Oxygen requirement of photosynthetic CO2 assimilation

In the absence of electron acceptors and of oxygen a proton gradient was supported across thylakoid membranes of intact spinach chloroplasts by far-red illumination. It was decreased by red light. Inhibition by red light indicates effective control of cyclic electron flow by Photosystem II. Inhibition was released by oxygen which supported a large proton gradient. Oxygen appeared to act as electron acceptor simultaneously preventing over-reduction of electron carriers of the cyclic electron transport pathway. It thus has an important regulatory function in electron transport. Under anaerobic conditions, the inhibition of electron transport caused by red illumination could also be released and a large proton gradient could be established by oxaloacetate, nitrite and 3-phosphoglycerate, but not by bicarbonate. In the absence of oxygen, ATP levels remained low in chloroplasts illuminated with red light even when bicarbonate was present. They increased when electron acceptors were added which could release the over-reduction of the electron transport chain. Inhibition of electron transport in the presence of bicarbonate was relieved and CO2-fixation was initiated by oxygen concentrations as low as about 10 microM. Once CO2 fixation was initiated, very low oxygen levels were sufficient to sustain it. The results support the assumption that pseudocyclic electron transport is necessary to poise the electron transport chain so that a proper balance of linear and cyclic electron transport is established to supply ATP for CO2 reduction.     

A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species

Various aspects of the biochemistry of photosynthetic carbon assimilation in C₃ plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO₂(p(CO₂)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O₂(p(O₂)), the dependence of net CO₂ assimilation rate on p(CO₂) and irradiance, and the influence of p(CO₂) and irradiance on the temperature dependence of assimilation rate.Abbreviations RuP₂ ribulose bisphosphate - PGA 3-phosphoglycerate - C=p(CO₂) partial pressure of CO₂ - O=p(O₂) partial pressure of O₂ - PCR photosynthetic carbon reduction - PCO photorespiratory carbon oxidation     

Analysing the responses of photosynthetic CO2 assimilation to long-term elevation of atmospheric CO2 concentration

Understanding how photosynthetic capacity acclimatises when plants are grown in an atmosphere of rising CO₂ concentrations will be vital to the development of mechanistic models of the response of plant productivity to global environmental change. A limitation to the study of acclimatisation is the small amount of material that may be destructively harvested from long-term studies of the effects of elevation of CO₂ concentration. Technological developments in the measurement of gas exchange, fluorescence and absorption spectroscopy, coupled with theoretical developments in the interpretation of measured values now allow detailed analyses of limitations to photosynthesisin vivo. The use of leaf chambers with Ulbricht integrating spheres allows separation of change in the maximum efficiency of energy transduction in the assimilation of CO₂ from changes in tissue absorptance. Analysis of the response of CO₂ assimilation to intercellular CO₂ concentration allows quantitative determination of the limitation imposed by stomata, carboxylation efficiency, and the rate of regeneration of ribulose 1:5 bisphosphate. Chlorophyll fluorescence provides a rapid method for detecting photoinhibition in heterogeneously illuminated leaves within canopies in the field. Modulated fluorescence and absorption spectroscopy allow parallel measurements of the efficiency of light utilisation in electron transport through photosystems I and IIin situ.Abbreviations A net rate of CO₂ uptke per unit leaf area (µmol m^–2 s^–1) - A_sat light-saturated A - A₈₂₀ change in absorptance of PSI on removal of illumination (OD) - c CO₂ concentration in air (µmol mol^–1) - c_a c in the bulk air; c_i, c in the intercellular spaces - ce carboxylation efficiency (mol m^–2 s^–1) - E transpiration per unit leaf area (mol m^–2 s^–1) - F fluorescence emission of PSII (relative units) - F_m maximal level of F - F_o minimal level of F upon illumination when PSII is maximally oxidised - F_s the steady-state F following the m peak - F_v the difference between F_m and F_o - F'_m maximal F' generated after the m peak by addition of a saturating light pulse - F'_o the minimal level of F' after the m peak determined by re-oxidising PSII by far-red light - g₁ leaf conductance to CO₂ diffusion in the gas phase (mol m^–2 s^–1) - g'₁ leaf conductance to water vapour diffusion in the gas phase (mol m^–2 s^–1) - k_c and k_o the Michaelis constants for CO₂ and O₂, respectively, (µmol mol^–1); - J_max the maximum rate of regeneration of rubP (µmol m^–2 s^–1) - l stomatal limitation to CO₂ uptake (dimensionless, 0–1) - LCP light compensation point of photosynthesis (µmol m^–2 s^–1) - o_i the intercellular O₂ concentration (mmol mol^–1) - Pi cytosol inorganic phosphate concentration - PSI photosystem I - PSII photosystem II - Q photon flux (µmol m^–2 s^–1) - Q_abs Q absorbed by the leaf - rubisCO ribulose 1:5 bisphosphate carboxylase/oxygenase; rubP, ribulose 1:5 bisphosphate; s, projected surface area of a leaf (m²) - V_c,max is the maximum rate of carboxylation (µmol m^–2 s^–1) - W_c the rubisCO limited rate of carboxylation (µmol m^–2 s¹) - W_j the electron transport limited rate of regeneration of rubP (µmol m^–2 s^–1) - W_p the inorganic phosphate limited rate of regeneration of rubP (µmol m^–2 s^–1) - absorptance of light (dimensionless, 0–1) - _a of standard black absorber ₁, of leaf - _s of integrating sphere walls - , CO₂ compensation point of photosynthesis (µmol mol^–1) - the specificity factor for rubisCO carboxylation (dimensionless) - , convexity of the response of A to Q (dimensionless 0–1) - the quantum yield of photosynthesis on an absorbed light basis (A/Q_abs; dimensionless) - the quantum yield of photosynthesis on an incident light basis (A/Q; dimensionless) - _app the maximum - _m the maximum - _m,app the photochemical efficiency of PSII (dimensionless, 0–1) - _PSII,m the maximum      

