Decreased ribulose-1,5-bisphosphate carboxylase/oxygenase in transgenic tobacco transformed with 'antisense'rbcS.VIII. Impact on photosynthesis and growth in tobacco growing under extreme high irradiance and high temperature

Wild-type and antisense rbcS tobacco (Nicotiana tabacum) plants were grown in a glasshouse in midsummer in Portugal with an irradiance of 1500–2000 μmol m⁻²s⁻¹ and daytime temperatures of 30–35 °C. The Rubisco content of the transformants was lower by 35, 80 and over 90% than that of the wild-type. Gas exchange was measured over three separate days. There was a near-linear relation between Rubisco content and photosynthetic rate during the period of high irradiance, allowing a flux control coefficient of 0.83–0.89 to be estimated. The relation deviated slightly from linearity, because the internal CO₂ concentration (c;) was higher in the transformants than in the wild-type (190 and 275 μmol mol⁻¹ in plants with 35 and 80% less Rubisco, respectively, compared with 175 μmol mol⁻¹ for wild-type), compensating to some extent for the decreased Rubisco content. This increase in c_i occurred because the stomatal conductance (g) remained unaltered or was even higher in plants with decreased Rubisco, despite the lower rate of CO₂ assimilation. As a consequence, water use efficiency declined. The decreased rate of photosynthesis was not accompanied by a stoichiometric decrease in apparent growth rate. These results are discussed in relation to earlier studies of the plant set in growth cabinets. It is concluded that tobacco can adjust over a wide range of growth conditions to avoid a onesided limitation by Rubisco, but that in extreme environmental conditions this capacity to adapt is exhausted.     

Effects of Cerium on Key Enzymes of Carbon Assimilation of Spinach Under Magnesium Deficiency

The mechanism of the fact that cerium improves the photosynthesis of plants under magnesium deficiency is poorly understood. The main aim of the study was to determine the role of cerium in the amelioration of magnesium deficiency effects in CO₂ assimilation of spinach. Spinach plants were cultivated in Hoagland’s solution. They were subjected to magnesium deficiency and to cerium chloride administered in the magnesium-present Hoagland’s media and magnesium-deficient Hoagland’s media. The results showed that the chlorophyll synthesis and oxygen evolution was destroyed, and the activities of Rubisco carboxylasae and Rubisco activase and the expression of Rubisco large subunit (rbcL), Rubisco small subunit (rbcS), and Rubisco activase subunit (rca) were significantly inhibited, then plant growth was inhibited by magnesium deficiency. However, cerium promotes the chlorophyll synthesis, the activities of two key enzymes in CO₂ assimilation, and the expression of rbcL, rbcS, and rca, thus leading to the enhancement of spinach growth under magnesium-deficient conditions.     

Decreased ribulose-1,5-bisphosphate carboxylase-oxygenase in transgenic tobacco transformed with ‘antisense’ rbcS

