Photosynthetic Electron Transport Rate Research Articles

Association of BrERF72 with methyl jasmonate-induced leaf senescence of Chinese flowering cabbage through activating JA biosynthesis-related genes

The ethylene response factor (ERF) and phytohormone jasmonate (JA) are reported to function in leaf senescence. The involvement of ERF in JA-mediated leaf senescence, however, needs to be elucidated. In the present work, we demonstrate a Chinese flowering cabbage ERF transcription factor (TF), BrERF72, that is associated with JA-promoted leaf senescence. Exogenous application of methyl jasmonate (MeJA)-accelerated leaf senescence of Chinese flowering cabbage, evidenced by the data that MeJA treatment led to the stronger reduction in the maximum quantum yield (Fv/Fm), photosynthetic electron transport rate (ETR), and total chlorophyll content, while significant induction in the expression of several senescence-associated genes (SAGs) including BrSAG12, BrSAG19, and chlorophyll catabolic genes (CCGs) BrPAO1, BrNYC1, BrPPH1, and BrSGR1. Increases in levels of endogenous JA and transcripts of JA biosynthetic genes BrLOX4, BrAOC3, and BrOPR3 were also found after MeJA treatment. BrERF72 was a MeJA-inducible, nucleus-localized protein, and possessed trans-activation ability. Transient overexpression of BrERF72 in tobacco leaves also promoted leaf senescence. More importantly, further experiments revealed that BrERF72 directly activated expression of BrLOX4, BrAOC3, and BrOPR3 through binding to their promoters via the GCC or DRE/CRT cis-element. Together, the novel JA-ERF association reported in our study uncovers a new insight into the transcriptional regulation of JA production mediated by ERF during JA-promoted leaf senescence in Chinese flowering cabbage.

Photoheterotrophy of photosynthetic bacteria ( Rhodopseudomonas palustris ) growing on oil palm and soybean cooking oils

Abstract Cooking oil wastes are environmentally damaging and costly to deal with. Photoheterotrophic growth of Rhodopseudomonas palustris (photosynthetic bacteria) (CGA 009) was used to decompose different types of cooking oil (palm and soybean oils; cooked and uncooked palm oil) under anaerobic and microaerobic conditions. Photosynthesis was measured in R. palustris using PAM technology (Pulse Amplitude Modulation Fluorometry): Optimum irradiance ≈ 284 μ mol photon m−2 s−1 and maximum photosynthetic electron transport rate (ETR) ≈ 341 μ mol e − g−1 BChl a s−1. R. palustris grew photoheterotrophically on both cooking oils as the sole carbon source grown anaerobically and microaerobically in PM media with a NH3-nitrogen source. R. palustris grew well on used palm oil. R. palustris produces small volumes of hydrogen gas from biodegradation of oil but only under nitrogen fixing conditions (anaerobic: no ammonia or organic nitrogen). N-fixing cells grew very slowly. Cooking oil waste residues are also produced at the production stage (Palm Oil Milling Effluent, POME) and in recycling them for reuse or to produce biodiesel. Hence, biodegradation of cooking oil wastes is an important issue at the production, domestic consumption and recycling stages of their use. A two-stage digestion using an anaerobic yeast first stage, followed by a R. palustris digestion was feasible. Two Line Capsule: The photosynthetic bacterium R. palustris grows photoheterotrophically on Palm Oil and Soybean Oil. Photosynthetic rates were measured using PAM technology.

