Photosynthetic response of wheat cultivar to long-term exposure to elevated temperature

Wheat (Triticum aestivum L. cv. HD 2285) was grown in control (C) and heated (H) open top chambers (OTCs) for entire period of growth and development till maturity. The mean maximum temperature of the entire period was 3 °C higher in H-compared to C-OTCs. Net photosynthetic rate (P _N) measured at different temperature (20–40 °C) of C-and H-grown plants showed greater sensitivity to high temperature in H-plants. P _N measured at respective growth temperature was lower in H-compared to C-plants. The CO₂ and irradiance response curves of photosynthesis also showed lesser response in H-compared to C-plants. The initial slope of P _N versus internal CO₂ concentration (P _N/C _i) curve was lower in H-than C-plants indicating ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) limitation. In irradiance response curve, the plateau was lower in H-compared to C-plants which is interpreted as RuBPCO limitation. RuBPCO content in the leaves of C-and H-plants, however, was not significantly different. Ribulose-1,5-bisphosphate carboxylase (RuBPC) initial activity was lower in H-plants, whereas activity of fully activated enzyme was not affected, indicating a decrease in activation state of the enzyme. This was further substantiated by the observed decrease in RuBPCO activase activity in H-compared to C-plants. RuBPCO activase was thus sensitive even to moderate heat stress. The decrease in P _N under moderate heat stress was mainly due to a decrease in activation state of RuBPCO catalysed by RuBPCO activase.     

Photosynthetic responses of radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis> var. <Emphasis Type="Italic">longipinnatus</Emphasis>) plants to infection by turnip mosaic virus

Plant growth, chlorophyll (Chl) content, photosynthetic gas exchange, ribulose-1,5-bisphosphate carboxylase (RuBPCO) enzyme activity, and Chl fluorescence in radish (Raphanus sativus var. longipinnatus) plants were examined after turnip mosaic virus (TuMV) infection. Plant fresh mass, dry mass, Chl content, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and RuBPCO activity were significantly lower in infected plants after 5 weeks of virus infection as compared to healthy plants. The 5-week virus infection did not induce significant differences in intercellular CO₂ concentration (C _i, photochemical efficiency of photosystem 2, PS2 (F_v/F_m), excitation capture efficiency of open PS2 reaction centres (F_v'/F_m'), effective quantum efficiency of photosystem 2 (ΔF/F_m'), and photochemical quenching (q_P), but non-photochemical quenching (q_N) and alternative electron sink (AES) were significantly enhanced. Thus the decreased plant biomass of TuMV-infected plants might be associated with the decreased photosynthetic activity mainly due to reduced RuBPCO activity.     

Differences in the Activation State of Ribulose-1,5-bisphosphate Carboxylase/Oxygenase in Barley,Pea, and Wheat at Two Altitudes

Activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) is an important parameter determining the rate of net photosynthesis (P _N) in situ for which no information is available with reference to altitude. We analyzed activation state along with P _N in three plant species and their cultivars grown at low (LA, 1 300 m) and high (HA, 4 200 m) altitudes. No significant change in P _N and the initial activity of RuBPCO was obtained with reference to altitude. However, activation state of RuBPCO was reduced significantly in the HA plants as compared to the LA ones. Hence low partial pressure of CO₂ prevailing at HA might be responsible for the lower activation state of RuBPCO.     

Differential inhibition of photosynthesis under drought stress in <Emphasis Type="Italic">Flaveria</Emphasis> species with different degrees of development of the C<Subscript>4</Subscript> syndrome

The effect of drought stress (DS) on photosynthesis and photosynthesis-related enzyme activities was investigated in F. pringlei (C₃), F. floridana (C₃–C₄), F. brownii (C₄-like), and F. trinervia (C₄) species. Stomatal closure was observed in all species, probably being the main cause for the decline in photosynthesis in the C₃ species under ambient conditions. In vitro ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and stromal fructose 1,6-bisphosphatase (sFBP) activities were sufficient to interpret the net photosynthetic rates (P _N), but, from the decreases in P _N values under high CO₂ (C _a = 700 μmol mol^{− 1}) it is concluded that a decrease in the in vivo rate of the RuBPCO reaction may be an additional limiting factor under DS in the C₃ species. The observed decline in the photosynthesis capacity of the C₃–C₄ species is suggested to be associated both to in vivo decreases of RuBPCO activity and of the RuBP regeneration rate. The decline of the maximum P _N observed in the C₄-like species under DS was probably attributed to a decrease in maximum RuBPCO activity and/or to decrease of enzyme substrate (RuBP or PEP) regeneration rates. In the C₄ species, the decline of both in vivo photosynthesis and photosynthetic capacity could be due to in vivo inhibition of the phosphoenolpyruvate carboxylase (PEPC) by a twofold increase of the malate concentration observed in mesophyll cell extracts from DS plants.     

