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Keywords:

  • leaf hydraulic conductance;
  • senescence;
  • phenology;
  • cavitation;
  • dry tropical forest;
  • Calycophyllum candidissimum;
  • Rhedera trinervis

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • • 
    The hydraulic conductance of leaves (kleaf) was examined to determine whether this little understood component of the water transport pathway plays a role in governing leaf phenology of two deciduous dry forest trees (Calycophyllum candidissimum and Rhedera trinervis).
  • • 
    kleaf was monitored in parallel with stem hydraulic conductivity (Kstem) during the transition from wet to dry season. The relationships between declining photosynthetic quantum yield during senescence and declining kleaf and Kstem were compared.
  • • 
    Divergent patterns were observed in the response of Kstem to seasonal drying; however, the behaviour of kleaf was essentially similar in both species. Large (five- to ten-fold) decreases in leaf hydraulic conductance occurred before, and during the later stages of leaf senescence. During senescence, declining kleaf, which continued until leaves were ultimately shed, was associated with a concomitant decline in quantum yield.
  • • 
    We conclude that, in these species, the loss of hydraulic conductance of the leaf vascular system is linked to, and possibly responsible for, the loss of photosynthetic capacity during leaf senescence.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The seasonally dry forest of northern Costa Rica provides a spectacular annual display of leaf shedding associated with the cessation of rainfall during the months of November and December. The majority of tree species shed leaves at the onset of the dry season and remain leafless for many months until rains begin to fall again at the end of May (Daubenmire, 1972; Frankie et al., 1974). This type of vegetation provides an excellent opportunity to focus on the interaction between water availability, water supply to leaves through the vascular system and photosynthesis during leaf senescence.

It has been demonstrated that the hydraulic capacity of the plant vascular system is a primary determinant of the photosynthetic properties of leaves (Hubbard et al., 1999; Brodribb & Field, 2000; Hubbard et al., 2001); however, there is little information on the integrity of this correlation during dynamic processes such as leaf shedding. A number of authors have demonstrated that xylem cavitation in the stem plays a role in drought-induced leaf shedding (Sobrado, 1986; Machado & Tyree, 1994), and recent evidence from a winter deciduous species suggests that leaf senescence is preceded by a decrease in the conductivity of the stem and petiole as a result of cavitation (Salleo et al., 2002). These observations have led to the hypothesis that annual patterns of leaf senescence are related to a loss in xylem hydraulic capacity. Interestingly, the small amount of data that has emerged from seasonally dry tropical forests to date has been unable to uncover a common pattern relating vascular and photosynthetic function during seasonal transitions (Sobrado, 1986, 1993; Prior & Eamus, 2000). While leaf shedding in some seasonally dry forest species is associated with xylem cavitation, in others stem conductivity remains high during leaf shedding (Brodribb et al., 2002a). Hence, we focus here on the hydraulic conductance of leaves with the goal of determining whether leaf, rather than stem hydraulic properties play a more definitive role in governing the phenology of leaf shedding.

Studies of plant vascular hydraulics have been dominated by work on stems. Hydraulic flow in stems is primarily axial, and hence the calculation of hydraulic fluxes, resistances or pressure gradients in the stem requires simple application of Ohms law (van den Honert, 1948; Tyree, 1997). However, there is increasing evidence that the largest restriction to water flow in plants occurs within the leaves, where the flow of water can no longer be treated as an axial network. In the few studies that have attempted to place the hydraulic properties of leaves within the context of the whole plant pathway, leaves account for between 40% and 80% of the whole-plant hydraulic resistance (Becker et al., 1999; Nardini & Tyree, 2000; Brodribb et al., 2002a). Furthermore, the hydraulic properties of leaves are likely to behave dynamically, given that the fine vein network appears to be highly susceptible to cavitation (Nardini et al., 2001; Salleo et al., 2001). Considering that the hydraulic capacity of the plant vascular system is a primary determinant of stomatal conductance (Sperry et al., 1993; Saliendra et al., 1995; Sperry, 2000) and maximum photosynthetic rate (Brodribb & Field, 2000; Hubbard et al., 2001) it is likely that the magnitude and dynamics of leaf hydraulic resistance also influence leaf longevity.