Effects of exogenous phenolic acids on photosystem functions and photosynthetic electron transport rate in strawberry leaves

Our study investigated the physiological and biochemical basis for the effects of exogenous phenolic acids on the function of the photosynthetic apparatus and photosynthetic electron transport rate in strawberry seedlings. Potted seedlings of the strawberry (Fragaria × ananassa Duch.) were used. Syringic acid inhibited net photosynthetic rate and water-use efficiency decreased. Additionally, primary quinone electron acceptor of the PSII reaction centre, the PSII reaction centre and the oxygen evolving complex were also impaired. Both the maximum quantum yield of the PSII primary photochemistry and the performance index on absorption basis were depressed, resulting in reduced function of the photosynthetic electron transport chain. Otherwise, low phthalic acid concentrations enhanced photosynthetic capacity, while high concentrations showed opposite effects. Syringic acid exhibited a higher toxic effect than that of phthalic acid which was more evident at higher concentrations.

Phytoplankton photosynthetic rate measurement using tunable pulsed light induced fluorescence kinetics.

The photosynthetic process of phytoplankton is the basis of the material circulation and energy flow of the ecosystem. The rapid and accurate measurement of phytoplankton photosynthesis rate is of great significance to water ecological environment monitoring, marine resource assessment and global climate change prediction. On the basis of "Bio-Optical" model, a photosynthetic rate measurement method based on tunable pulsed light induced fluorescence kinetics was put forward in this paper. The chlorophyll fluorescence was used as the probe of photosynthesis process, and the phytoplankton photosynthetic rate was evaluated by the photosynthetic electron transport rate. Comparative experiment results showed that the photosynthetic electron transport rate measured by fluorescence kinetic method under different conditions of DCMU, culture light and nutrients (nitrogen) were consistent with the photosynthetic oxygen evolution rate measured by oxygen evolution method, and the correlation coefficient R2 were 0.934, 0.957 and 0.955 respectively.

Biochemical and Physiological Characteristics of Photosynthesis in Plants of Two Calathea Species.

Plants of the genus Calathea possess many leaf colors, and they are economically important because they are widely used as ornamentals for interior landscaping. Physiological performances and photosynthetic capacities of C. insignis and C. makoyana were investigated. The photosynthetic efficiencies of C. insignis and C. makoyana were significantly increased when the photosynthetic photon flux density (PPFD) increased from 0 to 600 μmol photons·m−2·s−1 and became saturated with a further increase in the PPFD. The two Calathea species had lower values of both the light saturation point and maximal photosynthetic rate, which indicated that they are shade plants. No significant differences in predawn Fv/Fm values (close to 0.8) were observed between dark-green (DG) and light-green (LG) leaf sectors in all tested leaves. However, the effective quantum yield of photosystem II largely decreased as the PPFD increased. An increase in the apparent photosynthetic electron transport rate was observed in both species to a maximum at 600 μmol·m−2·s−1 PPFD, following by a decrease to 1500 μmol·m−2·s−1 PPFD. Compared to LG leaf extracts, DG leaf extracts contained higher levels of chlorophyll (Chl) a, Chl b, Chls a + b, carotenoids (Cars), anthocyanins (Ants), flavonoids (Flas), and polyphenols (PPs) in all plants, except for the Ant, Fla and PP contents of C. insignis plants. Calathea insignis also contained significantly higher levels of total protein than did C. makoyana. The adjusted normalized difference vegetation index (NDVI), photochemical reflectance index (PRI), red-green, and flavonol index (FlavI) were significantly correlated to leaf Chls a + b, Cars, Ants, and Flas in C. makoyana, respectively, and can be used as indicators to characterize the physiology of these plants.

Photosynthetic physio-ecological characteristics in soybean leaves at different CO2 concentrations.