Transgenic tobacco (Nicotiana tabacum L.) plants transformed with antisense rbcS to produce a series of plants with a progressive decrease in the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) have been used to investigate the contribution of Rubsico to the control of photosynthesis at different irradiance, CO₂ concentrations and vapour-pressure deficits. Assimilation rates, transpiration, the internal CO₂ concentration and chlorophyll fluorescence were measured in each plant. (i) The flux-control coefficient of Rubisco was estimated from the slope of the plot of Rubisco content versus assimilation rate. The flux-control coefficient had a value of 0.8 or more in high irradiance, (1050 mol·m^–2·s^–1), low-vapour pressure deficit (4 mbar) and ambient CO₂ (350 bar). Control was marginal in enhanced CO₂ (450 bar) or low light (310 mol·m^–2·s^–1) and was also decreased at high vapour-pressure deficit (17 mbar). No control was exerted in 5% CO₂. (ii) The flux-control coefficients of Rubisco were compared with the fractional demand placed on the calculated available Rubisco capacity. Only a marginal control on photosynthetic flux is exerted by Rubisco until over 50% of the available capacity is being used. Control increases as utilisation rises to 80%, and approaches unity (i.e. strict limitation) when more than 80% of the available capacity is being used. (iii) In low light, plants with reduced Rubisco have very high energy-dependent quenching of chlorophyll fluorescence (qE) and a decreased apparent quantum yield. It is argued that Rubisco still exerts marginal control in these conditions because decreased Rubisco leads to increased thylakoid energisation and high-energy dependent dissipation of light energy, and lower light-harvesting efficiency. (iv) The flux-control coefficient of stomata for photosynthesis was calculated from the flux-control coefficient of Rubisco and the internal CO₂ concentration, by applying the connectivity theorem. Control by the stomata varies between zero and about 0.25. It is increased by increased irradiance, decreased CO₂ or decreased vapour-pressure deficit. (v) Photosynthetic oscillations in saturating irradiance and CO₂ are suppressed in decreased-activity transformants before the steady-state rate of photosynthesis is affected. This provides direct evidence that these oscillations reveal the presence of excess Rubisco. (vi) Comparison of the flux-control coefficients of Rubisco with mechanistic models of photosynthesis provides direct support for the reliability of these models in conditions where Rubisco has a flux-control coefficient approach unity (i.e. limits photosynthesis), but also indicates that these models are less useful in conditions where control is shared between Rubisco and other components of the photosynthetic apparatus.Abbreviations A assimilation rate - C_i intercellular CO₂ concentration in the leaf - C_R flux-control coefficient of Rubisco for photosynthesis - qE high-energy-state-dependent quenching of chlorophyll fluorescence - Q_A primary acceptor of PSII - rbc S gene for the nuclear-encoded small subunit of Rubisco - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - Ru1,5bisP ribulose-1,5-bisphosphate - VPD vapour-pressure deficit     

Heterogeneous inhibition of photosynthesis over the leaf surface of Rosa rubiginosa L. during water stress and abscisic acid treatment: induction of a metabolic component by limitation of CO2 diffusion

The contribution of changes in stomatal conductance and metabolism in determining heterogeneous photosynthesis inhibition during dehydration and abscisic acid (ABA) feeding was investigated using detached leaves of Rosa rubiginosa L. The steady-state and maximal rates of electron transport under a transient high CO₂ concentration were monitored using chlorophyll fluorescence imaging. The decrease in electron transport rate induced by dehydration and ABA treatment almost reverted to the control rate under transient high CO₂ availability. Therefore, inhibition of photosynthesis was mainly mediated through stomatal closure. However, since reversion was not complete, a metabolic inhibition was also identified as a decrease in the maximal electron transport rate driven by carboxylation. Under dehydration or ABA feeding, as under low ambient CO₂ treatment, in 21% or 0.4% O₂, the lower the steady-state electron transport was, the lower was the maximal electron transport rate during transient high CO₂ availability. We conclude that low CO₂ availability reduced the capacity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) to drive electron transport. The potential contribution of Rubisco deactivation mediated by stomatal closure is discussed. Received: 1 February 1999 / Accepted: 15 June 1999     

Specific reduction of chloroplast glyceraldehyde-3-phosphate dehydrogenase activity by antisense RNA reduces CO2 assimilation via a reduction in ribulose bisphosphate regeneration in transgenic tobacco plants