Effect of oxygen and temperature on the efficiency of photosynthetic carbon assimilation in two microscopic algae

The CO₂ compensation points of Coccochloris peniocystis, a blue-green alga and Chlamydomonas reinhardtii, a green alga, were determined at pH 8.0 in a closed system by a gas chromatographic technique. The compensation point of Chlamydomonas increased markedly with temperature, rising from 0.79 microliter per liter CO₂ at 15 C to 2.5 microliters per liter CO₂ at 35 C. In contrast, the compensation point of Coccochloris at 20 C was 0.71 microliter per liter CO₂ and rose to only 0.95 microliter per liter CO₂ at 40 C.     

Effects of temperature and CO2 concentration on photosynthetic CO2 fixation by Chlorella

The rates of photosynthetic ¹⁴CO₂ fixation by Chlorella vulgarisllh, grown under high CO₂, were determined between 4 to 37°Cwith air containing from 300 to 13,000 ppm ¹⁴CO₂. When the CO₂level was increased, both the rate of photosynthesis and theoptimum temperature for maximum photosynthesis increased. Themaximum photosynthetic rate was reached at 12°C with 300ppm ^l4CO₂. Among the photosynthetic products fromed at 300 ppm ¹⁴CO₂, glycolatedecreased greatly when the temperature was raised from 20 to30°C. At 3,000 ppm ¹⁴CO₂ an insignificant amount of glycolatewas formed at all temperatures, whereas ¹⁴C-incorporation intothe insoluble fraction, sucrose, and the lipid fraction wassignificantly higher than at 300 ppm ¹⁴CO₂. The ¹⁴C in sucrosewas greatly increased and the radioactivity in the insolublefraction decreased when the temperature was raised from 28 to36°C. (Received April 8, 1980; )     

Effect of high temperature on active oxygen species,senescence and photosynthetic properties in cucumber leaves