Leaves present several problems in terms of measuring their hydraulic properties. First, the hydraulic pathway from leaf veins to the sites of evaporation is complicated, and may involve significant symplastic as well as apoplastic components (Canny, 1990; Kramer & Boyer, 1995). Second, the pressure distribution in transpiring leaves is unknown and the current measurement techniques that force water through the leaf at a constant driving gradient result in a homogeneous pressure distribution. Despite these unknowns, it has recently been shown that several simple techniques for measuring leaf conductance (kleaf) yield similar values (Sack et al., 2002) suggesting that, under some conditions, the distribution of hydraulic pressure in the leaf is relatively homogeneous. Curiously, in this and other recent studies of leaf hydraulics the variation of kleaf between species was relatively small considering the diverse range of leaf properties in the species investigated (Nardini, 2001; Sack et al., 2002).

Leaf hydraulic conductance is likely to be dynamic according to qualitative observations such as dye perfusion of water-stressed leaves (Salleo et al., 2001) and acoustic emission from leaves (Milburn, 1973; Nardini & Tyree, 2000), which suggest that the veins and perhaps the mesophyll apoplast are vulnerable to cavitation even at relatively mild water potentials. The corollary of this is that control over photosynthesis and productivity may be dominated by changes in the hydraulic conductivity of the leaf.

In this study, we made parallel observations of leaf and stem hydraulic conductance and photosynthetic quantum yield during leaf senescence to determine degree of coordination between hydraulics and seasonal patterns of photosynthesis during leaf shedding.

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Study site

This investigation was conducted in the Santa Rosa National Park, an area of 10 800 ha located on the Northern Pacific coast of Costa Rica (10°52′ N, 85°34′ W, 285 m above sea level). Mean annual rainfall in the park is 1528 mm; however, more than 95% of this falls between the months of May and December, resulting in a pronounced dry season (Janzen, 1983; Enquist & Leffler, 2001). The dry season is accompanied by strong trade winds, low relative humidity and high irradiance, all of which contribute to generate a high evaporative demand. Diurnal and seasonal temperature ranges are relatively small, with a mean annual temperature of 28°C. All measurements were made during the period mid-November to mid-December 2001, which corresponds to the early dry season. During this period all deciduous species shed their leaves, remaining leafless until late May.

Vegetation in the park comprises a heterogeneous mosaic consisting of various stages of regeneration from former pastures as well as some small areas of primary forest. Evergreen and deciduous species can be found at all successional stages, however, the cover by evergreen species is greatest in the mature forest, and deciduous species tend to be more dominant in earlier successional stages (Janzen, 1983).

Plant material

Two deciduous tree species were selected because of their differing ecological and hydraulic properties. Calycophyllum candidissimum (Rubiaceae) is a small tree that occurs in secondary forest. It produces simple mesic leaves, and stem xylem that has been show to be resistant to cavitation (50% loss of hydraulic conductivity at −2.9 MPa; Brodribb et al., 2002b). Stem conductivity in this species remains relatively high during the dry season despite the absence of leaves (Brodribb et al., 2002a). Rhedera trinervis is a small tree that dominates the open, early successional forest in Santa Rosa. This species bears simple leaves able to withstand low water potentials and sheds leaves over an extended period in the early dry season. The leaves of both species were small (less than 100 mm in any dimension) and only trees that were fully exposed to the sun and strong wind were selected. Leaf boundary layer conductances were assumed to be high.

Leaf hydraulic conductance

Leaf hydraulic conductance was measured in eight to ten individuals of each species during the period of senescence and leaf drop (November to December). Two methods were employed to measure kleafin situ and in vitro. Both techniques involved determination of the evaporative flux (E) and water potential drop from stem xylem (Ψx) to whole leaf (Ψl), thus enabling calculation of kleaf from Ohms law applied to hydraulic fluxes:

  • E = (Ψx − Ψl)kleaf(Eqn 1)

