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Keywords:

  • climate;
  • latitude;
  • maximum net photosynthesis;
  • optimum temperature;
  • photosynthetic acclimation

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • • 
    Photosynthetic responses to acclimation temperature were investigated in seedlings of eight Australian rainforest tree species. Australian rainforests extend over 33° of latitude, providing an opportunity to compare temperature responses of temperate and tropical species.
  • • 
    Net photosynthesis was measured in leaves developed under a constant (22°C : 14°C) or fluctuating (17°C : 9°C−27°C : 19°C) day/night temperature regime. These leaves were then subjected to a series of constant temperature regimes and net photosynthesis was measured 14 d after acclimation to each new regime.
  • • 
    Acclimation potential was not affected by the contrasting temperature regimes. The temperate species showed at least 80% of maximum net photosynthesis over a larger span of acclimation temperature than the tropical species.
  • • 
    The lack of an effect of the contrasting temperature regimes on acclimation potential may reflect either that adjustments were unnecessary for temperate species, which already have broad photosynthetic responses to temperature, and tropical species were incapable of adjustments, or that in general species respond to the mean temperature regime and not to the amount of fluctuation in the regime. The higher acclimation potential shown by the temperate species is consistent with the larger seasonal and day-to-day variation in temperature of the temperate climate compared with the tropical climate.

Introduction

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

There are many differences in the structure, species diversity and climate of tropical and temperate forests. However, there have been few direct comparisons of the physiology of their component species. Furthermore, these direct comparisons are often complicated by the inclusion of species with different growth forms (Schreiber & Riederer, 1996; Franks & Farquhar, 1999). The predominant changes in climate from temperate to tropical forests are the increase in temperature and the reduction in seasonality of temperatures (Archibold, 1995). Comparisons of species native to temperate and tropical forests will provide an understanding of how plants are adapted to these contrasting climates.

Rainforests occur across a latitudinal range of 33° in Australia, which includes climates from cool-temperate to tropical. These forests have a disjunct distribution along the eastern margin of Australia, being restricted to areas that have a high annual rainfall (> 1300 mm) and low fire frequency (Webb & Tracey, 1994; Specht & Specht, 1999). Therefore, they provide an opportunity to study the temperature responses of temperate and tropical species within the same forest type.

A recent study of some Australian rainforest tree species found that temperate species show maximum net photosynthesis at lower growth temperatures (the temperature at which leaves are developed) but maintain close to this rate over a larger range of growth temperatures than tropical species (Cunningham & Read, 2002). However, in the field, trees do not produce leaves continuously. Instead, leaves are produced during certain periods of the year and then exposed to the seasonal changes of temperature. The few studies of phenology in rainforest trees of Australia show that tropical species produce new leaves throughout the wet season from late spring to early autumn, whereas temperate species produce the majority of their leaves during spring and early summer (e.g. Read, 1989; Lowman, 1992). The ability to acclimate to new temperature conditions is likely to be more important in temperate trees due to their shorter growth period and the larger seasonal variation in temperature of their climates.

In the field, temperate species tend to maintain a similar rate of maximum net photosynthesis over the warmer months (e.g. Drew & Ledig, 1981; Pereira et al., 1986). In contrast, tropical species commonly show peaks in maximum net photosynthesis that are related to rainfall and not to temperature (Doley et al., 1987; Lugo et al., 1978). The seasonal responses of net photosynthesis in temperate species have been reproduced in plants exposed to a series of acclimation temperatures in controlled environment cabinets (e.g. Strain et al., 1976; Slatyer & Ferrar, 1977b; Mooney et al., 1978). Many of these studies have concluded that species from more variable climates have a higher acclimation potential – the ability to maintain maximum net photosynthesis when exposed to a wide range of acclimation temperatures (temperatures to which leaves are exposed after development, Berry & Björkman, 1980). Therefore, temperate species are likely to have a higher acclimation potential due to their exposure to larger seasonal variations in temperature than tropical species. However, most of these studies have only used two to three acclimation temperatures (e.g. Battaglia et al., 1996; Goldstein et al., 1996; Teskey & Will, 1999), which does not give an indication of their full acclimation potential.

