Regulation of isoprene emission in Populus trichocarpa leaves subjected to changing growth temperature

The hydrocarbon isoprene is emitted in large quantities from numerous plant species, and has a substantial impact on atmospheric chemistry. Temperature affects isoprene emission at several levels: the temperature at which emission is measured, the temperature at which leaves develop, and the temperatures to which a mature leaf is exposed in the days prior to emission measurement. The molecular regulation of the response to the last of these factors was investigated in this study. When plants were grown at 20 degrees C and moved from 20 to 30 degrees C and back, or grown at 30 degrees C and moved from 30 to 20 degrees C and back, their isoprene emission peaked within 3 h of the move and stabilized over the following 3 d. Trees that developed at 20 degrees C and experienced 30 degrees C episodes had higher isoprene emission capacities than did leaves grown exclusively at 20 degrees C, even 2 weeks after the last 30 degrees C episode. The levels and extractable activities of isoprene synthase protein, which catalyses the synthesis of isoprene, and those of dimethylallyl diphosphate (DMADP), its substrate, alone could not explain observed variations in isoprene emission. Therefore, we conclude that control of isoprene emission in mature leaves is shared between isoprene synthase protein and DMADP supply.     

Controls of the quantum yield and saturation light of isoprene emission in different‐aged aspen leaves

Leaf age alters the balance between the use of end‐product of plastidic isoprenoid synthesis pathway, dimethylallyl diphosphate (DMADP), in prenyltransferase reactions leading to synthesis of pigments of photosynthetic machinery and in isoprene synthesis, but the implications of such changes on environmental responses of isoprene emission have not been studied. Because under light‐limited conditions, isoprene emission rate is controlled by DMADP pool size (S_DMADP), shifts in the share of different processes are expected to particularly strongly alter the light dependency of isoprene emission. We examined light responses of isoprene emission in young fully expanded, mature and old non‐senescent leaves of hybrid aspen (Populus tremula x P. tremuloides) and estimated in vivo S_DMADP and isoprene synthase activity from post‐illumination isoprene release. Isoprene emission capacity was 1.5‐fold larger in mature than in young and old leaves. The initial quantum yield of isoprene emission (α_I) increased by 2.5‐fold with increasing leaf age primarily as the result of increasing S_DMADP. The saturating light intensity (Q_I90) decreased by 2.3‐fold with increasing leaf age, and this mainly reflected limited light‐dependent increase of S_DMADP possibly due to feedback inhibition by DMADP. These major age‐dependent changes in the shape of the light response need consideration in modelling canopy isoprene emission.     

Acclimation of the respiration/photosynthesis ratio to temperature: insights from a model

Based on short-term experiments, many plant growth models – including those used in global change research – assume that an increase in temperature stimulates plant respiration (R) more than photosynthesis (P), leading to an increase in the R/P ratio. Longer-term experiments, however, have demonstrated that R/P is relatively insensitive to growth temperature. We show that both types of temperature response may be reconciled within a simple substrate-based model of plant acclimation to temperature, in which respiration is effectively limited by the supply of carbohydrates fixed through photosynthesis. The short-term, positive temperature response of R/P reflects the transient dynamics of the nonstructural carbohydrate and protein pools; the insensitivity of R/P to temperature on longer time-scales reflects the steady-state behaviour of these pools. Thus the substrate approach may provide a basis for predicting plant respiration responses to temperature that is more robust than the current modelling paradigm based on the extrapolation of results from short-term experiments. The present model predicts that the acclimated R/P depends mainly on the internal allocation of carbohydrates to protein synthesis, a better understanding of which is therefore required to underpin the wider use of a constant R/P as an alternative modelling paradigm in global change research.     

