The relationship between steady-state gas exchange of bean leaves and the levels of carbon-reduction-cycle intermediates

The relationship between the gas-exchange characteristics of attached leaves of Phaseolus vulgaris L. and the pool sizes of several carbon-reduction-cycle intermediates was examined. After determining the rate of CO₂ assimilation at known intercellular CO₂ pressure, O₂ pressure and light, the leaf was rapidly killed (<0.1 s) and the levels of ribulose-1,5-bisphosphate (RuBP), 3-phosphoglyceric acid (PGA), fructose-1,6-bisphosphate, fructose-6-phosphate, glucose-6-phosphate, glyceraldehyde-3-phosphate, and dihydroxyacetone phosphate were measured. In 210 mbar O₂, photosynthesis appeared RuBP-saturated at low CO₂ pressure and RuBP-limited at high CO₂ pressure. In 21 mbar (2%) O₂, the level of RuBP always appeared saturating. Very high levels of PGA and other phosphate-containing compounds were found with some conditions, especially under low oxygen.Abbreviations and symbols C₁ intercellular CO₂ pressure - PGA 3-phosphoglyceric acid - RuBP ribulose-1,5-bisphosphate - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase     

Control of Photosynthesis in Wheat by CO2, O2 and Light Intensity

The regulation of photosynthesis in wheat leaves under varyingO₂, CO₂, and light was studied by analyzing certain metabolitepools and enzyme activities. Under high light when the rateof photosynthesis was limited by low intercellular levels ofCO₂ (C₁) there was a high level of ribulose-1,5-bisphosphate(RuBP) (about 100 nmols per mg chlorophyll). As C, increased,there was a parallel decrease in the ratios of RuBP/3-phosphoglycerate(PGA) (from 0.18 to 0.08 under 21% O₂) and triose-phosphate/PGA(from 0.16 to 0.07 under 21% O₂). The results suggest carboxylationis limited at low C_i, and that there is high carboxylation andlimited assimilatory power at high C_i. As photosynthesis increasedwith increasing Jight intensity under atmospheric levels ofCO₂ the ratios of RuBP/PGA and triosephosphate/PGA remainednearly constant (near 0.12 to 0.13) suggesting there may bea coordinate regulation by light of the different phases ofthe cycle. There was increasing activation of ribulose 1,5-bisphosphatecarboxylase oxygenase (Rubisco) and fructose 1,6-bisphosphatase(FBPase) with increasing light intensity. The ways in whichthe light activation of the enzymes Rubisco and FBPase may regulatecarbon metabolism in the cycle are discussed. ¹ Current address: Biological Sciences Center, Desert ResearchInstitute, PO Box 60220, Reno, Nevada 89506, U.S.A. (Received March 24, 1987; Accepted June 23, 1987)     

The photosynthetic induction response in wheat leaves: net CO2 uptake,enzyme activation,and leaf metabolites

