Search

Rising levels of atmospheric carbon dioxide (CO2 ) could, potentially, be exploited as a means to increase seed yield and maintain food security, especially for cereal grains. Although there have been multiple cultivar trials indicating that significant yield variation occurs, the basis for these differences has not been entirely elucidated. Here, we focus on two rice cultivars that differed in field trials to their yield sensitivity to elevated CO2 : Yangdao6hao (YD6), and Wuyunjing23 (W23) to assess whether observed yield differences (YD6 > W23) were associated with concurrent changes in leaf-level characteristics. At ambient levels of CO2 , leaf net photosynthesis (A) of YD6 was compatible with that of W23. However, at elevated CO2 , A was higher for YD6 relative to W23. The stability of leaf Rubisco content, biochemical characteristics (Vc,max, and Jmax ), nitrogen enzymatic activity, and chlorophyll concentration differed significantly, with greater values observed for YD6 relative to W23 at elevated CO2 . While such results are consistent with other studies, we also demonstrate that a higher ratio of carbon sinks (seed) to carbon sources (leaf), were linked to increases in cytokinins, and slower flag leaf senescence for the YD6 relative to the W23 cultivar at elevated CO2 . While additional data for a broader genetic selection are needed, the current study suggests a link between source/sink carbon assimilation, maintenance of photosynthetic biochemistry, and slower leaf senescence for rice cultivars that show a stronger yield response to projected CO2 levels. This information, in turn, may provide suitable metrics for future CO2 selection among rice cultivars.

Plants are increasingly exposed to events of elevated temperature and water deficit, which threaten crop productivity. Understanding the ability to rapidly recover from abiotic stress, restoring carbon assimilation and biomass production, is important to unravel crop climate resilience. This study compared the photosynthetic performance of two Triticum aestivum L. cultivars, Sokoll and Paragon, adapted to the climate of Mexico and UK, respectively, exposed to 1-week water deficit and high temperatures, in isolation or combination. Measurements included photosynthetic assimilation rate, stomatal conductance, in vitro activities of Rubisco (EC 4.1.1.39) and invertase (INV, EC 3.2.1.26), antioxidant capacity and chlorophyll a fluorescence. In both genotypes, under elevated temperatures and water deficit (WD38 degrees C), the photosynthetic limitations were mainly due to stomatal restrictions and to a decrease in the electron transport rate. Chlorophyll a fluorescence parameters clearly indicate differences between the two genotypes in the photoprotection when subjected to WD38 degrees C and showed faster recovery of Paragon after stress relief. The activity of the cytosolic invertase (CytINV) under these stress conditions was strongly related to the fast photosynthesis recovery of Paragon. Taken together, the results suggest that optimal sucrose export/utilization and increased photoprotection of the electron transport machinery are important components to limit yield fluctuations due to water shortage and elevated temperatures.

Phytopathogen infection alters primary metabolism status and plant development. The alternative oxidase (AOX) has been hypothesized to increase under pathogen attack preventing reductions, thus optimizing photosynthesis and growth. In this study, two genotypes of Medicago truncatula, one relatively resistant (Jemalong A17) and one susceptible (TN1.11), were infected with Fusarium oxysporum and Rhizoctonia solani. The in vivo foliar respiratory activities of the cytochrome oxidase pathway (COP) and the alternative oxidase pathway (AOP) were measured using the oxygen isotope fractionation. Gas exchange and photosynthesis-related parameters were measured and calculated together with antioxidant enzymes activities and organic acids contents. Our results show that the in vivo activity of AOX (valt ) plays a role under fungal infection. When infected with R. solani, the increase of valt in A17 was concomitant to an increase in net assimilation, in mesophyll conductance, to an improvement in the maximum velocity of Rubisco carboxylation and to unchanged malate content. However, under F. oxysporum infection, the induced valt was accompanied by an enhancement in the antioxidant enzymes, superoxide dismutase (SOD; EC1.15.1.1), catalase (CAT; EC1.11.1.6) and guaiacol peroxidase (GPX; EC1.11.1.7), activities and to an unchanged tricarboxylic acid cycle intermediates. These results provide new insight into the role of the in vivo activity of AOX in coordinating primary metabolism interactions that, partly, modulate the relative resistance of M. truncatula to diseases caused by soil-borne pathogenic fungi.