The majority of measurements were made in situ, with the in vitro method being used only in Rhedera trinervis as a means of independently verifying values of kleaf measured in the field. The in situ technique involved measuring the water potential drop across the whole leaf under conditions of stable transpiration. Fully sun-exposed, accessible, terminal leaf clusters (three to six leaves) were chosen such that one leaf could be covered and used to measure xylem water potential in the stem, while the immediately adjacent, transpiring leaves could be sampled for E and Ψl. Mean kleaf was expressed for individual trees from four measurements of kleaf. For each measurement of Kstem two leaves per tree were covered early in the morning using several layers of plastic film and a final layer of aluminium foil. Later, at 10 : 30–11 : 30 h (once atmospheric humidity had been stable for approximately 1 h) leaves were revisited and the evaporative flux and quantum yield of photosynthesis measured on two leaves adjacent to each of the covered leaves. Evaporative flux (E) was measured in situ with a Li-1600 porometer (Licor Inc., Lincoln, NE, USA). Because of the small leaf size and consistently high wind speed (4–5 m s−1 at the leaf) during the sample period, it was assumed that leaf boundary layer conductances were high, and that the transpiration fluxes measured in the Li-1600 cuvette were equivalent to those experienced before measurement. Leaf orientation, leaf temperature and ambient humidity were carefully conserved such that measured E was as close as possible to the actual E immediately before measurement. Following determination of E, the quantum yield of photosystem II electron transport (ØPSII) was measured in the light using a miniPAM portable fluorometer (Waltz, Effeltrich, Germany) operated in the field as described by Bilger et al. (1996). Measurements of ØPSII were made at the time of leaf collection under an actinic light of 2000 µmol m−2 s−1 photosynthetic photon flux density (PPFD), which was approximately the same as midday full sun at Santa Rosa. Mean ØPSII per tree was calculated from 10 measurements from different leaves including those sampled for kleaf.

Following determination of ØPSII, the four leaves sampled for E, and the two covered samples were removed and thoroughly sealed into plastic bags before being transported to the laboratory for determination of Ψl with a pressure chamber (PMS Instruments, Oregon, USA). Calculation of kleaf was made using Eqn 1, where Ψx was considered equal to the water potential of the covered leaf, and Ψl and E were measured. The units of kleaf were mmol m−2 s−1 MPa−1, giving a mean conductance per unit area for the entire leaf rather than a specific conductivity relative to the physical properties of the pathway of water movement through the leaf.

The second in vitro method of measuring kleaf involved applying a known pressure drop across excised leaves and measuring the flux of water into the leaf (Nardini et al., 2001). For this technique, leaves were cut underwater from sample trees at 10 : 30 h to 11 : 30 h and left to soak in water in the light for approximately 1 h to ensure that hydraulic and osmotic gradients in the leaf were in equilibrium with the transfusing liquid (water). Leaves were then attached by the petiole to a plastic tube that led to a reservoir on a computer-interfaced balance. Hydraulic gradients were generated by placing the leaf inside a glass Erlenmeyer flask that was evacuated to two vacuum pressures (40 Pa and 80 Pa). Leaves were placed underwater inside the flask to ensure that no evaporative water loss occurred during the measurements. Fluxes from the balance through the leaf were measured at 80, 40 and 0 Pa vacuum pressures and the slope of the linear regression through these data was taken as the mean conductance. To avoid problems with stomatal closure (Sack et al., 2002), leaves were maintained in the light before and during measurement. Leaf area was subsequently determined using a digital camera and image analysis software (Scion Image, National Institute of Health, USA) and kleaf expressed as above in mmol m−2 s−1 MPa−1. Four leaves per tree were collected and kleaf per tree expressed as the mean of these four samples.

Stem hydraulic conductivity was measured on segments excised from the distal ends of branches in all species. The size of excised segments was standardized such that diameters fell in the range 2–5 mm with the bark removed and lengths were 0.15–0.35 m. Care was taken to ensure that stem segments contained no through vessels (i.e. no vessels that were open at both ends). Vessel lengths were measured by injecting air at 0.1 MPa into the cut end of segments and cutting the distal end of the segment back until air bubbles were first seen to emerge from xylem vessels. Vessel lengths were surveyed every month in all species. Using segments that included the junction from stem to petiole was the safest way to ensure that all vessels contained at least one end-wall.

Branches were collected between 10 : 30 h and 11 : 30 h and cut under water to prevent air from being introduced into the measured segment. Branches were selected with a cluster of leaves at the tip such that when leaves were removed, the cut ends of the petioles were equidistant from the initial cut (this allowed the length of the segment to be simply expressed). Branches were then transferred to the laboratory where they were re-cut under water, leaves removed and saved for leaf area determination, and stems attached to a flowmeter for measurement of hydraulic conductivity. The flowmeter was similar to that described in Brodribb & Field (2000) and worked on the principle of measuring the decrease in water pressure across a capillary tube of known resistance connected in series with the segment to be measured. Water flowed from a head pressure of around 0.01 MPa, resulting in delivery pressures to the stem of approximately half this value. To avoid problems with ions affecting conductivity measurements (Zwieniecki et al., 2001) the stem perfusing solution was filtered (0.1 µm) and KCl added to make a concentration of 0.01 m. Once stems were attached to the flowmeter they were allowed to equilibrate (generally requiring less than 5 min) and the head pressure and delivery pressure recorded. From these two figures and the length of segment, its conductivity could be calculated (kg s−1 MPa−1 m−1). Leaf area distal to the excised segment was measured using the digital camera as above, and Kstem expressed by leaf area (kg s−1 MPa−1 m−1). Note that the units for Kstem differ from those of kleaf, because of the inability to characterize a single path-length for water flow through leaves. Measurements of Kstem were expressed as averages per tree from four branches.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