The photosynthetic induction response was studied in whole leaves of wheat (Triticum aestivum L.) following 5-min, 30-min and 10-h dark periods. After the 5-min dark treatment there was a rapid burst in the rate of photosynthesis upon illumination (half of maximum after 30s), followed by a slight decrease after 1.5 more min and then a gradual rise to the maximum rate. During this initial burst in photosynthesis, there was a rapid rise in the level of 3-phosphoglycerate (PGA) and a high PGA/triose-phosphate (triose-P) ratio was obtained. In addition, after the 5-min dark treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco, EC 4.1.1.39), ribulose-5-phosphate kinase (EC 2.7.1.19) and chloroplastic fructose-1,6-bisphosphatase (EC 3.1.3.11) maintained a relatively high state of activation, and maximum activation occurred within 1 min of illumination. The results indicate there is a high capacity for CO₂ fixation in the cycle upon illumination but attaining maximum rates requires an increase in the ribulose-1,5-bisphosphate (RuBP) pool (adjustment in triose-P utilization for carbohydrate synthesis versus RuBP synthesis). With both the 30-min and 10-h dark pretreatments there was only a slight rise in photosynthesis upon illumination, followed by a lag, then a gradual increase to steady-state (half-maximum rate after 6 min). In contrast to the 5-min dark treatment, the level of PGA was low and actually decreased initially, whereas the level of RuBP increased and was high during induction, indicating that Rubisco is limiting. This regulation via the carboxylase was not reflected in the initial extractable activity, which reached a maximum by 1 min after illumination. The light activation of chloroplastic fructose-1,6-bisphosphatase in leaves darkened for 30 min and 10 h prior to illumination was relatively slow (reaching a maximum after 8 min). However, this was not considered to limit carbon flux through the carbon-fixation cycle during induction since RuBP was not limiting. When photosynthesis approached the maximum steady-state rate, a high PGA/triose-P ratio and a high PGA/RuBP ratio were obtained. This may allow a high rate of photosynthesis by producing a favorable mass-action ratio for the reductive phase (the conversion of PGA to triose phosphate) while stimulating starch and sucrose synthesis.Abbreviations Chl chlorophyll - FBP fructose-1,6-bisphosphate - FBPase fructose-1,6-bisphosphatase - Fru6P fructose-6-phosphate - Glc6P glucose-6-phosphate - PGA 3-phosphoglycerate - Pi inoganic phosphate - Rubisco RuBP carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - Ru5P ribulose-5-phosphate - triose-P triose phosphates (dihydroxyacetone phosphate+glyceraldehyde-3-phosphate)     

Long- and short-term effects of decreased sink demand on carbohydrate levels and photosynthesis in wheat leaves

Wheat (Triticum aestivum L.) ears were removed to investigate long-term regulation of photosynthesis by sink demand at ambient CO₂ and 22 °C. The CO₂ level was also increased to 660 μmol mol^?1 and temperature was lowered to 5 °C to examine short-term responses of photosynthesis to low sink demand. Sink removal inhibited photosynthesis and increased leaf levels of glucose, fructose and ribulose-1, 5-bisphosphate (RuBP), and the glucose-6-phosphate (G6P)/fructose-6-phosphate (F6P) and RuBP/3-phosphoglycerate (PGA) ratios under growth conditions, but had no effect on the activity and activation state of ribulose-1, 5-bisphosphate carboxylase oxygenase (Rubisco) either under growth or short-term conditions, suggesting an inhibition of photosynthesis by decreased in vivo catalysis of Rubisco. Photosynthesis increased similarly in eared and earless shoots after a rise in CO₂ concentration, and the ratio of triose-phosphates (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP) to PGA was similar or higher for removed than intact ears, suggesting that feedback inhibition of photosynthesis was not caused by a limitation of ATP synthesis in chloroplasts. Under short-term conditions (660 μmol mol^?1 CO₂, 5 °C), TP and RuBP levels and the TP/PGA and TP/RuBP ratios were increased by sink removal, indicating an additional limitation of photosynthesis by the rate of RuBP regeneration.     

Changes in Levels of Intermediates of the C(4) Cycle and Reductive Pentose Phosphate Pathway under Various Light Intensities in Maize Leaves

The rate of CO₂ assimilation and levels of metabolites of the C₄ cycle and reductive pentose phosphate pathway in attached leaves of maize (Zea mays L.) were measured over a range of light intensity from 0 to 1,900 microEinsteins per square meter per second under a saturated CO₂ concentration of 350 microliters per liter and a limiting CO₂ concentration of 133 microliters per liter. The level of ribulose 1,5-bisphosphate (RuBP) stayed almost constant (around 60 nanomoles per milligram chlorophyll [Chl]) from low to high light intensities under 350 microliters per liter. Levels of 3-phosphoglycerate (PGA) increased from 100 to 650 nanomoles per milligram Chl under 350 microliters per liter CO₂ with increasing light intensity. The calculated RuBP concentration of 6 millimolar (corresponded to 60 nanomoles per milligram Chl) was about two times above the estimated RuBP binding-site concentration on ribulose bisphosphate carboxylase-oxygenase (Rubisco) of ~2.6 millimolar in maize bundle sheath chloroplasts in the light. The ratio of RuBP/PGA increased with decreasing light intensity under 350 microliters per liter CO₂. These results suggest that RuBP carboxylation is under control of light intensity possibly due to a limited supply of CO₂ to Rubisco through the C₄ cycle whose activity is highly dependent on light intensity. Pyruvate level increased with increasing light intensity as long as photosynthesis rate increased. A positive relationship between levels of PGA and those of pyruvate during steady-state photosynthesis under various conditions suggests that an elevated concentration of PGA increases the carbon input into the C₄ cycle through the conversion of PGA to PEP and consequently the level of total intermediates of the C₄ cycle can be raised to mediate higher photosynthesis rate.     

