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Keywords:

  • Eucalyptus;
  • high temperature;
  • photosynthesis;
  • sub-ambient and elevated CO2

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. REFERENCES

The unabated rise in atmospheric [CO2] is associated with increased air temperature. Yet, few CO2-enrichment studies have considered pre-industrial [CO2] or warming. Consequently, we quantified the interactive effects of growth [CO2] and temperature on photosynthesis of faster-growing Eucalyptus saligna and slower-growing E. sideroxylon. Well-watered and -fertilized tree seedlings were grown in a glasshouse at three atmospheric [CO2] (290, 400, and 650 µL L−1), and ambient (26/18 °C, day/night) and high (ambient + 4 °C) air temperature. Despite differences in growth rate, both eucalypts responded similarly to [CO2] and temperature treatments with few interactive effects. Light-saturated photosynthesis (Asat) and light- and [CO2]-saturated photosynthesis (Amax) increased by ∼50% and ∼10%, respectively, with each step-increase in growth [CO2], underpinned by a corresponding 6–11% up-regulation of maximal electron transport rate (Jmax). Maximal carboxylation rate (Vcmax) was not affected by growth [CO2]. Thermal photosynthetic acclimation occurred such that Asat and Amax were similar in ambient- and high-temperature-grown plants. At high temperature, the thermal optimum of Asat increased by 2–7 °C across [CO2] treatments. These results are the first to suggest that photosynthesis of well-watered and -fertilized eucalypt seedlings will remain strongly responsive to increasing atmospheric [CO2] in a future, warmer climate.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Research into the response of plants to rising atmospheric concentrations of carbon dioxide [(CO2)] has mainly focussed on the impacts of future, projected rises in atmospheric [CO2]. However, current ambient [CO2] (∼390 µL L−1) represents a 35% increase above the atmospheric [CO2] that prevailed 200 years ago at the outset of the Industrial Revolution – more than 20% of this increase has occurred in the last 50 years. The pre-industrial [CO2] of ∼280 µL L−1 persisted during the last 10 000 years, and atmospheric [CO2] oscillated between 180 and 300 µL L−1 for the last 1–3 million years. Therefore, the rise in atmospheric [CO2] since the early 1800s represents a significant change in [CO2] experienced by C3 plants (Sage & Cowling 1999). Consequently, studying the response of plants to the recent historical rise in atmospheric [CO2] can help us understand their response to the future rise in [CO2] (Dippery et al. 1995; Tissue et al. 1995; Ward 2005).

Many studies of the effects of elevated [CO2] do not take into consideration the associated impact of warming. In the past century, air temperature has increased by 1 °C. With atmospheric [CO2] predicted to approach 550–650 µL L−1 during the 21st century, temperatures are anticipated to increase a further 2–6 °C (Solomon et al. 2007). Therefore, it is important to investigate the interactive effects of elevated [CO2] and high temperature. To address this knowledge gap, we are investigating the physiological performance of eucalypt trees under past and future climatic conditions. In a previous study, we quantified interactive effects of [CO2] and temperature on the growth responses of faster (Eucalyptus saligna) and slower growing (E. sideroxylon) tree seedlings grown under well-watered and -fertilized conditions (Ghannoum et al. 2010). We found that sub-ambient [CO2] (290 µL L−1) reduced eucalypt plant growth under high temperature (ambient + 4 °C), but not under ambient temperature. We also found that both elevated [CO2] (650 µL L−1) and high temperature stimulated the growth of eucalypt seedlings. Thus, sub-ambient [CO2] and high temperature had an interactive effect on growth that was not observed in elevated [CO2] and high temperature treatments (Ghannoum et al. 2010). Consequently, the current study builds on our previous work by focussing on the photosynthetic responses of the same eucalypt tree seedlings to the main and interactive effects of past and future atmospheric [CO2] and higher air temperature.

The responses of C3 photosynthesis to short-term changes in [CO2] and temperature are generally well-understood. With short-term increases in [CO2], ribulose 1,5-bisphosphate (RuBP) carboxylation increases faster than RuBP regeneration. RuBP carboxylation increases because of both higher carboxylation and lower oxygenation, while RuBP regeneration increases because of lower oxygenation only. Reduced photorespiration with increasing [CO2] has two main consequences: (1) the photosynthetic thermal optimum (Topt) increases with [CO2]; and (2) the relative CO2-enhancement of light-saturated photosynthetic rates (Asat) increases with temperature (Long 1991). Therefore, we hypothesized that high temperature would have a stronger interactive effect on photosynthesis at sub-ambient relative to elevated [CO2].

