulacean acid metabolism. Plant Physiology 42: 977984.
  • Brulfert J, Guerrier D, Queiroz O. 1973. Photoperiodism and enzyme activity: Balance between inhibition and induction of the crassulacean acid metabolism. Plant Physiology 51: 220222.
  • Brulfert J, Guerrier D, Queiroz O. 1975. Photoperiodism and enzyme rhythms: kinetic characteristics of the photoperiodic induction of crassulacean acid metabolism. Planta 125: 3344.
  • Buchanan-Bollig IC, Kluge M. 1981. Crassulacean acid metabolism (CAM) in Kalanchoë daigremontiana: temperature response of phosphoenolpyruvate (PEP)-carboxylase in relation to allosteric effectors. Planta 152: 181188.
  • Buchanan-Bollig IC, Kluge M, Müller D. 1984. Kinetic changes with temperature of phosphoenolpyruvate carboxylase from a CAM plant. Plant, Cell & Environment 7: 6370.
  • Carter PJ, Wilkins MB, Nimmo HG, Fewson CA. 1995. Effects of temperature on the activity of phosphoenolpyruvate carboxylase and on the control of CO2 fixation in Bryophyllum fedtschenkoi. Planta 196: 375380.
  • Cirne P, Scarano FR. 2001. Resprouting and growth dynamics after fire of the clonal shrub Andira legalis (Leguminosae) in a sandy coastal plain in southeastern Brazil. Journal of Ecology 89: 351357.
  • Crayn DM, Winter K, Smith JAC. 2004. Multiple origins of crassulacean acid metabolism and the epiphytic habit in the neotropical family Bromeliaceae. Proceedings of the National Academy of Sciences, USA 101: 37033708.
  • Demmig-Adams B. 1990. Carotenoids and photoprotection: a role for the xanthophyll zeaxanthin cycle. Biochimica et Biophysica Acta 1020: 124.
  • Demmig-Adams B, Adams WW. 1992. Photoprotection and other responses of plants to high light stress. Annual Reviews of Plant Physiology and Plant Molecular Biology 43: 599626.
  • Dias ATC, Zaluar HLT, Ganade G, Scarano FR. 2005. Canopy composition influencing plant patch dynamics in a Brazilian sandy coastal plain. Journal of Tropical Ecology 21: 343347.
  • Duarte HM, Geßler A, Scarano FR, Franco AC, De Mattos EA, Nahm M, Rennenberg H, Rodrigues PJFP, Zaluar HTL, Lüttge U. 2005. Ecophysiology of six selected shrub species in different plant communities at the periphery of the Atlantic Forest of SE Brazil. Flora 200: 456476.
  • Ellenberg H. 1981. Ursachen des Vorkommens und Fehlens von Sukkulenten in den Trockengebieten der Erde. Flora 171: 114169.
  • Eller BM, Ferrari S, Ruess BR. 1992. Spatial and diel variations of water relations in leaves of the CAM-plant Senecio medley-woodii. Botanica Helvetica 102: 193200.
  • Eller BM, Ruess BR. 1986. Modulation of CAM and water balance of Senecio medley-woodii by environmental factors and age of leaf. Journal of Plant Physiology 125: 295309.
  • Engelmann S, Bläsing OE, Gowik U, Svensson P, Westhoff P. 2003. Molecular evolution of C4 phosphoenolpyruvate carboxylase in the genus Flaveria– a gradual increase from C3 to C4 characteristics. Planta 217: 717725.
  • Engler A. 1925. Guttiferae. In: EnglerA, PrantlK, eds. Die natürlichen Pflanzenfamilien, 2nd edn, Vol. 21A. Leipzig, Germany: Wilhelm Engelmann, 154237.
  • Fetene M, Lüttge U. 1991. Environmental influences on carbon recycling in a terrestrial CAM bromeliad, Bromelia humilis Jacq. Journal of Experimental Botany 42: 2531.