In the weeks following the end of the rains in late November, xylem and leaf water potential declined in trees of both species (Fig. 1). Leaf transpiration at midday also declined, but a large amount of variation was observed in leaves both within and between individuals (Fig. 1). Mean quantum yield and leaf transpiration rates of groups of four neighbouring leaves declined proportionally during leaf senescence (Fig. 2). Midday leaf to air vapour pressure deficit varied by less than 20% during this period, indicating that changes in E were mostly to changes in stomatal conductance. Transpiration rates and quantum yield declined proportionally indicating that stomatal conductance remained optimized with respect to photosynthesis during senescence. The water potential drop recorded from the stem xylem (Ψx) to the transpiring leaf (Ψl) varied between species, R. trinervis exhibited the largest range, while in C. candidissimum the potential drop across the leaf was more moderate (Fig. 3). Variation in E was poorly correlated with the observed water potential decrease across the leaf indicating that hydraulic conductance of leaves changed during leaf shedding (Fig. 3).

image

Figure 1. Time-course of mean xylem (open circles) and leaf (closed circles) water potential (± SD) from six sample trees in the 6 months before leaf shedding. A steady decline in water potential was observed after the cessation of rains (dotted line). Mean leaf transpiration also declined in the early dry season, although a large amount of variation was observed between trees during this period.

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image

Figure 2. Decreasing mean evapotranspiration and photochemical quantum yield (at 2000 µmol quanta m−2 s−1) during the 6 to 8 wk period of leaf senescence in six trees of Rhedera trinervis (open circles) and Calicophyllum candidissimum (closed circles). Each point represents a mean value per tree ± SD (n = 4).

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image

Figure 3. Variation in leaf transpiration and water potential drop from the stem xylem (Ψx) to leaf (Ψl) for the two species studied. Data from the entire study period, spanning the late wet season to early dry season, are shown.

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Both species experienced a marked decline in kleaf as leaf senescence progressed during the wet to dry season transition (Figs 4a and 5a). According to data collected using the pressure-drop technique, kleaf in Rhedera trinervis declined from peak values of more than 25 mmol m−2 s−1 MPa−1 at the end of the rainy season to minimum values around 3 mmol m−2 s−1 MPa−1 immediately before leaf drop. The relationship between kleaf and ØPSII was curvilinear in R. trinervis with declines in ØPSII only occurring once kleaf had fallen below approximately 15 mmol m−2 s−1 MPa−1 (Fig. 4a). A polynomial regression is fitted in Fig. 4(a) (R = 0.81) simply to illustrate the trend in the data. The relationship between these variables is unlikely to conform to a polynomial function. Values of kleaf collected using the applied vacuum technique corroborated the high sensitivity of ØPSII to leaf conductances below 15 mmol m−2 s−1 MPa−1. All values of kleaf measured using the in vitro method fell within the range measured in vivo, however, no leaves measured with the vacuum technique exhibited values of kleaf above 15 mmol m−2 s−1 MPa−1. This was probably the result of the in vitro method being first employed midway through in the study, once kleaf had already begun to decline.

image

Figure 4. (a) Changes in leaf conductance and quantum yield during the wet to dry season transition in Rhedera trinervis. The majority of data were collected using the in vivo method (closed triangles) with kleaf also measured in several trees using the in vitro method (open triangles) for verification of data collected in vivo. Data are means per tree (n = 4 for kleaf and 10 for ØPSII± SD). (b) The relationship between Kstem and ØPSII in Rhedera trinervis over the same period. Again, data are means per tree (n = 4 for kleaf and 10 for ØPSII± SD). Curves fitted are second-order polynomials in both cases.