The mechanisms underlying the photosynthetic responses to intercellular [CO2] (A/Ci curves) and leaf temperature (A/TL) are well characterized (Berry & Björkman 1980; Medlyn, Dreyer & Ellsworth 2002; Sage & Kubien 2007; Bernacchi et al. 2009; von Caemmerer, Berry & Farquhar 2009). At saturating light intensity, the initial slope of the A/Ci curve in C3 leaves is proportional to the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), while the saturated rate of the A/Ci curve depends on the rate of RuBP regeneration, which, in turn, depends on both the rates of electron transport and triose phosphate utilization (TPU) (von Caemmerer et al. 2009). As for the bell-shaped A/TL curves, the underlying biochemical limitations are more complex, particularly at and above the Topt where [CO2] exerts a strong influence (Sage & Kubien 2007). At ambient [CO2], photosynthesis is limited by TPU at low temperature, by Rubisco activity around the Topt and by electron transport capacity and Rubisco activation above the Topt (Crafts-Brandner & Salvucci 2004; Sage, Way & Kubien 2008). By stimulating Rubisco and electron transport activities, increased [CO2] produces a dual effect on the A/TL curves; TPU limits photosynthesis up to higher temperatures and electron transport capacity limits photosynthesis above the temperature where TPU is limiting (Sage & Kubien 2007).

In the long term, the response of photosynthesis to [CO2] and temperature also depends on the plant's capacity to acclimate to these two environmental factors (Berry & Björkman 1980; Drake, Gonzalez-Meler & Long 1997). In general, plants reduce their photosynthetic capacity in response to growth at elevated [CO2], and this reduction is associated with lower leaf nitrogen content [(N)] (Ainsworth & Long 2005; Ainsworth & Rogers 2007). Similarly, sub-ambient [CO2] is expected to up-regulate photosynthetic capacity, although very few studies have directly tested this hypothesis (Tissue et al. 1995). Sub-ambient [CO2] has been reported to stimulate Rubisco expression in rice (Gesch et al. 2000), and enhance photosynthetic capacity in Solanum dimidiatum (Anderson et al. 2001) but not in Phaseolus vulgaris (Sage & Reid 1992). Thus, we hypothesized that sub-ambient [CO2] will up-regulate photosynthetic capacity, while elevated [CO2] will down-regulate photosynthetic capacity. Photosynthetic acclimation to elevated [CO2] is usually stronger under sink-limited conditions (Tissue & Oechel 1987; Stitt & Krapp 1999; Tissue et al. 2001; Lewis et al. 2002; Ainsworth & Long 2005). Therefore, we hypothesized that photosynthetic acclimation to elevated [CO2] would be stronger in the slower-growing E. sideroxylon because of the slower rate of sink development.

Generally, thermal acclimation of photosynthesis culminates in shifting the Topt such that photosynthetic rates measured at growth temperature remain relatively unchanged (Berry & Björkman 1980; Battaglia, Beadle & Loughhead 1996). Minimal thermal adjustment of Topt has also been observed (Bunce 2000). Changes in Topt can be related to numerous factors, including alterations in RuBP carboxylation and regeneration, Rubisco activation, or membrane structure (Hikosaka 1997; Cen & Sage 2005; Yamori, Noguchi & Terashima 2005; Sage et al. 2008). Hence, we hypothesized that elevated [CO2] and temperature would both increase Topt. Consequently, this study compared the photosynthetic responses of two Eucalyptus species to short- and long-term changes in atmospheric [CO2] and temperature in order to better understand the mechanisms underlying the physiological responses of Eucalyptus to industrial-age climate change.

MATERIALS AND METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Plant culture

Seeds of Sydney blue gum (E. saligna Sm.) and red ironbark (E. sideroxylon A. Cunn. ex Woolls) were obtained from Ensis (Australian Tree Seed Centre, ACT, Australia) and were germinated in a nursery at ambient [CO2]. Four weeks after germination, seedlings were transplanted into the middle of 10 L cylindrical pots filled with 9 kg of air-dried loamy-sand soil and transferred to six adjacent, naturally lit glasshouse compartments. There were 50 pots of each species in each of the six [CO2] and temperature treatment combinations. Thirty days after planting (DAP), seedlings were thinned to one seedling per pot in all treatments. Tree seedlings were watered on a daily basis. Pots were irrigated on three occasions (30, 120 and 135 DAP) with a nutrient solution containing a commercial fertilizer (General Purpose, Thrive Professional, Yates, NSW, Australia) at a concentration of 0.2 g N L−1 (N:P:K:S:Fe:Mn:B 25:4.1:17.3:1.6:0.06:0.003:0.022%).