The reduction of 3-phosphoglycerate (PGA) to triose phosphate is a key step in photosynthesis linking the photochemical events of the thylakoid membranes with the carbon metabolism of the photosynthetic carbon-reduction (PCR) cycle in the stroma. Glyceraldehyde-3-phosphate dehydrogenase: NADP oxidoreductase (GAPDH) is one of the two chloroplast enzymes which catalyse this reversible conversion. We report on the engineering of an antisense RNA construct directed against the tobacco (Nicotiana tabacum L.) chloroplastlocated GAPDH (A subunit). The construct was integrated into the tobacco genome by Agrobacterium-mediated transformation of leaf discs. Of the resulting transformants, five plants were recovered with reduced GAPDH activities ranging from 11 to 24% of wild-type (WT) activities. Segregation analysis of the kanamycin-resistance character in self-pollinated T₁ seed from each of the five transformants revealed that one plant (GAP-R) had two active DNA inserts and the others had one insert. T₁ progeny from GAP-R was used to generate plants with GAPDH activities ranging from WT levels to around 7% of WT levels. These were used to study the effect of variable GAPDH activities on metabolite pools for ribulose1,5-bisphosphate (RuBP) and PGA, and the accompanying effects on the rate of CO₂ assimilation and other gasexchange parameters. The RuBP pool size was linearly related to GAPDH activity once GAPDH activity dropped below the range for WT plants, but the rate of CO₂ assimilation was not affected until RuBP levels dropped to 30–40% of WT levels. That is, the CO₂ assimilation rate fell when RuBP per ribulose-1,5-biphosphate carboxylase-oxygenase (Rubisco) site fell below 2 mol·(mol site)^–1 while the ratio for WT plants was 4–5 mol·m(mol site)^–1. Leaf conductance was not reduced in leaves with reduced GAPDH activities, resulting in an increase in the ratio of intercellular to ambient CO₂ partial pressure. Conductance in plants with reduced GAPDH activities was still sensitive to CO₂ and showed a normal decline with increases in CO₂ partial pressure. Although PGA levels did not fluctuate greatly, the effect of reduced GAPDH activity on RuBP-pool size and assimilation rate can be interpreted as being due to a blockage in the regeneration of RuBP. Concomitant gas-ex change and chlorophyll a fluorescence measurements indicated that photosynthesis changed from being Rubisco-limited to being RuBP-regeneration-limited at a lower CO₂ partial pressure in the antisense plants than in WT plants. Photosynthetic electron transport was down-regulated by the build-up of a large proton gradient and the electron-transport chain did not become over-reduced due to a shortage of NADP. Plants with severely reduced GAPDH activity were not photoinhibited despite the continuous presence of a large thylakoid proton gradient in the light. Along with plant size, Rubisco activity, leaf soluble protein and chlorophyll content were reduced in plants with the lowest GAPDH activities. We conclude that chloroplastic GAPDH activity does not appear to limit steady-state photosynthetic CO₂ assimilation at ambient CO₂. This is because WT leaves maintain the ratio of RuBP per Rubisco site about twofold higher than the level required to achieve a maximal rate of CO₂ assimilation.Abbreviations and Symbols bp base pairs - DHAP dihydroxy-acetone phosphate - GAPDH glyceraldehyde-3-phosphate dehy-drogenase - PCR photosynthetic carbon reduction - PGA 3-phosphoglycerate - p_i intercellular CO₂ partial pressure - q_NP non-photochemical fluorescence quenching - q_Q photochemicalfluorescence quenching - PSII quantum efficiency of electronflow through PSII - Rubisco ribulose-1,5-bisphosphate carboxy-lase-oxygenase - RuBP ribulose-1,5-bisphosphate - WT wild type We thank Karin Harrison, Prue Kell, Anne Gallagher and Barbara Setchell for excellent technical assistance. G.D.P. and S.V.C. acknowledge support from QE II Research Fellowships (Australian Research Council).     

Leaf Wilting Movement Can Protect Water-Stressed Cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.) Plants Against Photoinhibition of Photosynthesis and Maintain Carbon Assimilation in the Field

Under severe water stress, leaf wilting is quite general in higher plants. This passive movement can reduce the energy load on a leaf. This paper reports an experimental test of the hypothesis that leaf wilting movement has a protective function that mitigates against photoinhibition of photosynthesis in the field. The experiments exposed cotton (Gossypium hirsutum L.) to two water regimes: water-stressed and well-watered. Leaf wilting movement occurred in water-stressed plants as the water potential decreased to −4.1 MPa, reducing light interception but maintaining comparable quantum yields of photosystem II (PS II; Yield for short) and the proportion of total PS II centers that were open (qP). Predrawn F _v/F _m (potential quantum yield of PS II) as an indicator of overnight recovery of PS II from photoinhibition was higher than or similar to that in well-watered plants. Compared with water-stressed cotton leaves for which wilting movement was permitted, water-stressed cotton leaves restrained from such movement had significantly increased leaf temperature and instantaneous CO₂ assimilation rates in the short term, but reduced Yield, qP, and F _v/F _m. In the long term, predrawn F _v/F _m and CO₂ assimilation capacity were reduced in water-stressed leaves restrained from wilting movement. These results suggest that, under water stress, leaf wilting movement could reduce the incident light on leaves and their heat load, alleviate damage to the photosynthetic apparatus due to photoinhibition, and maintain considerable carbon assimilation capacity in the long term despite a partial loss of instantaneous carbon assimilation in the short term.     