  • Franco AC, Ball E, Lüttge U. 1990. Patterns of gas exchange and organic acid oscillations in tropical trees of the genus Clusia. Oecologia 85: 108114.
  • Franco AC, Ball E, Lüttge U. 1991. The influence of nitrogen, light and water stress on CO2 exchange and organic acid accumulation in the tropical C3-CAM tree, Clusia minor. Journal of Experimental Botany 42: 597603.
  • Franco AC, Ball E, Lüttge U. 1992. Differential effects of drought and light levels on accumulation of citric and malic acids during CAM in Clusia. Plant, Cell & Environment 15: 821829.
  • Franco AC, Haag-Kerwer A, Herzog B, Grams TEE, Ball E, De Mattos EA, Scarano FR, Barreto S, Garcia MA, Mantovani A, Lüttge U. 1996. The effect of light levels on daily patterns of chlorophyll fluorescence and organic acid accumulation in the tropical tree Clusia hilariana. Trees 10: 359365.
  • Franco AC, Herzog B, Hübner C, De Mattos EA, Scarano FR, Ball E, Lüttge U. 1999. Diurnal changes in chlorophyll a fluorescence, CO2-exchange and organic acid decarboxylation in the tropical tree Clusia hilariana. Tree Physiology 19: 635644.
  • Franco AC, Olivares E, Ball E, Lüttge U, Haag-Kerwer A. 1994. In situ studies of crassulacean acid metabolism in several sympatric species of tropical trees of the genus Clusia. New Phytologist 126: 203211.
  • Gehrig HH, Aranda J, Cushman MA, Virgo A, Cushman JC, Hammel BE, Winter K. 2003. Cladogram of Panamanian Clusia based on nuclear DNA: implications for the origins of crassulacean acid metabolism. Plant Biology 5: 5970.
  • Gehrig H, Gaussmann O, Marx H, Schwarzott D, Kluge M. 2001. Molecular phylogeny of the genus Kalanchoë (Crassulaceae) inferred from nucleotide sequences of the IST-1 and IST-2 regions. Plant Sciences 160: 827835.
  • Gehrig H, Brulfert J, Kluge M. 2000. In: LourencoWR, GoodmanSM, eds. Diversity and endemism in Madagascar. Paris, France: Mémoires de la Société de Biogéographie, 7582.
  • Gehrig HH, Rösicke H, Kluge M. 1997. Detection of DNA polymorphisms in the genus Kalanchoë by RAPD-PCR fingerprint and its relationships to infrageneric taxonomic position and ecophysiological photosynthetic behaviour of the species. Plant Science 125: 4151.
  • Gehrig HH, Wood JA, Cushman MA, Virgo A, Cushman JC, Winter K. 2005. Large gene family of phosphoenolpyruvate carboxylase in the crassulacean acid metabolism plant Kalanchoë pinnata (Crassulaceae) characterised by partial cDNA sequence analysis. Functional Plant Biology 32: 467472.
  • Geßler A, Duarte HM, Franco AC, Lüttge U, De Mattos EA, Nahm M, Rodrigues PJFP, Scarano FR, Rennenberg H. 2005b. Ecophysiology of selected tree species in different plant communities at the periphery of the Atlantic Forest of SE Brazil. III. Three legume trees in a semi-deciduous dry forest. Trees 19: 523530.
  • Geßler A, Duarte HM, Franco AC, Lüttge U, De Mattos EA, Nahm M, Scarano FR, Zaluar HLT, Rennenberg H. 2005a. Ecophysiology of selected tree species in different plant communities at the periphery of the Atlantic Forest of SE Brazil. II. Spatial and ontogenetic dynamics in Andira legalis, a deciduous legume tree. Trees 19: 510522.
  • Godwin H. 1977. Sir Arthur Tansley: the man and the subject. Journal of Ecology 65: 126.