Saving water and enhancing rice productivity are consensually the most important research goals globally. While increasing canopy cover would enhance growth rates by higher photosynthetic carbon gain, an accompanied increase in transpiration would have a negative impact on saving water as well as for sustainability under water-limited conditions. Increased water use efficiency (WUE) by virtue of higher carbon assimilatory capacity can significantly circumvent this trade-off. Here, we report leaf mass area (LMA) has an important canopy architecture trait which when combined with superior carboxylation efficiency (CE) would achieve higher water productivity in rice. A set of 130 ethyl methanesulfonate induced mutants of an upland cultivar Nagina-22 (N22), was screened for leaf morphological traits leading to the identification of mutants differing in LMA. The wild-type, N22, along with a selected low-LMA (380-4-3) and two high-LMA mutants (392-9-1 and 457-1-3), all with comparable total leaf area, were raised under well-watered (100% Field Capacity (FC)) and water-limited (60% FC) conditions. Low Δ13 C and a higher RuBisCO content in high-LMA mutants indicated higher carboxylation efficiency, leading to increased carbon gain. Single parent backcross populations developed by crossing high and the low-LMA mutants with N22, separately, were screened for LMA, Δ13 C and growth traits. Comparison of dry matter accumulation per unit leaf area among the progenies differing in LMA and Δ13 C reiterated the association of LMA with CE. Results illustrated that high-LMA when combined with higher CE (low Δ13 C) lead to increased WUE and growth rates.

The negative effects of water stress on rice can be alleviated by NH4 + nutrition. However, the effects of mixed nitrogen (N) nutrition (NO3 - + NH4 + ) on resistance to water stress are still not well known. To investigate the response of rice growth to water stress and its relationship with photosynthetic characteristics, a hydroponic experiment supplying different N forms was conducted. Compared with NO3 - nutrition, mixed-N and NH4 + nutrition greatly alleviated the reduction of leaf area, chlorophyll content, and photosynthesis under water stress, whilst subsequently maintaining higher biomass. In contrast, water stress inhibited the root-shoot ratios in NH4 + - and mixed-N-supplied plants, indicating reduced root growth and higher photosynthate availability to shoots. The following key observations were made: (1) a similar stomatal limitation and low proportion of activated Rubisco were observed among the three different N nutrition regimes; (2) increased mesophyll conductance in NH4 + - and mixed-N-supplied plants simultaneously stimulated leaf photosynthesis and improved the water use efficiency and (3), the maximum carboxylation rate and actual photochemical efficiency of photosystem II in NH4 + - and mixed-N-supplied plants were significantly higher than that in NO3 - -supplied plants, thus resulting in higher photochemical efficiency under water stress. In conclusion, mixed-N and NH4 + nutrition may be used to develop strategies for improved water stress resistance and stimulated biomass production under conditions of osmotic stress and possibly drought.

Rubisco, the enzyme that constitutes as much as half of the protein in a leaf, initiates either the photorespiratory pathway that supplies reductant for the assimilation of nitrate into amino acids or the C3 carbon fixation pathway that generates carbohydrates. The relative rates of these two pathways depend both on the relative extent to which O2 and CO2 occupies the active site of Rubisco and on whether manganese or magnesium is bound to the enzyme. This study quantified the activities of manganese and magnesium in isolated tobacco chloroplasts and the thermodynamics of binding of these metals to Rubisco purified from tobacco or a bacterium. In tobacco chloroplasts, manganese was less active than magnesium, but Rubisco purified from tobacco had a higher affinity for manganese. The activity of each metal in the chloroplast was similar in magnitude to the affinity of tobacco Rubisco for each. This indicates that, in tobacco chloroplasts, Rubisco associates almost equally with both metals and rapidly exchanges one metal for the other. Binding of magnesium was similar in Rubisco from tobacco and a bacterium, whereas binding of manganese differed greatly between the Rubisco from these species. Moreover, the ratio of leaf manganese to magnesium in C3 plants increased as atmospheric CO2 increased. These results suggest that Rubisco has evolved to improve the energy transfers between photorespiration and nitrate assimilation and that plants regulate manganese and magnesium activities in the chloroplast to mitigate detrimental changes in their nitrogen/carbon balance as atmospheric CO2 varies.

Rubisco activase (RCA) is an important enzyme that can catalyze the carboxylation and oxygenation activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is involved in the photosynthetic carbon reduction cycle. Here, we studied the effects of changes in RCA activity on photosynthesis, growth and development, as well as the low temperature and weak light tolerance of RCA overexpressing transgenic cucumber (Cucumis sativus) plants. CsRCA overexpression increased the plant height, leaf area and dry matter, and decreased the root/top ratio in transgenic cucumber plants compared with the wild-type (WT) plants. Low temperature and low light stress led to decreases in the CsRCA expression and protein levels, the photosynthetic rate (Pn) and the stomatal conductance (Gs), but an increase in the intercellular CO2 (Ci) concentration in cucumber leaves. The actual photochemical efficiency and maximal photochemical efficiency of photosystem II in cucumber seedlings also declined, but the initial fluorescence increased during low temperature and weak light stress. Transgenic plants showed a lower decrease in the CsRCA expression level and actual and maximal photochemical efficiencies, as well as increases in the Ci and initial fluorescence relative to the WT plants. Low temperature and low light stress resulted in a significant increase in the malondialdehyde (MDA) content; however, this increase was reduced in transgenic plants compared with that in WT plants. Thus, the overexpression of CsRCA may promote the growth and low temperature and low light tolerance of cucumber plants in solar greenhouses.