Photosynthetic Activity of Ripening Tomato Fruit

Gas exchanges, chlorophyll (Chl) a fluorescence and carboxylation activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and phosphoenolpyruvate carboxylase (PEPC) were determined in tomato (Lycopersicon esculentum Mill.) fruits picked at different developmental stages (immature, red-turning, mature, and over-ripe). The fruits did not show signs of CO₂ fixation. However, photochemical activity was detectable and an effective electron transport was observed, the values of Chl fluorescence parameters in green fruits being similar to those determined in the leaves. The RuBPCO activity, which was similar to those recorded in the leaves at the immature stage of the fruit, decreased as the fruit ripened. PEPC activity was always higher than RuBPCO activity.     

Long-term effect of elevated CO2 on spatial differentiation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in Norway spruce canopy

Total in vitro activity of RuBPCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) enzyme was assayed spectrophotometrically by the continuous measurement of 3-phosphoglycerate-dependent NADH oxidation in a coupled enzyme system. RuBPCO activities were found in the ranges 1.01–2.76 and 1.23–3.10 µmol(CO₂) m⁻² s^{− 1} in current Norway spruce needles growing in ambient (AC) and elevated (EC) CO₂ concentration, respectively. RuBPCO activity in AC needles from the upper layer (U) was 11–15 % higher compared to those from the middle (M) layer, and even 44–56 % higher compared to the lower (L) layer of spruce crown. Over the vegetation season, we observed a highly significant decrease of RuBPCO activity in the EC-U needles from 3.10 (July) to 1.60 (October) µmol(CO₂) m⁻² s⁻¹ as a consequence of downward feedback regulation. Moreover, this down-regulation was not caused by a non-specific decrease in total leaf nitrogen content.The work forms a part of the research supported by grants no. LN00A141 and OC E21.001 (Ministry of Education CR), VaV640/18/03 (Ministry of Environment CR), and by the Research Intention of ILE AS CR AV0Z6087904.     

Photosynthetic acclimation to CO2 enrichment related to ribulose-1,5-bisphosphate carboxylation limitation in wheat

Net photosynthetic rate (P _N) measured at the same CO₂ concentration, the maximum in vivo carboxylation rate, and contents of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (RuBPCO) and RuBPCO activase were significantly decreased, but the maximum in vivo electron transport rate and RuBP content had no significant change in CO₂-enriched [EC, about 200 μmol mol⁻¹ above the ambient CO₂ concentration (AC)] wheat leaves compared with those in AC grown wheat leaves. Hence photosynthetic acclimation in wheat leaves to EC is largely due to RuBP carboxylation limitation.     

The Effect of Sodium Chloride on the Balance between the C3- and C4-Carbon Fixation Pathways

Young leaves of salt-depleted Aeluropus litoralis Parl. plants show CO₂ fixation by the C₃-carbon fixation pathway. No detectable activity of phosphoenol pyruvate (PEP) carboxylase was found. When A. litoralis plants were exposed to a NaCl solution, the leaves showed a high activity of PEP carboxylase as well as a significant CO₂ fixation by the C₄-pathway. — Also in Zea mays L. and Chloris gayana Kunth., the presence of NaCl in the medium influences the balance between phosphoenol pyruvate carboxylase and ribulose-1,5-diphosphate carboxylase.     

Expression of Tobacco Carbonic Anhydrase in the C4 Dicot Flaveria bidentis Leads to Increased Leakiness of the Bundle Sheath and a Defective CO2-Concentrating Mechanism

