Over-expression of sucrose phosphate synthase in Ababidopsis thaliana results in increased foliar sucrose/starch ratios and favours decreased foliar carbohydrate accumulation in plants after prolonged growth with CO2 enrichment

Arabidopsis thaliana ecotype Columbia was transformed with a maize sucrose phosphate synthase (SPS) cDNA under the control of the promoter for the small subunit of ribulose-1,5-bisphosphate carboxylase from tobacco (rbcS). The effects of SPS over-expression were compared in plants of the T2 and T3 generations grown either in air or with CO2 enrichment (700 l 1^-1) for either 4 or 10 weeks. Maximal extractable foliar SPS activities were three times those of the untransformed controls in the highest rbcS-SPS expressing line. In untransformed Arabidopsis leaves SPS activity was not subject to light/dark regulation, but was modified by incubation with either the inhibitor, orthophosphate, or the activator, mannose. Photosynthesis (Amax) values were similar in all lines grown in air. After 10 weeks of CO2 enrichment a decrease in Amax in the untransformed controls, but not in the high SPS expressors, was observed. There was a strong correlation between the sucrose-to-starch ratio of the leaves and their SPS activity in both growth conditions. The total foliar carbohydrate contents of 4-week-old plants was similar in all lines whether plants were grown in air or with CO2 enrichment. After 10 weeks growth the leaves of the high rbcS-SPS expressors accumulated much less total carbohydrate than untransformed control leaves in both growth conditions. It was concluded that SPS over-expression causes increased foliar sucrose/starch ratios in Arabidopsis leaves and favours decreased foliar carbohydrate contents when plants are grown for long periods with CO2 enrichment.     

Regulation of photosynthesis in nitrogen deficient wheat seedlings

Nitrogen effects on the regulation of photosynthesis in wheat (Triticum aestivum L., cv Remia) seedlings were examined. Ribulose 1,5-bisphosphate carboxylase/oxygenase was rapidly extracted and tested for initial activity and for activity after incubation in presence of CO₂ and Mg²⁺. Freeze clamped leaf segments were extracted for determinations of foliar steady state levels of ribulose 1,5-bisphosphate, triose phosphate, 3-phosphoglycerate, ATP, and ADP. Nitrogen deficient leaves showed increased ATP/ADP and triose phosphate/3-phosphoglycerate ratios suggesting increased assimilatory power. Ribulose 1,5-bisphosphate levels were decreased due to reduced pentose phosphate reductive cycle activity. Nevertheless, photosynthesis appeared to be limited by ribulose 1,5-bisphosphate carboxylase/oxygenase, independent of nitrogen nutrition. Its degree of activation was increased in nitrogen deficient plants and provided for maximum photosynthesis at decreased enzyme protein levels. It is suggested that ribulose 1,5-bisphosphate carboxylase/oxygenase activity is regulated according to the amount of assimilatory power.     

The carboxylase activity of Rubisco and the photosynthetic performance in aquatic plants

Summary Activated carboxylase activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), as well as photosynthetic rates were measured for 42 species of freshwater and marine macrophytes. While the carboxylase activity varied greatly among the species investigated (0.2–12.5 mol CO₂ mg^–1 chlorophyll min^–1), the submersed freshwater plants showed significantly lower activities than emergent, floating leaved or secondary submersed forms. The variability in photosynthetic rates correlated with the carboxylase activity only for the marine macroalgae, and their photosynthesis to carboxylase activity ratios were close to 1. These plants also had a consistently high inorganic carbon transport capability, and it is suggested that ribulose-1,5-bisphosphate carboxylase/oxygenase activity is an important internal factor regulating the photosynthetic capacity within this plant group where, apparently, the internal CO₂ concentration is high and photorespiration is suppressed. Among the freshwater forms, it appears that their much lower inorganic carbon transport ability, rather than their carboxylase activity, limits the photosynthetic process.     