Brassinosteroids promote photosynthesis and growth by enhancing activation of Rubisco and expression of photosynthetic genes in <Emphasis Type="Italic">Cucumis sativus</Emphasis>

Brassinosteroids (BRs) are a new group of plant growth substances that promote plant growth and productivity. We showed in this study that improved growth of cucumber (Cucumis sativus) plants after treatment with 24-epibrassinolide (EBR), an active BR, was associated with increased CO₂ assimilation and quantum yield of PSII (Φ_PSII). Treatment of brassinazole (Brz), a specific inhibitor for BR biosynthesis, reduced plant growth and at the same time decreased CO₂ assimilation and Φ_PSII. Thus, the growth-promoting activity of BRs can be, at least partly, attributed to enhanced plant photosynthesis. To understand how BRs enhance photosynthesis, we have analyzed the effects of EBR and Brz on a number of photosynthetic parameters and their affecting factors, including the contents and activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Northern and Western blotting demonstrated that EBR upregulated, while Brz downregulated, the expressions of rbcL, rbcS and other photosynthetic genes. In addition, EBR had a positive effect on the activation of Rubisco based on increased maximum Rubisco carboxylation rates (V _c,max), total Rubisco activity and, to a greater extent, initial Rubisco activity. The accumulation patterns of Rubisco activase (RCA) based on immunogold-labeling experiments suggested a role of RCA in BR-regulated activation state of Rubisco. Enhanced expression of genes encoding other Calvin cycle genes after EBR treatment may also play a positive role in RuBP regeneration (J _max), thereby increasing maximum carboxylation rate of Rubisco (V _c,max). Thus, BRs promote photosynthesis and growth by positively regulating synthesis and activation of a variety of photosynthetic enzymes including Rubisco in cucumber.     

Photosynthesis and carbohydrate metabolism in tobacco leaves during an incompatible interaction with Phytophthora nicotianae

The metabolic and cellular changes in source leaves of Nicotiana tabacum L. cv SNN during an incompatible interaction with Phytophthora nicotianae van Breda de Haan were investigated and compared with defence reactions. Hypersensitive cell death was preceded by a rapid and highly localized shift to non-assimilatoric metabolism. During the first 6 h post infection (hpi), reactive oxygen species (ROS) accumulated. Callose was deposited at the interface of adjacent mesophyll cells (≥1 hpi), the export of sucrose collapsed and its content in the apoplast increased. Stomata closed and photosynthetic flux was reallocated from CO₂ assimilation in favour of photorespiration. This was accompanied by an increase in respiration, glucose-6-phosphate dehydrogenase (G6PDH) activity, apoplastic invertase and hexose content. Later (>6 hpi) the photosynthetic electron transport chain was interrupted and photosynthesis completely collapsed. This was accompanied by a further increase in apoplastic invertase and carbohydrates, respiration and oxidative pentose phosphate pathway (OPPP) and followed by further burst in ROS release. Hypersensitive cell death did not appear until photosynthesis completely declined. Photosynthesis was visualized by chlorophyll-a fluorescence imaging on a macro- and microscopic scale. Decline in photosynthesis and defence reactions were highly localized processes, which occur in single mesophyll cells. We propose that in photoautotrophic leaves, photosynthesis and assimilatory metabolism must be switched off to initiate respiration and other processes required for defence. An early blockage of intercellular sugar transportation, due to callose deposition, in conjunction with enhanced apoplastic invertase activity could facilitate this metabolic shift.     

Localization of photosynthetic metabolism in the parasitic angiosperm Cuscuta reflexa

Cells capable of photosynthesis in the parasitic angiosperm Cuscuta reflexa Roxb. (dodder) are highly localized. Immunolocalization of ribulose-1,5 bisphosphate carboxylase-oxygenase (Rubisco) and autofluorescence of chlorophyll in transverse sections of stems showed that they were largely restricted to a band of cells adjacent to the vascular bundles, consequently, the concentrations of Rubisco and chlorophyll were low per unit area or fresh weight. When ¹⁴CO₂ was supplied to stem segments of C. reflexa it preferentially accumulated in these cells adjacent to the vasculature. Although the conductance for CO₂ movement to the cells containing chlorophyll and Rubisco was very low, both the light reactions and dark reactions of photosynthesis appeared to be functional. De-epoxidation of the xanthophyll-cycle pigments after exposure to high light, and the chlorophyll fluorescence parameters, photochemical quenching (qP), non-photochemical quenching (NPQ) and the quantum efficiency of photosystem II (φPSII) responded normally to changes in photon flux density, indicating functional light-driven electron transport. The response of CO₂ exchange to photon flux density followed a typical hyperbolic curve, and positive rates of CO₂ fixation occurred when external CO₂ was increased to 5%. We propose that CO₂ for carbon assimilation is derived from internally respired CO₂ and that this layer of photosynthetic cells makes a positive contribution to the carbon budget of C. reflexa. Received: 23 October 1997 / Accepted: 16 December 1997     

