Photorespiration results from the oxygenase reaction catalysed by ribulose-1,5-bisphosphate carboxylase/oxygenase. In this reaction glycollate-2-phosphate is produced and subsequently metabolized in the photorespiratory pathway to form the Calvin cycle intermediate glycerate-3-phosphate. During this metabolic process, CO2 and NH3 are produced and ATP and reducing equivalents are consumed, thus making photorespiration a wasteful process. However, precisely because of this inefficiency, photorespiration could serve as an energy sink preventing the overreduction of the photosynthetic electron transport chain and photoinhibition, especially under stress conditions that lead to reduced rates of photosynthetic CO2 assimilation. Furthermore, photorespiration provides metabolites for other metabolic processes, e.g. glycine for the synthesis of glutathione, which is also involved in stress protection. In this review we describe the use of photorespiratory mutants to study the control and regulation of photorespiratory pathways. In addition, we discuss the possible role of photorespiration under stress conditions, such as drought, high salt concentrations and high light intensities encountered by alpine plants.

The effect of gradually-developing water-stress has been studied in Lupinus albus L., Helianthus annuus L., Vitis vinifera cv. Rosaki and Eucalyptus globulus Labill. Water was withheld and diurnal rhythms were investigated 4-8 d later, when the predawn water deficit was more negative than in watered plants, and the stomata closed almost completely early during the photoperiod. The contribution of 'stomatal' and 'non-stomatal' components to the decrease of photosynthetic rate was investigated by (1) comparing the changes of the rate of photosynthesis in air with the changes of stomatal conductance and (2) measuring photosynthetic capacity in saturating irradiance and 15% CO2. Three species (lupin, eucalyptus and sunflower) showed larger changes of stomatal conductance than photosynthesis in air, and showed little or no decrease of photosynthetic capacity in saturating CO2. Photosynthesis in air also recovered fully overnight after watering the plants in the evening. In grapevines, stomatal conductance and photosynthesis in air changed in parallel, there was a marked decrease of photosynthetic capacity, and photosynthesis and stomatal conductance did not recover overnight after watering water-stressed plants. Relative water content remained above 90% in grapevine. We conclude that non-stomatai components do not play a significant role in lupins, sunflower or eucalyptus, but could in grapevine. The effect of water-stress on partitioning of photosynthate was investigated by measuring the amounts of sucrose and starch in leaves during a diurnal rhythm, and by measuring the partitioning of "C-carbon dioxide between sucrose and starch. In all four species, starch was depleted in water-stressed leaves but sucrose was maintained at amounts similar to, or higher than, those in watered plants. Partitioning into sucrose was increased in lupins and eucalyptus, and remained unchanged in grapevine and sunflower. It is concluded that water-stressed leaves in all four species maintain high levels of soluble sugars in their Correspotidetiee: R. C. Leegood, Robert Hitt histitute. Departinent ofAtiimal and Platit .Scietiees, University of Sheffield, Sheffield SW 2TN, UK.leaves, despite having lower rates of field photosynthesis, decreased rates of export, and low amounts of starch in their leaves. in the intracellular spaces in the leaf; D, water vapour pressure deficit between the air and the leaf; DW, dry weight; FW, fresh weight; gs, stomatal conductance; RWC, relative water content; Tr, transpiration rate; ip, water potential.

The aim of this study was to determine the response of photosynthetic carbon metabolism in spinach and bean to low temperature. (a) Exposure of warm-grown spinach and bean plants to 10°C for 10 days resulted in increases in the total activities of a number of enzymes, including ribulose 1,5-bisphosphate carboxylase (Rubisco), stromal fructose 1,6 bisphosphatase (Fru 1,6-P2ase), sedoheptulose 1,7-bisphosphatase (Sed 1,7-P2ase), and the cytosolic Fru 1,6-P2ase. In spinach, but not bean, there was an increase in the total activity of sucrose-phosphate synthase. (b) The C02-saturated rates of photosynthesis for the coldacclimated spinach plants were 68% greater at 100C than those for warm-acclimated plants, whereas in bean, rates of photosynthesis at 10°C were very low after exposure to low temperature. (c) When spinach leaf discs were transferred from 27 to 100C, the stromal Fru 1,6-P2ase and NADP-malate dehydrogenase were almost fully activated within 8 minutes, and Rubisco reached 90% of full activation within 15 minutes of transfer. An initial restriction of Calvin cycle fluxes was evident as an increase in the amounts of ribulose 1,5-bisphosphate, glycerate-3-phosphate, Fru 1,6-P2, and Sed 1,7-P2. In bean, activation of stromal Fru 1,6-P2ase was weak, whereas the activation state of Rubisco decreased during the first few minutes after transfer to low temperature. However, NADP-malate dehydrogenase became almost fully activated, showing that no loss of the capacity for reductive activation occurred. (d) Temperature compensation in spinach evidently involves increases in the capacities of a range of enzymes, achieved in the short term by an increase in activation state, whereas long-term acclimation is achieved by an increase in the maximum activities of enzymes. The inability of bean to activate fully certain Calvin cycle enzymes and sucrose-phosphate synthase, or to increase nonphotochemical quenching of chlorophyll fluorescence at 100C, may be factors contributing to its poor performance at low temperature. complete in evergreen woody species that are subject to large seasonal variations in temperature, such as Eucalyptus species and the desert evergreen, Nerium oleander. For such plants acclimated to low temperature, temperature response curves for photosynthesis indicate an increased photosynthetic capacity over a wide range of temperatures (2, 7). The increases in photosynthetic capacity that result from acclimation to a lower growth temperature could be the result of a number of factors, as plants acclimating to low temperature show increases in, for example, soluble protein, the rate of electron transport, and in the activities of enzymes such as Rubisco and the stromal Fru 1,6-P2ase,2 which parallel the increase in photosynthetic capacity (2, 3).There are a number of other reports of increases in Rubisco at lower temperatures, for example, in the arctic-alpine species Oxyria digyna (5), in the C4 plant Atriplex lentiformis (24), and in the grass Dactylis glomerata (30). Gas-exchange studies also...