Detailed experimental set-up can be found in Ghannoum et al. (2010). Three glasshouse compartments were maintained at local ambient temperature and three glasshouse compartments were maintained at ambient + 4 °C (i.e. high temperature treatment). Air temperature was kept constant during the night and midday periods; temperature was stepped up and down in two 4 °C steps before and after the midday period. Average temperatures for the ambient and high temperature treatments were 26/18 and 30/22 °C (day/night), respectively. Within each temperature treatment, plants were grown at sub-ambient [CO2] (target 280 µL L−1), ambient [CO2] (target 400 µL L−1), and elevated [CO2] (target 640 µL L−1). Atmospheric [CO2] was controlled and monitored as described in Ghannoum et al. (2010). Average day-time [CO2] during the growth period for the sub-ambient, ambient and elevated treatments was 290, 400 and 650 µL L−1, respectively.

Leaf gas exchange analyses

Gas exchange measurements were carried out on attached, recently fully expanded leaves using a portable open gas exchange system (LI-6400, Li-Cor, Lincoln, NE, USA) supplying photosynthetic photon flux density by an in-built red/blue light-emitting diode source.

At 140 DAP, spot measurements were made at growth [CO2] and temperature just before the responses of assimilation rates to intercellular [CO2] (A/Ci curves) were measured. For the spot measurements, net photosynthesis at saturating light (Asat), stomatal conductance (gs), the ratio of intercellular to ambient [CO2] (Ci/Ca), and the ratio of photosynthesis to transpiration or leaf water use efficiency (WUEL) were measured at a PPFD of 1200 µmol m−2 s−1, target growth [CO2] (280, 400 or 640 µL L−1), mid-day growth temperature (28 or 32 °C), and leaf-to-air vapour pressure deficit of 1.8 kPa. Each leaf was allowed 5–10 min to equilibrate before measurements were made. There were five replicate tree seedlings measured per species and treatment.

The A/Ci curves were measured at a PPFD of 1200 µmol m−2 s−1, mid-day growth temperature (28 or 32 °C) and leaf-to-air vapour pressure deficit of 1.0–2.0 kPa. The A/Ci curves were measured by raising cuvette [CO2] in 10 steps (40, 70, 150, 230, 280, 400, 640, 900, 1200 and 1800 µL L−1). There were five replicate tree seedlings measured per species and treatment. Vcmax (apparent, maximal Rubisco-limited rate of photosynthesis) and Jmax (apparent, maximal electron transport-limited rate of photosynthesis) were estimated using an A/Ci curve fitting utility (version 0.4, updated in July 2007) without constraining mesophyll conductance (Sharkey et al. 2007). Similar results were obtained (but not shown) by fitting the initial slopes and the saturated part of the A/Ci curves separately using the Farquhar, von Caemmerer & Berry (1980) photosynthesis model and the temperature parameterization described in Bernacchi, Pimentel & Long (2003) and (Bernacchi et al. (2001). Using curves obtained by fitting the Farquhar, von Caemmerer & Berry (1980) model to each A/Ci data set, CO2-saturated assimilation rates (Amax) was determined at a Ci = 1500 µL L−1 and relative stomatal limitation Ls was calculated as Ls = (Ao − A)/Ao, where A denotes the net rate of CO2 assimilation and subscript zero denotes potential A if stomatal resistance were zero (i.e. Ci = Ca) (Farquhar & Sharkey 1982).

At 70 DAP, the responses of assimilation rates to leaf temperature (A/TL curves) were measured at a PPFD of 1500 µmol m−2 s−1 and target growth [CO2] (280, 400 or 640 µL L−1). The A/TL curves were measured by raising the block temperature of the leaf gas exchange chamber in six steps (15, 20, 25, 30, 35 and 42 °C). There were four replicate tree seedlings measured per species and treatment; all four replicates were measured at the same temperature before the block temperature was stepped up. Concurrently, the temperature of the glasshouse room where plants were measured was also raised to keep all plant leaves at the same temperature. The A/TL curves were fitted using a polynomial function (y = A + Bx + Cx2), and the value of Topt was taken as the temperature corresponding to maximal Asat. To describe the shape of the A/TL curves, the slopes of A versus leaf temperature were calculated 5 °C before and after Topt.

Leaf mass per area and nitrogen content

Leaf mass per area (LMA; g dry mass m−2) and leaf N content [(N); mg N g−1 dry mass] were determined on similar leaves to those used for the A/Ci curves. A leaf was removed and its area was measured using a portable leaf area meter (LI-3100A, Li-Cor). The leaf was oven-dried at 80 °C, weighed, then ground to a fine powder in a ball mill. Subsamples were analysed using a CN analyser (LECO TruSpec, LECO Corporation, St. Joseph, MI, USA). There were five replicate tree seedlings sampled per treatment.

Statistical analyses

Data were analysed using a general linear model, factorial analysis of variance (ANOVA) (Statistica, StatSoft Inc., Tulsa, OK, USA) with species, growth [CO2] and growth temperature as independent factors. Means were compared using Newman-Keuls post hoc test. A logarithmic transformation was applied to some data prior to ANOVA in order to normalize the distribution of variances.

RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Leaf mass per area and nitrogen content

LMA and leaf [N] expressed on both dry mass and area basis were greater in E. sideroxylon relative to E. saligna under all [CO2] and temperature treatments (Fig. 1, Table 1).

image

Figure 1. Leaf mass per area, LMA (a and b) and leaf nitrogen content on a dry mass (c and d) and area basis (e and f) of E. saligna (a, c and e; ○●) and E. sideroxylon (b, d and f; ▵▴) grown at ambient (○▵) and high temperature (●▴) and sub-ambient (290) ambient (400) and elevated (650 µL L−1) atmospheric [CO2]. Values are means ± SE of 5 leaves.

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Table 1.  Summary of 3-way ANOVA (species × growth [CO2] × growth temperature) for all leaf and gas exchange parameters measured in E. saligna and E. sideroxylon grown at three atmospheric [CO2] and two air temperatures
ParameterMain EffectsInteractions
Species[CO2]TempSpp. × [CO2]Spp. × Temp[CO2] × Temp
  • Spot measurements were made at target growth [CO2] (280, 400 and 640 µL L−1) and average midday air temperatures (28 and 32 °C). A/Ci curves were measured at average midday air temperatures (28 and 32 °C). A/TL curves were measured at target growth [CO2] and average daytime temperatures (26 or 30 °C). There were no three way species × [CO2]-temperature interactions. Significance levels are: ns = not significant (> 0.05);

  • *

    < 0.05;

  • **

    < 0.01;

  • ***

    < 0.001.

Leaf traits      
 LMA (g m−2)******nsnsns**
 Leaf N (mg g−1)*********ns*
 Leaf N (g m−2)***nsnsnsns*
Spot measurements      
 Asat (µmol m−2 s−1)******nsnsnsns
 gs (mol m−2 s−1)nsns**nsnsns
 Ci/Cans**nsnsns
 WUEL (mmol mol−1 s−1)ns******nsnsns
A/Ci curves      
 Amax (µmol m−2 s−1)****nsnsnsns
 Vcmax (µmol m−2 s−1)***ns***nsnsns
 Jmax (µmol m−2 s−1)****nsnsnsns
 Jmax/Vcmaxns**********ns
 Ls (%)ns***ns**nsns
A/TL curves      
 Asat at Topt (µmol m−2 s−1)******nsnsnsns
 Topt ( C)*******nsns*
 Curve parameter A****ns**nsns
 Curve parameter B*****ns*nsns
 Curve parameter C***nsnsnsnsns
 Slope before Topt****nsnsnsns
 Slope after Topt****nsnsnsns

At ambient temperature, LMA was greater at sub-ambient and elevated [CO2] relative to ambient [CO2] treatment in both species. At high temperature, LMA increased with increasing growth [CO2] in both species (Fig 1a & b, Table 1).

At ambient temperature, leaf [N] expressed on a dry mass basis [(N)mass] decreased between sub-ambient and ambient [CO2] in E. saligna only; and decreased between ambient and elevated [CO2] in both Eucalyptus species. At high temperature, leaf [N]mass decreased with increasing growth [CO2] in both eucalypts. High temperature reduced leaf [N]mass mainly in the elevated-[CO2]-grown eucalypt tree seedlings (Fig 1c & d, Table 1). Although there was a statistically significant [CO2]-temperature interaction for leaf [N] expressed on an area basis [(N)area], growth [CO2] and temperature had no effect on this parameter in either Eucalyptus species (Fig 1e & f, Table 1).

Gas exchange at growth conditions (Asat, gs, Ci/Ca and WUEL)

When measurements were made at growth temperature and [CO2], Asat was greater in E. sideroxylon relative to E. saligna plants. In contrast, gs, Ci/Ca and WUEL were similar between the two Eucalyptus species (Fig. 2, Table 1).

image

Figure 2. Light-saturated rates of photosynthesis, Asat (a and b), stomatal conductance, gs (c and d), the ratio of intercellular to ambient [CO2], Ci/Ca (e and f) and leaf water use efficiency, WUEL (g and h) of E. saligna (a, c, e and g) and E. sideroxylon (b, d, f and h) grown at two air temperatures and three atmospheric [CO2]. Values represent means ± SE of 5 leaves. Other details are as described for Fig. 1.