Factors influencing the capacity for photosynthetic carbon assimilation in barley leaves at low temperatures

The aim of this work was to examine the effect upon photosynthetic capacity of short-term exposure (up to 10 h) to low temperatures (5° C) of darkened leaves of barley (Hordeum vulgare L.) plants. The carbohydrate content, metabolite status and the photosynthetic rate of leaves were measured at low temperature, high light and higher than ambient CO₂. Under these conditions we could detect whether previous exposure of leaves to low temperature overcame the limitation by phosphate which occurs in leaves of plants not previously exposed to low temperatures. The rates of CO₂ assimilation measured at 8° C differed by as much as twofold, depending upon the pretreatment. (i) Leaves from plants which had previously been darkened for 24 h had a low content of carbohydrate, had the lowest CO₂-assimilation rates at low temperature, and photosynthesis was limited by carbohydrate, as shown by a large stimulation of photosynthesis by feeding glucose, (ii) Leaves from plants which had previously been illuminated for 24 h and which contained large carbohydrate reserves showed an accumulation of phosphorylated intermediates and higher CO₂-assimilation rates at low temperature, but nevertheless remained limited by phosphate, (iii) Maximum rates of CO₂ assimilation at low temperature were observed in leaves which had intermediate reserves of carbohydrate or in leaves which were rich in carbohydrate and which were also fed phosphate. It is suggested that carbohydrate reserves potentiate the system for the achievement of high rates of photosynthesis at low temperatures by accumulation of photosynthetic intermediates such as hexose phosphates, but that this potential cannot be realised if, at the same time, carbohydrate accumulation is itself leading to feedback inhibition of photosynthesis. This work was supported by the Agricultural and Food Research Council, UK (Research grant PG50/67) and by the Science and Engineering Reserach Council, UK. C.A.L. was supported by the British Council, by an Overseas Research Student Award and by the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), Brazil.     

Feedback-limited photosynthesis and regulation of sucrose-starch accumulation during cold acclimation and low-temperature stress in a spring and winter wheat

Regulation of sucrose-starch accumulation and its effect on CO₂ gas exchange and electron transport were studied in low-temperature-stressed and cold-acclimated spring (Katepwa) and winter (Monopol) cultivars of wheat (Triticum aestivum L.). Low-temperature stress of either the spring or winter cultivar was associated with feedback-limited photosynthesis as indicated by a 50–60% reduction in CO₂ assimilation rates, twofold lower ATP/ADP ratio, and threefold lower electron transport rate than 20°C-grown control plants. However, no limitations were evident at the level of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) in low-temperature-stressed plants. Cold acclimation of the spring cultivar resulted in similar feedback-limited photosynthesis observed during low-temperature stress. In contrast, cold acclimation of the winter cultivar resulted in an adjustment of CO₂ assimilation rates to that of control plants. However, we show, for the first time, that this capacity to adjust CO₂ assimilation still appeared to be associated with limited triose phosphate utilisation, a twofold lower ATP/ADP ratio, a reduction in electron transport rates but no restriction at the level of Rubisco compared to controls grown at 20°C. Thus, contrary to previous suggestions, we conclude that cold-acclimated Monopol appears to exhibit feedback limitations at the level of electron transport characteristic of cold-stressed plants despite the maintenance of high rates of CO₂ assimilation. Furthermore, the differential capacity of the winter cultivar to adjust CO₂ assimilation rates was associated with higher levels of sucrose accumulation and a threefold higher sucrose-phosphate synthase activity despite an apparent limitation in triose phosphate utilisation.Abbreviations AGPase ADP-glucose pyrophosphorylase - FBPase fructose-1,6-bisphosphatase - Fru 6-P fructose 6-phosphate - Fru 1,6-BP fructose 1,6-bisphosphate - Glc 6-P glucose 6-phosphate - PGA 3-phosphoglyceric acid - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - RuBP ribulose 1,5-bisphosphate - SPS sucrose-phosphate synthase - Triose-P triose phosphate     