Previous studies of Australian rainforest trees, using six to eight acclimation temperatures, have shown distinct differences in photosynthetic responses among species from different latitudes. A study of temperate rainforest trees found that species from lower latitudes showed maximum photosynthesis at higher acclimation temperatures than species from higher latitudes (Hill et al., 1988). Tropical and temperate species of Nothofagus showed little difference in the acclimation temperature for maximum photosynthesis (Read, 1990). Instead, the difference was the ability of temperate species to acclimate to a wider range of temperatures than tropical species. This is consistent with the tropical species of Nothofagus being from high altitude areas that experience similar maximum temperatures of the warmest month to temperate regions of Australasia where Nothofagus grows, but have smaller seasonal variation in temperature (Read, 1990). However, there is a positive relationship between the acclimation temperature for maximum photosynthesis and the maximum temperature of the warmest month among the temperate and tropical Nothofagus species of Australasia (Read & Hope, 1996). Whether these trends in acclimation are true of a broad range of temperate and tropical genera is the focus of this paper.

The majority of previous studies have used leaves developed under constant temperature conditions. However, in the field, plants are exposed to variation in day-to-day conditions. In addition, day-to-day variation in temperature is larger in temperate than tropical climates (Table 1). Therefore, leaves of temperate species in particular would be predicted to show a higher acclimation potential when developed under ambient conditions compared with constant conditions. A few studies have investigated leaves developed under ambient conditions and then exposed to a series of constant acclimation temperatures in controlled environment cabinets (e.g. Tranquillini et al., 1986; Hill et al., 1988). The acclimation potential of some species has been measured separately in leaves developed under ambient conditions and in controlled environment cabinets (e.g. Hill et al., 1988; Read & Busby, 1990; Gunderson et al., 2000). However, the effect of constant and fluctuating temperatures on their acclimation responses can not be discerned, as other factors besides temperature are likely to have varied between the growth conditions. Therefore, this study presents the first investigation under carefully controlled conditions of the effect of development under fluctuating vs. constant temperature on the acclimation potential of net photosynthesis. The study aimed to answer the following two questions:

Table 1.  Weekly standard deviations (WSD) in maximum temperature for different locations in eastern Australia
LocationMaximum temperature (°C)
Annual WSDSummer WSD
  1. Values are means of 2 yr of meteorological data (1995 and 1996) with standard errors in brackets.

Strahan (42° S)2.4 (0.1)3.0 (0.2)
Noojee (38° S)3.3 (0.0)4.2 (0.2)
Coffs Harbour (30° S)2.2 (0.1)2.2 (0.1)
Rockhampton (23° S)2.0 (0.0)2.1 (0.2)
Cairns (17° S)1.2 (0.1)1.4 (0.1)
  • 1
    Do temperate species show a greater ability to acclimate to new temperatures than tropical species?
  • 2
    Do leaves developed under fluctuating temperature conditions show a greater ability to acclimate to new temperatures than leaves developed under constant temperature conditions?

Materials and Methods

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

Species selection

Eight species were selected to cover the latitudinal range of rainforests in eastern Australia. Collection sites and distributional ranges of the species are given in Table 2 and their climate profiles are given in Table 3. Two species were selected from each of the four rainforest types (cool-temperate, warm-temperate, subtropical and tropical) defined by Webb (1968). Canopy dominants were chosen as previous research has shown that subcanopy species can have narrower photosynthetic responses to temperature than would be predicted from the macroclimate of their distribution (Read & Busby, 1990). All species were evergreen, ensuring that their leaves are exposed to the full seasonal changes of temperature. Species from different families were chosen where possible to minimise any confounding effects of phylogenetic relatedness. Species restricted to high altitudes were not included in this comparison.