Flaveria bidentis (L.) Kuntze, a C₄ dicot, was genetically transformed with a construct encoding the mature form of tobacco (Nicotiana tabacum L.) carbonic anhydrase (CA) under the control of a strong constitutive promoter. Expression of the tobacco CA was detected in transformant whole-leaf and bundle-sheath cell (bsc) extracts by immunoblot analysis. Whole-leaf extracts from two CA-transformed lines demonstrated 10% to 50% more CA activity on a ribulose-1,5-bisphosphate carboxylase/oxygenase-site basis than the extracts from transformed, nonexpressing control plants, whereas 3 to 5 times more activity was measured in CA transformant bsc extracts. This increased CA activity resulted in plants with moderately reduced rates of CO₂ assimilation (A) and an appreciable increase in C isotope discrimination compared with the controls. With increasing O₂ concentrations up to 40% (v/v), a greater inhibition of A was found for transformants than for wild-type plants; however, the quantum yield of photosystem II did not differ appreciably between these two groups over the O₂ levels tested. The quantum yield of photosystem II-to-A ratio suggested that at higher O₂ concentrations, the transformants had increased rates of photorespiration. Thus, the expression of active tobacco CA in the cytosol of F. bidentis bsc and mesophyll cells perturbed the C₄ CO₂-concentrating mechanism by increasing the permeability of the bsc to inorganic C and, thereby, decreasing the availability of CO₂ for photosynthetic assimilation by ribulose-1,5-bisphosphate carboxylase/oxygenase.     

Some relationships between contents of photosynthetic intermediates and the rate of photosynthetic carbon assimilation in leaves of Zea mays L.

The relationship between the gas-exchange characteristics of attached leaves of Zea mays L. and the contents of photosynthetic intermediates was examined at different intercellular partial pressure of CO₂ and at different irradiances at a constant intercellular partial pressure of CO₂. (i) The behaviour of the pools of the C₄-cycle intermediates, phosphoenolpyruvate and pyruvate, provides evidence for light regulation of their consumption. However, light regulation of phosphoenolpyruvate carboxylase does not influence the assimilation rate at limiting intercellular partial pressures of CO₂. (ii) A close correlation between the pools of phosphoenolpyruvate and glycerate-3-phosphate exists under many different flux conditions, consistent with the notion that the pools of C₄ and C₃ cycles are connected via the interconversion of glycerate-3-phosphate and phosphoenolpyruvate. (iii) The ratio of triose-phosphate to glycerate-3-phosphate is used as an indicator of the availability of ATP and NADPH. Changes of this ratio with CO₂ and with irradiance are compared with results obtained in C₃ leaves and indicate that the mechanism of regulation of carbon assimilation by light in leaves of C₄ plants may differ from that in C₃ plants. (iv) The behaviour of the ribulose-1,5-bisphosphate pool with CO₂ and irradiance is contrasted with the behaviour of these pools measured in leaves of C₃ plants.Abbreviations P _i intercellular CO₂ pressure - RuBP ribulose-1,5-bisphosphate - PEP phosphoenolpyruvate - triose-P triose phosphates - PGA glycerate-3-phosphate     

RuBPCO kinetics and the mechanism of CO2 entry in C3 plants

Abstract. The CO₂ partial pressure in the chloroplasts of intact photosynthetic C₃ leaves is thought to be less than the intercellular CO₂ partial pressure. The intercellular CO₂ partial pressure can be calculated from CO₂ and H₂O gas exchange measurements, whereas the CO₂ partial pressure in the chloroplasts is unknown. The conductance of CO₂ from the intercellular space to the chloroplast stroma and the CO₂ partial pressure in the chloroplast stroma can be calculated if the properties of photosynthetic gas exchange are compared with the kinetics of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPCO). A discrepancy between gas exchange and RuBPCO kinetics can be attributed to a deviation of CO₂ partial pressure in the chloroplast stroma from that calculated in the intercellular space. This paper is concerned with the following: estimation of the kinetic constants of RuBPCO and their comparison with the CO₂ compensation concentration; their comparison with differential uptake of ¹⁴CO₂ and ¹²CO₂; and their comparison with O₂ dependence of net CO₂ uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5 °C, a photosynthetic photon flux density (PPFD) of 550 μmol quanta m^?2 s^?1, and an ambient CO₂ partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO₂ partial pressure was decreased, temperature incresed and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO₂ across the chloroplast envelope which decreases with increasing temperature. At low CO₂ partial pressure, the diffusion resistance appears to be counterbalanced by active CO₂ (or HCO₃) transport with high affinity and low maximum velocity. At low PPFD, CO₂ partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO₂ flux.     

Photorespiration studies in Cycas circinalis L. and Cycas beddomei Dyer

Photosynthetic carboxylating enzymes and the effects of light and temperature on ¹⁴CO₂ efflux in two species of gymnosperm leaves were studied. The activity of RuBP carboxylase was high and that of PEP carboxylase was very low when compared to C₄ plants. The CO₂ compensation point was high. ¹⁴CO₂ efflux was greater in light than in darkness and the ratio (L/D) increased with increase in temperature and light intensity. The inhibitors of glycolate metabolism showed decreased ¹⁴CO₂ evolution in light while dark respiration was unaffected. It is concluded that both Cycas circinalis, L. and Cycas beddomei Dyer are C₃ plants.     