The availability of CO2, a substrate for photosynthesis, affects the photosynthesis process and photosynthate production. Using the Li-6400-40B, we measured the photosynthetic electron transport rate and the photosynthetic light-response curves of soybean (Glycine max) leaves at different CO2 concentrations (300, 400, 500 and 600 μmol·mol-1). By fitting these parameters with a mechanistic model characterizing the light response of photosynthesis, we obtained aseries of photosynthetic parameters, eco-physiological parameters, as well as the physical parameters of photosynthetic pigments. The results showed that the electronic use efficiency, maximum electron transport rate, and maximum net photosynthetic rate increased with the increase of CO2 concentration. The light compensation point and dark respiration rate decreased with the increase of CO2 concentration. In addition, the light-use efficiency and intrinsic (instantaneous) water-use efficiency increased with the increase of CO2 concentration, and their values differed significantly among different CO2 concentrations. There was no significant difference on the maximum carboxylation efficiency among different CO2 concentrations. Those results suggested that CO2 concentration could affect the primary light reaction of photosynthesis in soybean leaves, and thus higher CO2 concentration could decrease the minimum average lifespan of excitons at the lowest excited state, which would enhance the velocity of light energy transport and the use efficiency of photosynthetic electron flow.

Alleviation of Nitrogen and Sulfur Deficiency and Enhancement of Photosynthesis in Arabidopsis thaliana by Overexpression of Uroporphyrinogen III Methyltransferase (UPM1)

Siroheme, an iron-containing tetrapyrrole, is the prosthetic group of nitrite reductase (NiR) and sulfite reductase (SiR); it is synthesized from uroporphyrinogen III, an intermediate of chlorophyll biosynthesis, and is required for nitrogen (N) and sulfur (S) assimilation. Further, uroporphyrinogen III methyltransferase (UPM1), responsible for two methylation reactions to form dihydrosirohydrochlorin, diverts uroporphyrinogen III from the chlorophyll biosynthesis pathway toward siroheme synthesis. AtUPM1 [At5g40850] was used to produce both sense and antisense plants of Arabidopsis thaliana in order to modulate siroheme biosynthesis. In our experiments, overexpression of AtUPM1 signaled higher NiR (NII) and SiR gene and gene product expression. Increased NII expression was found to regulate and enhance the transcript and protein abundance of nitrate reductase (NR). We suggest that elevated NiR, NR, and SiR expression must have contributed to the increased synthesis of S containing amino acids in AtUPM1overexpressors, observed in our studies. We note that due to higher N and S assimilation in these plants, total protein content had increased in these plants. Consequently, chlorophyll biosynthesis increased in these sense plants. Higher chlorophyll and protein content of plants upregulated photosynthetic electron transport and carbon assimilation in the sense plants. Further, we have observed increased plant biomass in these plants, and this must have been due to increased N, S, and C assimilation. On the other hand, in the antisense plants, the transcript abundance, and protein content of NiR, and SiR was shown to decrease, resulting in reduced total protein and chlorophyll content. This led to a decrease in photosynthetic electron transport rate, carbon assimilation and plant biomass in these antisense plants. Under nitrogen or sulfur starvation conditions, the overexpressors had higher protein content and photosynthetic electron transport rate than the wild type (WT). Conversely, the antisense plants had lower protein content and photosynthetic efficiency in N-deficient environment. Our results clearly demonstrate that upregulation of siroheme biosynthesis leads to increased nitrogen and sulfur assimilation, and this imparts tolerance to nitrogen and sulfur deficiency in Arabidopsis thaliana plants.

基于光合电子传递速率的浮游植物初级生产力测量

基于光合电子传递速率的浮游植物初级生产力测量

Non-destructive phenomic tools for the prediction of heat and drought tolerance at anthesis in Brassica species