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Asat increased linearly with growth [CO2] but was not affected by growth temperature in both Eucalyptus species (Fig 2a & b, Table 1). gs increased in response to growth at high temperature. Although gs was generally not affected by growth [CO2], there was a non-significant trend for lower gs in sub-ambient [CO2]-high temperature-grown eucalypts relative to their ambient and elevated [CO2] counterparts (Fig 2c & d, Table 1). Increasing growth [CO2] and temperature led to small, positive but non-interactive effects on Ci/Ca, partly due to the temperature effect on gs and [CO2] effect on Asat. WUEL decreased at high temperature and increased with increasing [CO2] in both E. saligna and E. sideroxylon (Fig 2g & h, Table 1).

Analyses of the A/Ci curves (Amax, Vcmax, Jmax and Ls)

When measurements were made at growth temperature, E. saligna plants had lower Amax, Vcmax and Jmax when compared with E. sideroxylon (Figs 3–5, Table 1).

image

Figure 3. The response of CO2 assimilation rates to intercellular [CO2] in E. saligna (a, c and e) and E. sideroxylon (b, d and f) grown at ambient (▿○▵) and high temperature (▾●▴) and sub-ambient (290; ▿▾), ambient (400; ○●) and elevated (650 µL L−1; ▵▴) atmospheric [CO2]. Measurements were made at respective growth temperatures (corresponding to the average midday air temperature, 28 or 32 °C) and 1200 µmol quanta m−2 s−1. Values represent means ± SE of 5 leaves. The dashed (ambient temperature) and solid (high temperature) curves represent the output by the Farquhar et al. (1980) model of C3 photosynthesis using average Vcmax and Jmax (shown in Fig. 5) for each treatment, as described in the Materials and Methods.

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image

Figure 4. Light- and CO2-saturates rates of photosynthesis, Amax (a and b) and relative stomatal limitation, Ls (c and d) of E. saligna (a and c; ○●) and E. sideroxylon (b and d; ▵▴) grown at two air temperatures and three atmospheric [CO2]. Amax and Ls were estimated from the A/Ci curves shown in Fig. 3, as described in the Materials and Methods. Values represent means ± SE of 5 leaves. Other details are as described for Fig. 1.

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image

Figure 5. Apparent, maximal Rubisco-limited rate of photosynthesis, Vcmax (a and b), apparent, maximal electron transport-limited rate of photosynthesis, Jmax (c and d) and the ratio Jmax/Vcmax (e and f) of E. saligna (a, c and e) and E. sideroxylon (b, d and f) grown at two air temperatures and three atmospheric [CO2]. Vcmax and Jmax were estimated from the A/Ci curves shown in Fig. 3, as described in the Materials and Methods. Values represent means ± SE of 5 leaves. Other details are as described for Fig. 1.

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At both ambient and high temperatures, Amax increased and Ls decreased with increasing growth [CO2] in both Eucalyptus species; for E. sideroxylon grown at ambient temperature, Ls was similar for sub-ambient and ambient [CO2] treatments. Growth temperature had no effect on Amax or Ls in either eucalypt (Fig. 4, Table 1).

Growth [CO2] and temperature had contrasting effects on Vcmax and Jmax (Figs 3 and 5, Table 1). Vcmax increased at the higher growth temperature while Jmax was up-regulated by increasing growth [CO2] in both eucalypts. The ratio Jmax/Vcmax increased with growth [CO2] in E. sideroxylon only, and decreased more at high temperature in E. sideroxylon than in E. saligna (Fig. 5, Table 1). Both [CO2] and temperature had strong but non-interactive effects on Jmax/Vcmax (Fig. 5, Table 1).

Analyses of the A/TL curves (Asat at Topt, Topt and shape)

E. saligna plants had lower Asat at Topt and lower Topt relative to E. sideroxylon for all [CO2] and temperature treatments. In addition, the A/TL response curves were broader in E. saligna than in E. sideroxylon plants as indicated by the slopes of the A/TL curves around the Topt (Figs 6–8, Tables 1 and 2).

image

Figure 6. The response of CO2 assimilation rates to leaf temperature (A/TL) in E. saligna (a and c) and E. sideroxylon (b and d) grown at two air temperatures and three atmospheric [CO2]. Measurements were made at the respective, target growth [CO2] and 1500 µmol quanta m−2 s−1. Values are means ± SE of 4 leaves. The dashed (ambient temperature) and solid (high temperature) curves represent the output of the average polynomial fits (Asat = A + B*Tl + C*Tl2, where Tl is leaf temperature and A, B and C are the fitted parameters shown in Table 2) for each treatment. Other details are as described for Fig. 3.

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image

Figure 7. The simulated responses of CO2 assimilation rates to leaf temperature (A/TL) in E. saligna (a and b) and E. sideroxylon (c and d) grown at ambient (a and c) and high temperature (b and d) temperature, and sub-ambient (290 µL L−1; dashed and dotted lines), ambient (400 µL L−1; solid lines) and elevated (650 µL L−1; dashed lines) atmospheric [CO2]. Curves represent the output of the average polynomial fits (Asat = A + B*TL + C*TL2, where TL is leaf temperature and A, B and C are the fitted parameters shown in Table 2) for each treatment. Arrows indicate the Topt of the simulated A/TL curves.