Effects of growth under elevated UV-B on photosynthesis and isoprene emission in Quercus gambelii and Mucuna pruriens

Increasing surface levels of UV-B resulting from stratospheric ozone reduction directly affect tropospheric photochemistry. There may also be indirect tropospheric effects due to changes in emission of organic compounds from vegetation. We treated woody and herbaceous isoprene-emitting species in the field with supplemental UV-B simulating 30% ozone depletion. For Quercus gambelii, photosynthesis and isoprene emission were significantly greater in elevated UV-B treatments when expressed on a leaf area basis, but not on a leaf mass basis. Leaves of Mucuna pruriens, however, showed no significant differences in photosynthesis or isoprene emission between treatments, nor when exposed for 45 min to acute high levels of UV-B. Elevated UV-B during growth did not elicit significant isoprene emission from Acer platanoides, a non-emitting species. Other potential UV-B effects, such as changes in leaf area or species composition, which may influence regional isoprene emissions, should be examined.     

Acclimation of photosynthesis and respiration is asynchronous in response to changes in temperature regardless of plant functional group

Gas exchange, fluorescence, western blot and chemical composition analyses were combined to assess if three functional groups (forbs, grasses and evergreen trees/shrubs) differed in acclimation of leaf respiration (R) and photosynthesis (A) to a range of growth temperatures (7, 14, 21 and 28 degrees C). When measured at a common temperature, acclimation was greater for R than for A and differed between leaves experiencing a 10-d change in growth temperature (PE) and leaves newly developed at each temperature (ND). As a result, the R : A ratio was temperature dependent, increasing in cold-acclimated plants. The balance was largely restored in ND leaves. Acclimation responses were similar among functional groups. Across the functional groups, cold acclimation was associated with increases in nonstructural carbohydrates and nitrogen. Cold acclimation of R was associated with an increase in abundance of alternative and/or cytochrome oxidases in a species-dependent manner. Cold acclimation of A was consistent with an initial decrease and subsequent recovery of thylakoid membrane proteins and increased abundance of proteins involved in the Calvin cycle. Overall, the results point to striking similarities in the extent and the biochemical underpinning of acclimation of R and A among contrasting functional groups differing in overall rates of metabolism, chemical composition and leaf structure.     

Seasonal changes in temperature and nutrient control of photosynthesis, respiration and growth of natural phytoplankton communities

1. To investigate the influence of elevated temperatures and nutrients on photosynthesis, respiration and growth of natural phytoplankton assemblages, water was collected from a eutrophic lake in spring, summer, autumn, winter and the following spring and exposed to ambient temperature and ambient +2, +4 and +6 °C for 2 weeks with and without addition of extra inorganic nutrients. 2. Rates of photosynthesis, respiration and growth generally increased with temperature, but this effect was strongly enhanced by high nutrient availability, and therefore was most evident for nutrient amended cultures in seasons of low ambient nutrient availability. 3. Temperature stimulation of growth and metabolism was higher at low than high ambient temperature showing that long‐term temperature acclimation of the phytoplankton community before the experiments was of great importance for the measured rates. 4. Although we found distinct responses to relatively small temperature increases, the interaction between nutrient availability, time of the year and, thus, ambient temperature was responsible for most of the observed variability in phytoplankton growth, photosynthesis and respiration. 5. Although an increase in global temperature will influence production and degradation of organic material in lakes, the documented importance of ambient temperatures and nutrient conditions suggests that effects will be most pronounced during winter and early spring, while the remaining part of the growth season will be practically unaffected by increasing temperatures.     

Acclimation of photosynthesis and respiration to elevated atmospheric CO2 in two Scrub Oaks