Abiotic stresses lead to an array of physiological and biochemical changes in crops which affect their growth and development leading to grain yield reductions. Oilseed Brassica species are vulnerable to heat and drought stress, especially in the early reproductive stage. We evaluated the use of several non-destructive phenomic tools to predict heat and drought tolerance at anthesis in three Brassica species, including five B. rapa, five B. napus and two B. juncea genotypes. Heat, drought or combined heat and drought treatments were applied for seven days from the beginning of anthesis, following which plants were returned to normal temperatures and well-watered conditions. Leaf and floral bud temperatures increased relative to ambient, and leaf conductance decreased in response to seven days of drought or combined treatments, but not to heat or control treatments. The drought and/or heat treatment significantly affected three photosynthesis-related parameters: the maximum carboxylation rate allowed by Rubisco (Vcmax), the photosynthetic electron transport rate and the rate of triose phosphate use. One day after the beginning of the heat and combined heat and drought treatments, Vcmax increased to more than double the basal level but remained low in the drought and control treatments. Vcmax on the first day of heat and combined treatments was positively correlated to the harvested seed yield of genotypes (r = 0.670 and 0.878, respectively), suggesting that a rapid increase in Vcmax in response to heat stress during anthesis was a positive predictor of final grain yield. The tissue volume of the whole plant, detected by plant growth imaging using a digital imaging system, was positively correlated with the above-ground fresh weight of genotypes after seven days in all treatments and was, therefore, an excellent surrogate for above-ground biomass of the whole plant. The volume of the yellow flowers after seven days in all treatments was positively correlated with the harvested seed yield of genotypes, and is a potential non-destructive phenomic screen for heat and drought stress tolerance in Brassica germplasm.

Exogenous spermidine enhances the photosynthetic and antioxidant capacity of rice under heat stress during early grain-filling period.

High temperature has adverse effects on rice growth by inhibiting the flag leaf photosynthetic and antioxidant capacity, which can be alleviated by various exogenous chemicals such as spermidine (Spd). However, the role of Spd in conferring heat tolerance in rice is not well documented. Conventional japonica rice varieties Wuyunjing 24 and Ningjing 3 were treated with high temperatures at 37.5/27.0°C (day/night) and foliar sprayed with 1mmolL-1 Spd after flowering. Results showed activities of superoxide dismutase (SOD) and peroxidase (POD) activities were deceased during high temperature treatment and eventually lead to the malondialdehyde (MDA) accumulation. Exogenous Spd significantly increased both SOD and POD activities at the later stage of high-temperature treatment, and reduced MDA accumulation were identified in both rice varieties. Application of Spd further increased the amount of soluble sugars during high temperature stress and that maintained the osmotic equilibrium of rice leaves. Spd significantly increased photosystem II (ΦPSII), photosynthetic electron transport rate (ETR), variable fluorescence/maximum fluorescence ratio (Fv'/Fm'), stomatal conductance and the photochemical reaction of light energy ratio (Pr), and ultimately improved the photosynthetic and transpiration rate during high temperature stress. In conclusion, exogenous Spd can effectively alleviate the adverse consequences of high temperature and could be further applied to provide strategies in mitigating the challenges of global warming-induced yield loss and other possible relevant issues.

Impacts of Brassinolide on the Photosynthetic Characteristics of 'Qiyuexian' Jujube

The jujube cultivar ‘Qiyuexian’ was chosen as object in this study. Effects of different concentrations of brassinolide (BL) on its characteristic parameters of light/CO 2 response curve were investigated. The results indicated that the mentioned parameters were significantly affected by the concentrations of BL. The Light Saturation Point ( LSP ) was significantly increased by 17.42% after being treated by 0.1 mg/L BL. After spraying 0.2 mg/L BL, the maximum net photosynthetic rate ( Pnmax ), apparent quantum efficiency ( AQY ), LSP, maximum carboxylation rate ( V cmax ), maximum rate of photosynthetic electron transport ( J max ) and Triose Phosphate Utilization rate ( TPU ) were significantly increased by 21.14, 20.26, 21.95, 21.14, 20.26 and 21.95%, respectively. The AQY , LCP , LSP , V cmax and J max were significantly increased by 12.55, 60.75, 19.00, 20.96 and 20.06% respectively after being treated by 0.3 mg/L BL, while the LCP and LSP were significantly increased by 65.53% and 25.79% respectively after being treated by 0.4 mg/L BL. In conclusion, 0.2 mg/L BL treatment could significantly accelerate the overall photosynthetic ability of “Qiyuexian” jujube, promoting its accumulation of assimilative products, which is beneficial to increase the production and quality of jujube fruits.

