Temperature Response Of Photosynthesis Research Articles

The stomatal response to vapor pressure deficit drives the apparent temperature response of photosynthesis in tropical forests.

As temperature rises, net carbon uptake in tropical forests decreases, but the underlying mechanisms are not well understood. High temperatures can limit photosynthesis directly, for example by reducing biochemical capacity, or indirectly through rising vapor pressure deficit (VPD) causing stomatal closure. To explore the independent effects of temperature and VPD on photosynthesis we analyzed photosynthesis data from the upper canopies of two tropical forests in Panama with Generalized Additive Models. Stomatal conductance and photosynthesis consistently decreased with increasing VPD, and statistically accounting for VPD increased the optimum temperature of photosynthesis (Topt) of trees from a VPD-confounded apparent Topt of c. 30-31°C to a VPD-independent Topt of c. 33-36°C, while for lianas no VPD-independent Topt was reached within the measured temperature range. Trees and lianas exhibited similar temperature and VPD responses in both forests, despite 1500 mm difference in mean annual rainfall. Over ecologically relevant temperature ranges, photosynthesis in tropical forests is largely limited by indirect effects of warming, through changes in VPD, not by direct warming effects of photosynthetic biochemistry. Failing to account for VPD when determining Topt misattributes the underlying causal mechanism and thereby hinders the advancement of mechanistic understanding of global warming effects on tropical forest carbon dynamics.

Contrasting responses of two C4 desert shrubs to drought but consistent decoupling of photosynthesis and stomatal conductance at high temperature

How desert shrubs respond to drought and heat remains poorly understood. We investigated how drought affected the morphology and physiology of two C4 species with different root architecture, Calligonum arborescens and Haloxylon ammodendron, in the Taklamakan Desert. We measured leaf traits, chlorophyll fluorescence and temperature responses of photosynthesis after ∼3 years of experimental drought and asked: (1) Do both species adjust similarly to drought? (2) Does transpirational cooling increase at high temperature, and if so, how is this affected by drought? Drought plants of H. ammodendron had shorter and thinner leaves than controls, and lower chlorophyll content, photochemical efficiency, and electron transport rate. In contrast, C. arborescens leaves became shorter and thicker. Drought reduced maximum photosynthesis by 63 % and 21 % in H. ammodendron and C. arborescens, respectively. The optimum temperature did not change significantly in H. ammodendron and decreased by ∼2 °C in C. arborescens. Surprisingly, above 40–45 °C, stomatal conductance (gsw) and transpiration increased—even under drought stomata partially reopened. This consistent uncoupling of photosynthesis and gsw suggests that widely-implemented stomatal optimization models may poorly reflect high-temperature behaviors in dryland ecosystems. Our study established the basis for predicting the eco-physiological responses of C4 species in hyper-arid ecosystems to climate change.

Solar-induced chlorophyll fluorescence imperfectly tracks the temperature response of photosynthesis in winter wheat.

Solar-induced fluorescence (SIF) is a promising proxy for photosynthesis, but it is unclear whether it performs well in tracking the gross primary productivity (GPP) under different environmental conditions. In this study, we investigated the dynamics of the two parameters from October 2020 to June 2021 in field-grown winter wheat (Triticum aestivum) and found that the ability of SIF to track GPP was weakened at low temperatures. Accounting for the coupling of light and temperature at a seasonal scale, we found that SIF yield showed a lower temperature sensitivity and had a lower but broader optimal temperature range compared with light-use efficiency (LUE), although both SIF yield and LUE decreased in low-temperature conditions. The discrepancy between the temperature responses of SIF yield and GPP caused an increase in the ratio of SIF/GPP in winter, which indicated the variation in the relationship between them during this period. The results of our study highlight the impact of low temperature on the relationship between SIF and GPP and show the necessity of reconsidering the dynamics of energy distribution inside plants under changing environments.

Temperature responses of photosynthesis and respiration in evergreen trees from boreal to tropical latitudes.