Demonstration of C3-photosynthesis in a bluegreen alga,Coccochloris peniocystis

Air-grown cells of the cyanobacterium, Coccochloris peniocystis Kutz were exposed to [¹⁴C] bicarbonate in the light for periods of 0.5 to 2.0 s followed by longer exposures to unlabelled bicarbonate. Although C₄ acids are among the initial products of photosynthesis, the kinetics of tracer movement during the pulse-chase experiments demonstrate that the principal mechanism of CO₂ fixation in this alga is the C₃-pathway.Abbreviations PGA 3-phophoglyceric acid - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate     

Effects of water stress on photosynthetic electron transport,photophosphorylation, and metabolite levels of Xanthium strumarium mesophyll cells

Several component processes of photosynthesis were measured in osmotically stressed mesophyll cells of Xanthium strumarium L. The ribulose-1,5-bisphosphate regeneration capacity was reduced by water stress. Photophoshorylation was sensitive to water stress but photosynthetic electron transport was unaffected by water potentials down to-40 bar (-4 MPa). The concentrations of several intermediates of the photosynthetic carbon-reduction cycle remained relatively constant and did not indicate that ATP supply was limiting photosynthesis in the water-stressed cells.Abbreviations Hepes 4-(2-hydroxyethyl)-1-piperazinepropanesulfonic acid - PGA 3-phosphoglyceric acid - RuBP ribulose-1,5-bisphosphate     

An improved model of C3 photosynthesis at high CO2: Reversed O2 sensitivity explained by lack of glycerate reentry into the chloroplast

Current models of C₃ photosynthesis incorporate a phosphate limitation to carboxylation which arises when the capacity for starch and sucrose synthesis fails to match the capacity for the production of triose phosphates in the Calvin cycle. As a result, the release of inorganic phosphate in the chloroplast stroma fails to keep pace with its rate of sequestration into triose phosphate, and phosphate becomes limiting to photosynthesis. Such a model predicts that when phosphate is limiting, assimilation becomes insensitive to both CO₂ and O₂, and is thus incapable of explaining the experimental observation that assimilation, under phosphate-limited conditions, frequently exhibits reversed sensitivity to both CO₂ and O₂, i.e., increasing O₂ stimulates assimilation and increasing CO₂ inhibits assimilation. We propose a model which explains reversed sensitivity to CO₂ and O₂ by invoking the net release of phosphate in the photorespiratory oxidation cycle. In order for this to occur, some fraction of the glycollate carbon which leaves the stroma and which is recycled to the chloroplast by the photorespiratory pathway as glycerate must remain in the cytosol, perhaps in the form of amino acids. In that case, phosphate normally used in the stromal glycerate kinase reaction to generate PGA from glycerate is made available for photophosphorylation, stimulating RuBP regeneration and assimilation. The model is parameterized for data obtained on soybean and cotton, and model behavior in response to CO₂, O₂, and light is demonstrated.Abbreviations PFD photon flux density - PGA 3-phosphoglycerate - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - TPU triose phosphate utilization     

Oxygen and Carbon Dioxide Effects on the Pool Size of Some Photosynthetic and Photorespiratory Intermediates in Soybean (Glycine max [L.] Merr.)

The levels of ribulose 1,5-bisphosphate (RuBP), 3-phosphoglyceric acid (PGA), glycolate, glycine, and serine were measured in soybean leaflets during photosynthesis in atmospheres ranging from 1 to 60% O₂ and from 0 to 500 microliters per liter CO_2.     

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).     