Physiological and molecular characterisation for high temperature stress in Lens culinaris.

In the present study, 11 lentil (Lens culinaris Medik) genotypes including heat tolerant and heat sensitive genotypes identified after a screening of 334 accessions of lentil for traits imparting heat tolerance, were characterised based on physiological traits and molecular markers. Results showed a higher reduction in pollen viability among sensitive genotypes (up to 52.3%) compared with tolerant genotypes (up to 32.4%) at 43°C. Higher photosynthetic electron transport rate was observed among heat tolerant genotypes and two heat tolerant lentil genotypes, IG 4258 (0.43) and IG 3330 (0.38) were having highest Fv/Fm values. However, membrane stability was significantly higher in only one heat tolerant genotype, ILL 10712, indicating that different mechanisms are involved to control heat tolerance in lentil. The molecular characterisation of lentil genotypes with 70 polymorphic SSR and genic markers resulted into distinct clusters in accordance with their heat stress tolerance. A functional marker ISM11257 (intron spanning marker) amplifying an allele of 205bp in size was present only among heat tolerant genotypes, and could be further used in a breeding program to identify heat tolerant lentil genotypes. The findings of this study will contribute to the development of heat tolerant lentil cultivars.

Plants increase CO2 uptake by assimilating nitrogen via the photorespiratory pathway.

Photorespiration is a major bioengineering target for increasing crop yields as it is often considered a wasteful process. Photorespiratory metabolism is integrated into leaf metabolism and thus may have certain benefits. Here, we show that plants can increase their rate of photosynthetic CO2 uptake when assimilating nitrogen de novo via the photorespiratory pathway by fixing carbon as amino acids in addition to carbohydrates. Plants fed NO3- had higher rates of CO2 assimilation under photorespiratory than low-photorespiratory conditions, while plants lacking NO3- nutrition exhibited lower stimulation of CO2 uptake. We modified the widely used Farquhar, von Caemmerer and Berry photosynthesis model to include the carbon and electron requirements for nitrogen assimilation via the photorespiratory pathway. Our modified model improves predictions of photosynthetic CO2 uptake and of rates of photosynthetic electron transport. The results highlight how photorespiration can improve photosynthetic performance despite reducing the efficiency of Rubisco carboxylation.

Kaolin modulates ABA and IAA dynamics and physiology of grapevine under Mediterranean summer stress

The foliar exogenous application of kaolin, a radiation-reflecting inert mineral, has proven to be an effective short-term climate change mitigation strategy for Mediterranean vineyards. In this work, we address the hypothesis that kaolin could improve both the hormonal dynamics and physiological responses of grapevines growing in Douro Region, northern Portugal. For this purpose, the leaf water potential, gas exchange and chlorophyll a fluorescence parameters were monitored, as well as the abscisic acid (ABA) and indole-3-acetic acid (IAA) quantification and immunolocalization were assessed. The study revealed a slight decrease in ABA and an increase in IAA in the kaolin treatment, which in turn were associated with the improvement of physiological performance. A month after spraying, kaolin improves the water potential respectively, 30% and 17% in the predawn and midday periods. Besides, plants treated with kaolin showed higher values of stomatal conductance, net CO2 assimilation rate and intrinsic water use efficiency. Kaolin also ameliorates the effective PSII efficiency (67%), as well as the maximum quantum efficiency of photosystem II and the photosynthetic electron transport rate (>73%). These results were consistent with the higher photochemical quenching and the lower non-photochemical quenching observed in treated leaves and with the better performance obtained by the JIP test parameters. Physiological and hormonal analysis confirmed that kaolin effectively enhance grapevine summer stress tolerance.

The OsPS1-F gene regulates growth and development in rice by modulating photosynthetic electron transport rate.