Water deficit is a major environmental constraint on crop productivity and performance and nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. This study aims to test the hypothesis that leaf spraying of S-nitrosoglutathione (GSNO), an NO donor, improves the antioxidant defense in both roots and leaves of sugarcane plants under water deficit, with positive consequences for photosynthesis. In addition, the roles of key photosynthetic enzymes ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) in maintaining CO2 assimilation of GSNO-sprayed plants under water deficit were evaluated. Sugarcane plants were sprayed with water or GSNO 100 μM and subjected to water deficit, by adding polyethylene glycol (PEG-8000) to the nutrient solution. Sugarcane plants supplied with GSNO presented increases in the activity of antioxidant enzymes such as superoxide dismutase in leaves and catalase in roots, indicating higher antioxidant capacity under water deficit. Such adjustments induced by GSNO were sufficient to prevent oxidative damage in both organs and were associated with better leaf water status. As a consequence, GSNO spraying alleviated the negative impact of water deficit on stomatal conductance and photosynthetic rates, with plants also showing increases in Rubisco activity under water deficit.

Our previous studies demonstrated that chloroplastic CO2 concentration (Cc) is not sufficient under high-nitrogen (N) supply in rice plants. In this research, we studied how aquaporins- (AQPs) mediated Cc under different N-supply levels. A hydroponic experiment was conducted in a greenhouse with three different N levels (low N, 0.71 mM; intermediate N, 2.86 mM; and high N, 7.14 mM) in a rice cultivar (Oryza sativa cv. Shanyou 63) and with an ospip1;1 mutant (Oryza sativa cv. Nipponbare). The photosynthetic nitrogen-use efficiency (PNUE) decreased with increasing leaf-N content. Under high-N supply, the estimated Cc was significantly lower than the theoretical Cc and the specific Rubisco activity (carboxylation efficiency/Rubisco content, CE/Rubisco) decreased, because of a decrease of relative CO2 diffusion conductance (total CO2 diffusion conductance/leaf-N content, gt /N) in mesophyll cells. Real Time Quantitative PCR (Q-RT-PCR) showed that most OsPIP1s and OsPIP2s expression were downregulated under the high-N supply. Furthermore, Cc and gm decreased in the ospip1;1 mutant line compared with that of the wild-type plant. It was concluded that under high-N supply, the decreased PNUE was associated with non-sufficient Cc, mediated by AQP in mesophyll conductance.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is highly regulated in response to fluctuations in the environment, including changes in irradiance. However, no complex data are available on Rubisco regulatory mechanisms triggered in plants which are submitted to moderate-low irradiance shift. Therefore, we investigated in a comprehensive way the changes at the level of amount of Rubisco protein, its structural organization and carboxylase activity of the holoenzyme as triggered by exposure of moderate irradiance-grown Arabidopsis thaliana plants to low irradiance conditions. An exposure of moderate irradiance-grown plants to low irradiance for a single photoperiod caused the exclusion of a certain pool of Rubisco under altered conditions owing to oxidative modifications resulting in the formation of protein aggregates involving Rubisco large subunit (LS). As a result, both initial and total Rubisco carboxylase activities were reduced, whereas Rubisco activation state remained largely unchanged. The results of the determination of reactive oxygen species indicated that a moderate/low irradiance transition had stimulated (1) O2 accumulation and we strongly suggest that Rubisco oxidative modifications leading to formation of aggregates encompassing Rubisco-LS were triggered by (1) O2 . When moderate irradiance regime was resumed, the majority of Rubisco-LS containing aggregates tended to be resolubilized, and this allowed Rubisco carboxylation activities to be largely recovered, without changes in the activation state of the enzyme. In the longer term, these results allow us to better understand a complexity of Rubisco regulatory mechanisms activated in response to abiotic stresses and during recovery from the stresses.