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image

Figure 8. Asat at the temperature optimum (a and b) and the temperature optimum of Asat, Topt (c and d) of E. saligna (a and c) and E. sideroxylon (b and d) grown at two air temperatures and three atmospheric [CO2]. Values (means ± SE of 4 leaves) were estimated from the A/TL curves shown in Fig. 6, as described in the Materials and Methods. Other details are as described for Fig. 1.

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Table 2.  Characterization of the A/TL curves of E. saligna and E. sideroxylon grown at three atmospheric [CO2] and two air temperatures
Growth [CO2µL L−1E. salignaE. sideroxylon
290400650290400650
ParameterGrowth temp      
  1. Shown are outputs of the polynomial fits (Asat = A + B*TL + C*TL2, where TL is leaf temperature and A, B and C are the fitted parameters); and the slopes of the A/TL curves over 5 °C before and after Topt. Values are treatment averages ± SE of four leaves which were fitted separately.

Curve parameter AAmbient−2.7 ± 1.5−14.5 ± 5.6−14.9 ± 1.5−59.1 ± 13.7−15.7 ± 5.9−38.7 ± 13.7
High−1.1 ± 1.8−13.2 ± 3.8−21.0 ± 6.5−21.7 ± 5.1−25.4 ± 5.1−48.2 ± 19.5
Curve parameter BAmbient1.1 ± 0.182.1 ± 0.342.2 ± 0.164.7 ± 0.902.9 ± 0.414.8 ± 1.02
High0.9 ± 0.181.8 ± 0.212.3 ± 0.622.7 ± 0.383.5 ± 0.314.5 ± 1.50
Curve parameter CAmbient−0.022 ± 0.004−0.038 ± 0.005−0.038 ± 0.003−0.072 ± 0.017−0.048 ± 0.006−0.074 ± 0.017
High−0.017 ± 0.004−0.030 ± 0.003−0.036 ± 0.012−0.045 ± 0.005−0.056 ± 0.006−0.063 ± 0.025
Slope before ToptAmbient0.094 ± 0.0090.192 ± 0.0300.190 ± 0.0250.289 ± 0.0600.236 ± 0.0330.367 ± 0.075
High0.083 ± 0.0180.144 ± 0.0090.185 ± 0.0650.239 ± 0.0280.281 ± 0.0320.443 ± 0.104
Slope after ToptAmbient0.122 ± 0.0320.184 ± 0.0250.193 ± 0.0130.264 ± 0.0380.242 ± 0.0290.378 ± 0.091
High0.084 ± 0.0200.158 ± 0.0190.177 ± 0.0560.210 ± 0.0240.277 ± 0.0270.369 ± 0.087

Asat at Topt was enhanced by increasing growth [CO2] in both E. saligna and E. sideroxylon, and this enhancement was independent of growth temperature. For both Eucalyptus species, high temperature had no effect on Asat at Topt for all [CO2] treatments (Figs 6–8a & b, Table 1).

At ambient temperature, Topt was not significantly affected by increasing [CO2] in either Eucalyptus species. At high temperature, Topt increased by 2–3 °C between sub-ambient and ambient [CO2], and increased by 5–7 °C between ambient and elevated [CO2] for both eucalypts. High temperature increased Topt at elevated [CO2] only (Fig 8c & d, Table 1).

The slopes of the A/TL curves increased with increasing growth [CO2], as indicated by the higher slopes around the Topt. The shape of the A/TL curve was not affected by growth temperature (Figs 6–8, Tables 1 and 2).

DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Effects of sub-ambient [CO2] on photosynthetic capacity

Pre-industrial [CO2] imposes significant short-term limitations on C3 photosynthesis, particularly at warm temperatures (Farquhar et al. 1980). This was apparent in the low Asat and high Ls measured at sub-ambient [CO2] and high temperature for the two Eucalyptus species in this study, as well as in the reduced growth reported for the same species in a previous study (Ghannoum et al. 2010). Our gas exchange results at sub-ambient [CO2] concur with what has been reported in the literature for a number of C3 species grown at pre-industrial or glacial [CO2] (Polley, Johnson & Mayeux 1992; Tissue et al. 1995; Ward et al. 1999; Sage & Coleman 2001).