Abstract For two species of oak, we determined whether increasing atmospheric CO₂ concentration (C_a) would decrease leaf mitochondrial respiration (R) directly, or indirectly owing to their growth in elevated C_a, or both. In particular, we tested whether acclimatory decreases in leaf‐Rubisco content in elevated C_a would decrease R associated with its maintenance. This hypothesis was tested in summer 2000 on sun and shade leaves of Quercus myrtifolia Willd. and Quercus geminata Small. We also measured R on five occasions between summer 1999 and 2000 on leaves of Q. myrtifolia. The oaks were grown in the field for 4 years, in either current ambient or elevated (current ambient + 350 µmol mol^?1) C_a, in open‐top chambers (OTCs). For Q. myrtifolia, an increase in C_a from 360 to 710 µmol mol^?1 had no direct effect on R at any time during the year. In April 1999, R in young Q. myrtifolia leaves was significantly higher in elevated C_a—the only evidence for an indirect effect of growth in elevated C_a. Leaf R was significantly correlated with leaf nitrogen (N) concentration for the sun and shade leaves of both the species of oak. Acclimation of photosynthesis in elevated C_a significantly reduced maximum RuBP‐saturated carboxylation capacity (V_{c max}) for both the sun and shade leaves of only Q. geminata. However, we estimated that only 11–12% of total leaf N was invested in Rubisco; consequently, acclimation in this plant resulted in a small effect on N and an insignificant effect on R. In this study measurements of respiration and photosynthesis were made on material removed from the field; this procedure had no effect on gas exchange properties. The findings of this study were applicable to R expressed either per unit leaf area or unit dry weight, and did not support the hypothesis that elevated C_a decreases R directly, or indirectly owing to acclimatory decreases in Rubisco content.     

Stimulation of isoprene emissions and electron transport rates as key mechanisms of thermal tolerance in the tropical species Vismia guianensis

Tropical forests absorb large amounts of atmospheric CO₂ through photosynthesis, but high surface temperatures suppress this absorption while promoting isoprene emissions. While mechanistic isoprene emission models predict a tight coupling to photosynthetic electron transport (ETR) as a function of temperature, direct field observations of this phenomenon are lacking in the tropics and are necessary to assess the impact of a warming climate on global isoprene emissions. Here we demonstrate that in the early successional species Vismia guianensis in the central Amazon, ETR rates increased with temperature in concert with isoprene emissions, even as stomatal conductance (g_s) and net photosynthetic carbon fixation (P_n) declined. We observed the highest temperatures of continually increasing isoprene emissions yet reported (50°C). While P_n showed an optimum value of 32.6 ± 0.4°C, isoprene emissions, ETR, and the oxidation state of PSII reaction centers (q_L) increased with leaf temperature with strong linear correlations for ETR (? = 0.98) and q_L (? = 0.99) with leaf isoprene emissions. In contrast, other photoprotective mechanisms, such as non‐photochemical quenching, were not activated at elevated temperatures. Inhibition of isoprenoid biosynthesis repressed P_n at high temperatures through a mechanism that was independent of stomatal closure. While extreme warming will decrease g_s and P_n in tropical species, our observations support a thermal tolerance mechanism where the maintenance of high photosynthetic capacity under extreme warming is assisted by the simultaneous stimulation of ETR and metabolic pathways that consume the direct products of ETR including photorespiration and the biosynthesis of thermoprotective isoprenoids. Our results confirm that models which link isoprene emissions to the rate of ETR hold true in tropical species and provide necessary “ground‐truthing” for simulations of the large predicted increases in tropical isoprene emissions with climate warming.     

Seasonal changes in temperature response of photosynthesis and its contribution to annual carbon gain in Daphniphyllum humile,an evergreen understorey shrub

We evaluated seasonal variation in photosynthetic temperature dependence and its contribution to annual carbon gain in an evergreen understorey shrub, Daphniphyllum humile Maxim, growing at the forest border and in the understorey of a deciduous forest. Plants at both sites exhibited similar optimal temperatures for photosynthesis (T_opt). The activation energy for ribulose‐1,5‐bisphosphate (RuBP) carboxylation (H_aV) at both sites tended to be higher in summer than in spring or autumn, suggesting that H_aV may be the controlling factor in the T_opt shift in D. humile. In contrast to the seasonal changes in T_opt, the maximum photosynthetic rate at the optimal temperature (P_opt) differed between the two sites: it was lower in autumn than in summer at the forest border, but was the same in summer and autumn in the understorey. In the understorey plants, nitrogen content (N_area) increased in autumn, but this was not the case for forest border plants. In addition, Rubisco content increased significantly in autumn in the understorey leaves but decreased distinctly in forest border leaves. Increased N_area and Rubisco in understorey leaves resulted in increased in photosynthesis in autumn. Annual carbon gain was 30.8 mol·m^?2 in forest border leaves and 5.8 mol·m^?2 in understorey leaves. Carbon gain in understorey leaves during the short period after overstorey leaf fall and before snow accumulation was approximately 49% of annual carbon gain. Furthermore, autumn carbon gain calculated using activation energy of summer with autumn photosynthetic parameters underestimated the autumn carbon gain by as much as 31%. In conclusion, photosynthetic temperature acclimation may be a key factor in increasing annual carbon gain in understorey D. humile.     