Ds insertion in rice OsPS1-F gene results in semi-dwarf plants with reduced tiller number and grain yield, while genetic complementation with OsPS1-F rescued the mutant phenotype. Photosynthetic electron transport is regulated in the chloroplast thylakoid membrane by multi-protein complexes. Studies about photosynthetic machinery and its subunits in crop plants are necessary, because they could be crucial for yield enhancement in the long term. Here, we report the characterization of OsPS1-F (encoding Oryza sativa PHOTOSYSTEM 1-F subunit) using a single copy Ds insertion rice mutant line. The homozygous mutant (osps1-f) showed striking difference in growth and development compared to the wild type (WT), including, reduction in plant height, tiller number, grain yield as well as pale yellow leaf coloration. Chlorophyll concentration and electron transport rate were significantly reduced in the mutant compared to the WT. OsPS1-F gene was highly expressed in rice leaves compared to other tissues at different developmental stages tested. Upon complementation of the mutant with proUBI::OsPS1-F, the observed mutant phenotypes were rescued. Our results illustrate that OsPS1-F plays an important role in regulating proper growth and development of rice plants.

Overexpression of plastidic maize NADP-malate dehydrogenase (ZmNADP-MDH) in Arabidopsis thaliana confers tolerance to salt stress.

The plastidic C4 Zea mays NADP-malate dehydrogenase (ZmNADP-MDH), responsible for catalysis of oxaloacetate to malate, was overexpressed in Arabidopsis thaliana to assess its impact on photosynthesis and tolerance to salinity stress. Different transgenic lines were produced having ~3-6-fold higher MDH protein abundance and NADP-MDH enzyme activity than vector control. The overexpressors had similar chlorophyll, carotenoid, and protein content as that of vector control. Their photosynthetic electron transport rates, carbon assimilation rate, and consequently fresh weight and dry weight were almost similar. However, these overexpressors were tolerant to salt stress (150mM NaCl). In saline environment, the Fv/Fm ratio, yield of photosystem II, chlorophyll, and protein content were higher in ZmNADP-MDH overexpressor than vector control. Under identical conditions, the generation of reactive oxygen species (H2O2) and production of malondialdehyde, a membrane lipid peroxidation product, were lower in overexpressors. In stress environment, the structural distortion of granal organization and swelling of thylakoids were less pronounced in ZmNADP-MDH overexpressing plants as compared to the vector control. Chloroplastic NADP-MDH in consort with cytosolic and mitochondrial NAD-MDH plays an important role in exporting reducing power (NADPH) and exchange of metabolites between different cellular compartments that maintain the redox homeostasis of the cell via malate valve present in chloroplast envelope membrane. The tolerance of NADP-MDH overexpressors to salt stress could be due to operation of an efficient malate valve that plays a major role in maintaining the cellular redox environment.

Kleptoplast photosynthesis is nutritionally relevant in the sea slug Elysia viridis

Several sacoglossan sea slug species feed on macroalgae and incorporate chloroplasts into tubular cells of their digestive diverticula. We investigated the role of the “stolen” chloroplasts (kleptoplasts) in the nutrition of the sea slug Elysia viridis and assessed how their abundance, distribution and photosynthetic activity were affected by light and starvation. Elysia viridis individuals feeding on the macroalga Codium tomentosum were compared with starved specimens kept in dark and low light conditions. A combination of variable Chl a fluorescence and hyperspectral imaging, and HPLC pigment analysis was used to evaluate the spatial and temporal variability of photopigments and of the photosynthetic capacity of kleptoplasts. We show increased loss of weight and body length in dark-starved E. viridis as compared to low light-starved sea slugs. A more pronounced decrease in kleptoplast abundance and lower photosynthetic electron transport rates were observed in dark-starved sea slugs than in low light-starved animals. This study presents strong evidence of the importance of kleptoplast photosynthesis for the nutrition of E. viridis in periods of food scarcity. Deprived of photosynthates, E. viridis could accelerate the breakdown of kleptoplasts in the dark to satisfy its’ energy requirements.