  • Gould SJ. 2002. The structure of evolutionary theory. Cambridge, MA, USA: Harvard University Press.
  • Grams TEE, Haag-Kerwer A, Olivares E, Ball E, Arndt S, Popp M, Medina E, Lüttge U. 1997. Comparative measurements of chlorophyll a fluorescence, acid accumulation and gas exchange in exposed and shaded plants of Clusia minor L. and Clusia multiflora H.B.K. in the field. Trees 11: 240247.
  • Guralnick LJ, Jackson MD. 2001. The occurrence and phylogenetics of Crassulacean acid metabolism in the Portulacaceae. International Journal of Plant Science 162: 257262.
  • Guralnick LJ, Ting IP, Lord EM. 1986. Crassulacean acid metabolism in the Gesneriaceae. American Journal of Botany 53: 336345.
  • Gustafsson MHG, Bittrich V. 2003. Evolution of morphological diversity and resin secretion in flowers of Clusia L. (Clusiaceae): insights from ITS sequence variation. Nordic Journal of Botany 22: 183203.
  • Gustafsson MHG, Bittrich V, Stevens PF. 2002. Phylogeny of Clusiaceae based on rbcL sequences. International Journal of Plant Science 163: 10451054.
  • Gustafsson MHG, Winter K, Bittrich V. 2006. Diversity, phylogeny and classification of Clusia. In: LüttgeU, ed. Clusia: a woody neotropical genus of remarkable plasticity and diversity. Ecological Studies. Berlin, Heidelberg, New York: Springer. (In press.)
  • Haag-Kerwer A, Franco AC, Lüttge U. 1992. The effect of temperature and light on gas exchange and acid accumulation in the C3-CAM plant Clusia minor L. Journal of Experimental Botany 43: 345352.
  • Haag-Kerwer A, Grams TEE, Olivares E, Ball E, Arndt S, Popp M, Medina E, Lüttge U. 1996. Comparative measurements of gas exchange, acid accumulation and chlorophyll a fluorescence of different species of Clusia showing C3 photosynthesis, or crassulacean acid metabolism, at the same field site in Venezuela. New Phytologist 134: 215226.
  • Hager A. 1980. The reversible, light-induced conversions of xanthophylls in the chloroplast. In: CzyganFC, ed. Pigments in plants. Stuttgart, Germany: G. Fischer, 5779.
  • Hartenburg W. 1937. Der Wasser- und Kohlensäurehaushalt tropischer Regenwaldpflanzen in sommerlicher Gewächshauskultur. Jahrbücher Wissenschaftlicher Botanik 85: 641697.
  • Herzog B, Hoffmann S, Hartung W, Lüttge U. 1999. Comparison of photosynthetic responses of the sympatric tropical C3 species Clusia multiflora H.B.K. and the C3-CAM intermediate species Clusia minor L. to irradiance and drought stress in a phytotron. Plant Biology 1: 460470.
  • Holbrook NM, Putz FE. 1996. From epiphyte to tree: differences in leaf structure and leaf water relations associated with the transition in growth form in eight species of hemiepiphytes. Plant, Cell & Environment 19: 631642.
  • Holtum JAM, Aranda J, Virgo A, Gehrig HH, Winter K. 2004. δ13C values and crassulacean acid metabolism in Clusia species from Panama. Trees 18: 658668.
  • Holtum JAM, Smith JAC, Neuhaus HE. 2005. Intracellular transport and pathways of carbon flow in plants with crassulacean acid metabolism. Functional Plant Biology 32: 429449.
  • Horton P, Ruban AV, Walters RG. 1994. Regulation of light harvesting in green plants. Indication by nonphotochemical quenching of chlorophyll fluorescence. Plant Physiology 106: 415420.
  • Kluge M. 2005. Options of photosynthesis in the genus Kalanchoë: an approach integrating the levels of biochemistry, ecophysiology, molecular taxonomy, and phytogeographic distribution of the species. Nova Acta Leopoldina Neue Folge 92, number 342: 195205.