Short-term waterlogging and chronic elevated temperature occur concomitantly in the cotton (Gossypium hirsutum) growing season. While previous research about co-occurring waterlogging and elevated temperature has focused primarily on cotton fiber, no studies have investigated carbohydrate metabolism of the subtending leaf (a major source leaf for boll development) cross-acclimation to aforementioned stressors. To address this, plants were exposed to ambient (31.6/26.5°C) and elevated (34.1/29.0°C) temperatures during the whole flowering and boll formation stage, and waterlogging (0, 3, 6 days) beginning on the day of anthesis. Both waterlogging and high temperature limited boll biomass (reduced by 1.19-32.14%), but effects of different durations of waterlogging coupled with elevated temperature on carbohydrate metabolism in the subtending leaf were quite different. The 6-day waterlogging combined with elevated temperature had the most negative impact on net photosynthetic rate (Pn) and carbohydrate metabolism of any treatment, leading to upregulated GhSusA and GhSusC expression and enhanced sucrose synthase (SuSy, EC 2.4.1.13) activity for sucrose degradation. A prior exposure to waterlogging for 3 days improved subtending leaf performance under elevated temperature. Pn, sucrose concentrations, Rubisco (EC 4.1.1.39) activity, and cytosolic fructose-1,6-bisphosphatase (cy-FBPase, EC 3.1.3.11) activity in the subtending leaf significantly increased, while SuSy activity decreased under 3 days waterlogging and elevated temperature combined relative to elevated temperature alone. Thus, we concluded that previous exposure to a brief (3 days) waterlogging stress improved sucrose composition and accumulation cross-acclimation to high temperature later in development not only by promoting leaf photosynthesis but also inhibiting sucrose degradation.

In order to find some basis of salinity resistance in the chloroplastic metabolism, a halophytic Thellungiella salsuginea was compared with glycophytic Arabidopsis thaliana. In control T.s. plants the increased ratios of chlorophyll a/b and of fluorescence emission at 77 K (F_{730}/F_{685}) were documented, in comparison to A.t.. This was accompanied by a higher YII and lower NPQ (non-photochemical quenching) values, and by a more active PSI (photosystem I). Another prominent feature of the photosynthetic electron transport (PET) in T.s. was the intensive production of H_2O_2 from PQ (plastoquinone) pool. Salinity treatment (0.15 and 0.30 M NaCl for A.t. and T.s., respectively) led to a decrease in ratios of chl a/b and F_{730}/F_{685}. In A.t., a salinity-driven enhancement of YII and NPQ was found, in association with the stimulation of H_2O_2 production from PQ pool. In contrast, in salinity-treated T.s., these variables were similar as in controls. The intensive H_2O_2 generation was accompanied by a high activity of PTOX (plastid terminal oxidase), whilst inhibition of this enzyme led to an increased H_2O_2 formation. It is hypothesized, that the intensive H_2O_2 generation from PQ pool might be an important element of stress preparedness in Thellungiella plants. In control T.s. plants, a higher activation state of carboxylase ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) was also documented in concert with the attachment of Rubisco activase (RCA) to the thylakoid membranes. It is supposed, that a closer contact of RCA with PSI in T.s. enables a more efficient Rubisco activation than in A.t.

The ability of the freshwater alga, Chlorella kessleri, to maintain a carbon concentrating mechanism when grown at acid pH was investigated. The alga grows over the pH range 4.0-9.0 and was found to take up bicarbonate and CO2 actively when grown at pH 6.0. However, when grown at acid pH (below 5.5), it does not have active CO2 uptake. The acidotolerant species maintained an internal pH of 6.1-7.5 over the external pH range 4.5-7.5, thus the pH difference between the cell interior and the external medium was large enough to allow for the diffusive uptake of CO2 at acid external pH. Mass spectrometric monitoring of O2 and CO2 fluxes by suspensions of C. kessleri, grown at acid pH, and maintained at pH 7.5 showed that the rates of O2 evolution did not exceed those of CO2 uptake. The final CO2 compensation concentrations of 14.0-17.7 µM reached by photosynthetic cells were above the CO2 equilibrium concentration in the external medium, indicating a lack of active CO2 uptake at acid pH. Chlorella kessleri accumulated CO2 with internal concentrations that were 9.9, 18.7 and 22.7-fold that of the external medium for cells grown, respectively, at pH 4.5, 5.0 and 5.5. The ability of C. kessleri cells to accumulate high intracellular concentrations of inorganic carbon at acid pH would provide a sufficiently high concentration of CO2 at the active site of Rubisco thus allowing the alga to maintain growth rates similar to those at alkaline pH.