Altered photosynthesis,flowering, and fruiting in transgenic tomato plants that have an increased capacity for sucrose synthesis

Photosynthesis, leaf assimilate partitioning, flowering, and fruiting were examined in two lines of Lycopersicon esculentum Mill. transformed with a gene coding for sucrose-phosphate synthase (SPS) (EC 2.3.1.14) from Zea mays L. expressed from a tobacco ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit promoter. Plants were grown at either 35 or 65 Pa CO₂ and high light (1000 mol photons·m^–2·s^–1). Limiting and maximum SPS activities were significantly greater (up to 12 times) in the leaves of SPS-transformed lines for all treatments. Partitioning of carbon into sucrose increased 50% for the SPS transformants. Intact leaves of the control lines exhibited CO₂-insensitivity of photosynthesis at high CO₂ levels, whereas the SPS transformants did not exhibit CO₂-insensitivity. The O₂-sensitivity of photosynthesis was also greater for the SPS-transformed lines compared to the untransformed control when measured at 65 Pa CO₂. These data indicate that the SPS transformants had a reduced limitation on photosynthesis imposed by end-product synthesis. Growth at 65 Pa CO₂ resulted in reduced photosynthetic capacity for control lines but not for SPS-transformed lines. When grown at 65 Pa CO₂, SPS transformed lines had a 20% greater photosynthetic rate than controls when measured at 65 Pa CO₂ and a 35% greater rate when measured at 105 Pa CO₂. Photosynthetic rates were not different between lines when grown at 35 Pa CO₂. The time to 50% blossoming was reduced and the total number of inflorescences was significantly greater for the SPS transformants when grown at either 35 or 65 Pa CO₂. At 35 Pa CO₂, the total fruit number of the SPS transformants was up to 1.5 times that of the controls, the fruit matured earlier, and there was up to a 32% increase in total fruit dry weight. Fruit yield was not significantly different between the lines when grown at 65 Pa CO₂. Therefore, there was not a strict relationship between yield and leaf photosynthesis rate. Flowering and fruit development of the SPS-transformed lines grown at 35 Pa CO₂ showed similar trends to the controls grown at 65 Pa CO₂. Incidences of blossom-end rot were also reduced in the SPS-transformed lines. These data indicate that altering starch/sucrose partitioning by increasing the capacity for sucrose synthesis can affect acclimation to elevated CO₂ partial pressure and flowering and fruiting in tomato.Abbreviations DAS days after seeding - nptII neomycin phos-photransferase - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - SPS sucrose-phosphate synthase - SSU Rubisco small subunit This research was supported by U.S. Department of Energy grant FG02-87ER13785. B.J.M. thanks the Natural Sciences and Engineering Research Council of Canada for financial support. We are grateful to Toni A. Voelker (Calgene Inc.) for supplying tomato seeds and valuable advice.     

The Association of d-Ribulose- 1,5-Bisphosphate Carboxylase/Oxygenase with Phosphoribulokinase

When Ribulose- 1,5-bisphosphate carboxylase/oxygenase was purified from spinach leaves (Spinacia oleracea) using precipitation with polyethylene glycol and MgCl₂ followed by DEAE cellulose chromatography, 75% of phosphoribulokinase and 7% of phosphoriboisomerase activities copurified with ribulose- 1,5-bisphosphate carboxylase/oxygenase. This enzyme preparation showed ribose-5-phosphate and ribulose-5-phosphate dependent carboxylase and oxygenase activities which were nearly equivalent to its corresponding ribulose- 1,5-bisphosphate dependent activity. The ribose-5-phosphate and ribulose-5-phosphate dependent reaction rates were stable and linear for much longer time periods than the ribulose- 1,5-bisphosphate dependent rates. When sucrose gradients were used to purify ribulose- 1,5-bisphosphate carboxylase/oxygenase from crude stromal extracts, phosphoribulokinase was found to cosediment with ribulose- 1,5-bisphosphate carboxylase. Under these conditions most of the phosphoriboisomerase activity remained with the slower sedimenting proteins. Ammonium sulfate precipitation resulted in separation of the ribulose- 1,5-bisphosphate carboxylase peak from phosphoribulokinase peak. Crude extracts of peas Pisum sativum and spinach contained 0.725 to 0.730 milligram of phosphoribulokinase per milligram of chlorophyll, respectively, based on an enzyme-linked immunosorbent assay.     