Regulation of ribulose-1,5-bisphosphate carboxylase activity by the activase system in lysed spinach chloroplasts

Ribulose-1,5-bisphosphate (RuBP) carboxylase in lysed spinach (Spinacia oleracea L. cv virtuosa) chloroplasts that had been partly inactivated at low CO₂ and Mg²⁺ by incubating in darkness with 4 millimolar partially purified RuBP was reactivated by light. If purified RuBP was used to inhibit dark activation of the enzyme, reactivation by light was not observed unless fructose-1,6-bisphosphate, ATP, or ADP plus inorganic phosphate were also added. Presumably, ADP plus inorganic phosphate acted as an ATP-generating system with a requirement for the generation of ΔpH across the thylakoid membrane. When the RuBP obtained from Sigma Chemical Co. was used, light did not reactivate the enzyme. There was no direct correlation between ΔpH and activation. Therefore, thylakoids are required in the ribulose-1,5-bisphosphate carboxylase activase system largely to synthesize ATP. Inactivation of RuBP carboxylase in isolated chloroplasts or in the lysed chloroplast system was not promoted simply by a transition from light to dark conditions but was caused by low CO₂ and Mg²⁺.     

The role of enzyme activation state in limiting carbon assimilation under variable light conditions

The mechanisms regulating transient photosynthesis by soybean (Glycine max) leaves were examined by comparing photosynthetic rates and carbon reduction cycle enzyme activities under flashing (saturating 1 s lightflecks separated by low photon flux density (PFD) periods of different durations) and continuous PFD. At the same mean PFD, the mean photosynthetic rates were reduced under flashing as compared to continuous light. However, as the duration of the low PFD period lengthened, the CO₂ assimilation attributable to a lightfleck increased. This enhanced lightfleck CO₂ assimilation was accounted for by a greater postillumination CO₂ fixation occurring after the lightfleck. The induction state of photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), fructose 1,6-bisphosphatase (FBPase) and ribulose 5-phosphate kinase (Ru5P kinase) activities all responded similarly and were all lower under flashing as compared to constant PFD of the same integrated mean value. However, the fast phase of induction and FBPase and Ru5P kinase activities were reduced more than were the slow phase of induction and rubisco activity. This was consistent with the role of the former enzymes in the fast induction component that limited RuBP regeneration. Competition for reducing power between carbon metabolism and thioredoxin-mediated enzyme activation may have resulted in lower enzyme activation states and hence lower induction states under flashing than continuous PFD, especially at low lightfleck frequencies (low mean PFD).Abbreviations FBPase fructose 1,6-bisphosphatase (EC 3.1.3.11) - LUE lightfleck use efficiency - P-glycerate 3-phosphoglycerate - PICF post-illumination CO₂ fixation - Ru5P kinase ribulose 5-phosphate kinase (EC 2.7.1.19) - RuBP ribulose 1,5-bisphosphate - rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) - SBpase sedoheptulose 1,7-bisphosphatase (EC 3.1.3.37)     

A Model Describing the Regulation of Ribulose-1,5-Bisphosphate Carboxylase, Electron Transport, and Triose Phosphate Use in Response to Light Intensity and CO(2) in C(3) Plants

A model of the regulation of the activity of ribulose-1,5-bisphosphate carboxylase, electron transport, and the rate of orthophosphate regeneration by starch and sucrose synthesis in response to changes in light intensity and partial pressures of CO₂ and O₂ is presented. The key assumption behind the model is that nonlimiting processes of photosynthesis are regulated to balance the capacity of limiting processes. Thus, at CO₂ partial pressures below ambient, when a limitation on photosynthesis by the capacity of rubisco is postulated, the activities of electron transport and phosphate regeneration are down-regulated in order that the rate of RuBP regeneration matches the rate of RuBP consumption by rubisco. Similarly, at subsaturating light intensity or elevated CO₂, when electron transport or Pi regeneration may limit photosynthesis, the activity of rubisco is downregulated to balance the limitation in the rate of RuBP regeneration. Comparisons with published data demonstrate a general consistency between modelled predictions and measured results.     

The in-vivo response of the ribulose-1,5-bisphosphate carboxylase activation state and the pool sizes of photosynthetic metabolites to elevated CO2 inPhaseolus vulgaris L.