Reduction of ribulose-1,5-bisphosphate carboxylase/oxygenase content by antisense RNA reduces photosynthesis in transgenic tobacco plants

A complementary DNA for the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was cloned from tobacco (Nicotiana tabacum) and fused in the antisense orientation to the cauliflower mosaic virus 35S promoter. This antisense gene was introduced into the tobacco genome, and the resulting transgenic plants were analyzed to assess the effect of the antisense RNA on Rubisco activity and photosynthesis. The mean content of extractable Rubisco activity from the leaves of 10 antisense plants was 18% of the mean level of activity of control plants. The soluble protein content of the leaves of anti-small subunit plants was reduced by the amount equivalent to the reduction in Rubisco. There was little change in phosphoribulokinase activity, electron transport, and chlorophyll content, indicating that the loss of Rubisco did not affect these other components of photosynthesis. However, there was a significant reduction in carbonic anhydrase activity. The rate of CO₂ assimilation measured at 1000 micromoles quanta per square meter per second, 350 microbars CO₂, and 25°C was reduced by 63% (mean value) in the antisense plants and was limited by Rubisco activity over a wide range of intercellular CO₂ partial pressures (p_i). In control leaves, Rubisco activity only limited the rate of CO₂ assimilation below a p_i of 400 microbars. Despite the decrease in photosynthesis, there was no reduction in stomatal conductance in the antisense plants, and the stomata still responded to changes in p_i. The unchanged conductance and lower CO₂ assimilation resulted in a higher p_i, which was reflected in greater carbon isotope discrimination in the leaves of the antisense plants. These results suggest that stomatal function is independent of total leaf Rubisco activity.     

Determination of carbon-reduction-cycle intermediates in leaves of Arbutus unedo L. suffering depressions in photosynthesis after application of abscisic acid or exposure to dry air

Gas exchange and contents of photosynthetic intermediates of leaves of Arbutus unedo L. were determined with the aim of recognizing the mechanisms of inhibition that were responsible for the midday depression of photosynthesis following exposure to dry air, and the decline in photosynthetic capacity following application of abscisic acid (ABA). Rapidly killed (<0.1 s) leaf samples were taken when gas analysis showed reduced CO₂ assimilation. Determination of the contents of 3-phosphoglyceric acid (PGA), ribulose 1,5-bisphosphate (RuBP), triose phosphates, fructose 1,6-bisphosphate and hexose phosphates in the samples showed that significant variation occurred only in the level of PGA. As a result, the ratio PGA/RuBP decreased with increasing inhibition of photosynthesis, particularly when application of ABA had been the cause. A comparison of metabolite patterns did not bring out qualitative differences that would have indicated that effects of ABA and of dry air had been caused by separate mechanisms. Depression of photosynthesis occurred in the presence of sufficient RuBP which indicated that the carboxylation reaction of the carbon-reduction-cycle was inhibited after application of ABA or exposure to dry air.Abbreviations and symbols ABA abscisic acid - C _a partial pressure of CO₂ in the ambient air - C _i partial pressure of CO₂ in the intercellular spaces - I quantum flux - PGA 3-phosphoglyceric acid - RuBP ribulose 1,5-bisphosphate - I _L leaf temperature - w water-vapor pressure difference between leaf and air     

Co-overproducing Rubisco and Rubisco activase enhances photosynthesis in the optimal temperature range in rice