Exchange Properties of the Activator CO(2) of Spinach Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase

The exchange properties of the activator CO₂ of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase were characterized both in vitro with the purified enzyme, and in situ within isolated chloroplasts. Carboxyarabinitol-1,5-bisphosphate, a proposed reaction intermediate analog for the carboxylase activity of the enzyme, was used to trap the activator CO₂ on the enzyme both in vitro and in situ. Modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in intact chloroplasts during a light/dark cycle was associated with a similar modulation in carboxyarabinitol-1,5-bisphosphate-trapped CO₂. The exchange kinetics of the activator CO₂ were monitored by activation of the enzyme to steady state in the presence of ¹²CO₂, followed by addition of ¹⁴CO₂ and determination of the amount of labeled CO₂ trapped on the enzyme by carboxyarabinitol-1,5-bisphosphate. Rate constants (K_obs) for exchange with both the purified enzyme (0.45 min⁻¹) and in illuminated chloroplasts (0.18 min⁻¹) were comparable to the observed rate constants for enzyme activation under the two conditions. A similar exchange of the activator CO₂ was not observed in chloroplasts in the dark. Kinetic analysis of the exchange properties of the purified enzyme were consistent with an equilibrium between active and inactive forms of the enzyme during steady state activation.     

Regulation of Ribulose-1,5-bisphosphate Carboxylase/Oxygenase in vivo: Control by High CO2 Concentration

High CO₂ concentrations (HC) in air induce partial deactivation of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO, EC 4.1.1.39). Under saturating irradiance, increase in [CO₂] to 1 200 cm³ m^–3 reduces the concentration of operating carboxylation centres by 20–30 %. At a further increase in [CO₂], the activity remained on the same level. Under limiting irradiance, the lowest activity was reached at 600 cm³(CO₂) m^–3. The presence of oxygen diminished deactivation, but O₂ failed to stimulate reactivation under high CO₂. Conditions that favour oxygenation of ribulose-1,5-bisphosphate (RuBP) facilitated reactivation. Even HC did not act as an inhibitor. HC induces deactivation of RuBPCO by increasing the concentration of free reaction centres devoid of the substrate, which are more vulnerable to inhibition than the centres filled with substrates or products.     

Photosynthesis of Ulva sp: III. O(2) Effects, Carboxylase Activities, and the CO(2) Incorporation Pattern

Ulva, a common green seaweed, performs at the biochemical level as a typical C₃ plant. Over 90% of label was found in glycerate 3-phosphate following a 3 second ¹⁴C pulse in the light, and the label was subsequently transferred to sugars. Also, the level of ribulose-1,5-bisphosphate carboxylase activity in crude extracts was about 10 times higher than that of phosphoenolpyruvate carboxylase. Concerning gas exchange, photosynthetic rates of Ulva showed no O₂ sensitivity, indicating that photorespiratory CO₂ losses are repressed as in C₄ plants. This apparent anomaly could be explained by the efficient HCO₃⁻ uptake system of Ulva which might concentrate CO₂ to the chloroplasts, thus suppressing the oxygenase activity of ribulose-1,5-bisphosphate carboxylase.     