Table 2.  The species used in the study, grouped into climate groups and with details of their collection sites and distributional ranges
 Collection siteDistribution range
 LatitudeLongitudeAltitude (m)LatitudeAltitude (m)
Temperate species
 Eucryphia lucida (Labill.) Baill. (Eucryphiaceae)41°10′ S144°57′ E140  41–43.5° S 5–1000
 Nothofagus cunninghamii (Hook.) Oerst. (Fagaceae)41°09′ S145°01′ E180  37–43.5° S 0–1440
 Tristaniopsis laurina (Sm.) Wilson & Waterhouse (Myrtaceae)37°42′ S147°22′ E15025.5–38° S 5–1035
 Acmena smithii var. smithii (Poir.) Merrill & Perry (Myrtaceae)37°25′ S149°49′ E20024.5–39° S 0–1270
Tropical species
 Sloanea woollsii F. Muell. (Elaeocarpaceae)30°43′ S152°43′ E 60  26–32° S20–1200
 Heritiera trifoliolata (F. Muell.) Kosterm. (Sterculiaceae)28°36′ S152°43′ E540  17–30° S10–1075
 Castanospermum australe Cunn. & C. Fraser ex Hook. (Fabaceae)26°38′ S153°38′ E 4012.5–30° S 5–1150
 Alstonia scholaris (L.) R. Br. (Apocynaceae)16°13′ S145°52′ E 2010.5–22° S 0–1300
Table 3.  Climate profiles for the study species
 nMean annual temperature (°C)Max. temp. hottest quarter (°C)Max. temp. coldest month (°C)Max. temp. annual range (°C)Max. temp. range hottest 6 months (°C)Mean diurnal range (°C)Mean annual precipitation (mm)
  1. Values are means, with standard errors in brackets, of climate variables produced by ANUCLIM 5.0 (Houlder et al., 1999), or calculated from those variables, for n site locations. Species are presented in order from highest to lowest latitudinal origin.

E. lucida112 9.3 (0.1)18.0 (0.1) 8.7 (0.2)10.2 (0.1)6.0 (0.1) 8.6 (0.1)2072 (52)
N. cunninghamii354 9.2 (0.1)18.7 (0.1) 7.8 (0.1)11.7 (0.1)6.4 (0.1) 8.6 (0.1)1764 (28)
T. laurina13716.2 (0.2)26.1 (0.2)16.3 (0.2)10.2 (0.1)4.0 (0.1)11.2 (0.2)1318 (34)
A. smithii29116.4 (0.1)26.1 (0.1)16.0 (0.2)10.5 (0.1)4.2 (0.1)10.7 (0.2)1320 (24)
S. woollsii14015.8 (0.2)26.0 (0.1)15.9 (0.2)10.6 (0.1)3.8 (0.0)11.6 (0.2)1395 (27)
H. trifoliolata 9818.3 (0.2)27.3 (0.2)18.6 (0.2) 9.1 (0.1)3.2 (0.1)10.7 (0.2)1750 (64)
C. australe12320.7 (0.2)29.0 (0.2)21.4 (0.3) 8.1 (0.1)2.8 (0.1)10.4 (0.3)1655 (59)
A. scholaris 6123.0 (0.3)30.2 (0.2)23.5 (0.3) 7.1 (0.2)2.3 (0.1) 9.0 (0.3)1978 (94)

Establishment conditions

Seedlings of all species were from natural populations, collected as recently germinated seedlings or in the case of the two tropical species Alstonia scholaris (L.) R. Br. and Castanospermum australe Cunn. & C. Fraser ex Hook. raised from seed. Seedlings were grown in sandy loam soil in glasshouses for 2 yr before the experiments. Seedlings were watered every 2 d and fertiliser was added every 14 d in the form of FOGG-IT fish emulsion fertilizer (FOGG-IT Nozzle Company, San Francisco, CA, USA) diluted 1 : 500 with water to provide 98 mg l−1 of nitrogen, 20 mg l−1 of potassium, and 31 mg l−1 of phosphorus.

Response of net photosynthesis whilst growing under constant or fluctuating temperature treatments

Seedlings were grown under two contrasting temperature regimes in controlled environment cabinets. One set of seedlings was grown under a constant day/night temperature regime of 22°C : 14°C over a 16 : 8-h cycle. The other set of seedlings was grown under a fluctuating temperature regime in which the day/night temperature was changed daily to a random temperature regime between 17°C : 9°C and 27°C : 19°C, always with an 8°C diurnal difference. The temperature program included an equal number of all unit temperatures (17, 18, 19, 20, 21, 22, 23, 24, 25, 26 and 27°C), so that the mean day : night temperature over the experimental period was 22°C : 14°C. Each treatment was replicated in two cabinets with four seedlings of each species in each cabinet. The watering and fertilizer regime applied in the glasshouses was continued in the cabinets. Seedlings were raised on stands so that the first set of leaves initiated was exposed to an irradiance (PPFD) of 600–800 µmol quanta m−2 s−1 and the desired temperature regime. This also helped avoid shading of crown leaves by neighbouring seedlings. Plants were grown under these conditions for 8 wk until enough leaves had fully expanded for the photosynthetic measurements to be carried out.