Rubisco limits C₃ photosynthesis under some conditions and is therefore a potential target for improving photosynthetic efficiency. The overproduction of Rubisco is often accompanied by a decline in Rubisco activation, and the protein ratio of Rubisco activase (RCA) to Rubisco (RCA/Rubisco) greatly decreases in Rubisco-overproducing plants (RBCS-ox). Here, we produced transgenic rice (Oryza sativa) plants co-overproducing both Rubisco and RCA (RBCS-RCA-ox). Rubisco content in RBCS-RCA-ox plants increased by 23%–44%, and RCA/Rubisco levels were similar or higher than those of wild-type plants. However, although the activation state of Rubisco in RBCS-RCA-ox plants was enhanced, the rates of CO₂ assimilation at 25°C in RBCS-RCA-ox plants did not differ from that of wild-type plants. Alternatively, at a moderately high temperature (optimal range of 32°C–36°C), the rates of CO₂ assimilation in RBCS-ox and RBCS-RCA-ox plants were higher than in wild-type plants under conditions equal to or lower than current atmospheric CO₂ levels. The activation state of Rubisco in RBCS-RCA-ox remained higher than that of RBCS-ox plants, and activated Rubisco content in RCA overproducing, RBCS-ox, RBCS-RCA-ox, and wild-type plants was highly correlated with the initial slope of CO₂ assimilation against intercellular CO₂ pressures (A:Ci) at 36°C. Thus, a simultaneous increase in Rubisco and RCA contents leads to enhanced photosynthesis within the optimal temperature range.

A simultaneous increase in Rubisco and RCA contents in transgenic rice leads to an enhancement of photosynthesis at moderately high temperatures within the optimal temperature range.     

Simultaneous and independent effects of abscisic acid on stomata and the photosynthetic apparatus in whole leaves

(±)-Abscisic acid (ABA) at 10^-5 M was added to the transpiration stream of leaves of 16 species (C₃ and C₄, monocotyledons and dicotyledons). Stomatal responses followed one of three patterns: i) stomata that were wide and insensitive to CO₂ initially, closed partially and became sensitive to CO₂; ii) for stomata that were sensitive to CO₂ before the application of ABA, the range of highest sensitivity to CO₂ shifted from high to low intercellular partial pressures of CO₂, for instance in leaves of Zea mays from 170–350 to 70–140 bar; iii) when stomata responded strongly to ABA, their conductance was reduced to a small fraction of the initial conductance, and sensitivity to CO₂ was lost. The photosynthetic apparatus was affected by applications of ABA to various degrees, from no response at all (in agreement with several previous reports on the absence of effects of ABA on photosynthesis) through a temporary decrease of its activity to a lasting reduction. Saturation curves of photosynthesis with respect to the partial pressure of CO₂ in the intercellular spaces indicated that application of ABA could produce three phenomena: i) a reduction of the initial slope of the saturation curve (which indicates a diminished carboxylation efficiency); ii) a reduction of the level of the CO₂-saturated rate of assimilation (which indicates a reduction of the ribulose-1,5-bisphosphate regeneration capacity); and iii) an increase of the CO₂ compensation point. Photosynthesis of isolated mesophyll cells was not affected by ABA treatments. Responses of the stomatal and photosynthetic apparatus were usually synchronous and often proportional to each other, with the result that the partial pressure of CO₂ in the intercellular spaces frequently remained constant in spite of large changes in conductance and assimilation rate. Guard cells and the photosynthetic apparatus were able to recover from effects of ABA applications while the ABA supply continued. Recovery was usually partial, in the case of the photosynthetic apparatus occasionally complete. Abscisic acid did not cause stomatal closure or decreases in the rate of photosynthesis when it was applied during a phase of stomatal opening and induction of photosynthesis that followed a transition from darkness to light.Abbreviations and symbols A rate of CO₂ assimilation - ABA (±)-abscisic acid - c _a partial pressure of CO₂ in the ambient air or in the gas supplied to the leaf chambers - c _i partial pressure of CO₂ in the intercellular spaces of a leaf - e _a partial pressure of H₂O in the air - g conductance for water vapor - J quantum flux - T ₁ leaf temperature     

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol⁻¹ CO₂) and elevated CO₂ (700 μmol mol⁻¹ CO₂) at two irradiances (150 and 500 μmol m⁻² s⁻¹) for 35 days at which point the plants were harvested. Elevated CO₂ did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO₂ and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO₂, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO₂ enrichment, particularly at the higher growth irradiance. Elevated-CO₂-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO₂-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO₂ enrichment and low light. We conclude that high CO₂ will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO₂-induced enhancement of photosynthesis. Received: 23 February 1999 / Accepted: 15 June 1999     

Ear of durum wheat under water stress: water relations and photosynthetic metabolism