Acclimation of photosynthesis to temperature in eight cool and warm climate herbaceous C3 species: Temperature dependence of parameters of a biochemical photosynthesis model

To determine how parameters of a Farquhar-type photosynthesis model varied with measurement temperature and with growth temperature, eight cool and warm climate herbaceous crop and weed species were grown at 15 and 25 °C and single leaf carbon dioxide and water vapor exchange rates were measured over the range of 15 – 35 °C. Photosynthetic parameters examined were the initial slope of the response of assimilation rate (A) to substomatal carbon dioxide concentration (C_i), A at high C_i, and stomatal conductance. The first two measurements allow calculation of V_Cmax, the maximum rate of carboxylation of ribulose bisphosphate carboxylase and J_max, the maximum rate of photosynthetic electron transport, of Farquhar-type photosynthesis models. In all species, stomatal conductance increased exponentially with temperature over the whole range of 15 – 35 °C, even when A decreased at high measurement temperature. There were larger increases in conductance over this temperature range in the warm climate species (4.3 ×) than in the cool climate species (2.5 ×). The initial slope of A vs. C_i exhibited an optimum temperature which ranged from 20 to 30 °C. There was a larger increase in the optimum temperature of the initial slope at the warmer growth temperature in the cool climate species than in the warm climate species. The optimum temperature for A at high C_i ranged from 25 to 30 °C among species, but changed little with growth temperature. The absolute values of both the initial slope of A vs. C_i and A at high C_i were increased about 10% by growth at the warmer temperature in the warm climate species, and decreased about 20% in the cool climate species. The ratio of J_max — V_Cmax normalized to 20 °C varied by more than a factor of 2 across species and growth temperatures, but differences in the temperature response of photosynthesis were more related to variation in the temperature dependencies of J_max and V_Cmax than to the ratio of their normalized values.This revised version was published online in October 2005 with corrections to the Cover Date.     

Strong correlation between isoprene emission and gross photosynthetic capacity during leaf phenology of the tropical tree species Hymenaea courbaril with fundamental changes in volatile organic compounds emission composition during early leaf development

Changes of the volatile organic compounds (VOC) emission capacity and composition of different developmental stages of the tropical tree species Hymenaea courbaril were investigated under field conditions at a remote Amazonian rainforest site. The basal emission capacity of isoprene changed considerably over the course of leaf development, from young to mature and to senescent leaves, ultimately spanning a wide range of observed isoprene basal emission capacities from 0.7 to 111.5 µg C g^?1 h^?1 during the course of the year. By adjusting the standard emission factors for individual days, the diel courses of instantaneous isoprene emission rates could nevertheless adequately be modelled by a current isoprene algorithm. The results demonstrate the inadequacy of using one single standard emission factor to represent the VOC emission capacity of tropical vegetation for an entire seasonal cycle. A strong linear correlation between the isoprene emission capacity and the gross photosynthetic capacity (GP_max) covering all developmental stages and seasons was observed. The present results provide evidence that leaf photosynthetic properties may confer a valuable basis to model the seasonal variation of isoprenoid emission capacity; especially in tropical regions where the environmental conditions vary less than in temperate regions. In addition to induction and variability of isoprene emission during early leaf development, considerable amounts of monoterpenes were emitted in a light‐dependent manner exclusively in the period between bud break and leaf maturity. The fundamental change in emission composition during this stage as a consequence of resource availability (supply side control) or as a plant's response to the higher defence demand of young emerging leaves (demand‐side control) is discussed. The finding of a temporary emergence of monoterpene emission may be of general interest in understanding both the ecological functions of isoprenoid production and the regulatory processes involved.     