Evergreen species are widespread across the globe, representing two major plant functional forms in terrestrial models. We reviewed and analysed the responses of photosynthesis and respiration to warming in 101 evergreen species from boreal to tropical biomes. Summertime temperatures affected both latitudinal gas exchange rates and the degree of responsiveness to experimental warming. The decrease in net photosynthesis at 25°C (Anet25 ) was larger with warming in tropical climates than cooler ones. Respiration at 25°C (R25 ) was reduced by 14% in response to warming across species and biomes. Gymnosperms were more sensitive to greater amounts of warming than broadleaved evergreens, with Anet25 and R25 reduced c. 30-40% with > 10°C warming. While standardised rates of carboxylation (Vcmax25 ) and electron transport (Jmax25 ) adjusted to warming, the magnitude of this adjustment was not related to warming amount (range 0.6-16°C). The temperature optimum of photosynthesis (ToptA ) increased on average 0.34°C per °C warming. The combination of more constrained acclimation of photosynthesis and increasing respiration rates with warming could possibly result in a reduced carbon sink in future warmer climates. The predictable patterns of thermal acclimation across biomes provide a strong basis to improve modelling predictions of the future terrestrial carbon sink with warming.

Reduced photosynthetic thermal acclimation capacity under elevated ozone in poplar (Populus tremula) saplings.

The sensitivity of photosynthesis to temperature has been identified as a key uncertainty for projecting the magnitude of the terrestrial carbon cycle response to future climate change. Although thermal acclimation of photosynthesis under rising temperature has been reported in many tree species, whether tropospheric ozone (O3 ) affects the acclimation capacity remains unknown. In this study, temperature responses of photosynthesis (light-saturated rate of photosynthesis (Asat ), maximum rates of RuBP carboxylation (Vcmax ), and electron transport (Jmax ) and dark respiration (Rdark ) of Populus tremula exposed to ambient O3 (AO3 , maximum of 30ppb) or elevated O3 (EO3 , maximum of 110ppb) and ambient or elevated temperature (ambient +5°C) were investigated in solardomes. We found that the optimum temperature of Asat (ToptA ) significantly increased in response to warming. However, the thermal acclimation capacity was reduced by O3 exposure, as indicated by decreased ToptA , and temperature optima of Vcmax (ToptV ) and Jmax (ToptJ ) under EO3 . Changes in both stomatal conductance (gs ) and photosynthetic capacity (Vcmax and Jmax ) contributed to the shift of ToptA by warming and EO3 . Neither Rdark measured at 25°C ( ) nor the temperature response of Rdark was affected by warming, EO3 , or their combination. The responses of Asat , Vcmax , and Jmax to warming and EO3 were closely correlated with changes in leaf nitrogen (N) content and N use efficiency. Overall, warming stimulated growth (leaf biomass and tree height), whereas EO3 reduced growth (leaf and woody biomass). The findings indicate that thermal acclimation of Asat may be overestimated if the impact of O3 pollution is not taken into account.

Temperature responses of photosynthesis and stomatal conductance in rice and wheat plants

The mechanisms for the different temperature (T) responses of leaf photosynthetic rate (A) and stomatal conductance (gs) have not been thoroughly elucidated, and the uncertainties and limitations facing the optimal-stomata models have not been systematically investigated. In the present study, the temperature responses of A, gs, and mesophyll conductance (gm) were studied in the selected rice and wheat genotypes, and the C3 photosynthesis model and three widely used optimal-stomata models were introduced to investigate these two questions. The photosynthetic limiting process varied significantly between rice and wheat plants. The photosynthetic limitation and gm had strong impacts on the temperature response of λ, the key parameter for the optimal-stomata models. λ was increased with T when photosynthesis was limited by the RuBP carboxylation, while it was decreased with T when it was limited by the RuBP regeneration. Because of the negative correlation between λ and gs, the differing λ-T relationships may explain the different gs-T relationships between the selected genotypes. This is the first study using the optimal-stomata models to interpret the differential gs-T relationships and highlights the importance of estimating gm and the photosynthetic limitations before applying the optimal-stomata models.