  • Kluge M, Brulfert J, Ravelomanana D, Ziegler H. 1991. Crassulacean acid metabolism in Kalanchoë species collected in various climatic zones of Madagascar: a survey by δ13C analysis. Oecologia 88: 407414.
  • Kluge M, Lange OL, Eichmann M, Schmid R. 1973. Diurnaler Säurerhythmus bei Tillandsia usneoides: Untersuchungen über den Weg des Kohlenstoffs sowie die Abhängigkeit des CO2-Gaswechsels von Lichtintensität, Temperatur und Wassergehalt der Pflanze. Planta 112: 357372.
  • Kluge M, Ting IP. 1978. Crassulacean acid metabolism. Analysis of an ecological adaptation. Ecological Studies, Vol. 30. Berlin, Heidelberg, New York: Springer.
  • Krätz O. 2001. Alexander von Humboldt (1769–1859) auf Pflanzenjagd. In: Palmengarten, Heft 35, Palmengarten, Frankfurt M, 3346.
  • Lacerda LD, Araujo DSD, Maciel NC. 1993. Dry coastal ecosystems of the tropical Brazilian coast. In: Van der MaarelE, ed. Dry coastal ecosystems: Africa, America, Asia and Oceania. Amsterdam, the Netherlands: Elsevier, 477493.
  • Lee HSJ, Schmitt AK, Lüttge U. 1989. The response of the C3-CAM tree Clusia rosea to light and water stress. II. Internal CO2 concentration and water use efficiency. Journal of Experimental Botany 40: 171179.
  • Liebig M, Scarano FR, De Mattos EA, Zaluar HLT, Lüttge U. 2001. Ecophysiological and floristic implications of sex expression in the dioecious neotropical CAM tree Clusia hilariana Schltdl. Trees 15: 278288.
  • Lüttge U. 1986. Nocturnal water storage in plants having crassulacean acid metabolism. Planta 168: 287289.
  • Lüttge U. 1987. Carbon dioxide and water demand: crassulacean acid metabolism (CAM), a versatile ecological adaptation exemplifying the need for integration in ecophysiological work. New Phytologist 106: 593629.
  • Lüttge U. 1988. Day–night changes of citric-acid levels in crassulacean acid metabolism: phenomenon and ecophysiological significance. Plant, Cell & Environment 11: 445451.
  • Lüttge U. 1991. Morphogenetische, physiologische und biochemische Strategien von Baumwürgern im tropischen Wald. Naturwissenschaften 78: 4958.
  • Lüttge U. 1995a. Ecophysiological basis of the diversity of tropical plants: the example of the genus Clusia. Scientia Guaianae 5: 2336.
  • Lüttge U. 1995b. Clusia: ein Modellfall der ökophysiologischen Plastizität in einer tropischen Gattung. In: Rundgespräche der Kommission für Ökologie der Bayerischen Akademie der Wissenschaften, Vol. 10. Bayerische TropenforschungEinst und jetzt. Munich, Germany: Dr. Pfeil, 173186.
  • Lüttge U. 1996. Clusia: plasticity and diversity in a genus of C3/CAM intermediate tropical trees. In: WinterK, SmithJAC, eds. Crassulacean acid metabolism: biochemistry, ecophysiology and evolution. Ecological Studies, Vol. 114. Berlin, Heidelberg, New York: Springer, 296311.
  • Lüttge U. 1999. One morphotype, three physiotypes: Sympatric species of Clusia with obligate C3 photosynthesis, obligate CAM and C3-CAM intermediate behaviour. Plant Biology 1: 138148.
  • Lüttge U. 2002. CO2-concentrating: consequences in crassulacean acid metabolism. Journal of Experimental Botany 53: 21312142.