Growth temperature influences the underlying components of relative growth rate: an investigation using inherently fast- and slow-growing plant species

We examined the effect of growth temperature on the underlying components of growth in a range of inherently fast‐ and slow‐growing plant species. Plants were grown hydroponically at constant 18, 23 and 28 °C. Growth analysis was conducted on 16 contrasting plant species, with whole plant gas exchange being performed on six of the 16 species. Inter‐specific variations in specific leaf area (SLA) were important in determining variations in relative growth rate (RGR) amongst the species at 23 and 28 °C but were not related to variations in RGR at 18 °C. When grown at 18 °C, net assimilation rate (NAR) became more important than SLA for explaining variations in RGR. Variations in whole shoot photosynthesis and carbon concentration could not explain the importance of NAR in determining RGR at the lower temperatures. Rather, variations in the degree to which whole plant respiration per unit leaf area acclimated to the different growth temperatures were responsible. Plants grown at 28 °C used a greater proportion of their daily fixed carbon in respiration than did the 18 and 23 °C‐grown plants. It is concluded that the relative importance of the underlying components of growth are influenced by growth temperature, and the degree of acclimation of respiration is of central importance to the greater role played by NAR in determining variations in RGR at declining growth temperatures.     

Acclimation of respiration to temperature and CO2 in seedlings of boreal tree species in relation to plant size and relative growth rate

The role of acclimation of dark respiration to temperature and CO₂ concentration and its relationship to growth are critical in determining plant response to predicted global change. We explored temperature acclimation of respiration in seedlings of tree species of the North American boreal forest. Populus tremuloides, Betula papyrifera, Larix laricina, Pinus banksiana, and Picea mariana plants were grown from seed in controlled-environments at current and elevated concentrations of CO₂ (370 and 580 μmol mol^–1) in combination with three temperature treatments of 18/12, 24/18, and 30/24 °C (light/dark period). Specific respiration rates of roots and shoots acclimated to temperature, damping increases in rates across growth-temperature environments compared to short-term temperature responses. Compared at a standard temperature, root and shoot respiration rates were, on average, 40% lower in plants grown at the highest compared to lowest growth temperature. Broad-leaved species had a lower degree of temperature acclimation of respiration than did the conifers. Among species and treatment combinations, rates of respiration were linearly related to size and relative growth rate, and relationships were comparable among growth environments. Specific respiration rates and whole-plant respiratory CO₂ efflux as a proportion of daily net CO₂ uptake increased at higher growth temperatures, but were minimally affected by CO₂ concentration. Whole-plant specific respiration rates were two to three times higher in broad-leaved than coniferous species. However, compared to faster-growing broad-leaved species, slower-growing conifers lost a larger proportion of net daily CO₂ uptake as respiratory CO₂ efflux, especially in roots. Interspecific variation in acclimation responses of dark respiration to temperature is more important than acclimation of respiration to CO₂ enrichment in modifying tree seedling growth responses to projected increases in CO₂ concentration and temperature.     