Instantaneous temperature dependence (ITD) curves were measured on leaves expanded under the treatment conditions. A total of eight plants were measured for each species within each treatment. Photosynthesis was measured in the laboratory using an ADC LCA4 infrared gas analyser (ADC, United Kingdom), which is an open gas-exchange system. Each experimental leaf was equilibrated in the leaf chamber at an air temperature of 22 ± 0.1°C, a CO2 concentration of 350 ± 5 µl l−1, a PPFD of 800 ± 20 µmol quanta m−2 s−1 and a vapour pressure deficit of 1.1 ± 0.1 kPa until a steady rate was reached. After the initial photosynthetic measurement at 22°C, measurements were taken sequentially at 12°C, 17°C, 22°C, 27°C and 32°C. The leaf was equilibrated for 10 min at each new temperature before measurement. The VPD was maintained at a constant 1.1 kPa at all air temperatures, with the exception of 32°C at which the lowest VPD that could be maintained without condensation building up in the lines was 1.5 kPa. The area of leaf within the gas chamber was traced and the traces were measured using image analysis (BIOSCAN™ Image Analyser).

Response of net photosynthesis to acclimation temperatures

After the initial ITD curves were measured, one seedling of each species was removed from each cabinet to reduce crowding. Seedlings from both treatments were then subjected to a sequence of constant temperature regimes (22°C : 14°C, 30°C : 22°C, 26°C : 18°C, 22°C : 14°C, 18°C : 10°C, 14°C : 6°C and 22°C : 14°C). It was decided that the acclimation temperatures should start at the highest temperature and then be progressively lowered for several reasons. First, the lowest temperature was the most likely to damage leaves of the tropical species and therefore would affect acclimation to subsequent temperatures. Second, the initial change from 22°C to 30°C was the largest and therefore would give a conservative estimate of how long leaves would take to acclimate. Finally, the sequence is representative of the seasonal changes in temperature experienced in climates of the temperate species, that is summer through to winter.

The acclimation response of seedlings was determined from measurements of the maximum rate of net photosynthesis (Pmax) and the optimum instantaneous temperature for net photosynthesis (Topt). Pmax and Topt were measured by changing the air temperature in 1°C increments until a distinct maximum was recorded. The time taken to acclimate to a new temperature regime was determined when seedlings were exposed to the 30°C : 22°C temperature regime. Previous studies have found full acclimation has taken up to 2 wk (Slatyer & Ferrar, 1977b; Badger et al., 1982; Hill et al., 1988). Consequently, Pmax was measured in the same leaf after 10 and 14 d of acclimation to the new temperature regime. Photosynthetic rates of leaves showed no significant difference (F = 0.32, P = 0.60) between readings taken after 10 and 14 d.

After the photosynthetic measurements were taken at the acclimation temperature of 30°C : 22°C, any shoots that were shading the measurement leaves were pruned. The seedlings were then pruned after each measurement period. Partial defoliation has been found to found to increase the net photosynthetic rate (Reich et al., 1993; Pinkard & Beadle, 1998) and arrest the effects of aging on net photosynthesis (Mooney & Chiariello, 1984) in the remaining leaves. To determine the effect of aging and pruning on net photosynthesis, the leaves were acclimated to 22°C : 14°C three times over the experiment and their photosynthetic rate measured. The measurement leaves of seedlings of C. australe began to pale and drop off during the second acclimation to 22°C : 14°C, presumably in response to pruning. Consequently, seedlings of C. australe were removed from the experiment after photosynthetic measurements were taken for the second acclimation to 22°C : 14°C.