  • Lüttge U. 2004. Ecophysiology of crassulacean acid metabolism (CAM). Annals of Botany 93: 629652.
  • Lüttge U. 2005. Genotypes – phenotypes – ecotypes: relations to crassulacean acid metabolism. Nova Acta Leopoldina Neue Folge 92, number 342: 177193.
  • Lüttge U. 2006a. Photosynthesis. In: LüttgeU, ed. Clusia: a woody neotropical genus of remarkable plasticity and diversity. Ecological Studies. Berlin, Heidelberg, New York: Springer. (In press.)
  • Lüttge U. 2006b. Physiological ecology. In: LüttgeU, ed. Clusia: a woody neotropical genus of remarkable plasticity and diversity. Ecological Studies. Berlin, Heidelberg, New York: Springer. (In press.)
  • Lüttge U. 2006c. Clusia: a woody neotropical genus of remarkable plasticity and diversity. Ecological Studies. Berlin, Heidelberg, New York: Springer. (In press.)
  • Lüttge U, Scarano FR. 2004. Ecophysiology. Revista Brasileira de Bôtanica 27: 110.
  • Lüttge U, Smith JAC, Marigo G, Osmond CM. 1981. Energetics of malate accumulation in the vacuoles of Kalanchoë tubiflora cells. FEBS Letters 126: 8184.
  • De Mattos EA, Herzog B, Lüttge U. 1999. Chlorophyll fluorescence during CAM-phases in Clusia minor L. under drought stress. Journal of Experimental Botany 50: 253261.
  • De Mattos EA, Lüttge U. 2001. Chlorophyll fluorescence and organic acid oscillations during transition from CAM to C3-photosynthesis in Clusia minor L. (Clusiaceae). Annals of Botany 88: 457463.
  • Maxwell K. 2002. Resistance is useful: diurnal patterns of photosynthesis in C3 and crassulacean acid metabolism epiphytic bromeliads. Functional Plant Biology 29: 679687.
  • Maxwell C, Griffiths H, Borland AM, Young AJ, Broadmeadow MSJ, Fordham MC. 1995. Short-term photosynthetic responses of the C3-CAM epiphyte Guzmania monostachia var. monostachia to tropical seasonal transitions under field conditions. Australian Journal of Plant Physiology 22: 771778.
  • Maxwell C, Griffiths H, Young AJ. 1994. Photosynthetic acclimation to light regime and water stress by the C3-CAM epiphyte Guzmania monostachia: gas exchange characteristics, photochemical efficiency and the xanthophyll cycle. Functional Ecology 8: 746754.
  • Maxwell K, Marrison JL, Leech RM, Griffiths H, Horton P. 1999. Chloroplast acclimation in leaves of Guzmania monostachia in response to high light. Plant Physiology 121: 8995.
  • Medina E, Delgado M, Troughton JH, Medina JD. 1977. Physiological ecology of CO2 fixation in Bromeliaceae. Flora 166: 137152.
  • Menninger EA. 1967. Fantastic trees. New York, NY, USA: The Viking Press.
  • Murphy R, Smith JAC. 1998. Determination of cell water-relation parameters using the pressure probe: extended theory and practice of the pressure-clamp technique. Plant, Cell & Environment 21: 637657.
  • Neales TF. 1973. Effect of temperature on the assimilation of carbon dioxide by mature pineapple plants, Ananas comosus (L.) Merr. Australian Journal of Biological Science 26: 539546.
  • Nobel PS. 1988. Environmental biology of agaves and cacti. Cambridge, UK: Cambridge University Press.
  • Olivares E. 1997. Prolonged leaf senescence in Clusia multiflora H.B.K. Trees 11: 370377.
  • Olivares E, Faist K, Kluge M, Lüttge U. 1993. 14CO2 pulse-chase labelling in Clusia minor L. Journal of Experimental Botany 44: 15271533.