To gain a physiological understanding of the effects of high temperatures on cucumber (Cucumis sativus L.), we subjected seedlings to heat treatment at daytime temperatures of 28 °C, 32 °C, and 36 °C for 7 h a day for 30 days. The amount of active oxygen species, indicators of senescence, and photosynthetic properties in the second and third leaves were determined at the start of temperature treatment and on the 15th and 30th days of treatment. The amount of active oxygen species superoxide in leaves was greatest in the high temperature zones on the 15th day of treatment, and the amount of hydrogen peroxide was greatest in the high temperature zones on both the 15th and 30th days of treatment. The reduction in the amount of protein and the increase in the amount of malondialdehyde, both indicators of senescence, were greatest in the high temperature zones on both the 15th and 30th days of treatment, and the amount of chlorophyll was lowest in the 36 °C zone on the 15th day, and lower in the high temperature zones on the 30th day. It is clear from these results that a large amount of active oxygen species is generated and accumulated in the leaves at high temperatures, and senescence is significantly accelerated. The photosynthetic properties of stomatal conductance, sub-stomatal CO₂ concentration, and transpiration rate were at the same level on both the 15th and 30th days of treatment in all three temperature treatment zones. No significant difference was seen in the net photosynthesis rate between the 28 °C and 32 °C zones, was lower in the 36 °C zone than the 32 °C zone on the 15th day, and lowest in the 36 °C zone on the 30th day. CO₂ intake and water absorption are only mildly affected by high temperatures, and the reduction in net photosynthesis rate due to the 36 °C high temperature stress suggests that the large amount of active oxygen species induces inhibition of photosynthesis and damage to the mechanism of photosynthesis.     

Steady-state room temperature fluorescence and CO2 assimilation rates in intact leaves

Steady-state room temperature variable fluorescence from leaves was measured as a function of CO₂ pressure in Xanthium strumarium L. and Phaseolus vulgaris L. Measurements were made in a range of light intensities, at normal and low O₂ parital pressure and over a range of temperatures. At low CO₂ pressure fluorescence increased with increasing CO₂. At higher CO₂ pressure fluorescence usually decreased with increasing CO₂ but occasionally increased slightly. The transition CO₂ pressure between the responses could be changed by changing light, O₂ pressure, or temperature. This breakpoint in the fluorescence-CO₂ curve was a reliable indicator of the transition between ribulose 1,5-bisphosphate (RuBP) saturated assimilation and RuBP regeneration limited assimilation. The fluorescence signal was not a reliable indicator of O₂-insensitive assimilation in these C₃ species.     

Chloroplastic ATP synthase optimizes the trade-off between photosynthetic CO2 assimilation and photoprotection during leaf maturation

In the present study, we studied the role of chloroplastic ATP synthase in photosynthetic regulation during leaf maturation. We measured gas exchange, chlorophyll fluorescence, P700 redox state, and the electrochromic shift signal in mature and immature leaves. Under high light, the immature leaves displayed high levels of non-photochemical quenching (NPQ) and P700 oxidation ratio, and higher values for proton motive force (pmf) and proton gradient (ΔpH) across the thylakoid membranes but lower values for the activity of chloroplastic ATP synthase (g_H⁺) than the mature leaves. Furthermore, g_H⁺ was significantly and positively correlated with CO₂ assimilation rate and linear electron flow (LEF), but negatively correlated with pmf and ΔpH. ΔpH was significantly correlated with LEF and the P700 oxidation ratio. These results indicated that g_H⁺ was regulated to match photosynthetic capacity during leaf maturation, and the formation of pmf and ΔpH was predominantly regulated by the alterations in g_H⁺. In the immature leaves, the high steady-state ΔpH increased lumen acidification, which, in turn, stimulated photoprotection for the photosynthetic apparatus via NPQ induction and photosynthetic control. Our results highlighted the importance of chloroplastic ATP synthase in optimizing the trade-off between CO₂ assimilation and photoprotection during leaf maturation.     