Experimental and theoretical study on high temperature induced changes in chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence oscillations in barley leaves upon 2 % CO<Subscript>2</Subscript>

Oscillations in many of photosynthetic quantities with a period of about 1 min can be routinely measured with higher plant leaves after perturbation of the steady state by sudden change in gas phase. Among all hypotheses suggested so far to explain the oscillations, an effect of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activation status to control the oscillations is highly probable, at least upon high temperature (HT) treatment when in vivo RuBPCO activity controlled by RuBPCO activase (RuBPCO-A) decreases. Therefore, we measured the oscillations in fluorescence signal coming from barley leaves (Hordeum vulgare L. cv. Akcent) after their exposure for various time intervals to different HTs in darkness. We also evaluated steady state fluorescence and CO₂ exchange parameters to have an insight to functions of electron transport chain within thylakoid membrane and Calvin cycle before initiation of the oscillations. The changes in period of the oscillations induced by moderate HT (up to 43 °C) best correlated with changes in non-photochemical fluorescence quenching (q_N) that in turn correlated with changes in gross photosynthetic rate (P _G) and rate of RuBPCO activation (k_act). Therefore, we suggest that changes in period of the oscillations caused by moderate HT are mainly controlled by RuBPCO activation status. For more severe HT (45 °C), the oscillations disappeared which was probably caused by an insufficient formation of NADPH by electron transport chain within thylakoid membrane as judged from a decrease in photochemical fluorescence quenching (q_P). Suggestions made on the basis of experimental data were verified by theoretical simulations of the oscillations based on a model of Calvin cycle and by means of a control analysis of the model.     

Hormonal Regulation of Photosynthetic Enzymes in Cotton under Water Stress

Activities of ribulose bisphosphate carboxylase/oxygenase (RuBPCO), phosphoenolpyruvate carboxylase (PEPC), and carbonic anhydrase (CA) were determined in leaves of cotton (Gossypium hirsutum L. cv. H-777) subjected to 8-d waterlogging (WL) at the vegetative stage, or to drought (D) at the reproductive stage, or to interaction of both stresses. The soil moisture of control plants was kept at field capacity. One day prior to stress various growth hormones (5 μM) were sprayed up to runoff. WL reduced RuBPCO and CA activities, while PEPC activity increased. Upon D, RuBPCO and PEPC activities were reduced while CA activity was increased. Imposition of both stresses increased activities of all three enzymes. Effect of stresses on enzyme activity was alleviated by benzylaminopurine (BAP), but indol-3-yl-acetic acid was more promoting under interactive stress. No CA activity with BAP was observed during interactive stress. This revised version was published online in August 2006 with corrections to the Cover Date.     

A study of photorespiratory ammonia production in the C4 plant Amaranthus edulis, using mutants with altered photosynthetic capacities

Previous studies have indicated that the rate of photorespiration in C₄ plants is low or negligible. In this study, wild-type and mutant leaves of the C₄ plant Amaranthus edulis were treated with the glutamine synthetase inhibitor, phosphinothricin and the glycine decarboxylase inhibitor, aminoacetonitrile, at different concentrations of CO₂. The time course of ammonia accumulation in leaves of the wild type was compared with a mutant lacking phosphoenolpyruvate carboxylase activity (EC 4.1.1.31), and with three different mutants that accumulated glycine. The increase in the concentration of ammonia in the leaves, stimulated by the treatments was used as a measurement of the rate of photorespiration in C₄ plants. The application of glutamine and glycine maintained the rate of photorespiratory ammonia production for a longer period in the wild type, and increased the rate in a mutant lacking phosphoenolpyruvate carboxylase suggesting that there was a lack of amino donors in these plants. The calculated rate of photorespiration in Amaranthus edulis wild-type leaves when the supply of amino donors was enough to maintain the photorespiratory nitrogen flow, accounted for approximately 6% of the total net photosynthetic CO₂ assimilation rate. In a mutant lacking phosphoenolpyruvate carboxylase, however, this rate increased to 48%, when glutamine was fed to the leaf, a value higher than that found in some C₃ plants. In mutants of Amaranthus edulis that accumulated glycine, the rate of photorespiration was reduced to 3% of the total net CO₂ assimilation rate. The rate of ammonia produced during photorespiration was 60% of the total produced by all metabolic reactions in the leaves. The data suggests that photorespiration is an active process in C₄ plants, which can play an important role in photosynthetic metabolism in these plants.     