In the long term, sub-ambient [CO2] is expected to elicit an acclimation response. For example, Gesch et al. (2000) reported increased Rubisco activity in developing rice leaves exposed to sub-ambient [CO2]. In our study, Amax, Vcmax and Jmax were somewhat similar between sub-ambient and ambient [CO2] eucalypts grown at ambient temperature. These results suggest that there was a preservation, rather than absolute up-regulation, of photosynthetic capacity at sub-ambient [CO2] and ambient temperature. This response may partially explain the lack of growth reduction in the Eucalyptus tree seedlings grown at sub-ambient [CO2] and ambient temperature, when compared with their ambient-[CO2]-grown counterparts (Ghannoum et al. 2010). This view is supported by the observation that, at ambient temperature, LMA was similar for the sub-ambient and ambient [CO2] treatments. Given that [CO2]-induced changes in LMA are usually related to parallel changes in carbohydrates, it may be concluded that leaves of sub-ambient-[CO2]-grown eucalypts were not more carbon-limited than their ambient-[CO2]-grown counterparts, at ambient temperature.

Effects of elevated [CO2] on photosynthetic capacity

In response to growth at elevated [CO2], photosynthetic capacity of many plant species is either down-regulated or unchanged (Wong, Kriedemann & Farquhar 1992; Idso & Idso 1994; Tissue, Griffin & Ball 1999; Evans et al. 2000; Ellsworth et al. 2004; Ainsworth & Rogers 2007). Accordingly, we hypothesized that elevated [CO2] will lead to photosynthetic acclimation that is stronger in the slower growing E. sideroxylon species. Both hypotheses were rejected in this study. Growth at elevated [CO2] stimulated Amax in both Eucalyptus species and at both growth temperatures. Although E. sideroxylon grows more slowly than E. saligna, the former species exhibited greater biomass responses to elevated [CO2] (Ghannoum et al. 2010). Increased Amax at elevated [CO2] was underpinned by an up-regulation of Jmax and disconnected from changes in leaf [N]area, which was insensitive to elevated [CO2] due to corresponding changes in LMA.

At warm temperatures, increasing [CO2] shifts C3 photosynthesis from a state of RuBP carboxylation limitation to a state of electron transport limitation (Farquhar et al. 1980; Sharkey 1985; Bernacchi et al. 2009). Plants grown at elevated [CO2] may redress this shift by up-regulating Jmax, with or without a concomitant down-regulation of Vcmax (Sage 1994). Hence, the up-regulation of Jmax observed in E. saligna and E. sideroxylon is a plausible response which has been reported in other studies (e.g. Sage, Sharkey & Seemann 1989; Ziska et al. 1991). By selectively up-regulating Jmax, increasing [CO2] resulted in greater Jmax/Vcmax ratio for the two eucalypts. Although Jmax/Vcmax tends to remain constant at elevated [CO2] (Turnbull et al. 1998; Medlyn, Badeck & De Pury 1999; Lewis et al. 2004; Sholtis et al. 2004; Crous, Walters & Ellsworth 2008), a number of studies have reported an effect of elevated [CO2] on Jmax/Vcmax (Sage et al. 1989; Lewis, Tissue & Strain 1996; Li et al. 1999). It is possible that the Jmax/Vcmax ratio is conserved when rising [CO2] leads to photosynthetic acclimation. However, when rising [CO2] exerts a stimulatory effect on photosynthetic capacity, it is more efficient to up-regulate Jmax selectively, as there will be limited gain in up-regulating Rubisco activity under elevated [CO2].

Effects of high temperature on photosynthetic capacity

In our study, growth at high temperature led to a homeostatic, thermal acclimation of photosynthesis (Hikosaka et al. 2006), such that Asat measured at growth [CO2] and temperature was similar for ambient- and high-temperature-grown eucalypts. The thermal acclimation of Asat may in part reflect a loss of Rubisco activation at high temperature, particularly above the thermal optimum (Crafts-Brandner & Salvucci 2004; Sage et al. 2008). For both Eucalyptus species, measurement temperatures were higher than Topt in the high-temperature-sub-ambient [CO2] and high-temperature-ambient [CO2] treatments. Although measurement temperatures were lower than Topt for the elevated [CO2] treatments, the combination of elevated [CO2] and high temperature can significantly reduce Rubisco activation state (Crafts-Brandner & Salvucci 2004; Sage & Kubien 2007). Higher growth temperatures led to increased Vcmax but did not affect Jmax, consequently decreasing Jmax/Vcmax in both Eucalyptus species. These results are in agreement with findings from numerous studies in response to short-term (Dreyer et al. 2001; Leuning 2002; Medlyn et al. 2002) and long-term (Hikosaka, Murakami & Hirose 1999; Onoda, Hikosaka & Hirose 2005; Yamori et al. 2005; Kattge & Knorr 2007) increases in temperature. Changes in Jmax/Vcmax with growth temperature can be due to differences in the temperature dependence of Jmax and Vcmax and/or changes in nitrogen partitioning within the photosynthetic apparatus (Onoda et al. 2005; Hikosaka et al. 2006).