Cyclic electron flow around photosystem I via chloroplast NAD(P)H dehydrogenase (NDH) complex performs a significant physiological role during photosynthesis and plant growth at low temperature in rice

The role of NAD(P)H dehydrogenase (NDH)-dependent cyclic electron flow around photosystem I in photosynthetic regulation and plant growth at several temperatures was examined in rice (Oryza sativa) that is defective in CHLORORESPIRATORY REDUCTION 6 (CRR6), which is required for accumulation of sub-complex A of the chloroplast NDH complex (crr6). NdhK was not detected by Western blot analysis in crr6 mutants, resulting in lack of a transient post-illumination increase in chlorophyll fluorescence, and confirming that crr6 mutants lack NDH activity. When plants were grown at 28 or 35°C, all examined photosynthetic parameters, including the CO(2) assimilation rate and the electron transport rate around photosystems I and II, at each growth temperature at light intensities above growth light (i.e. 800 μmol photons m(-2) sec(-1)), were similar between crr6 mutants and control plants. However, when plants were grown at 20°C, all the examined photosynthetic parameters were significantly lower in crr6 mutants than control plants, and this effect on photosynthesis caused a corresponding reduction in plant biomass. The F(v)/F(m) ratio was only slightly lower in crr6 mutants than in control plants after short-term strong light treatment at 20°C. However, after long-term acclimation to the low temperature, impairment of cyclic electron flow suppressed non-photochemical quenching and promoted reduction of the plastoquinone pool in crr6 mutants. Taken together, our experiments show that NDH-dependent cyclic electron flow plays a significant physiological role in rice during photosynthesis and plant growth at low temperature.     

Comparison of temperate and tropical rainforest tree species: growth responses to temperature

Abstract Aim To investigate whether the latitudinal distribution of rainforest trees in Australia can be explained by their growth responses to temperature. Methods The rainforest canopy trees Acmena smithii (Poir.) Merrill & Perry, Alstonia scholaris (L.) R. Br., Castanospermum australe Cunn. & C. Fraser ex Hook., Eucryphia lucida (Labill.) Baill., Heritiera trifoliolata (F. Muell.) Kosterm., Nothofagus cunninghamii (Hook.) Oerst., Sloanea woollsii F. Muell. and Tristaniopsis laurina (Sm.) Wilson & Waterhouse were selected to cover the latitudinal range of rainforests in eastern Australia. Seedlings of these species were grown under a range of day/night temperature regimes (14/6, 19/11, 22/14, 25/17, and 30/22 °C) in controlled‐environment cabinets. These seedlings were harvested after 16 weeks to determine differences in growth rate and biomass allocation among species and temperature regimes. Results The temperate species showed maximum growth at lower temperatures than the tropical species. However, there was considerable overlap in the growth rates of the temperate and tropical rainforest types across the temperature range used. Maximum growth of the tropical rainforest types was associated with changes in biomass allocation whereas the temperate rainforest types showed no significant changes in biomass allocation across the temperature range. Main conclusions All species showed temperatures for maximum growth that were considerably higher than those previously shown for maximum net photosynthesis. The growth responses to temperature of the rainforest species under these experimental conditions provided limited evidence for their restriction to certain latitudes. These growth responses to temperature showed that the physiological assumptions used in various types of vegetation‐climate models may not be true of Australian rainforest trees.     

Acclimation to irradiance of leaf photosynthesis and associated nitrogen reallocation in photosynthetic apparatus in the year following thinning of a young stand of Chamaecyparis obtusa

In order to quantify the effects of thinning on photosynthetic parameters and associated change in leaf nitrogen (N) contents, half of the trees in a 10-year-old Chamaecyparis obtusa (Sieb. et Zucc.) Endl. stand (36° 3′N, 140°7′E) were removed, giving a final density of 1 500 trees ha⁻¹, in May 2004. Photosynthetic photon flux density (PPFD) and leaf N and carbon (C) contents in the lower (L), middle (M), and upper (U) crowns were monitored one, three, and five months after thinning in both the thinned stand and a non-thinned control stand. In addition, leaves’ photosynthetic responses to CO₂ concentration were simultaneously measured in situ to estimate the maximum rates of carboxylation (V_cmax) and electron transport (J_max). Thinning increased PPFD in the L and M crowns but not in the U crown. V_cmax in both the L and M crowns of the thinned stand increased significantly in comparison with the same crown position of the control stand in the three and five months following thinning. In addition, the thinned stand exhibited an increase in N partitioned to ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) in the L and M crowns relative to the control stand three and five months after thinning, indicating that N had been redistributed within the photosynthetic machinery. Thinning did not affect N per unit area at any of the crown positions, but significantly increased the content of N as a fraction of the total leaf dry mass in the L and M crowns three and five months after thinning. This was a consequence of a decrease in leaf dry mass due to rapid shoot growth. Thus thinning did not cause a redistribution of N between leaves. Thinning improved irradiance in the L and M crowns of C. obtusa, leading to photosynthetic acclimation. Photosynthetic acclimation in the first year mainly occurred via redistribution of N within but not between leaves.     