The biochemical mechanisms underlying the involvement of cytosolic ascorbate peroxidases (cAPXs) in photosynthesis are still unknown. In this study, rice plants doubly silenced in these genes (APX1/2) were exposed to moderate light (ML) and high light (HL) to assess the role of cAPXs in photosynthetic efficiency. APX1/2 mutants that were exposed to ML overexpressed seven and five proteins involved in photochemical activity and photorespiration, respectively. These plants also increased the pheophytin and chlorophyll levels, but the amount of five proteins that are important for Calvin cycle did not change. These responses in mutants were associated with Rubisco carboxylation rate, photosystem II (PSII) activity and potential photosynthesis, which were similar to non-transformed plants. The upregulation of photochemical proteins may be part of a compensatory mechanism for APX1/2 deficiency but apparently the finer-control for photosynthesis efficiency is dependent on Calvin cycle proteins. Conversely, under HL the mutants employed a different strategy, triggering downregulation of proteins related to photochemical activity, Calvin cycle and decreasing the levels of photosynthetic pigments. These changes were associated to strong impairment in PSII activity and Rubisco carboxylation. The upregulation of some photorespiratory proteins was maintained under that stressful condition and this response may have contributed to photoprotection in rice plants deficient in cAPXs. The data reveal that the two cAPXs are not essential for photosynthesis in rice or, alternatively, the deficient plants are able to trigger compensatory mechanisms to photosynthetic acclimation under ML and HL conditions. These mechanisms involve differential regulation in protein expression related to photochemistry, Calvin cycle and photorespiration.

Although shoot N depletion in plants exposed to elevated [CO] has already been reported on several occasions, some uncertainty remains about the mechanisms involved. This study illustrates (1) the importance of characterizing root–shoot interactions and (2) the physiological, biochemical and gene expression mechanisms adopted by nitrate-fed Arabidopsis thaliana plants grown under elevated [CO]. Elevated [CO] increases biomass and photosynthetic rates; nevertheless, the decline in total soluble protein, Rubisco and leaf N concentrations revealed a general decrease in leaf N availability. A transcriptomic approach (conducted at the root and shoot level) revealed that exposure to 800 ppm [CO] induced the expression of genes involved in the transport of nitrate and mineral elements. Leaf N and mineral status revealed that N assimilation into proteins was constrained under elevated [CO]. Moreover, this study also highlights how elevated [CO] induced the reorganization of nitrate assimilation between tissues; root nitrogen assimilation was favored over leaf assimilation to offset the decline in nitrogen metabolism in the leaves of plants exposed to elevated [CO]., This work has been funded by the Spanish MICIIN (AGL2009-13339-C02-02, AGL2011-30386-C02-02 and AGL2012-37815-C05-05). I. Jauregui has been holder of an FPI fellowship from the Spanish Ministry of Economy and Competitiveness. S. Sakalauskiene has been holder of a Postdoctoral fellowship funded by the Research Council of Lithuania by the project ‘Postdoctoral fellowship Implementation in Lithuania’. I. Aranjuelo was the recipient of Ramón y Cajal research grant (Ministerio de Economía y Competitividad).

Acclimation of plant photosynthesis to light irradiance (photoacclimation) involves adjustments in levels of pigments and proteins and larger scale changes in leaf morphology. To investigate the impact of rising atmospheric CO₂ on crop physiology, we hypothesize that elevated CO₂ interacts with photoacclimation in rice (Oryza sativa). Rice was grown under high light (HL: 700 µmol m⁻² s⁻¹), low light (LL: 200 µmol m⁻² s⁻¹), ambient CO₂ (400 µl l⁻¹) and elevated CO₂ (1000 µl l⁻¹). Leaf six was measured throughout. Obscuring meristem tissue during development did not alter leaf thickness indicating that mature leaves are responsible for sensing light during photoacclimation. Elevated CO₂ raised growth chamber photosynthesis and increased tiller formation at both light levels, while it increased leaf length under LL but not under HL. Elevated CO₂ always resulted in increased leaf growth rate and tiller production. Changes in leaf thickness, leaf area, Rubisco content, stem and leaf starch, sucrose and fructose content were all dominated by irradiance and unaffected by CO₂. However, stomata responded differently; they were significantly smaller in LL grown plants compared to HL but this effect was significantly suppressed under elevated CO₂. Stomatal density was lower under LL, but this required elevated CO₂ and the magnitude was adaxial or abaxial surface-dependent. We conclude that photoacclimation in rice involves a systemic signal. Furthermore, extra carbohydrate produced under elevated CO₂ is utilized in enhancing leaf and tiller growth and does not enhance or inhibit any feature of photoacclimation with the exception of stomatal morphology.