ATP synthase repression in tobacco restricts photosynthetic electron transport, CO2 assimilation, and plant growth by overacidification of the thylakoid lumen

Tobacco (Nicotiana tabacum) plants strictly adjust the contents of both ATP synthase and cytochrome b(6)f complex to the metabolic demand for ATP and NADPH. While the cytochrome b(6)f complex catalyzes the rate-limiting step of photosynthetic electron flux and thereby controls assimilation, the functional significance of the ATP synthase adjustment is unknown. Here, we reduced ATP synthase accumulation by an antisense approach directed against the essential nuclear-encoded γ-subunit (AtpC) and by the introduction of point mutations into the translation initiation codon of the plastid-encoded atpB gene (encoding the essential β-subunit) via chloroplast transformation. Both strategies yielded transformants with ATP synthase contents ranging from 100 to <10% of wild-type levels. While the accumulation of the components of the linear electron transport chain was largely unaltered, linear electron flux was strongly inhibited due to decreased rates of plastoquinol reoxidation at the cytochrome b(6)f complex (photosynthetic control). Also, nonphotochemical quenching was triggered at very low light intensities, strongly reducing the quantum efficiency of CO(2) fixation. We show evidence that this is due to an increased steady state proton motive force, resulting in strong lumen overacidification, which in turn represses photosynthesis due to photosynthetic control and dissipation of excitation energy in the antenna bed.     

Leaf construction cost, nutrient concentration, and net CO2 assimilation of native and invasive species in Hawaii

To examine the predictability of leaf physiology and biochemistry from light gradients within canopies, we measured photosynthetic light-response curves, leaf mass per area (LMA) and concentrations of nitrogen, phosphorus and chlorophyll at 15–20 positions within canopies of three conifer species with increasing shade tolerance, ponderosa pine [Pinus ponderosa (Laws.)], Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], and western hemlock [Tsuga heterophylla (Raf.) Sarg.]. Adjacent to each sampling position, we continuously monitored photosynthetically active photon flux density (PPFD) over a 5-week period using quantum sensors. From these measurements we calculated FPAR: integrated PPFD at each sampling point as a fraction of full sun. From the shadiest to the brightest canopy positions, LMA increased by about 50% in ponderosa pine and 100% in western hemlock; Douglas fir was intermediate. Canopy-average LMA increased with decreasing shade tolerance. Most foliage properties showed more variability within and between canopies when expressed on a leaf area basis than on a leaf mass basis, although the reverse was true for chlorophyll. Where foliage biochemistry or physiology was correlated with FPAR, the relationships were non-linear, tending to reach a plateau at about 50% of full sunlight. Slopes of response functions relating physiology and biochemistry to ln(FPAR) were not significantly different among species except for the light compensation point, which did not vary in response to light in ponderosa pine, but did in the other two species. We used the physiological measurements for Douglas fir in a model to simulate canopy photosynthetic potential (daily net carbon gain limited only by PPFD) and tested the hypothesis that allocation of carbon and nitrogen is optimized relative to PPFD gradients. Simulated photosynthetic potential for the whole canopy was slightly higher (<10%) using the measured allocation of C and N within the canopy compared with no stratification (i.e., all foliage identical). However, there was no evidence that the actual allocation pattern was optimized on the basis of PPFD gradients alone; simulated net carbon assimilation increased still further when even more N and C were allocated to high-light environments at the canopy top. Received: 12 August 1998 / Accepted: 25 March 1999     

Effect of carbon dioxide and temperature on photosynthetic CO2 uptake and photorespiratory CO2 evolution in sunflower leaves

Using an open gas-exchange system, apparent photosynthesis, true photosynthesis (TPS), photorespiration (PR) and dark respiration of sunflower (Helianthus annuus L.) leaves were determined at three temperatures and between 50 and 400 l/l external CO₂. The ratio of PR/TPS and the solubility ratio of O₂/CO₂ in the intercellular spaces both decreased with increasing CO₂. The rate of PR was not affected by the CO₂ concentration in the leaves and was independent of the solubility ratio of oxygen and CO₂ in the leaf cell. At photosynthesis-limiting concentrations of CO₂, the ratio of PR/TPS significantly increased from 18 to 30°C and the rate of PR increased from 4.3 mg CO₂ dm^-2 h^-1 at 18°C to 8.6 mg CO₂ dm^-2 h^-1 at 30°C. The specific activity of photorespired CO₂ was CO₂-dependent but temperature-independent, and the carbon traversing the glycolate pathway appeared to be derived both from recently fixed assimilate and from older reserve materials. It is concluded that PR as a percentage of TPS is affected by the concentrations of O₂ and CO₂ around the photosynthesizing cells, but the rate of PR may also be controlled by other factors.Abbreviations APS apparent photosynthesis (net CO₂ uptake) - PR photorespiration (CO₂ evolution in light) - RuBP ribulose-1,5-bisphosphate - TPS true photosynthesis (true CO₂ uptake)     