Light Quality Affects Chloroplast Electron Transport Rates Estimated from Chl Fluorescence Measurements.

Chl fluorescence has been used widely to calculate photosynthetic electron transport rates. Portable photosynthesis instruments allow for combined measurements of gas exchange and Chl fluorescence. We analyzed the influence of spectral quality of actinic light on Chl fluorescence and the calculated electron transport rate, and compared this with photosynthetic rates measured by gas exchange in the absence of photorespiration. In blue actinic light, the electron transport rate calculated from Chl fluorescence overestimated the true rate by nearly a factor of two, whereas there was closer agreement under red light. This was consistent with the prediction made with a multilayer leaf model using profiles of light absorption and photosynthetic capacity. Caution is needed when interpreting combined measurements of Chl fluorescence and gas exchange, such as the calculation of CO2 partial pressure in leaf chloroplasts.

Cyclic electron flow, NPQ and photorespiration are crucial for the establishment of young plants of Ricinus communis and Jatropha curcas exposed to drought.

Although plant physiological responses to drought have been widely studied, the interaction between photoprotection, photorespiration and antioxidant metabolism in water-stressed plants is scarcely addressed. This study aimed to evaluate the physiological adjustments preserving photosynthesis and growth in two plant species with different tolerance to drought: Jatropha curcas and Ricinus communis. We measured stress indicators, gas exchange, photochemistry of PSII and PSI, antioxidant enzymes, cyclic electron flow and photorespiration. Physiological stress indicators associated with reduction in growth confirmed R.communis as sensitive and J.curcas as tolerant to drought. Drought induced loss of photosynthesis in R.communis, whereas J.curcas maintained higher leaf gas exchange and photochemistry under drought. In addition, J.curcas showed higher dissipation of excess energy and presented higher cyclic electron flow when exposed to drought. Although none of these mechanisms have been triggered in R.communis, this species showed increases in photorespiration. R.communis displayed loss of Rubisco content while the Rubisco relative abundance did not change in J.curcas under drought. Accordingly, the invivo maximum Rubisco carboxylation rate (Vcmax ) and the maximum photosynthetic electron transport rate driving RuBP regeneration (Jmax ) were less affected in J.curcas. Both species displayed an efficient antioxidant mechanism by increasing activities of ascorbate peroxidase (APX) and superoxide dismutase (SOD). Overall, we suggest that the modulation of different photoprotective mechanisms is crucial to mitigate the effects caused by excess energy, maintaining photosynthetic apparatus efficiency and promoting the establishment of young plants of these two species under drought.

Photosynthetic acclimation to warming in tropical forest tree seedlings.

Tropical forests have a mitigating effect on man-made climate change by acting as a carbon sink. For that effect to continue, tropical trees will have to acclimate to rising temperatures, but it is currently unknown whether they have this capacity. We grew seedlings of three tropical tree species over a range of temperature regimes (TGrowth = 25, 30, 35 °C) and measured the temperature response of photosynthetic CO2 uptake. All species showed signs of acclimation: the temperature-response curves shifted, such that the temperature at which photosynthesis peaked (TOpt) increased with increasing TGrowth. However, although TOpt shifted, it did not reach TGrowth at high temperature, and this difference between TOpt and TGrowth increased with increasing TGrowth, indicating that plants were operating at supra-optimal temperatures for photosynthesis when grown at high temperatures. The high-temperature CO2 compensation point did not increase with TGrowth. Hence, temperature-response curves narrowed with increasing TGrowth. TOpt correlated with the ratio of the RuBP regeneration capacity over the RuBP carboxylation capacity, suggesting that at high TGrowth photosynthetic electron transport rate associated with RuBP regeneration had greater control over net photosynthesis. The results show that although photosynthesis of tropical trees can acclimate to moderate warming, carbon gain decreases with more severe warming.