To determine the effects of elevated CO(2) concentration ([CO(2)]) on the temperature-dependent photosynthetic properties, we measured gas exchange and chlorophyll fluorescence at various leaf temperatures (15, 20, 25, 30, 35 and 40°C) in 1-year-old seedlings of the Japanese white birch (Betula platyphylla var. japonica), grown in a phytotron under natural daylight at two [CO(2)] levels (ambient: 400 µmol mol(-1) and elevated: 800 µmol mol(-1)) and limited N availability (90 mg N plant(-1)). Plants grown under elevated [CO(2)] exhibited photosynthetic downregulation, indicated by a decrease in the carboxylation capacity of Rubisco. At temperatures above 30°C, the net photosynthetic rates of elevated-CO(2)-grown plants exceeded those grown under ambient [CO(2)] when compared at their growth [CO(2)]. Electron transport rates were significantly lower in elevated-CO(2)-grown plants than ambient-CO(2)-grown ones at temperatures below 25°C. However, no significant difference was observed in the fraction of excess light energy [(1 - q(P))× F(v)'/F(m)'] between CO(2) treatments across the temperature range. The quantum yield of regulated non-photochemical energy loss was significantly higher in elevated-CO(2)-grown plants than ambient, when compared at their respective growth [CO(2)] below 25°C. These results suggest that elevated-CO(2)-induced downregulation might not exacerbate the temperature-dependent susceptibility to photoinhibition, because reduced energy consumption by electron transport was compensated for by increased thermal energy dissipation at low temperatures.

To clarify the relationship between cultivar difference in the sensitivity of net photosynthesis to ozone (O(3)) and the reactive oxygen species (ROS) scavenging system in wheat (Triticum aestivum), we investigated the effects of chronic exposure to ambient levels of O(3) on gas exchange rates, activity and concentration of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), activity of ROS scavenging enzymes and concentration of antioxidants of the flag leaf in two Japanese winter wheat cultivars (Norin 61 and Shirogane-komugi). Although the net photosynthetic rate of the flag leaf in Norin 61 was not significantly reduced by exposure to O(3), that in Shirogane-komugi was significantly reduced by the exposure to O(3) during the anthesis and early grain-filling stages. In the two cultivars, stomatal diffusive conductance to H(2) O of the flag leaf was not significantly affected by the exposure to O(3). The exposure to O(3) induced significant reductions in the activity and concentration of Rubisco, activities of catalase (CAT) and monodehydroascorbate reductase (MDAR) and concentrations of reduced form of ascorbate and total glutathione of the flag leaf in Shirogane-komugi. It was concluded that the sensitivity of net photosynthesis of flag leaf to O(3) is higher in Shirogane-komugi than in Norin 61, and the difference in the sensitivity to O(3) between the two cultivars is mainly due to that in the effects of O(3) on the detoxification ability of ROS, mainly determined by the activity of ROS scavenging enzymes, such as CAT and MDAR.

Oryza meridionalis is a wild species of rice, endemic to tropical Australia. It shares a significant genome homology with the common domesticated rice Oryza sativa. Exploiting the fact that the two species are highly related but O. meridionalis has superior heat tolerance, experiments were undertaken to identify the impact of temperature on key events in photosynthesis. At an ambient CO(2) partial pressure of 38 Pa and irradiance of 1500 µmol quanta m(-2) s(-1), the temperature optimum of photosynthesis was 33.7 ± 0.8°C for O. meridionalis, significantly higher than the 30.6 ± 0.7°C temperature optimum of O. sativa. To understand the basis for this difference, we measured gas exchange and rubisco activation state between 20 and 42°C and modeled the response to determine the rate-limiting steps of photosynthesis. The temperature response of light respiration (R(light)) and the CO(2) compensation point in the absence of respiration (Γ(*)) were determined and found to be similar for the two species. C3 photosynthesis modeling showed that despite the difference in susceptibility to high temperature, both species had a similar temperature-dependent limitation to photosynthesis. Both rice species were limited by ribulose-1,5-bisphosphate (RuBP) regeneration at temperatures of 25 and 30°C but became RuBP carboxylation limited at 35 and 40°C. The activation state of rubisco in O. meridionalis was more stable at higher temperatures, explaining its greater heat tolerance compared with O. sativa.

In this study, we tested for the temporal occurrence of photosynthetic acclimation to elevated [CO₂] in the flag leaf of two important cereal crops, rice and wheat. In order to characterize the temporal onset of acclimation and the basis for any observed decline in photosynthetic rate, we characterized net photosynthesis, g(s) , g(m) , C(i) /C(a) , C(i) /C(c) , V(cmax) , J(max) , cell wall thickness, content of Rubisco, cytochrome (Cyt) f, N, chlorophyll and carbohydrate, mRNA expression for rbcL and petA, activity for Rubisco, sucrose phosphate synthase (SPS) and sucrose synthase (SS) at full flag expansion, mid-anthesis and the late grain-filling stage. No acclimation was observed for either crop at full flag leaf expansion. However, at the mid-anthesis stage, photosynthetic acclimation in rice was associated with RuBP carboxylation and regeneration limitations, while wheat only had the carboxylation limitation. By grain maturation, the decline of Rubisco content and activity had contributed to RuBP carboxylation limitation of photosynthesis in both crops at elevated [CO₂]; however, the sharp decrease of Rubisco enzyme activity played a more important role in wheat. Although an increase in non-structural carbohydrates did occur during these later stages, it was not consistently associated with changes in SPS and SS or photosynthetic acclimation. Rather, over time elevated [CO₂] appeared to enhance the rate of N degradation and senescence so that by late-grain fill, photosynthetic acclimation to elevated [CO₂] in the flag leaf of either species was complete. These data suggest that the basis for photosynthetic acclimation with elevated [CO₂] may be more closely associated with enhanced rates of senescence, and, as a consequence, may be temporally dynamic, with significant species variation.