Reconstitution of chlorophylls and photosynthetic CO2 assimilation upon rehydration of the desiccated poikilochlorophyllous plant Xerophyta scabrida (Pax) Th. Dur. et Schinz

Resynthesis of the photosynthetic apparatus and resumption of CO₂ assimilation upon rehydration is reported for the monocotyledonous and poikilochlorophyllous desiccation-tolerant (PDT) plant Xerophyta scabrida (Pax) Th. Dur. et Schinz (Velloziaceae). During desiccation there was a complete breakdown of chlorophylls whereas the total carotenoid content of air-dried leaves was reduced to about 22% of that of functional leaves. The prerequisites for the resynthesis of photosynthetic pigments and functional thylakoids were the reappearance of turgor and maximum leaf water content at 2 and 10 h after rehydration, respectively. The period of increased initial respiration after rewetting leaves (rehydration respiration) lasted up to 30 h and was thus 6 to 10 times longer than in homoiochlorophyllous desiccation-tolerant plants (HDTs) in which chlorophylls are retained during desiccation. Accumulation of chlorophylls a + b and total carotenoids (xanthophylls and carotene) started 10 h after rehydration. Normal levels of chlorophyll and carotenoids were obtained 72 h after rehydration. Values for the variable-fluorescence decrease ratio (Rfd690 values), an indicator of photochemical activity, showed that photochemical function started 10 h after rehydration, but normal values of 2.7 were reached only 72 h after rehydration. Net CO₂ assimilation started 24 h after rewetting and normal rates were reached after 72 h, at the same time as normal values of stomatal conductance were obtained. The increasing rates of net CO₂ assimilation were paralleled by decreasing values of the intercellular CO₂ concentration. All photosynthetic parameters investigated showed values normal for functional chloroplasts by 72 h after the onset of rehydration. Fully regreened leaves of the presumed C₃ plant X. scabrida exhibited a net CO₂ assimilation rate which was in the same range as that of other C₃ plants and higher than that of recovered HDT plants. The fundamental difference between air-dried PDT plants, such as X. scabrida, which have to resynthesize the photosynthetic pigment apparatus, and air-dried HDT plants, which only undergo a functional recovery, is discussed.Abbreviations c -carotene - c_i intercellular CO₂ concentration - Car x + c total carotenoid content x + c - Chl a + b total chlorophyll a + b content - g_s stomatal conductance - HDT homoiochlorophyllous desiccation tolerant - LWC leaf-water content - P_N net photosynthesis rate - PDT poikilochloro phyllous desiccation tolerant - R_d dark respiration - Rfd variable fluorescence decrease ratio (Rfd = fd/fs) - x xanthophylls The senior author thanks the Deutschem Akademischem Auslandsdienst (Bonn, Germany), Soros Foundation (Budapest, Hungary) and European Community (Brussels, Belgium) for providing fellowships for research periods at Karlsruhe. The research was also supported by the Hungarian Scientific Research Foundation (OTKA I/848, OTKA I/3.1545 and OTKA I/4.F.5359). We wish to thank Professor T. Pocs (Eger, Hungary — Morogoro, Tanzania) for collecting the plant material and to the linguist Mr. A. Jackson for correcting the English.     