The short-term, in-vivo response to elevated CO₂ of ribulose-1,5-bisphosphate carboxylase (RuBPCase, EC 4.1.1.39) activity, and the pool sizes of ribulose 1,5-bisphosphate, 3-phosphoglyceric acid, triose phosphates, fructose 1,6-bisphosphate, glucose 6-phosphate and fructose 6-phosphate in bean were studied. Increasing CO₂ from an ambient partial pressure of 360–1600 bar induced a substantial deactivation of RuBPCase at both saturating and subsaturating photon flux densities. Activation of RuBPCase declined for 30 min following the CO₂ increase. However, the rate of photosynthesis re-equilibrated within 6 min of the switch to high CO₂, indicating that RuBPCase activity did not limit photosynthesis at high CO₂. Following a return to low CO₂, RuBPCase activation increased to control levels within 10 min. The photosynthetic rate fell immediately after the return to low CO₂, and then increased in parallel with the increase in RuBPCase activation to the initial rate observed prior to the CO₂ increase. This indicated that RuBPCase activity limited photosynthesis while RuBPCase activation increased. Metabolite pools were temporarily affected during the first 10 min after either a CO₂ increase or decrease. However, they returned to their original level as the change in the activation state of RuBPCase neared completion. This result indicates that one role for changes in the activation state of RuBPCase is to regulate the pool sizes of photosynthetic intermediates.Abbreviations and symbols A net CO₂ assimilation rate - C_a ambient CO₂ partial pressure - C_i intercellular CO₂ partial pressure - CABP 2-carboxyarabinitol 1,5-bisphosphate - k_cat catalytic turnover rate per RuBPCase molecule - PFD photon flux density (400 to 700 nm on an area basis) - PGA 3-phosphoglyceric acid - P_i orthophosphate - RuBP ribulose 1,5-bisphosphate - RuBPCase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39)     

Photometric method for routine determination of kcat and carbamylation of rubisco

Ribulose bisphosphate carboxylase (rubisco) is the first enzyme in photosynthetic CO₂ assimilation. It is also the single largest sink for nitrogen in plants. Several parameters of rubisco activity are often measured including initial activity upon extraction, degree of carbamylation, catalytic constant of the enzyme (k_cat), and the total amount of enzyme present in a leaf. We report here improvements of the photometric assay of rubisco in which rubisco activity is coupled to NADH oxidation which is continuously monitored in a photometer. The initial lag usually found in this assay was eliminated by assaying rubisco activity at pH 8.0 instead of 8.2, using a large amount of phosphoglycerate kinase, and adding monovalent cations to the assay buffer. We found that when using the photometric assay, the ratio of activity found initially upon extraction divided by the activity after incubating with CO₂ and Mg²⁺ reflects the degree of carbamylation as determined by ¹⁴carboxyarabinitol bisphosphate/¹²carboxyarabinitol bisphosphate competition. We developed methods for measuring the catalytic constant of rubisco as well as the total amount of enzyme present using the photometric assay and carboxyarabinitol 1,5-bisphosphate. We believe that the photometric assay for activity will prove more useful than the ¹⁴CO₂ assay in many studies.Abbreviations CA1P 2-carboxyarabinitol 1-phosphate - GAP glyceraldehyde 3-phosphate - OD optical density - PGA 3-phosphoglycerate - rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose 1,5-bisphosphate     

光合作用的CO_2阶跃响应动态生化模型

针对CO2阶跃变化下光合动态响应的振荡现象,依据经典的光合系统酶触反应动力学,初步尝试构建了光合系统反馈控制动态生化模型。该模型以卡尔文环的核酮糖-1,5-二磷酸羧化/加氧酶(ribulose-1,5-bisphosphate carboxylase/oxygenase,Rubisco)接触反应的酶动力学进程作为核心,以磷酸甘油酸(phosphorylglyceric acid,PGA)还原和接续的核酮糖-1,5-二磷酸(ribulose-1,5-bisphosphate,RuBP)再生的多级过程高度简化为复合酶接触反应的酶动力学进程为反馈,构成反馈控制系统。采用经典的控制系统传递函数分析手段,将反馈控制系统表达为光合动态生化模型传递函数。据此模型将实测羧化速率Vc振荡动态进行仿真拟合,呈现出很高的拟合度(r=0.9377)。这表明,在卡尔文环或者光合系统反馈环中,因RuBP的消耗和再生补充不平衡引起的光合振荡现象,与光合系统酶接触反应动力学参数(k)造成各个中间产物再生的"滞后"效应有关。从而在机理上解释了Laisk和Walker(1986)的无机磷(inorganic phosphate,Pi)再生供应的光合动态生化模型中,需要假定代谢途径中蔗糖合成"滞后15～20s"才能表现出光合振荡效果的现象。     