Ribulose-1,5-bisphosphate regeneration limitation in rice leaf photosynthetic acclimation to elevated CO2

Our previous study has demonstrated that both RuBP carboxylation limitation and RuBP regeneration limitation exist simultaneously in rice grown under free-air CO₂ enrichment (FACE, about 200 μmol mol⁻¹ above the ambient air CO₂ concentration) conditions [G.-Y. Chen, Z.-H. Yong, Y. Liao, D.-Y. Zhang, Y. Chen, H.-B. Zhang, J. Chen, J.-G. Zhu, D.-Q. Xu, Photosynthetic acclimation in rice leaves to free-air CO₂ enrichment related to both ribulose-1,5-bisphosphate carboxylase limitation and ribulose-1,5-bisphosphate regeneration limitation. Plant Cell Physiol. 46 (2005) 1036–1045]. To explore the mechanism for forming of RuBP regeneration limitation, we conducted the gas exchange measurements and some biochemical analyses in FACE-treated and ambient rice plants. Net CO₂ assimilation rate (A_net) in FACE leaves was remarkably lower than that in ambient leaves when measured at the same CO₂ concentration, indicating that photosynthetic acclimation to elevated CO₂ occurred. In the meantime the maximum electron transport rate (ETR) (J_max), maximum carboxylation rate (V_cmax) in vivo, and RuBP contents decreased significantly in FACE leaves. The whole chain electron transport rate and photophosphorylation rate reduced significantly while ETR of photosystem II (PSII) did not significantly decrease and ETR of photosystem I (PSI) was significantly increased in the chloroplasts from FACE leaves. Further, the amount of cytochrome (Cyt) f protein, a key component localized between PSII and PSI, was remarkably declined in FACE leaves. It appears that during photosynthetic acclimation the decline in the Cyt f amount is an important cause for the decreased RuBP regeneration capacity by decreasing the whole chain electron transport in FACE leaves.     

The drelationship between CO2-assimilation rate,Rubisco carbamylation and Rubisco activase content in activase-deficient transgenic tobacco suggests a simple model of activase action

Transgenic tobacco (Nicotiana tabacum L. cv. W38) plants with an antisense gene directed against the mRNA of ribulose-1,5-bisphosphate carboxylase/ oxygenase (Rubisco) activase were used to examine the relationship between CO₂-assimilation rate, Rubisco carbamylation and activase content. Plants used were those members of the r₁ progeny of a primary transformant with two independent T-DNA inserts that could be grown without CO₂ supplementation. These plants had from < 1% to 20% of the activase content of control plants. Severe suppression of activase to amounts below 5% of those present in the controls was required before reductions in CO₂-assimilation rate and Rubisco carbamylation were observed, indicating that one activase tetramer is able to service as many as 200 Rubisco hexadecamers and maintain wild-type carbamylation levels in vivo. The reduction in CO₂-assimilation rate was correlated with the reduction in Rubisco carbamylation. The anti-activase plants had similar ribulose-1,5-bisphosphate pool sizes but reduced 3-phosphoglycerate pool sizes compared to those of control plants. Stomatal conductance was not affected by reduced activase content or CO₂-assimilation rate. A mathematical model of activase action is used to explain the observed hyperbolic dependence of Rubisco carbamylation on activase content.Abbreviations CA1P 2-carboxyarabinitol-1-phosphate - P_ip_a intercellular, ambient partial pressure of CO₂ - PGA 3-phospho-glycerate - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - SSU small subunit of Rubisco     

Mass-spectrometric evidence for the double-carboxylation pathway of malate synthesis by Crassulacean acid metabolism plants in light