Temperature responses of photosynthesis and leaf hydraulic conductance in rice and wheat.

Studies on the temperature (T) responses of photosynthesis and leaf hydraulic conductance (Kleaf ) are important to plant gas exchange. In this study, the temperature responses of photosynthesis and Kleaf were studied in Shanyou 63 (Oryza sativa) and Yannong 19 (Triticum aestivum). Leaf water potential (Ψleaf ) was insensitive to T in Shanyou 63, while it significantly decreased with T in Yannong 19. The differential Ψleaf - T relationship partially accounted for the differing gm -T relationships, where gm was less sensitive to T in Yannong 19 than in Shanyou 63. With different gm -T and Ψleaf -T relationships, the temperature responses of photosynthetic limitations were surprisingly similar between the two lines, and the photosynthetic rate was highly correlated with gm . With the increasing T, Kleaf increased in Shanyou 63 while it decreased in Yannong 19. The different Kleaf -T relationships were related to different Ψleaf -T relationships. When excluding the effects of water viscosity and Ψleaf , Kleaf was insensitive to T in both lines. gm and Kleaf were generally not coordinated across different temperatures. This study highlights the importance of Ψleaf on leaf carbon and water exchanges, and the mechanisms for the gm -T and Kleaf -T relationships were discussed.

Variation in Responses of Photosynthesis and Apparent Rubisco Kinetics to Temperature in Three Soybean Cultivars.

Recent in vivo assays of the responses of Rubisco to temperature in C3 plants have revealed substantial diversity. Three cultivars of soybean (Glycine max L. Merr.), Holt, Fiskeby V, and Spencer, were grown in indoor chambers at 15, 20, and 25 °C. Leaf photosynthesis was measured over the range of 15 to 30 °C, deliberately avoiding higher temperatures which may cause deactivation of Rubisco, in order to test for differences in temperature responses of photosynthesis, and to investigate in vivo Rubisco kinetic characteristics responsible for any differences observed. The three cultivars differed in the optimum temperature for photosynthesis (from 15 to 30 °C) at 400 μmol mol−1 external CO2 concentration when grown at 15 °C, and in the shapes of the response curves when grown at 25 °C. The apparent activation energy of the maximum carboxylation rate of Rubisco differed substantially between cultivars at all growth temperatures, as well as changing with growth temperature in two of the cultivars. The activation energy ranged from 58 to 84 kJ mol−1, compared with the value of 64 kJ mol−1 used in many photosynthesis models. Much less variation in temperature responses occurred in photosynthesis measured at nearly saturating CO2 levels, suggesting more diversity in Rubisco than in electron transport thermal properties among these soybean cultivars.

Exploring high temperature responses of photosynthesis and respiration to improve heat tolerance in wheat.

High temperatures account for major wheat yield losses annually and, as the climate continues to warm, these losses will probably increase. Both photosynthesis and respiration are the main determinants of carbon balance and growth in wheat, and both are sensitive to high temperature. Wheat is able to acclimate photosynthesis and respiration to high temperature, and thus reduce the negative affects on growth. The capacity to adjust these processes to better suit warmer conditions stands as a potential avenue toward reducing heat-induced yield losses in the future. However, much remains to be learnt about such phenomena. Here, we review what is known of high temperature tolerance in wheat, focusing predominantly on the high temperature responses of photosynthesis and respiration. We also identify the many unknowns that surround this area, particularly with respect to the high temperature response of wheat respiration and the consequences of this for growth and yield. It is concluded that further investigation into the response of photosynthesis and respiration to high temperature could present several methods of improving wheat high temperature tolerance. Extending our knowledge in this area could also lead to more immediate benefits, such as the enhancement of current crop models.

Acclimation and adaptation components of the temperature dependence of plant photosynthesis at the global scale.