Balancing carboxylation and regeneration of ribulose-1,5- bisphosphate in leaf photosynthesis: temperature acclimation of an evergreen tree, Quercus myrsinaefolia

Changes in the temperature dependence of the photosynthetic rate depending on growth temperature were investigated for a temperate evergreen tree, Quercus myrsinaefolia . Plants were grown at 250 μ mol quanta m^–2 s^–1 under two temperature conditions, 15 and 30 °C. The optimal temperature that maximizes the light-saturated rate of photosynthesis at 350 μ L L^–1 CO₂ was found to be 20–25 and 30–35 °C for leaves grown at 15 and 30 °C, respectively. We focused on two processes, carboxylation and regeneration of ribulose-1,5-bisphosphate (RuBP), which potentially limit photosynthetic rates. Because the former process is known to limit photosynthesis at lower CO₂ concentrations while the latter limits it at higher CO₂ concentrations, we determined the temperature dependence of the photosynthetic rate at 200 and 1000 μ L L^–1 CO₂ under saturated light. It was revealed that the temperature dependence of both processes varied depending on the growth temperature. Using a biochemical model, we estimated the capacity of the two processes at various temperatures under ambient CO₂ concentration. It was suggested that, in leaves grown at low temperature (15 °C), the photosynthetic rate was limited solely by RuBP carboxylation under any temperature. On the other hand, it was suggested that, in leaves grown at high temperature (30 °C), the photosynthetic rate was limited by RuBP regeneration below 22 °C, but limited by RuBP carboxylation above 22 °C. We concluded that: (1) the changes in the temperature dependence of carboxylation and regeneration of RuBP and (2) the changes in the balance of these two processes altered the temperature dependence of the photosynthetic rate.     

Functional diversity of photosynthesis during drought in a model tropical rainforest – the contributions of leaf area, photosynthetic electron transport and stomatal conductance to reduction in net ecosystem carbon exchange

The tropical rainforest mesocosm within the Biosphere 2 Laboratory, a model system of some 110 species developed over 12 years under controlled environmental conditions, has been subjected to a series of comparable drought experiments during 2000–2002. In each study, the mesocosm was subjected to a 4–6 week drought, with well‐defined rainfall events before and after the treatment. Ecosystem CO₂ uptake rate (A_eco) declined 32% in response to the drought, with changes occurring within days and being reversible within weeks, even though the deeper soil layers did not become significantly drier and leaf‐level water status of most large trees was not greatly affected. The reduced A_eco during the drought reflected both morphological and physiological responses. It is estimated that the drought‐induced 32% reduction of A_eco has three principal components: (1) leaf fall increased two‐fold whereas leaf expansion growth of some canopy dominants declined to 60%, leading to a 10% decrease in foliage coverage of the canopy. This might be the main reason for the persistent reduction of A_eco after rewatering. (2) The maximum photosynthetic electron transport rate at high light intensities in remaining leaves was reduced to 71% for three of the four species measured, even though no chronic photo‐inhibition occurred. (3) Stomata closed, leading to a reduced ecosystem water conductance to water vapour (33% of pre‐drought values), which not only reduced ecosystem carbon uptake rate, but may also have implications for water and energy budgets of tropical ecosystems. Additionally, individual rainforest trees responded differently, expressing different levels of stress and stress avoiding mechanisms. This functional diversity renders the individual response heterogeneous and has fundamental implications to scale leaf level responses to ecosystem dynamics.