Changes in chlorophyll a fluorescence, photosynthetic CO2 assimilation and xanthophyll cycle interconversions during dehydration in desiccation-tolerant and intolerant liverworts

The interactions among water content, chlorophyll a fluorescence emission, xanthophyll interconversions and net photosynthesis were analyzed during dehydration in desiccation-tolerant Frullania dilatata (L.) Dum. and desiccation-intolerant Pellia endiviifolia (Dicks) Dum. Water loss led to a progressive suppression of photosynthetic carbon assimilation in both species. Their chlorophyll fluorescence characteristics at low water content were: low photosynthetic quantum conversion efficiency, high excitation pressure on photosystem II and strong non-photochemical quenching. However, dissipation activity was lower in P. endiviifolia and was not accompanied by a rise in the concentration of de-epoxidised xanthophylls as F. dilatata. The photosynthetic apparatus of F. dilatata remained fully and speedily recuperable after desiccation in as indicated by the restoration of chlorophyll fluorescence parameters to pre-desiccation values upon rehydration. A lack of recovery upon remoistening of P. endiviifolia indicated permanent and irreversible damage to photosystem II. The results suggest that F. dilatata possesses a desiccation-induced zeaxanthin-mediated photoprotective mechanism which might aid photosynthesis recovery when favourable conditions are restored by alleviating photoinhibitory damage during desiccation. This avoidance mechanism might have evolved as an adaptative response to repeated cycles of desiccation and rehydration that represent a real threat to photosynthetic viability. Received: 12 January 1998 / Accepted: 14 July 1998     

CO2 is the first species formed upon CO oxidation by CO dehydrogenase from Pseudomonas carboxydovorans

The active species of CO₂, i.e. CO₂ or HCO ₃ ^- , formed in the CO dehydrogenase reaction was determined using the pure enzyme from the carboxydotrophic bacterium Pseudomonas carboxydovorans. Employing an assay system similar to that used to test for carbonic anhydrase, data were obtained which are quite compatible with those expected if CO₂ is the first species formed. In addition, carbonic anhydrase activity was not detected in P. carboxydovorans.     

Light-harvesting mutants show differential gene expression upon shift to high light as a consequence of photosynthetic redox and reactive oxygen species metabolism

The amount of light energy that is harvested and directed to the photosynthetic machinery is regulated in order to control the production of reactive oxygen species (ROS) in leaf tissues. ROS have important roles as signalling factors that instigate and mediate a range of cellular responses, suggesting that the mechanisms regulating light-harvesting and photosynthetic energy transduction also affect cell signalling. In this study, we exposed wild-type (WT) Arabidopsis and mutants impaired in the regulation of photosynthetic light-harvesting (stn7, tap38 and npq4) to transient high light (HL) stress in order to study the role of these mechanisms for up- and downregulation of gene expression under HL stress. The mutants, all of which have disturbed regulation of excitation energy transfer and distribution, responded to transient HL treatment with surprising similarity to the WT in terms of general ‘abiotic stress-regulated’ genes associated with hydrogen peroxide and 12-oxo-phytodienoic acid signalling. However, we identified distinct expression profiles in each genotype with respect to induction of singlet oxygen and jasmonic acid-dependent responses. The results of this study suggest that the control of excitation energy transfer interacts with hormonal regulation. Furthermore, the photosynthetic pigment–protein complexes appear to operate as receptors that sense the energetic balance between the photosynthetic light reactions and downstream metabolism.     

The midday depression of CO2 assimilation in leaves of Arbutus unedo L.: diurnal changes in photosynthetic capacity related to changes in temperature and humidity