The temperature response of photosynthesis is one of the key factors determining predicted responses to warming in global vegetation models (GVMs). The response may vary geographically, owing to genetic adaptation to climate, and temporally, as a result of acclimation to changes in ambient temperature. Our goal was to develop a robust quantitative global model representing acclimation and adaptation of photosynthetic temperature responses. We quantified and modelled key mechanisms responsible for photosynthetic temperature acclimation and adaptation using a global dataset of photosynthetic CO2 response curves, including data from 141 C3 species from tropical rainforest to Arctic tundra. We separated temperature acclimation and adaptation processes by considering seasonal and common-garden datasets, respectively. The observed global variation in the temperature optimum of photosynthesis was primarily explained by biochemical limitations to photosynthesis, rather than stomatal conductance or respiration. We found acclimation to growth temperature to be a stronger driver of this variation than adaptation to temperature at climate of origin. We developed a summary model to represent photosynthetic temperature responses and showed that it predicted the observed global variation in optimal temperatures with high accuracy. This novel algorithm should enable improved prediction of the function of global ecosystems in a warming climate.

Temperature Responses of Photosynthesis and Respiration of Maize (Zea mays) Plants to Experimental Warming

Understanding the key processes and mechanisms of photosynthetic and respiratory acclimation of maize (Zea mays L.) plants in response to experimental warming may further shed lights on the changes in the carbon exchange and Net Primary Production (NPP) of agricultural ecosystem in a warmer climate regime. In the current study, we examined the temperature responses and sensitivity of foliar photosynthesis and respiration for exploring the mechanisms of thermal acclimation associated with physiological and biochemical processes in the North China Plain (NCP) with a field manipulative warming experiment. We found that thermal acclimation of An as evidenced by the upward shift of An-T was determined by the maximum velocity of Rubisco carboxylation (Vcmax), the maximum rate of electron transport (Jmax), and the stomatal- regulated CO2 diffusion process (gs), while the balance between respiration and photosynthesis (Rd/Ag), and/or regeneration of RuBP and the Rubisco carboxylation (Jmax/Vcmax) barely affected the thermal acclimation of An. We also found that the temperature response and sensitivity of Rd was closely associated with the changes in foliar N concentration induced by warming. These results suggest that the leaf-level thermal acclimation of photosynthesis and respiration may mitigate or even offset the negative impacts on maize from future climate warming, which should be considered to improve the accuracy of process-based ecosystem models under future climate warming.

A common thermal niche among geographically diverse populations of the widely distributed tree species Eucalyptus tereticornis: No evidence for adaptation to climate-of-origin.

Impacts of climate warming depend on the degree to which plants are constrained by adaptation to their climate-of-origin or exhibit broad climatic suitability. We grew cool-origin, central and warm-origin provenances of Eucalyptus tereticornis in an array of common temperature environments from 18 to 35.5°C to determine if this widely distributed tree species consists of geographically contrasting provenances with differentiated and narrow thermal niches, or if provenances share a common thermal niche. The temperature responses of photosynthesis, respiration, and growth were equivalent across the three provenances, reflecting a common thermal niche despite a 2,200km geographic distance and 13°C difference in mean annual temperature at seed origin. The temperature dependence of growth was primarily mediated by changes in leaf area per unit plant mass, photosynthesis, and whole-plant respiration. Thermal acclimation of leaf, stem, and root respiration moderated the increase in respiration with temperature, but acclimation was constrained at high temperatures. We conclude that this species consists of provenances that are not differentiated in their thermal responses, thus rejecting our hypothesis of adaptation to climate-of-origin and suggesting a shared thermal niche. In addition, growth declines with warming above the temperature optima were driven by reductions in whole-plant leaf area and increased respiratory carbon losses. The impacts of climate warming will nonetheless vary across the geographic range of this and other such species, depending primarily on each provenance's climate position on the temperature response curves for photosynthesis, respiration, and growth.

Short-term acclimation to warmer temperatures accelerates leaf carbon exchange processes across plant types.