Data analysis

ITD curves tended to be asymmetrical and the following regression curve was found to be an appropriate fit (Ratkowsky et al., 1983):

  • P   =  { b ( T  −  Tmin )  ×  [1 − exp( c ( T  −  Tmax ))]} 2(Eqn 1)

where P is the net photosynthetic rate (µmol m−2 s−1), T is the air temperature (°K), Tmin and Tmax are the minimum and maximum temperatures at which the net photosynthetic rate is zero, and b and c are fitting parameters. Tmin and Tmax were simply parameters estimated to fit the curve and their values were believed to have no physiological significance. These regression curves were used to estimate Pmax, Topt, and the temperature span over which at least 80% of Pmax was shown (Tspan). The mean values of these parameters for each species within each treatment were used as replicates for statistical analysis of the climate groups.

For each plant, the values of Pmax for each acclimation temperature regime were regressed against acclimation temperature using the above equation. The maximum rates of Pmax (PATmax), the optimum acclimation temperatures for Pmax (ATopt) and the span of temperatures over which at least 80% of PATmax was shown (ATspan) were determined from these regressions. Any plants that showed less than 80% of their original Pmax at 22°C : 14°C during the second acclimation to 22°C : 14°C were removed from the analysis. This resulted in Heritiera trifoliolata, Sloanea woollsii and Tristaniopsis laurina having only one replicate cabinet for one of the temperature treatments. Species’ means of parameters for each treatment were used in statistical analyses of the climate groups. Sloanea woollsii was removed from this analysis as the regression for the fluctuating treatment produce a straight line and therefore none of parameters could be derived.

Results

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

Response of net photosynthesis to instantaneous temperatures whilst growing under constant or fluctuating temperature treatments

Overall, there was no significant difference in the response of net photosynthesis to instantaneous temperatures shown by the climate groups between the constant and fluctuating temperature regimes (Table 4). However, among the individual species there were distinct differences in the response of net photosynthesis to the temperature treatments. The temperate species Eucryphia lucida showed a significantly higher Pmax when grown under the fluctuating temperature regime compared with the constant temperature regime (F = 36.7, P = 0.03). In contrast, the tropical species Castanospermum australe showed a significantly lower Pmax in leaves grown under the fluctuating temperature regime compared with leaves grown under the constant temperature regime (F = 54.7, P = 0.02).

Table 4. Maximum rates of net photosynthesis ( Pmax ), optimum instantaneous temperatures for net photosynthesis ( Topt ) and the span of instantaneous temperatures over which at least 80% of Pmax was shown ( Tspan ) for climate groups grown under constant (C) and fluctuating (F) temperature regimes
  Pmax (µmol m −2 s −1 ) Topt (°C) Tspan (°C)
  1. Values are means of the four species in each climate group with standard errors in brackets.

TemperateC8.5 (1.9)20.8 (1.7)26.0 (1.7)
F9.4 (1.2)20.0 (1.7)23.2 (1.4)
TropicalC6.4 (0.9)23.0 (0.6)16.4 (2.2)
F6.0 (1.2)22.9 (0.9)16.9 (1.4)
ANOVA
ClimateF4.06 4.9121.6
p0.07 0.05<0.001
TreatmentF0.05 0.17 0.49
p0.83 0.68 0.50
Climate × treatmentF0.24 0.12 0.95
p0.63 0.74 0.35

The climate groups had consistently different responses of net photosynthesis to instantaneous temperatures under both the constant and fluctuating temperature regimes (Table 4). Maximum net photosynthesis was shown at 20.4°C by the temperate group compared with 22.9°C in the tropical group. The temperate group showed at least 80% of maximum net photosynthesis over an instantaneous temperature span of 24.6°C, which was significantly higher than the temperature span of 16.7°C shown by the tropical group (F = 21.2, P < 0.001).