Stomatal closure and metabolic impairment under drought stress limits photosynthesis. The objective of this study was to determine major stomatal and metabolic factors involved in photosynthetic responses to drought and recovery upon re-watering in a C(3) perennial grass species, Kentucky bluegrass (Poa pratensis L.). Two genotypes differing in drought resistance, 'Midnight' (tolerant) and 'Brilliant' (sensitive), were subjected to drought stress for 15 days and then re-watered for 10 days in growth chambers. Single-leaf net photosynthetic rate (A), stomatal conductance (g(s)) and transpiration rate (Tr) decreased during drought, with a less rapid decline in 'Midnight' than in 'Brilliant'. Photochemical efficiency, Rubisco activity and activation state declined during drought, but were significantly higher in 'Midnight' than in 'Brilliant'. The relationship between A and internal leaf CO(2) concentration (A/Ci curve) during drought and re-watering was analyzed to estimate the relative influence of stomatal and non-stomatal components on photosynthesis. Stomatal limitation (Ls %), non-stomatal limitation (Lns %), CO(2) compensation point (CP) and dark respiration (Rd) increased with stress duration in both genotypes, but to a lesser extent in 'Midnight'. Maximum CO(2) assimilation rate (A(max)), carboxylation efficiency (CE) and mesophyll conductance (g(m)) declined, but 'Midnight' had significantly higher levels of A(max), CE and g(m) than 'Brilliant'. Maximum carboxylation rate of Rubisco (V(cmax)) and ribulose-1,5-bisphospate (RuBP) regeneration capacity mediated by maximum electron transport rate (J(max)) decreased from moderate to severe drought stress in both genotypes, but to a greater extent in 'Brilliant' than in 'Midnight'. After re-watering, RWC restored to about 90% of the control levels in both genotypes, whereas A, g(s), Tr and Fv/Fm was only partially recovered, with a higher recovery level in 'Midnight' than in 'Brilliant'. Rubisco activity and activation state restored to the control level after re-watering, with more rapid increase in 'Midnight' than in 'Brilliant'. The values of Ls, Lns, CP and Rd declined, and A(max), CE, V(cmax), J(max) and g(m) increased after re-watering, with more rapid change in all parameters in 'Midnight' than in 'Brilliant'. These results indicated that the maintenance of higher A and A(max) under drought stress in drought-tolerant Kentucky bluegrass could be attributed to higher Rubico activation state, higher CE and less stomatal limitation. The ability to resume metabolic activity (A(max), CE, Fv/Fm and Rubisco) was observed in the drought-tolerant genotype and is the most likely cause for the increased recuperative ability of photosynthesis. Incomplete recovery of photosynthesis upon re-watering could be attributable to lasting stomatal limitations caused by severe drought damage in both genotypes. Promoting rapid stomatal recovery from drought stress may be critical for plants to resume full photosynthetic capacity in C(3) perennial grass species.

Studies on some plant species have shown that increasing the growth temperature gradually or pretreating with high temperature can lead to obvious photosynthetic acclimation to high temperature. To test whether this acclimation arises from heat adaptation of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) activation mediated by Rubisco activase (RCA), gene expression of RCA large isoform (RCA(L)) and RCA small isoform (RCA(S)) in rice was determined using a 4-day heat stress treatment [40/30 degrees C (day/night)] followed by a 3-day recovery under control conditions [30/22 degrees C (day/night)]. The heat stress significantly induced the expression of RCA(L) as determined by both mRNA and protein levels. Correlative analysis indicated that RCA(S) protein content was extremely significantly related to Rubisco initial activity and net photosynthetic rate (Pn) under both heat stress and normal conditions. Immunoblot analysis of the Rubisco-RCA complex revealed that the ratio of RCA(L) to Rubisco increased markedly in heat-acclimated rice leaves. Furthermore, transgenic rice plants expressing enhanced amounts of RCA(L) exhibited higher thermotolerance in Pn and Rubisco initial activity and grew better at high temperature than wild-type (WT) plants and transgenic rice plants expressing enhanced amounts of RCA(S). Under normal conditions, the transgenic rice plants expressing enhanced amounts of RCA(S) showed higher Pn and produced more biomass than transgenic rice plants expressing enhanced amounts of RCA(L) and wild-type plants. Together, these suggest that the heat-induced RCA(L) may play an important role in photosynthetic acclimation to moderate heat stress in vivo, while RCA(S) plays a major role in maintaining Rubisco initial activity under normal conditions.