While temperature responses of photosynthesis and plant respiration are known to acclimate over time in many species, few studies have been designed to directly compare process-level differences in acclimation capacity among plant types. We assessed short-term (7day) temperature acclimation of the maximum rate of Rubisco carboxylation (Vcmax ), the maximum rate of electron transport (Jmax ), the maximum rate of phosphoenolpyruvate carboxylase carboxylation (Vpmax ), and foliar dark respiration (Rd ) in 22 plant species that varied in lifespan (annual and perennial), photosynthetic pathway (C3 and C4 ), and climate of origin (tropical and nontropical) grown under fertilized, well-watered conditions. In general, acclimation to warmer temperatures increased the rate of each process. The relative increase in different photosynthetic processes varied by plant type, with C3 species tending to preferentially accelerate CO2 -limited photosynthetic processes and respiration and C4 species tending to preferentially accelerate light-limited photosynthetic processes under warmer conditions. Rd acclimation to warmer temperatures caused a reduction in temperature sensitivity that resulted in slower rates at high leaf temperatures. Rd acclimation was similar across plant types. These results suggest that temperature acclimation of the biochemical processes that underlie plant carbon exchange is common across different plant types, but that acclimation to warmer temperatures tends to have a relatively greater positive effect on the processes most limiting to carbon assimilation, which differ by plant type. The acclimation responses observed here suggest that warmer conditions should lead to increased rates of carbon assimilation when water and nutrients are not limiting.

Insitu temperature response of photosynthesis of 42 tree and liana species in the canopy of two Panamanian lowland tropical forests with contrasting rainfall regimes.

Tropical forests contribute significantly to the global carbon cycle, but little is known about the temperature response of photosynthetic carbon uptake in tropical species, and how this varies within and across forests. We determined insitu photosynthetic temperature-response curves for upper canopy leaves of 42 tree and liana species from two tropical forests in Panama with contrasting rainfall regimes. On the basis of seedling studies, we hypothesized that species with high photosynthetic capacity - light-demanding, fast-growing species - would have a higher temperature optimum of photosynthesis (TOpt ) than species with low photosynthetic capacity - shade-tolerant, slow-growing species - and that, therefore, TOpt would scale with the position of a species on the slow-fast continuum of plant functional traits. TOpt was remarkably similar across species, regardless of their photosynthetic capacity and other plant functional traits. Community-average TOpt was almost identical to mean maximum daytime temperature, which was higher in the dry forest. Photosynthesis above TOpt appeared to be more strongly limited by stomatal conductance in the dry forest than in the wet forest. The observation that all species in a community shared similar TOpt values suggests that photosynthetic performance is optimized under current temperature regimes. These results should facilitate the scaling up of photosynthesis in relation to temperature from leaf to stand level in species-rich tropical forests.

Transgenically altered lignin biosynthesis affects photosynthesis and water relations of field-grown Populus trichocarpa

Concerns over energy security and environmental sustainability have stimulated interest in development of high-yield, low-lignin trees for bioenergy. Black cottonwood (Populus trichocarpa) has been targeted as a potential bioenergy species due to its high productivity, but it is unclear how transgenically altered lignin biosynthesis will affect plant function. We investigated the physiology of two transgenic P. trichocarpa genotypes grown in short rotation woody cropping systems at two sites in southeastern USA: (1) mesic mountain site and (2) warmer, drier Piedmont site. Our results suggest that lignin is fundamental for tree growth and survival in field environments. Lignin deficiency can decrease biochemical photosynthetic processes and interfere with the temperature-response of photosynthesis. Significantly, hydraulic conductivity of transgenic genotypes was 15–25% that of wildtype trees, resulting in decreased leaf-specific whole-plant hydraulic conductance. In the Piedmont, decreased hydraulic efficiency drastically reduced productivity of low-lignin genotypes by 50–70% relative to wildtype. Transgenic trees at the mountain site recovered stem lignin concentrations to levels observed in wildtype trees, but still had severely impaired hydraulic traits, highlighting the major consequences of genetic transformation on whole-plant function. Surprisingly, substantial loss of hydraulic conductivity had only minor effects on productivity at the mesic site and resulted in an alternative advantage for bioenergy systems – lower water consumption. In the hottest month (July), higher intrinsic water use efficiency resulted in total water savings of roughly 1 kg d−1 per transgenic tree without sacrificing productivity. Decreased hydraulic conductivity could therefore be a promising trait for selection of water-efficient genotypes in Populus.