Response of net photosynthesis to acclimation temperatures

Most species showed similar responses of net photosynthesis to acclimation temperature between the constant and fluctuating temperature regimes (Fig. 1). The most notable difference was a 63% increase in the maximum rate of net photosynthesis in E. lucida when grown under the fluctuating temperature regime compared with the constant temperature regime (Table 5). Four species showed increases in the temperature span over which at least 80% of PATmax was maintained (ATspan) but this increase was only significant for Alstonia scholaris. Similarly, Sloanea woollsii showed a curvilinear response when grown under the constant temperature regime and a broader linear response when grown under the fluctuating temperature regime (Fig. 1e). The climate groups showed no significant difference in the response of net photosynthesis to acclimation temperature between the constant and fluctuating temperature regimes (Table 6). The temperate group showed a significantly larger ATspan of 14.5°C than the tropical group, which only maintained PATmax over a span of 9.2°C.

image

Figure 1. Relationship between maximum net photosynthesis and acclimation temperature for the individual species developed under a constant (C, solid circles) and a fluctuating (F, open circles) temperature regime. Values of Pmax are means for each cabinet with Heritiera trifoliolata, Sloanea woollsii and Tristaniopsis laurina having only one replicate cabinet in one of the treatments. Mean-corrected r2 values are given for each regression.

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Table 5.  Maximum rates of Pmax ( PATmax ), optimum acclimation temperatures for Pmax ( ATopt ) and the span of acclimation temperatures over which at least 80% of PATmax was shown ( ATspan ) for individual species developed under constant (C) and fluctuating (F) temperature regimes
 PATmaxAToptATspan
 CFCFCF
  1. Values are means of two cabinets with standard errors in brackets or means of replicates from a single run without standard errors. Significant differences between treatments within a species are indicated in bold. na = parameters could not be derived due to response being a straight line.

Temperate species
 E. lucida 5.44 (0.18)8.86 (0.75)23.4 (0.1)22.5 (0.4)15.7 (0.8)17.8 (0.7)
 N. cunninghamii 7.45 (0.31) 8.92 (0.30)22.8 (0.8)25.8 (0.4)16.7 (1.5)17.1 (2.9)
 T. laurina15.8716.8 (0.3)27.127.9 (0.2)13.810.6 (2.1)
 A. smithii 8.76 (0.24) 9.15 (1.45)25.5 (0.3)26.6 (0.4) 9.9 (0.5)14.3 (3.3)
Tropical species
 S. woollsii 9.15 (0.24)na27.2 (0.6)na 8.6 (0.5)na
 H. trifoliolata 8.95 8.74 (0.6)28.926.9 (0.4) 8.6 8.3 (1.6)
 C. australe 5.70 (0.85) 4.48 (0.53)26.1 (0.1)25.5 (0.0) 7.6 (0.5)13.3 (1.8)
 A. scholaris10.62 (1.70) 9.18 (0.43)26.4 (0.5)27.1 (0.4) 7.3 (0.2) 9.9 (0.5)
Table 6.  Maximum rates of Pmax ( PATmax ), optimum acclimation temperatures for Pmax ( ATopt ) and the span of acclimation temperatures over which at least 80% of P ATmax was shown ( GTspan ) for climate groups developed under constant (C) and fluctuating (F) temperature regimes
  PATmax (µmol m −2 s −1 ) ATopt (°C) ATspan (°C)
  1. Values are means of the four species in each climate group with standard errors in brackets.

TemperateC 9.4 (2.3)24.7 (1.0)14.0 (1.5)
F10.9 (2.0)25.7 (1.2)15.0 (1.6)
TropicalC 8.4 (1.4)27.1 (0.9) 7.8 (0.4)
F 7.5 (1.5)26.5 (0.5)10.5 (1.5)
ANOVA
ClimateF 1.25 2.6513.3
p 0.29 0.14 0.01
TreatmentF 0.02 0.03 1.51
p 0.88 0.86 0.25
Climate × treatmentF 0.40 0.68 0.36
p 0.54 0.43 0.56

All species showed significant increases in the instantaneous temperature for maximum net photosynthesis (Topt) with increasing acclimation temperature in leaves developed under at least one temperature regime (Fig. 2). The exceptions were E. lucida, S. woollsii and C. australe in the constant temperature regime and Nothofagus cunninghamii in the fluctuating temperature regime. There was no trend of species showing larger shifts in Topt when developed under the fluctuating temperature regime compared with the constant temperature regime (F = 1.01, P = 0.33). Furthermore, there was no significant difference in the magnitude of the shift in Topt between the temperate and tropical groups (F = 1.12, P = 0.31).

image

Figure 2. Relationship between the instantaneous temperature for maximum net photosynthesis (T opt ) and acclimation temperature for the individual species developed under a constant (C, solid circles) and a fluctuating (F, open circles) temperature regime. Values of T opt are means for each cabinet.