Phyllodia of the Crassulacean acid metabolism (CAM) plant Kalanchoë tubiflora were allowed to fix ¹³CO₂ in light and darkness during phase IV of the diurnal CAM cycle, and during prolongation of the regular light period. After ¹³CO₂ fixation in darkness, only singly labelled [¹³C]malate molecules were found. Fixation of ¹³CO₂ under illumination, however, produced singly labelled malate as well as malate molecules which carried label in two, three or four carbon atoms. When the irradiance during ¹³CO₂ fixation was increased, the proportion of singly labelled malate decreased in favour of plurally labelled malate. The irradiance, however, did not change either the ratio of labelled to unlabelled malate molecules found in the tissue after the ¹³CO₂ application, or the magnitude of malate accumulation during the treatment with label. The ability of the tissue to store malate and the labelling pattern changed throughout the duration of the prolonged light period. The results indicate that malate synthesis by CAM plants in light can proceed via a pathway containing two carboxylation steps, namely ribulose-1,5-bisphosphate-carboxylase/oxygenase (EC 4.1.1.39) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) which operate in series and share common intermediates. It can be concluded that, in light, phosphoenolpyruvate carboxylase can also synthesize malate independently of the proceeding carboxylation step by ribulose-1,5-bisphosphate carboxylase/oxygenase.Abbreviations CAM Crassulacean acid metabolism - PEP phosphoenolpyruvate - PEPCase phosphoenolpyruvate carboxylase (EC 4.1.1.31) - RuBPCase ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) - TMS trimethylsilyl     

Photorespiration and carbon concentrating mechanisms: two adaptations to high O2, low CO2 conditions

This review presents an overview of the two ways that cyanobacteria, algae, and plants have adapted to high O₂ and low CO₂ concentrations in the environment. First, the process of photorespiration enables photosynthetic organisms to recycle phosphoglycolate formed by the oxygenase reaction catalyzed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Second, there are a number of carbon concentrating mechanisms that increase the CO₂ concentration around Rubisco which increases the carboxylase reaction enhancing CO₂ fixation. This review also presents possibilities for the beneficial modification of these processes with the goal of improving future crop yields.     

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

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

Modulation of carbon and nitrogen metabolism, and of nitrate reductase, in untransformed and transformed Nicotiana plumbaginifolia during CO2 enrichment of plants grown in pots and in hydroponic culture

Transformed plants of Nicotiana plumbaginifolia Viv. constitutively expressing nitrate reductase (35S-NR) or β-glucuronidase (35S-GUS) and untransformed controls were grown for two weeks in a CO₂-enriched atmosphere. Whereas CO₂ enrichment (1000 μl · l⁻¹) resulted in an increase in the carbon (C) to nitrogen (N) ratio of both the tobacco lines grown in pots with vermiculite, the C/N ratio was only slightly modified when plants were grown in hydroponic culture in high CO₂ compared to those grown in air. Constitutive nitrate reductase (NR) expression per se did not change the C/N ratio of the shoots or roots. Biomass accumulation was similar in both types of plant when hydroponic or pot-grown material, grown in air or high CO₂, were compared. Shoot dry matter accumulation was primarily related to the presence of stored carbohydrate (starch and sucrose) in the leaves. In the pot-grown tobacco, growth at elevated CO₂ levels caused a concomitant decrease in the N content of the leaves involving losses in NO⁻ ₃ and amino acid levels. In contrast, the N content and composition were similar in all plants grown in hydroponic culture. The 35S-NR plants grown in air had higher foliar maximum extractable NR activities and increased glutamine levels (on a chlorophyll or protein basis) than the untransformed controls. These increases were maintained following CO₂ enrichment when the plants were grown in hydroponic culture, suggesting that an increased flux through nitrogen assimilation was possible in the 35S-NR plants. Under CO₂ enrichment the NR activation state in the leaves was similar in all plants. When the 35S-NR plants were grown in pots, however, foliar NR activity and glutamine content fell in the 35S-NR transformants to levels similar to those of the untransformed controls. The differences in NR activity between untransformed and 35S-NR leaves were much less pronounced in the hydroponic than in the pot-grown material but the difference in total extractable NR activity was more marked following CO₂ enrichment. Foliar NR message levels were decreased by CO₂ enrichment in all growth conditions but this was much more pronounced in pot-grown material than in that grown hydroponically. Since β-glucuronidase (GUS) activity and message levels in 35S-GUS plants grown under the same conditions of CO₂ enrichment (to test the effects of CO₂ enrichment on the activity of the 35S promoter) were found to be constant, we conclude that NR message turnover was specifically accelerated in the 35S-NR plants as well as in the untransformed controls as a result of CO₂ enrichment. The molecular and metabolic signals involved in increased NR message and protein turnover are not known but possible effectors include NO₃ ⁻, glutamine and asparagine. We conclude that plants grown in hydroponic culture have greater access to N than those grown in pots. Regardless of the culture method, CO₂ enrichment has a direct effect on NR mRNA stability. Received: 17 October 1996 / Accepted: 11 February 1997     