By affecting chloroplastic [CO2], mesophyll conductance (gm) may influence the estimates of Vcmax and Jmax (Bernacchi et al. 2002). Mesophyll conductance is known to increase with both growth and measurement temperature (Bernacchi et al. 2002; Yamori et al. 2006; Warren 2008). For example, over a temperature range similar to that used in our study (c. 4 °C), gm of some Eucalyptus species varied by 10–15% when determined at 30 °C (Warren & Dreyer 2006; Warren 2008). Hence, there is a degree of uncertainty in our estimates of Vcmax and Jmax due to gm variations. Currently, it is not possible to quantify this uncertainty due to our lack of knowledge about the temperature dependence of gm and Rubisco kinetics in eucalypts.

Effects of growth [CO2] on the photosynthetic thermal optimum

For both E. saligna and E. sideroxylon, growth [CO2] had little effect on Topt in the ambient-temperature-grown tree seedlings. At high temperature, Topt increased to a greater extent between ambient and elevated [CO2] than between sub-ambient and ambient [CO2]. Topt estimated from A/TL response curves depends on Rubisco kinetics as well as on the balance between RuBP carboxylation and RuBP regeneration. Topt of Asat represents the lowest Topt of these two limiting processes (Long 1991; Leuning 2002; Hikosaka et al. 2006; Sage & Kubien 2007). Short term increases in [CO2] lead to a greater stimulation of RuBP carboxylation relative to RuBP regeneration. This would result in greater Topt of Asat because Topt of Jmax is greater than that of Vcmax. These short-term responses are modulated by the long-term acclimation responses. In our study, growth at increasing [CO2] led to up-regulation of Jmax. Therefore, while short-term increases in [CO2] were expected to reduce Jmax/Vcmax and hence increase Topt of Asat, long-term acclimation to increasing [CO2] led to increased Jmax/Vcmax and hence decreased Topt. The outcome of these opposing responses was that Topt of Asat measured at growth [CO2] changed little at ambient temperature. A similar dynamic was also expected to occur at high temperature. However, at high temperature, the relative short-term stimulatory effect of increasing [CO2] on Topt was more pronounced (due to higher photorespiration), thus resulting in increased Topt of Asat in response to higher growth [CO2].

Effects of growth temperature on the photosynthetic thermal optimum

For many plants, growth at high temperature increases the Topt of Asat (Berry & Björkman 1980). For example, Topt increased by 0.34–0.54 °C °C−1 in some Eucalyptus species exposed to large seasonal temperature fluctuations (7–19 °C) in the field (Battaglia et al. 1996). In our study, high temperature stimulated Vcmax and reduced Jmax/Vcmax. As a result, Topt of Asat was expected to increase with growth temperature. However, for both Eucalyptus species, high temperature enhanced Topt of Asat by 1.3–1.8 °C °C−1 in elevated-[CO2]-grown tree seedlings only. Similar trends were reported by Alonso et al. (2008) and Badger, Bjorkman & Armond (1982). At low [CO2], Asat is limited by RuBP carboxylation, which is not very sensitive to temperature at low [CO2]. This occurs because high temperature-stimulation of Vcmax is counterbalanced by increased photorespiration and the greater increase in O2 solubility relative to that for CO2. At high [CO2], these counterbalancing effects are less pronounced (Hikosaka et al. 2006).

CONCLUSIONS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. REFERENCES

Photosynthesis of two Eucalyptus species acclimated to a warming of 4 °C, such that Asat, measured at growth [CO2] and temperature, was similar for ambient- and high-temperature-grown tree seedlings. For both species and at both growth temperatures, Asat doubled between sub-ambient and ambient [CO2] and between ambient and elevated [CO2]. Increasing [CO2] stimulated Amax by 10% due to an equivalent up-regulation of Jmax. The thermal optimum of Asat increased by 2–7 °C with increasing [CO2] at high temperature. In conclusion, when water and nutrient supplies are non-limiting, photosynthesis of eucalypt seedlings is expected to strongly increase with rising [CO2] and temperature. These results partly explain the strong growth responses to elevated [CO2] and temperature observed in a previous study with the same eucalypt seedlings (Ghannoum et al. 2010).

ACKNOWLEDGMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. REFERENCES

We appreciate the technical assistance of Renee Smith, Renee Attard, Roslyn Woodfield and Kaushal Tewari with plant growth and chemical analyses. This research was supported by an Australian Research Council Discovery Project grant (DP0879531; DT, JC, BL, NP), a University of Western Sydney International Research grant (71827; NP) and a travel grant from the Grua/O'Connell endowment at Bowdoin College (MS).

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. CONCLUSIONS
  8. ACKNOWLEDGMENTS
  9. REFERENCES