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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

Does development under fluctuating temperatures improve acclimation potential compared with development under constant temperature?

Overall, the temperate and tropical groups showed no difference in their ability to acclimate to instantaneous temperatures or new growth temperatures between the constant and fluctuating temperature regimes (Tables 4 and 6). However, the temperate species Eucryphia lucida showed a 63% increase in maximum net photosynthesis when grown under the fluctuating temperature regime compared with the constant temperature regime, whereas the tropical species Castanospermum australe showed a 21% decrease.

This increased photosynthetic rate of Eucryphia lucida under the fluctuating temperature regime compared with the constant temperature regime suggests development under more optimal conditions. The fluctuating temperature regime included temperatures closer to the growth temperature for maximum net photosynthesis of 19°C shown by E. lucida (Cunningham & Read, 2002) than the constant regime of 22°C. However, leaves developed under a temperature regime of 19°C : 11°C showed maximum net photosynthetic rates of 5 µmol CO2 m−2 s−1, which is similar to that shown under the present constant temperature regime but considerably less than the 9 µmol CO2 m−2 s−1 shown under the fluctuating temperature regime. Therefore, the mechanism for the enhancement of net photosynthesis in E. lucida under the fluctuating temperature regime remains uncertain. The reduction of net photosynthesis of the tropical species C. australe under the fluctuating temperature regime compared with the constant temperature regime may be a result of exposure to low temperatures during development.

Studies of Californian shrubs have shown similar rates of net photosynthesis between the field and controlled environments (Björkman et al., 1975; Mooney et al., 1976) although Atriplex lentiformis showed higher rates in the field (Pearcy & Harrison, 1974; Pearcy, 1977). In contrast, the alpine species Eucalyptus pauciflora showed lower rates of photosynthesis in the field compared with rates under constant temperature regimes representative of field temperatures (Slatyer & Ferrar, 1977a; Slatyer & Morrow, 1977). Of course, these differences in photosynthetic rates between the field and controlled environments may reflect differences in water stress, nutrient availability, irradiance levels and leaf age rather than temperature conditions. Studies where these factors were not limiting have shown photosynthetic rates of temperate species in the field are higher or similar in the field compared with controlled environments (Battaglia et al., 1996; Gunderson et al., 2000). These findings suggest temperate species commonly have higher photosynthetic rates when grown under fluctuating temperatures compared with constant temperatures. Therefore, growing species from variable climates, such as E. lucida, under constant temperature regimes is potentially underestimating their maximum net photosynthetic rate. The enhancement of net photosynthesis in leaves of temperate species developed under fluctuating temperature conditions is likely to be an adaptation to the larger day-to-day variation in temperature of temperate climates compared with tropical climates (Table 1).

Species showed no significant difference in the span of instantaneous temperatures over which maximum net photosynthesis was maintained (Tspan) between the contrasting or fluctuating temperature regimes (Table 4). In addition, species did not increase the span of acclimation temperatures over which maximum net photosynthesis was shown (ATspan) when developed under a fluctuating temperature regime compared with a constant temperature regime (Table 6). Leaves developed under fluctuating temperature conditions were expected to have larger temperature spans than those developed under constant temperature conditions, due to the larger range of temperatures. However, the temperate species already showed close to maximum net photosynthesis over large temperature spans in leaves developed under a constant temperature regime (Tspan= 26°C, ATspan= 14°C), making adjustments under fluctuating conditions unnecessary. The tropical species were not expected to adjust to the fluctuating temperature regime, as they do not experience such large temperature fluctuations in the field.

There was no significant difference in the instantaneous temperature for maximum net photosynthesis between leaves developed under a constant or a fluctuating temperature regime (Table 4). Therefore, the Topt of leaves did not respond to the amount of fluctuation in temperature but instead to the mean daily temperature. This is consistent with the finding in Eucalyptus pauciflora that the temperature for maximum net photosynthesis was strongly correlated with the mean maximum temperature of the 10 d prior to measurement (Slatyer & Morrow, 1977). Similarly, leaves of Quercus rubra showed the same Topt when acclimated to three constant temperature regimes with the same mean daily temperature but different amplitudes of diurnal change (Chabot & Lewis, 1976). Furthermore, when leaves of these species were developed under different constant temperature regimes (Cunningham & Read, 2002), most showed minor shifts in the instantaneous temperature for maximum net photosynthesis. Therefore, it is unlikely that shifts in the temperature for maximum net photosynthesis would occur under the daily temperature changes of the fluctuating temperature regime.