Diverging temperature responses of CO2 assimilation and plant development explain the overall effect of temperature on biomass accumulation in wheat leaves and grains.

There is a growing consensus in the literature that rising temperatures influence the rate of biomass accumulation by shortening the development of plant organs and the whole plant and by altering rates of respiration and photosynthesis. A model describing the net effects of these processes on biomass would be useful, but would need to reconcile reported differences in the effects of night and day temperature on plant productivity. In this study, the working hypothesis was that the temperature responses of CO2 assimilation and plant development rates were divergent, and that their net effects could explain observed differences in biomass accumulation. In wheat (Triticum aestivum) plants, we followed the temperature responses of photosynthesis, respiration and leaf elongation, and confirmed that their responses diverged. We measured the amount of carbon assimilated per "unit of plant development" in each scenario and compared it to the biomass that accumulated in growing leaves and grains. Our results suggested that, up to a temperature optimum, the rate of any developmental process increased with temperature more rapidly than that of CO2 assimilation and that this discrepancy, summarised by the CO2 assimilation rate per unit of plant development, could explain the observed reductions in biomass accumulation in plant organs under high temperatures. The model described the effects of night and day temperature equally well, and offers a simple framework for describing the effects of temperature on plant growth.

Conditional variations in temperature response of photosynthesis, mesophyll and stomatal control of water use in rice and winter wheat

Environmental responses of photosynthesis and CO2 diffusive conductance as fundamental information for photosynthesis-transpiration coupled model have been increasingly concerned while are still research areas with unanswered questions in cereal crops. Photosynthesis (A), light utilization efficiency (α), mesophyll conductance (gm) and stomatal coefficient (gfac) in temperate rice and winter wheat were investigated. There were no seasonal trend, no inter- and intra-species differences in relative temperature responses (activation energy ΔHa) of Vcmax or Jmax. A phenomenon that grain-filling plants generally decreased gm and had a lower Q10 as compared to that at early growth stage existed, particularly in rice. Analyses of environmental influences indicated that optimal temperatures (Topt) in Vcmax/Jmax depended on the prevailing temperature environment. Although prevailing temperature dependence in Topt of gm was not as profound as that of Vcmax or Jmax, Topt of gm in winter wheat was significantly lower than rice. Temperature response of Rdark in all sampled leaves shared a common trajectory. α were almost invariant, while, high sensitivity to soil desiccation was observed. gfac in sunlit leaves was conservative during wet soil conditions. Shaded leaves with lower Na had higher gfac, resulting in a negative correlation between Na and gfac in canopy profiles. gfac was susceptible to fluctuations in soil water potential (ψs), rapidly declined at a threshold of top-layer ψs approx. −0.1MPa. Numerical analyses regarding gm/Vcmax/Jmax effects on photosynthetic performance in rice and between rice and winter wheat documented that in context of climate change, consider growth environment-induced differences in temperature responses of photosynthetic parameters among cereal crops is indispensable to better predict interactions among soil-plant-atmosphere consortium.

The effects of CO2 and nutrient fertilisation on the growth and temperature response of the mangrove Avicennia germinans.

In order to understand plant responses to both the widespread phenomenon of increased nutrient inputs to coastal zones and the concurrent rise in atmospheric CO2 concentrations, CO2-nutrient interactions need to be considered. In addition to its potential stimulating effect on photosynthesis and growth, elevated CO2 affects the temperature response of photosynthesis. The scarcity of experiments testing how elevated CO2 affects the temperature response of tropical trees hinders our ability to model future primary productivity. In a glasshouse study, we examined the effects of elevated CO2 (800ppm) and nutrient availability on seedlings of the widespread mangrove Avicennia germinans. We assessed photosynthetic performance, the temperature response of photosynthesis, seedling growth and biomass allocation. We found large synergistic gains in both growth (42%) and photosynthesis (115%) when seedlings grown under elevated CO2 were supplied with elevated nutrient concentrations relative to their ambient growing conditions. Growth was significantly enhanced under elevated CO2 only under high-nutrient conditions, mainly in above-ground tissues. Under low-nutrient conditions and elevated CO2, root volume was more than double that of seedlings grown under ambient CO2 levels. Elevated CO2 significantly increased the temperature optimum for photosynthesis by ca. 4°C. Rising CO2 concentrations are likely to have a significant positive effect on the growth rate of A. germinans over the next century, especially in areas where nutrient availability is high.