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image

Figure 5. (a) Changes in leaf conductance and quantum yield during the wet to dry season transition in Calycophyllum candidissimum. All data were collected using the in vivo method and represent means per tree (n = 4 for kleaf and 10 for ØPSII± SD). (b) The relationship between Kstem and ØPSII in Calycophyllum candidissimum over the same period. Data are means per tree (n = 4 for kleaf and 10 for ØPSII± SD).

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Changes in stem hydraulic conductivity in Rhedera closely matched the pattern observed in kleaf (Fig. 4b). Quantum yield appeared insensitive to the initial decline in Kstem, and only decreased consistently once Kstem fell to below 50% of its maximum. Stems remained conductive right up until leaf shedding, however, Kstem immediately before leaf shedding was less than 0.5 × 10−4 kg s−1 MPa−1 m−1 (approximately 15% of maximum values recorded during the rainy season).

A similar pattern of decreasing leaf hydraulic conductance was observed in Calycophyllum candidissimum where kleaf fell from wet season values of 15–18 mmol m−2 s−1 MPa−1 down to mean minimum values 2.1–4.2 mmol m−2 s−1 MPa−1 just before leaf shedding (Fig. 5a). The shape of the relationship between kleaf and ØPSII in Calycophyllum was very similar to that in Rhedera, although maximum ØPSII was slightly lower in Calycophyllum. By contrast to Rhedera, stem conductivity in Calycophyllum did not decline significantly between the wet season and the time of leaf shedding (Fig. 5b). Mean Kstem ranged from 0.69 to 1.36 × 10−4 kg s−1 MPa−1 m−1, although no significant trend was observed relating this variation to changes in photosynthetic rate.

No evidence for a relationship between kleaf and leaf water potential was observed in either species investigated. Data for Rhedera is shown in Fig. 6, illustrating that despite a large range in recorded leaf water potentials (from −1.0 MPa to −3.7 MPa) there was no dependence of kleaf on Ψl in transpiring leaves. However, this result may be confounded by the fact that data from leaves of all ages were pooled together.

image

Figure 6. Mean leaf hydraulic conductance (kleaf) vs leaf water potential (Ψl) (n = 4) in Rhedera trinervis during the transition from wet to dry season. An absence of correlation between these variables may have been due to differences in leaf age in the sample population.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

In both species, leaf shedding was preceded by a large decline in kleaf, while the behaviour of Kstem varied between species such that only in Rhedera was leaf shedding associated with declining branch conductivity. A recent study into changes in stem and leaf hydraulics during leaf shedding in a winter deciduous species concluded that declining stem or root conductivity were more important than leaf conductance in driving leaf senescence (Salleo et al., 2002). By contrast, we found here that in the absence of photoperiodic or freezing cues, leaf hydraulics provided the only common relationship between vascular function and leaf shedding in species with otherwise disparate hydraulic behaviour in the stem.

The cause of the decline in leaf conductance remains unknown, although physiological changes in the vasculature or mesophyll tissue of the leaf appear good candidates. Our measure of leaf conductance included the petiole, and hence we are unable to distinguish between petiole, leaf venation and mesophyll as sites of declining conductivity. We consider that the most likely physiological explanation is that the leaf hydraulic conductance becomes progressively reduced owing to cavitation of the vascular system. The combination of falling Ψl (Fig. 1), increased evaporative demand and soil drying during the onset of the dry season, as well as increased xylem vulnerability caused by cavitation fatigue (Hacke et al., 2001) and possibly reduced embolism repair capacity (due to reduced leaf metabolic rate) are likely to have induced strong xylem cavitation in the leaf, resulting in large reductions in kleaf.

Although there was no evidence for a relationship between Ψl and kleaf (Fig. 6), these data are confounded by the fact that leaf senescence was influenced by a combination of soil drying and leaf age, and our study included several cohorts of leaves, possibly with different physiological characteristics. Several studies have used dye injection (Salleo et al., 2001) and acoustic emissions in leaves (Milburn, 1973; Nardini & Tyree, 2000) to demonstrate that leaf veins, and particularly the tertiary vein network are susceptible to cavitation at only moderate water potentials. An alternative to cavitation-induced loss of conductivity is the possibility that developmental changes in the leaf may result in decreased conductivity of the xylem. Deposition of tyloses in the stem xylem of trees undergoing leaf senescence have been reported (Fahn, 1990; Salleo et al., 2002), although it is probable that these tyloses develop after xylem cavitation rather than as part of a programmed xylem shutdown.