  • Osmond CB. 1978. Crassulacean acid metabolism: a curiosity in context. Annual Reviews of Plant Physiology 29: 379414.
  • Patiño S, Tyree MT, Herre EA. 1995. Comparison of hydraulic architecture of woody plants of differing phylogeny and growth form with special reference to free-standing and hemi-epiphytic Ficus species from Panama. New Phytologist 129: 125134.
  • Pfündel E, Bilger W. 1994. Regulation and possible function of the violaxanthin cycle. Photosynthesis Research 42: 89109.
  • Pipoly JJ, Kearns DM, Berry PE. 1998. Clusia. In: BerryPE, HolstBK, YatskievychK, eds. Flora of the Venezuelan Guayana Caesalpiniaceae – Ericaceae, Vol. 4. St Louis, MO, USA: Missouri Botanical Garden Press, 260294
  • Popp M, Kramer D, Lee H, Diaz M, Ziegler H, Lüttge U. 1987. Crassulacean acid metabolism in tropical dicotyledonous trees of the genus Clusia. Trees 1: 238247.
  • Reinert F, Roberts A, Wilson MJ, De Ribas L, Cardinot G, Griffiths H. 1997. Graduation in nutrient composition and photosynthetic pathways across the restinga vegetation of Brazil. Botanica Acta 110: 135142.
  • Roberts A, Borland AM, Maxwell K, Griffiths H. 1998. Ecophysiology of the C3-CAM intermediate Clusia minor L. in Trinidad: seasonal and short-term photosynthetic characteristics of sun and shade leaves. Journal of Experimental Botany 49: 15631573.
  • Roberts A, Griffiths H, Borland AM, Reinert F. 1996. Is crassulacean acid metabolism activity in sympatric species of hemi-epiphytic stranglers such as Clusia related to carbon cycling as a photoprotective process? Oecologia 106: 2838.
  • Ruess BR, Ferrari S, Eller BM. 1988. Water economy and photosynthesis of the CAM plant Senecio medley-woodii during increasing drought. Plant, Cell & Environment 11: 583589.
  • Scarano FR. 2002. Structure, function and floristic relationships of plant communities in stressful habitats marginal to the Brazilian Atlantic Rainforest. Annals of Botany 90: 517542.
  • Scarano FR, Cirne P, Nascimento MT, Sampaio MC, Villela D, Wendt T, Zaluar HLT. 2004. Ecologia vegetal: integrando ecosistema, comunidades, populações e organismos. In: RochaCFD, EstevesFA, ScaranoFR, eds. Pesquisas de longa duração na restinga de Jurubatiba: ecologia, história natural e conservação. São Carlos, Brazil: Editoria Rima, 7797.
  • Scarano FR, Duarte HM, Franco AC, Geßler A, De Mattos EA, Nahm M, Rennenberg H, Zaluar HLT, Lüttge U. 2005. Ecophysiology of selected tree species in different plant communities at the periphery of the Atlantic Forest of SE Brazil. I. Performance of three different species of Clusia in an array of plant communities. Trees 19: 497509.
  • Schindler C, Lichtenthaler HK. 1996. Photosynthetic CO2-assimilation, chlorophyll fluorescence and zeaxanthin accumulation in field grown maple trees in the course of a sunny and a cloudy day. Journal of Plant Physiology 148: 399412.
  • Schmitt A, Lee HSJ, Lüttge U. 1988. The response of the C3-CAM tree, Clusia rosea, to light and water stress. Journal of Experimental Botany 39: 15811590.
  • Smith JAC. 1989. Epiphytic bromeliads. In: LüttgeU, ed. Vascular plants as epiphytes: evolution and ecophysiology. Ecological Studies, Vol. 76. Berlin, Heidelberg, New York: Springer, 109138.
  • Spalding MH, Stumpf DK, Ku MSB, Burris RH, Edwards GE. 1979. Crassulacean acid metabolism and diurnal variations of internal CO2 and O2-concentrations in Sedum praealtum DC. Australian Journal of Plant Physiology 6: 557567.