Leaf-age effects on temperature responses of photosynthesis and respiration of an alpine oak, Quercus aquifolioides, in southwestern China.

Temperature responses and sensitivity of photosynthesis (A(n_)T) and respiration for leaves at different ages are crucial to modeling ecosystem carbon (C) cycles and productivity of evergreen forests. Understanding the mechanisms and processes of temperature sensitivity may further shed lights on temperature acclimation of photosynthesis and respiration with leaf aging. The current study examined temperature responses of photosynthesis and respiration of young leaves (YLs) (fully expanded in current growth season) and old leaves (OLs) (fully expanded in last growth season) of Quercus aquifolioides Rehder and E.H. Wilson in an alpine oak forest, southwestern China. Temperature responses of dark respiration (R(dark)), net assimilation (A(n)), maximal velocity of carboxylation (V(cmax)) and maximum rate of electron transport (J(max)) were significantly different between the two leaf ages. Those differences implied different temperature response parameters should be used for leaves of different ages in modeling vegetation productivity and ecosystem C cycles in Q. aquifolioides forests and other evergreen forests. We found that RuBP carboxylation determined the downward shift of A(n_)T in OLs, while RuBP regeneration and the balance between Rubisco carboxylation and RuBP regeneration made little contribution. Sensitivity of stomatal conductance to vapor pressure deficit changed in OLs and compensated part of the downward shift. We also found that OLs of Q. aquifolioides had lower An due to lower stomatal conductance, higher stomatal conductance limitation and deactivation of the biochemical processes. In addition, the balance between R(dark) and A(n) changed between OLs and YLs, which was represented by a higher R(dark)/A(n) ratio for OLs.

Intraspecific variation in thermal acclimation of photosynthesis across a range of temperatures in a perennial crop.

Interest in the thermal acclimation of photosynthesis has been stimulated by the increasing relevance of climate change. However, little is known about intra-specific variations in thermal acclimation and its potential for breeding. In this article, we examined the difference in thermal acclimation between alfalfa (Medicago sativa) cultivars originating from contrasting origins, and sought to analyze the mechanisms in play. A series of experiments was carried out at seven growth temperatures between 5 and 35 °C using four cultivars from temperate and Mediterranean origin. Leaf traits, the photosynthetic rate at 25 °C (A400 (25)), the photosynthetic rate at optimal temperature (A400 (opt)), the thermal optimum of photosynthesis (Topt), and the photosynthetic parameters from the Farqhuar model were determined. Irrespective of cultivar origin, a clear shift in the temperature responses of photosynthesis was observed as a function of growth temperature, affecting thermal optimum of photosynthesis, photosynthetic rate at optimal temperature and photosynthetic rate at 25 °C. For both cultivars, Topt values increased linearly in leaves grown between 5 and 35 °C. Relative homeostasis of A400 (25) and A400 (opt) was found between 10 °C and 30 °C growth temperatures, but sharp declines were recorded at 5 and 35 °C. This homeostasis was achieved in part through modifications to leaf nitrogen content, which increased at extreme temperatures. Significant changes were also recorded regarding nitrogen partitioning in the photosynthetic apparatus and in the temperature dependence of photosynthetic parameters. The cultivars differed only in terms of the temperature response of photosynthetic parameters, with Mediterranean genotypes displaying a greater sensitivity of the maximum rate of Rubisco carboxylation to elevated temperatures. It was concluded that intra-specific variations in the temperature acclimation of photosynthesis exist among alfalfa cultivars, but that Mediterranean genotypes presented no evidence of superior performance at high temperatures.