Limited data are available on the effects of phosphorus (P) and aluminum (Al) interactions on Citrus spp. growth and photosynthesis. Sour pummelo (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 50, 100, 250 and 500 microM KH(2)PO(4)x 0 and 1.2 mM AlCl(3). 6H(2)O. Thereafter, P and Al in roots, stems and leaves, and leaf chlorophyll (Chl), CO(2) assimilation, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and Chl a fluorescence (OJIP) transients were measured. Under Al stress, P increased root Al, but decreased stem and leaf Al. Shoot growth is more sensitive to Al than root growth, CO(2) assimilation and OJIP transients. Al decreased CO(2) assimilation, Rubisco activity and Chl content, whereas it increased or did not affect intercellular CO(2) concentration. Al affected CO(2) assimilation more than Rubisco and Chl under 250 and 500 microM P. Al decreased root, stem and leaf P, leaf maximum quantum yield of primary photochemistry (F(v)/F(m)) and total performance index (PI(tot,abs)), but increased leaf minimum fluorescence (F(o)), relative variable fluorescence at K- and I-steps. P could alleviate Al-induced increase or decrease for all these parameters. We conclude that P alleviated Al-induced inhibition of growth and impairment of the whole photosynthetic electron transport chain from photosystem II (PSII) donor side up to the reduction of end acceptors of photosystem I (PSI), thus preventing photosynthesis inhibition through increasing Al immobilization in roots and P level in roots and shoots. Al-induced impairment of the whole photosynthetic electron transport chain may be associated with growth inhibition.

In temperate regions, evergreen species are exposed to large seasonal changes in air temperature and irradiance. They change photosynthetic characteristics of leaves responding to such environmental changes. Recent studies have suggested that photosynthetic acclimation is strongly constrained by leaf anatomy such as leaf thickness, mesophyll and chloroplast surface facing the intercellular space, and the chloroplast volume. We studied how these parameters of leaf anatomy are related with photosynthetic seasonal acclimation. We evaluated differential effects of winter and summer irradiance on leaf anatomy and photosynthesis. Using a broad-leaved evergreen Aucuba japonica, we performed a transfer experiment in which irradiance regimes were changed at the beginning of autumn and of spring. We found that a vacant space on mesophyll surface in summer enabled chloroplast volume to increase in winter. The leaf nitrogen and Rubisco content were higher in winter than in summer. They were correlated significantly with chloroplast volume and with chloroplast surface area facing the intercellular space. Thus, summer leaves were thicker than needed to accommodate mesophyll surface chloroplasts at this time of year but this allowed for increases in mesophyll surface chloroplasts in the winter. It appears that summer leaf anatomical characteristics help facilitate photosynthetic acclimation to winter conditions. Photosynthetic capacity and photosynthetic nitrogen use efficiency were lower in winter than in summer but it appears that these reductions were partially compensated by higher Rubisco contents and mesophyll surface chloroplast area in winter foliage.

The temperature dependence of C(3) photosynthesis may be altered by the growth environment. The effects of long-term growth in elevated CO(2) on photosynthesis temperature response have been investigated in wheat (Triticum aestivum L.) grown in controlled chambers with 370 or 700 mumol mol(-1) CO(2) from sowing through to anthesis. Gas exchange was measured in flag leaves at ear emergence, and the parameters of a biochemical photosynthesis model were determined along with their temperature responses. Elevated CO(2) slightly decreased the CO(2) compensation point and increased the rate of respiration in the light and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) V(cmax), although the latter effect was reversed at 15 degrees C. With elevated CO(2), J(max) decreased in the 15-25 degrees C temperature range and increased at 30 and 35 degrees C. The temperature response (activation energy) of V(cmax) and J(max) increased with growth in elevated CO(2). CO(2) enrichment decreased the ribulose 1,5-bisphosphate (RuBP)-limited photosynthesis rates at lower temperatures and increased Rubisco- and RuBP-limited rates at higher temperatures. The results show that the photosynthesis temperature response is enhanced by growth in elevated CO(2). We conclude that if temperature acclimation and factors such as nutrients or water availability do not modify or negate this enhancement, the effects of future increases in air CO(2) on photosynthetic electron transport and Rubisco kinetics may improve the photosynthetic response of wheat to global warming.