  • Svensson P, Bläsing OE, Westhoff P. 2003. Evolution of C4 phosphoenolpyruvate carboxylase. Archives of Biochemistry and Biophysics 414: 180188.
  • Taybi T, Nimmo HG, Borland AM. 2004. Expression of phosphoenol-pyruvate carboxylase and phosphoenol-pyruvate carboxylase kinase genes. Implications for genotypic capacity and phenotypic plasticity in the expression of crassulacean acid metabolism. Plant Physiology 135: 587598.
  • Ting IP, Lord EM, Sternberg L da SL, DeNiro MJ. 1985. Crassulacean acid metabolism in the strangler Clusia rosea Jacq. Science 229: 969971.
  • Tinoco Ojanguren C, Vazquez-Yanez C. 1983. Especies CAM in la selva humeda tropical de Los Tuxtlas, Veracruz. Boletin Sociedad Botanica de Mexico. 45: 150153.
  • Ule E. 1901. Die Vegetation von Cabo Frio an der Küste von Brasilien. Botanische Jahrbücher für Systematik. Pflanzengeschichte und Pflanzengeographie 28: 511528.
  • Vaasen A, Begerow D, Hampp R. 2006. Phosphoenolpyruvate carboxylase (PEPC) genes in C3, CAM and C3/CAM intermediate species of the genus Clusia: rapid reversible C3/CAM switches are based on the C3 housekeeping gene. Plant, Cell & Environment . (In press.)
  • Vaasen A. 2005. Verwandtschaftsbeziehungen, Photosynthesemechanismen und Phosphoenolpyruvat-Carboxylase bei der südamerikanischen Pflanzengattung Clusia. PhD thesis. Universität Tübingen, Tübingen, Germany.
  • Vaasen A, Begerow D, Lüttge U, Hampp R. 2002. The genus Clusia L. Molecular evidence for independent evolution of photosynthetic flexibitity. Plant Biology 4: 8693.
  • Vareschi V. 1980. Vegetationsökologie der Tropen. Stuttgart, Germany: Eugen Ulmer.
  • Westhoff P, Gowik U. 2004. Evolution of C4 phosphoenolpyruvate carboxylase. Genes and proteins: a case study with the genus Flaveria. Annals of Botany 93: 1323.
  • Winter K, Aranda J, Holtum JAM. 2005. Carbon isotope composition and water-use efficiency in plants with crassulacean acid metabolism. Functional Plant Biology 32: 381388.
  • Winter K, Holtum JAM. 2002. How closely do δ13C values of crassulacean acid metabolism plants reflect the proportion of CO2 fixed during day and night? Plant Physiology 129: 18431851.
  • Winter K, Lesch M, Diaz M. 1990. Changes in xanthophyll-cycle components and in fluorescence yield in leaves of a crassulacean- acid-metabolism plant Clusia rosea Jacq., throughout a 12 hour photoperiod of constant irradiance. Planta 182: 181185.
  • Ziegler H. 1994. Stable isotopes in plant physiology and ecology. Progress in Botany 56: 124.
  • Zotz G, Hietz P. 2001. The physiological ecology of vascular epiphytes: current knowledge, open questions. Journal of Experimental Botany 52: 20672078.
  • Zotz G, Patiño S, Tyree MT. 1997. Water relations and hydraulic architecture of woody hemi-epiphytes. Journal of Experimental Botany 48: 18251833.
  • Zotz G, Winter K. 1994a. Annual carbon balance and nitrogen-use efficiency in tropical C3 and CAM epiphytes. New Phytologist 126: 481492.
  • Zotz G, Winter K. 1994b. A one year study on carbon, water and nutrient relationships in a tropical C3-CAM hemi-epiphyte, Clusia uvitana Pittier. New Phytologist 127: 4560.