The technique of measuring kleaf employed here did not allow differentiation of xylem and mesophyll contributions to kleaf and hence it is also possible that changes in the hydraulic conductivity of the mesophyll tissue may have been responsible for the observed reduction in leaf conductance. Increased leaf hardness has been noted in many species during latter part of the wet season in the Santa Rosa National Park (Janzen & Waterman, 1984), although the underlying cause of this does not appear to be related to changes in leaf lignin or water content. Such changes in the tissue characteristics of leaves may affect the hydraulic conductivity of either the leaf vascular system, or symplastic flow within the leaf.

Although previous work has emphasized a link between stem xylem dysfunction and leaf shedding, it is clear from the data here that this is not generally applicable. Calycophyllum candidissimum was found here to shed leaves while stem conductivity remained relatively high. This is in agreement with previously published data showing the stem xylem to be resistant to cavitation (50% loss of hydraulic conductivity at −2.9 MPa; Brodribb et al., 2002b) while leaves were mesic (turgor loss at −1.3 MPa; Brodribb et al., 2002b). Such characteristics suggest that water is conserved in the stem throughout the dry season, and under such conditions only changes in the conductance of the leaf were correlated with decreasing photosynthetic capacity during senescence (Fig. 5). The conservation of stem water would probably allow C. candidissimum to transfer water rapidly to the growing regions of the plant upon soil rewetting, and evidence of this is that Calycophyllum is one of the first to leaf-out after the commencement of rains (T. Brodribb, unpubl. data).

The range of kleaf observed during the lifecycle of these species was impressively large. Over the period of a few months leaf conductivities of each species covered a range larger than that measured in a morphologically diverse selection of mature evergreen and deciduous leaves from a Mediterranean environment (Nardini, 2001). All species here exhibited a five- to ten-fold decrease in kleaf between the time of maximum quantum yield and the period immediately before leaf shedding. An unusual feature of this decline in leaf hydraulic conductance was that the ØPSII was initially insensitive to falling kleaf (Figs 4 and 5). Initial consideration of this pattern suggests some degree of redundancy in leaf hydraulic conductance, however, this ‘excess conductance’ could conceivably confer advantages to the plant. The most likely advantage is that it would buffer the stomata from changes in the ambient vapour pressure deficit (VPD) during the wet season, enhancing CO2 uptake during this period of reliable water supply. This argument is supported by measurements indicating that the deciduous species examined here are relatively insensitive to fluctuations in VPD compared with associated evergreens (C. Chinchilla & T. J. Brodribb, unpubl. data).

Conversely, decreasing leaf hydraulic conductance may also confer an advantage to the plant. As leaf hydraulic conductance decreases, leaf water potential, and hence stomatal aperture, becomes more responsive to changes in the evaporative environment. As a result, water-use efficiency is likely to increase, and other parts of the plant which may be potentially more difficult to refill after cavitation are protected by stomatal closure. Unfortunately for the plant, this increased hydraulic protection will decrease intercellular CO2 concentration in leaves, resulting in excess light energy that cannot be dissipated by the usual process of electron transport for CO2 fixation. This excess energy is likely to lead to the production of damaging photo-oxidative compounds that likely accelerate the process of leaf senescence (Horton et al., 1996; Richter et al., 1999).

An important question that remains unanswered by these data is whether declining kleaf is causal in the process of leaf senescence or simply a response to physical changes in the characteristics of the leaf during senescence. Given that kleaf begins to decline relatively early in the life cycle of the leaf before any signs of leaf senescence (hence the plateau in the relationship between kleaf and ØPSII; Figs 4 and 5) it appears unlikely that kleaf is driven by leaf senescence. Further, the good relationship between kleaf and ØPSII as photosynthesis was declining supports the idea of a hydraulic limitation of photosynthesis.

These data lead us to suggest that the large seasonal fluctuations in the conductivity of leaf xylem in these species are responsible for driving the process of leaf senescence and ultimately leaf shedding. It is still unknown whether decreasing kleaf occurs in response to changes in soil or atmospheric moisture, or whether kleaf declines as a result of developmental changes in the leaf, and further work will be necessary to resolve these issues.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank the research and administrative staff of Santa Rosa National Park for their contributions to this study. This research was supported by grants from The National Science Foundation (IBN: 0212792), The Arnold Arboretum, Harvard University, and The Andrew W. Mellon Foundation.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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