Isolation and Characterization of High CO(2)-Requiring-Mutants of the Cyanobacterium Synechococcus PCC7942 : Two Phenotypes that Accumulate Inorganic Carbon but Are Apparently Unable to Generate CO(2) within the Carboxysome

A total of 24 high CO₂-requiring-mutants of the cyanobacterium Synechococcus PCC7942 have been isolated and partially characterized. These chemically induced mutants are able to grow at 1% CO₂, on agar media, but are incapable of growth at air levels of CO₂. All the mutants were able to accumulate inorganic carbon (C_i) to levels similar to or higher than wild type cells, but were apparently unable to generate intracellular CO₂. On the basis of the rate of C_i release following a light (5 minutes) → dark transition two extreme phenotypes (fast and slow release mutants) and a number of `intermediate' mutants (normal release) were identified. Compared to wild-type cells, Type I mutants had the following characteristics: fast C_i release, normal internal C_i pool, normal carbonic anhydrase (CA) activity in crude extracts, reduced internal exchange of ¹⁸O from ¹⁸O-labeled CO₂, 1% CO₂ requirement for growth in liquid media, normal affinity of carboxylase for CO₂, and long, rod-like carboxysomes. Type II mutants had the following characteristics: slow C_i release, increased internal C_i pool, normal CA activity in crude extracts, normal internal ¹⁸O exchange, a 3% CO₂ requirement for growth in liquid media, high carboxylase activity, normal affinity of carboxylase for CO₂, and normal carboxysome structure but increased in numbers per cell. Both mutant phenotypes appear to have genetic lesions that result in an inability to convert intracellular HCO₃⁻ to CO₂ inside the carboxysome. The features of the type I mutants are consistent with a scenario where carboxysomal CA has been mistargeted to the cytosol. The characteristics of the type II phenotype appear to be most consistent with a scenario where CA activity is totally missing from the cell except for the fact that cell extracts have normal CA activity. Alternatively the type II mutants may have a lesion in their capacity for H⁺ import during photosynthesis.     

Expression of the large isoform of ribulose-1,5-bisphosphate carboxylase/oxygenase activase gene driven by rbcS promoter in Oryza sativa enhances the photosynthetic capacity

In order to investigate the effect of large isoform of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activase (RuBPCO-A) on photosynthesis, cDNA of the enzyme (rca) was transferred to rice cultivars (Oryza sativa f. japonica cv. Nipponbare) under the control of RuBPCO small subunit gene promoter (rbcS) via Agrobacterium tumefaciens-mediated transformation. Transgenic rice plants were identified by polymerase chain reaction (PCR) and Southern and Western blot analyses. Net photosynthetic rate (P _N) values of the T₁ transgenic lines 34 (T34) and 40 (T40) were 45.26 and 46.32 % higher than that of the control plants, respectively. At the same time, their carboxylation efficiency and RuBPCO initial activity, quantum yield of electron transport in photosystem 2 (Φ_PS2), and steady state photochemical fluorescence quenching (q_P) increased. In addition, heading time of the transgenic rice was advanced. Thus increasing the amount of large isoform of RuBPCO-A in the transgenic rice might have a stimulatory effect on both photosynthesis and plant growth.     

On the significance of C3—C4 intermediate photosynthesis to the evolution of C4 photosynthesis