The photosynthetic characteristics of the ear and flag leaf of well-watered (WW) and water-stressed (WS) durum wheat (Triticum turgidum L. var. durum) were studied in plants grown under greenhouse and Mediterranean field conditions. Gas exchange measurements simultaneously with modulated chlorophyll fluorescence were used to study the response of the ear and flag leaf to CO₂ and O₂ during photosynthesis. C₄ metabolism was identified by assessing the sensitivity of photosynthetic rate and electron transport to oxygen. The presence of CAM metabolism was assessed by measuring daily patterns of stomatal conductance and net CO₂ assimilation. In addition, the histological distribution of Rubisco protein in the ear parts was studied by immunocytochemical localisation. Relative water content (RWC) and osmotic adjustment (osmotic potential at full turgor) were also measured in these organs. Oxygen sensitivity of the assimilation rate and electron transport, the lack of Rubisco compartmentalisation in the mesophyll tissues and the gas-exchange pattern at night indicated that neither C₄ nor CAM metabolism occurs in the ear of WW or WS plants. Nevertheless, photosynthetic activity of the flag leaf was more affected by WS conditions than that of the ear, under both growing conditions. The lower sensitivity under water stress of the ear than of the flag leaf was linked to higher RWC and osmotic adjustment in the ear bracts and awns. We demonstrate that the better performance of the ear under water stress (compared to the flag leaf) is not related to C₄ or CAM photosynthesis. Rather, drought tolerance of the ear is explained by its higher RWC in drought. Osmotic adjustment and xeromorphic traits of ear parts may be responsible.     

The efficiency of water use in water stressed plants is increased due to ABA induced stomatal closure

Gas exchange and abscisic acid content of Digitalis lanata EHRH. have been examined at different levels of plant water stress. Net photosynthesis, transpiration and conductance of attached leaves declined rapidly at first, then more slowly following the withholding of irrigation. The intercellular partial pressure of CO₂ decreased slightly. The concentration of 2-cis(S)ABA increased about eight-fold in the leaves of non-irrigated plants as compared with well-watered controls. A close linear correlation was found between the ABA content of the leaves and their conductance on a leaf area basis. In contrast, the plot of net assimilation versus ABA concentration was curvilinear, leading to an increased efficiency of water use during stress. After rewatering, photosynthesis reached control values earlier than transpiration, leaf conductance and ABA content. From these data it is concluded that transpiration through the stomata is directly controlled by the ABA content, whereas net photosynthesis is influenced additionally by other factors.Possible reasons for the responses of photosynthesis and water use efficiency to different stress and ABA levels are discussed.Abbreviations A net CO₂ assimilation - ABA abscisic acid - C_i intercellular CO₂ concentration - g stomatal conductance - T transpiration - WUE water use efficiency     

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m^?2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO₂ concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bis-phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO₂-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO₂ assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO₂ assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO₂ assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO₂ assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO₂ assimilation.     

Control of steady-state photosynthesis in sunflowers growing in enhanced CO2

Control coefficients were used to describe the degree to which ribulose bisphosphate carboxylase/oxygenase (Rubisco) limits the steady-state rate of CO₂ assimilation in sunflower leaves from plants grown at high (800 μmol mol⁻¹) and low (350 μmol mol⁻¹) CO₂. The magnitude of a control coefficient is approximately the percentage change in the flux that would result from a 1% rise in enzyme active site concentration. In plants grown at low CO₂, leaves of different ages varied considerably in their photosynthetic capacities. In a saturating light flux and an ambient CO₂ concentration of 350 μmol mol⁻¹, the Rubisco control coefficient was about 0.7 in all leaves, indicating that Rubisco activity largely limited the assimilation flux. The Rubisco control coefficient for leaves grown at 350 μmol mol⁻¹ CO₂ dropped to about zero when the ambient CO₂ concentration was raised to 800 μmol mol⁻¹. In relatively young, fully expanded leaves of plants grown at high CO₂, the Rubisco control coefficient was also about 0.7 at a saturating light flux and at the CO₂ concentration at which the plants were grown (800 μmol mol⁻¹). This apparently resulted from a decrease in the concentration of Rubisco active sites. In older leaves, however, the control coefficient was about 0.2. Because, on the whole, Rubisco activity still largely limits the assimilation flux in plants grown at high CO₂, the kinetics of this enzyme can still be used to model photosynthesis under these conditions. The relatively high Rubisco control coefficient under enhanced CO₂ indicates that the young sunflower leaves have the capacity to acclimate their photosynthetic biochemistry in a way consistent with an optimal use of protein resources.