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     

Involvement of abscisic acid in photosynthetic process in Hordeum vulgare L. during salinity stress

In Hordeum vulgare L. plants, NaCl stress imposed through the root medium for a period of 8 days decreased the rate of CO₂ assimilation, the chlorophyll and protein leaf content, and the activity of ribulose-1,5-bisphosphate carboxylase. The activity of phosphoenolpyruvate carboxylase was twofold over the control. Pretreatment with abscisic acid (ABA) for 3 days before salinization diminished the inhibitory effect of NaCl on the rate of CO₂ fixation. The leaf Na⁺ and Cl^– content decreased in ABA-pretreated plants. Both ABA and NaCl treatments led to an increase in the endogenous level of ABA in the plant leaves. Patterns of total proteins extracted from the leaves of control or ABA- and salt-treated plants were compared. Both ABA and NaCl induced marked quantitative and qualitative changes in the polypeptide profiles concerning mainly the proteins with approximately equal mobility. The results are discussed in terms of a possible role of ABA in increasing the salt tolerance when ABA is applied to the plants for a short period before exposure to salinity stress, thus improving the invulnerability to unfavorable conditions.Abbreviations RuBPC ribulose-1,5-bisphosphate carboxylase - PSII photosystem II - ABA abscisic acid - PEPC phosphoenolpyruvate carboxylase - DTTr dithiothreitol - BSA bovine serum albumin - ELISA enzyme-linked immunosorbent assay - SDS sodium dodecyl sulfate - PAGEr polyacrylamide gel electrophoresis     

Whole Leaf Carbon Exchange Characteristics of Phosphate Deficient Soybeans (Glycine max L.)

Low phosphate nutrition results in increased chlorophyll fluorescence, reduced photosynthetic rate, accumulation of starch and sucrose in leaves, and low crop yields. This study investigated physiological responses of soybean (Glycine max [L.] Merr.) leaves to low inorganic phosphate (Pi) conditions. Responses of photosynthesis to light and CO₂ were examined for leaves of soybean grown at high (0.50 millimolar) or low (0.05 millimolar) Pi. Leaves of low Pi plants exhibited paraheliotropic orientation on bright sunny days rather than the normal diaheliotropic orientation exhibited by leaves of high Pi soybeans. Leaves of plants grown at high Pi had significantly higher light saturation points (1000 versus 630 micromole photons [400-700 nanometers] per square meter per second) and higher apparent quantum efficiency (0.062 versus 0.044 mole CO₂ per mole photons) at ambient (34 pascals) CO₂ than did low Pi leaves, yet stomatal conductances were similar. High Pi leaves also had significantly higher carboxylation efficiency (2.90 versus 0.49 micromole CO₂ per square meter per second per pascal), a lower CO₂ compensation point (6.9 versus 11.9 pascals), and a higher photosynthetic rate at 34 pascals CO₂ (19.5 versus 6.7 micromoles CO₂ per square meter per second) than did low Pi leaves. Soluble protein (0.94 versus 0.73 milligram per square centimeter), ribulose-1,5-bisphosphate carboxylase/oxygenase content (0.33 versus 0.25 milligram per square centimeter), and ribulose-1,5-bisphosphate carboxylase/oxygenase specific activity (25.0 versus 16.7 micromoles per square meter per second) were significantly greater in leaves of plants in the high Pi treatment. The data indicate that Pi stress alters the plant's CO₂ reduction characteristics, which may in turn affect the plant's capacity to accommodate normal radiation loads.     