Do temperate species show a greater ability to acclimate to new temperatures than tropical species?

The temperate group showed maximum net photosynthesis over a 15°C span of acclimation temperatures, which was significantly larger than the 9°C span shown by the tropical group (Table 6). A comparison of Nothofagus species also found that the temperate species showed maximum photosynthesis over a larger span of acclimation temperatures than the tropical species (Read, 1990). Similarly, the temperate group showed close to its highest maximum net photosynthetic rate over a larger span of growth temperatures than the tropical group (Cunningham & Read, 2002).

The tropical species Alstonia scholaris showed the largest shifts in the instantaneous temperature for maximum net photosynthesis (Topt) with acclimation temperature (Fig. 2). However, these larger shifts in Topt were associated with larger declines in maximum net photosynthesis at low acclimation temperatures than the temperate species. Therefore, these larger shifts are not a reflection of a higher acclimation potential. A larger span of instantaneous temperatures at which maximum net photosynthesis occurs (Tspan) was recorded in temperate species than tropical species, and this diminishes the importance of shifting Topt towards the new growth temperature. The same trend was found for shifts in the instantaneous temperature for maximum net photosynthesis with growth temperature (Cunningham & Read, 2002).

It is often argued that acclimation is more likely to occur in evergreen species that occupy a wide range of thermal environments or are exposed to large seasonal variations in temperature (Berry & Björkman, 1980). By comparison, species from environments in which growth is restricted to one season, or seasonal temperature variations are small, show a limited ability to acclimate to new temperatures (Kemp & Williams, 1980; Goldstein et al., 1996). Therefore, the greater ability of the temperate species to acclimate maximum net photosynthesis to new temperatures compared with the tropical species is likely to be an adaptation to the larger seasonal fluctuations in temperature of the temperate climate. Similarly, the ability of the temperate species to maintain maximum net photosynthesis over a larger span of growth temperatures than tropical species (Cunningham & Read, 2002) is likely to be an adaptation to the larger temperature fluctuations experienced during leaf development in the field.

Differences in acclimation potential between temperate and tropical species reflect the combination of differences in their phenology and temperature conditions. Temperate rainforest species of Australia tend to produce new leaves during spring and early summer (e.g. Ashton & Frankenberg, 1976; Read, 1989). By comparison, tropical species produce leaves over a longer period during the wet season from spring to autumn (Frith & Frith, 1985; Lowman, 1992). The climates of rainforest species of Australia differ in the larger seasonal and day-to-day variation of temperature experienced by temperate species compared with tropical species. Consequently, leaves of temperate species develop under more variable temperature conditions, and over their life span experience larger divergences from their development conditions, than those of tropical species. Therefore, the strategy of temperate species may be to produce leaves in spring that can acclimate to the large temperature changes from spring through to autumn. In contrast, tropical species may rely less on acclimation as they are exposed to smaller seasonal variations in temperature and produce new leaves over a much longer period.

The present work and other investigations (Read, 1990; Cunningham & Read, 2002) have shown the wider temperature tolerance of net photosynthesis in temperate rainforest trees compared with tropical rainforest trees. Furthermore, temperate rainforest species show maximum growth at temperatures higher than the average summer temperatures of their native climates (Cunningham & Read, in press). These findings suggest that many temperate trees may adjust adequately to the predicted 1°C to 4.5°C temperature increase by 2100 (Kattenberg et al., 1996). However, the narrower temperature tolerance of tropical tree species may make them more susceptible than temperate tree species to the predicted increases in global temperatures.

Acknowledgements

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

We thank Alia Thomas and Fiona Clissold for their assistance in data collection. Permission was given to collect seedlings by State Forests of N.S.W. and the Department of Natural Resources and Environment Victoria. This research was supported by a Monash Postgraduate Research Scholarship awarded to S.C. and an Australian Research Council Large Grant awarded to J.R.

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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