Abstract Evidence is drawn from previous studies to argue that C₃—C₄ intermediate plants are evolutionary intermediates, evolving from fully-expressed C₃ plants towards fully-expressed C₄ plants. On the basis of this conclusion, C₃—C₄ intermediates are examined to elucidate possible patterns that have been followed during the evolution of C₄ photosynthesis. An hypothesis is proposed that the initial step in C₄-evolution was the development of bundle-sheath metabolism that reduced apparent photorespiration by an efficient recycling of CO₂ using RuBP carboxylase. The CO₂-recycling mechanism appears to involve the differential compartmentation of glycine decarboxylase between mesophyll and bundle-sheath cells, such that most of the activity is in the bundlesheath cells. Subsequently, elevated phosphoenolpyruvate (PEP) carboxylase activities are proposed to have evolved as a means of enhancing the recycling of photorespired CO₂. As the activity of PEP carboxylase increased to higher values, other enzymes in the C₄-pathway are proposed to have increased in activity to facilitate the processing of the products of C₄-assimilation and provide PEP substrate to PEP carboxylase with greater efficiency. Initially, such a ‘C₄-cycle’ would not have been differentially compartmentalized between mesophyll and bundlesheath cells as is typical of fully-expressed C₄ plants. Such metabolism would have limited benefit in terms of concentrating CO₂ at RuBP carboxylase and, therefore, also be of little benefit for improving water- and nitrogen-use efficiencies. However, the development of such a limited C₄-cycle would have represented a preadaptation capable of evolving into the leaf biochemistry typical of fully-expressed C₄ plants. Thus, during the initial stages of C₄-evolution it is proposed that improvements in photorespiratory CO₂-loss and their influence on increasing the rate of net CO₂ assimilation per unit leaf area represented the evolutionary ‘driving-force’. Improved resourceuse efficiency resulting from an efficient CO₂-concentrating mechanism is proposed as the driving force during the later stages.     

Effects of Nitrogen Deficiency on Gas Exchange,Chlorophyll Fluorescence,and Antioxidant Enzymes in Leaves of Rice Plants

Gas exchange, chlorophyll (Chl) fluorescence, and contents of photosynthetic pigments, soluble proteins (ribulose-1,5-bisphosphate carboxylase/oxygenase, RuBPCO), and antioxidant enzymes were characterized in the fully expanded 6^th leaves in rice seedlings grown on either complete (CK) or on nitrogen-deficient nutrient (N-deficiency) solutions during a 20-chase period. Compared with the control plants, the lower photosynthetic capacity at saturation irradiance (P _max) was accompanied by an increase in intercellular CO₂ concentration (C_i), indicating that in N-deficient plants the decline in P _max was not due to stomatal limitation but due to the reduced carboxylation efficiency. The fluorescence parameters _PS2, F_v/F_m, electron transport rate (ETR), and q_P showed the same tendency as P _max in N-deficient plants. Correspondingly, a higher q_N paralleled the rise of the ratio of carotenoid (Car) to Chl contents. However, F_v/F_m was still diminished, suggesting that photoinhibition did occur in the photosystem 2 (PS2) reaction centres. In addition, the activities of antioxidant enzymes on a fresh mass basis were gradually lowered, leading to the aggravation of membrane lipid peroxidation with the proceeding N-deficiency. The accumulation of malonyldialdehyde resulted in the lessening of Chl and soluble protein content. Analyses of regression showed PS2 excitation pressure (1 - q_P) was linearly correlated with the content of Chl and inversely with soluble protein (particularly RuBPCO) content. There was a lag phase in the increase of PS2 excitation pressure compared to the decrease of RuBPCO content. Therefore, the increased excitation pressure under N-deficiency is probably the result of saturation of the electron transport chain due to the limitation of the use of reductants by the Calvin cycle. Rice plants responded to N-deficiency and high irradiance by decreasing light-harvesting capacity and by increasing thermal dissipation of absorbed energy.     

Photosynthetic characteristics and tolerance to photo-oxidation of transgenic rice expressing C4 photosynthesis enzymes

The photosynthetic characteristics of four transgenic rice lines over-expressing rice NADP-malic enzyme (ME), and maize phosphoenolpyruvate carboxylase (PC), pyruvate,orthophosphate dikinase (PK), and PC+PK (CK) were investigated using outdoor-grown plants. Relative to untransformed wild-type (WT) rice, PC transgenic rice exhibited high PC activity (25-fold increase) and enhanced activity of carbonic anhydrase (more than two-fold increase), while the activity of ribulose-bisphosphate carboxylase/oxygenase (Rubisco) and its kinetic property were not significantly altered. The PC transgenic plants also showed a higher light intensity for saturation of photosynthesis, higher photosynthetic CO₂ uptake rate and carboxylation efficiency, and slightly reduced CO₂ compensation point. In addition, chlorophyll a fluorescence analysis indicates that PC transgenic plants are more tolerant to photo-oxidative stress, due to a higher capacity to quench excess light energy via photochemical and non-photochemical means. Furthermore, PC and CK transgenic rice produced 22–24% more grains than WT plants. Taken together, these results suggest that expression of maize C₄ photosynthesis enzymes in rice, a C₃ plant, can improve its photosynthetic capacity with enhanced tolerance to photo-oxidation. This revised version was published online in June 2006 with corrections to the Cover Date.