Suboptimal nitrogen status sensitizes the photosynthetic apparatus in willow leaves to long term but not short term water stress

The response to drought was compared for willow plants of optimal leaf nitrogen content (100 N) and those of 86% of this content (86 N). Gas exchange measurements revealed that the carboxylation efficiency (CE) of photosynthesis was more sensitive to drought than the photosynthetic capacity in both N regimes. Since the leaf content of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was found to be much more resistant it is suggested that a decreased specific activity of Rubisco underlies the decreased CE. Although the rate of water consumption was the same for 86 N and 100 N plants the photosynthetic apparatus responded much more rapidly in the 86 N leaves. This increased sensitivity of 86 N leaves was not due to accelerated senescence as judged by comparison with parallel plants subjected to discontinued fertilization; the two categories of treatments resulted in the same loss of leaf nitrogen and Rubisco but drought induced a much more rapid photosynthetic depression. In contrast to the drought situation, 86 N and 100 N plants behaved similarly when compared under short term water stress. First, when single attached leaves were exposed to a sudden drop in air humidity the capacity of CO₂ uptake in both N regimes decreased about 20% over 10 min while the leaf water potential remained high. Second, in freely transpiring leaf discs cut from 86 N and 100 N leaves the same relationship between capacity of O₂ evolution and extent of dehydration was observed. The possible mechanisms underlying the increased susceptibility of 86 N leaves to drought is discussed; the water status of the roots not the leaves is suggested to be the determining factor.Abbreviations CE carboxylation efficiency - 100 N optimal nitrogen regime - 86 N suboptimal nitrogen regime with 86% of the optimal leaf nitrogen content, Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase     

Cold-hardening results in increased activity of enzymes involved in carbon metabolism in leaves of winter rye (Secale cereale L.)

Light- and CO₂-saturated photosynthesis of nonhardened rye (Secale cereale L. cv. Musketeer) was reduced from 18.10 to 7.17 mol O₂·m^–2·s^–1 when leaves were transferred from 20 to 5°C for 30 min. Following cold-hardening at 5°C for ten weeks, photosynthesis recovered to 15.05 mol O₂·m^–2·s^–1,comparable to the nonhardened rate at 20°C. Recovery of photosynthesis was associated with increases in the total activity and activation of enzymes of the photosynthetic carbon-reduction cycle and of sucrose synthesis. The total hexose-phosphate pool increase by 30% and 120% for nonhardened and cold-hardened leaves respectively when measured at 5°C. The large increase in esterified phosphate in coldhardened leaves occurred without a limitation in inorganic phosphate supply. In contrast, the much smaller increase in esterified phosphate in nonhardened leaves was associated with an inhibition of ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose-phosphate synthase activation. It is suggested that the large increases in hexose phosphates in cold-hardened leaves compensates for the higher substrate threshold concentrations needed for enzyme activation at low temperatures. High substrate concentrations could also compensate for the kinetic limitations imposed by product inhibition from the accumulation of sucrose at 5°C. Nonhardened leaves appear to be unable to compensate in this fashion due to an inadequate supply of inorganic phosphate.Abbreviations DHAP dihydroxyacetone phosphate - Fru6P fructose-6-phosphate - Fru 1,6BP fructose-1,6-bisphosphate - Fru1,6BPase fructose-1,6-bisphosphatase - Glc6P glucose-6-phosphate - PGA 3-phosphoglycerate - PPFD photosynthetic photon flux density - CH cold-hardened rye grown at 5°C - NH nonhardened rye grown at 24°C - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - SPS sucrose-phosphate synthase - UDPGlc uridine 5-diphosphoglucose This work was supported by operating grants from the Swedish Natural Sciences Research Council to G.Ö. and P.G.