Reversible heat-inactivation of the calvin cycle: A possible mechanism of the temperature regulation of photosynthesis

Photosynthetic CO₂ fixation rates in leaves and intact chloroplasts of spinach measured at 18°–20° C are substantially decreased by pretreatment at temperatures exceeding 20° C. Mild heating which causes 80% inhibition of CO₂ fixation does not affect phosphoglyceroacid reduction and causes increases in the ATP/ADP ratio and the light-induced transthylakoid proton gradient. The inactivation of the CO₂ fixation is completely reversible with half-times of recovery in the order of 15–20 min. Comparison of steady-state patterns of ¹⁴C labeled Calvin cycle intermediates of heat-treated and control samples reveals a large increase in the ribulose-1,5-bisphosphate/phosphoglyceroacid ratio and a large decrease in the phosphoglyceroacid/triosephosphate ratio. It is concluded that inactivation of CO₂ fixation occurring at elevated temperatures is caused by inhibition of the ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). Measurements of light-induced light scattering changes of thylakoids and of the light-induced electrochromic absorption shift show that these signals are affected by mild heating in a way which is strictly correlated with the inactivation of the CO₂ fixation. It is proposed that the function of the ribulose-1,5-bisphosphate carboxylase in vivo requires a form of activation that involves properties of the thylakoid membrane which are affected by the heat treatment. The fact that these changes in thylakoid membrane properties and of ribulose-1,5-bisphosphate carboxylase activity are already affected at elevated temperatures which can still be considered physiological, and the reversible nature of these changes, suggest that they may play a role in temperature regulation of the overall photosynthetic process.Abbreviations 9-AA 9-aminoacridine - DMO 5,5-dimethyloxazolidine-2,4-dione - FBP fructose-1,6-bisphosphate - HEPES N-2-hydroxyethylpiperazine N-2-ethane sulfonic acid - HMP hexose monophosphates - PGA 3-phosphoglycerate - PMP pentose monophosphates - RuBP ribulose-1,5-bisphosphate - SBP seduheptulose-1,7-bisphosphate - TP triose monophosphates     

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     

Long- and short-term responses of leaf carbohydrate levels and photosynthesis to decreased sink demand in soybean

Axillary buds and the apical portion of shoots of soybean [Glycine max (L.) Merr. cultivar Turchina] plants were trimmed to investigate long-term regulation of photosynthesis by sink demand at ambient CO₂ and 22 °C. Also, in intact and trimmed shoots, the CO₂ level was increased to 660 μmol mol^?1 and temperature was lowered to 5°C to examine the superimposed short-term responses of photosynthesis to low sink demand. Under growth conditions, trimming the shoots increased leaf photosynthesis and the levels of sucrose, glucose-6-phosphate (G6P) and 3-phosphoglycerate (PGA), as well as the G6P/fructose-6-phosphate (F6P) and sucrose/starch ratios, while it decreased the level of starch and the triose-phosphate (glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, TP)/PGA ratio. Photosynthesis enhancement was accompanied by increased chlorophyll contents and ribulose-l,5-bisphosphate carboxylase oxygenase (Rubisco) activity. Sink removal consistently increased photosynthesis measured under a variety of conditions (growth CO₂ or a short-term change to 660 μmol mol_-1 CO₂; growth temperature or a short-term change to 5 °C), except when low temperature was combined with ambient CO₂; the increase in photosynthesis was higher under short-term elevated CO₂ than at ambient CO₂. In contrast with its effect at ambient CO₂, shoot trimming increased the levels of TP and ribulose-1,5-bisphosphate (RuBP) and the TP/PGA ratio under high-CO₂ conditions.