Parts of attached leaves of the sclerophyllous shrub Arbutus unedo were subjected to simulated mediterranean days. Gas exchange was recorded in order to recognize the causes of the midday depression in CO₂ assimilation. Depressions could be induced in part of a leaf: they were local responses. The CO₂-saturation curves of photosynthesis, determined during the morning and afternoon maxima of CO₂ assimilation and during the minimum at midday, established that depressions in CO₂ assimilation were in one-half of the investigated cases totally caused by reversible reductions in the photosynthetic capacity of the leaves, and in the other half almost totally caused by such reductions. An analysis of 37 daily courses showed that morning reductions and afternoon recoveries of stomatal conductance and rate of photosynthesis occurred simultaneously and in proportion to each other, with the result that the partial pressure of CO₂ in the intercellular spaces remained more or less constant. Midday depressions occurred also in detached leaves standing in water. The initiation of a midday depression was not caused by a circadian rhythm, nor was high quantum flux or high temperature a requirement. There was no correlation between the rate of water loss from the leaves, or the amount of water lost, with the degree of reduction of the photosynthetic capacity. However, depressions occurred if an apparent threshold in the water-vapor pressure difference between leaf and air was exceeded. This critical value varied between about 20 and 30 mbar, depending on the leaf investigated. The dominating role of humidity in the induction of the midday depression was further demonstrated when leaf temperature was held constant and the vapor-pressure difference was made to follow the pattern of the mediterranean day: depressions occurred. Depressions however were hardly noticeable when the water-vapor pressure difference was held constant and leaf temperature was allowed to vary. In another set of experiments, leaves were subjected to variations in temperature and humidity independent of the time of the day, under otherwise constant conditions. Photosynthetic capacity and stomatal conductance proved to be almost insensitive to changes in temperature (in a range extending from 20 to 37° C) as long as the water vapor-pressure difference was held constant. If it was not, the rate of photosynthesis began to decline with increasing temperature after a threshold water-vapor pressure difference was exceeded. The position of the resulting apparent temperature optimum of photosynthesis depended on the humidity of the air. We suggest that the ability of A. unedo to respond to a dry atmosphere with a reversible reduction of its photosynthetic capacity (by a still unknown mechanism) is the result of a co-evolution with the development of a strong stomatal sensitivity to changes in humidity.     

Effects of temperature on the regulation of photosynthetic carbon assimilation in leaves of maize and barley

The aim of this work was to examine the effect of temperature in the range 5 to 30 ° C upon the regulation of photosynthetic carbon assimilation in leaves of the C₄ plant maize (Zea mays L.) and the C₃ plant barley (Hordeum vulgare L.). Measurements of the CO₂-assimilation rate in relation to the temperature were made at high (735 bar) and low (143 bar) intercellular CO₂ pressure in barley and in air in maize. The results show that, as the temperature was decreased, (i) in barley, pools of phosphorylated metabolites, particularly hexose-phosphate, ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate, increased in high and low CO₂; (ii) in maize, pools of glycerate 3-phosphate, triose-phosphate, pyruvate and phosphoenolpyruvate decreased, reflecting their role in, and dependence on, intercellular transport processes, while pools of hexose-phosphate, ribulose 1,5-bis phosphate and fructose 1,6-bisphosphate remained approximately constant; (iii) the redox state of the primary electron acceptor of photosystem II (Q_A) increased slightly in barley, but rose abruptly below 12° C in maize. Non-photochemical quenching of chlorophyll fluorescence increased slightly in barley and increased to high values below 20 ° C in maize. The data from barley are consistent with the development of a limitation by phosphate status at low temperatures in high CO₂, and indicate an increasing regulatory importance for regeneration of ribulose 1,5-bisphosphate within the Calvin cycle at low temperatures in low CO₂. The data from maize do not show that any steps of the C₄ cycle are particularly cold-sensitive, but do indicate that a restriction in electron transport occurs at low temperature. In both plants the data indicate that regulation of product synthesis results in the maintenance of pools of Calvin-cycle intermediates at low temperatures.Abbreviations Glc6P glucose-6-phosphate - Fru6P fructase-6-phosphate - Frul,6bisP fructose-1,6-bisphosphate - PGA glycerate-3-phosphate - p _i intercellular partial pressure of CO₂ - RuBP ribulose-1,5-bisphosphate - triose-P sum of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate We thank the Agricultural and Food Research Council, UK (Research grant PG50/67) and the Science and Engineering Research Council, UK for financial support. C.A.L. was supported by the British Council, by the Conselho Nacional de Desenvolvimento Cientiflco e Tecnologico (CNPq), Brazil and by an Overseas Research Student Award. We also thank Mark Stitt (Bayreuth, FRG) and Debbie Rees for helpful discussions.