The significance of photorespiration in drought-stressed plants was studied by withholding water from wild-type barley (Hordeum vulgare L.) and from heterozygous mutants with reduced activities of chloroplastic glutamine synthetase (GS-2), glycine decarboxylase (GDC) or serine : glyoxylate aminotransferase (SGAT). Wellwatered plants of all four genotypes had identical rates of photosynthesis. Under moderate drought stress (leaf water potentials between -1 and -2 MPa), photosynthesis was lower in the mutants than in the wild type, indicating that photorespiration was increased under these conditions. Analysis of chlorophyll a fluorescence revealed that, in the GDC and SGAT mutants, the lower rates of photosynthesis coincided with a decreased quantum efficiency of photosystem II and increased non-photochemical dissipation of excitation energy. Correspondingly, the de-epoxidation state of xanthophyll-cycle carotenoids was increased several-fold in the drought-stressed GDC and SGAT mutants compared with the wild type. Accumulation of glycine in the GDC mutant was further evidence for increased photorespiration in droughtstressed barley. The effect of drought on the photorespiratory enzymes was determined by immunological detection of protein abundance. While the contents of GS-2 and Pand H-protein of the GDC complex remained unchanged as drought stress developed, the content of NADH-dependent hydroxypyruvate reductase increased. Enzymes of the Benson-Calvin cycle, on the other hand, were either not affected (ribulose-1,5-bisphosphate carboxylase-oxygenase and plastidic fructose-1,6-bisphosphatase) or declined (sedoheptulose-1,7-bisphosphatase and NADPdependent glyceraldehyde-3-phosphate dehydrogenase). These data demonstrate that photorespiration was enhanced during drought stress in barley and that the control exerted by photorespiratory enzymes on the rate of photosynthetic electron transport and CO 2 fixation was increased.Key-words: drought stress; glutamine synthetase; glycine decarboxylase; hydroxypyruvate reductase; mutants; photorespiration; photosynthesis; serine : glyoxylate aminotransferase; xanthophyll cycle.Abbreviations: C i , intercellular CO 2 concentration; F v /F m , quantum efficiency of excitation capture by open photosystem II centres; FBPase, fructose-1,6-bisphosphatase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GDC, glycine decarboxylase; GS-2, chloroplastic glutamine synthetase; HPR, hydroxypyruvate reductase; PFD, photon flux density; ΦCO 2 , quantum efficiency of CO 2 assimilation; ΦPSII, quantum efficiency of photosystem II electron transport; ψ, water potential; q N , non-photochemical chlorophyll a fluorescence quenching; q P , photochemical chlorophyll a fluorescence quenching; RuBP, ribulose-1,5-bisphosphate; Rubisco, ribulose-1,5-bisphosphate carboxylase-oxygenase; SBPase, sedoheptulose-1,7-bisphosphatase; SGAT, serine : glyoxylate aminotransferase. INTRODUCTIONDrought stress leads to a substantial reduction in the rate of photosynthetic CO 2 assimilation. Under mild to...

Marine diatoms are responsible for up to 20% of global CO 2 fixation. Their photosynthetic efficiency is enhanced by concentrating CO 2 around Rubisco, diminishing photorespiration, but the mechanism is yet to be resolved. Diatoms have been regarded as C 3 photosynthesizers, but recent metabolic labeling and genome sequencing data suggest that they perform C 4 photosynthesis. We studied the pathways of photosynthetic carbon assimilation in two diatoms by short-term metabolic 14 C labeling. In Thalassiosira weissflogii, both C3 (glycerate-P and triose-P) and C4 (mainly malate) compounds were major initial (2-5 s) products, whereas Thalassiosira pseudonana produced mainly C3 and C6 (hexose-P) compounds. The data provide evidence of C 3 -C 4 intermediate photosynthesis in T. weissflogii, but exclusively C 3 photosynthesis in T. pseudonana. The labeling patterns were the same for cells grown at near-ambient (380 mL L 21 ) and low (100 mL L 21 ) CO 2 concentrations. The lack of environmental modulation of carbon assimilatory pathways was supported in T. pseudonana by measurements of gene transcript and protein abundances of C 4 -metabolic enzymes (phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase) and Rubisco. This study suggests that the photosynthetic pathways of diatoms are diverse, and may involve combined CO 2 -concentrating mechanisms. Furthermore, it emphasizes the requirement for metabolic and functional genetic and enzymic analyses before accepting the presence of C 4 -metabolic enzymes as evidence for C 4 photosynthesis.

C(4) photosynthesis has a number of distinct properties that enable the capture of CO(2) and its concentration in the vicinity of Rubisco, so as to reduce the oxygenase activity of Rubisco, and hence the rate of photorespiration. The aim of this review is to discuss the properties of this CO(2)-concentrating mechanism, and thus to indicate the minimum requirements of any genetically-engineered system. In particular, the Kranz leaf anatomy of C(4) photosynthesis and the division of the C(4)-cycle between two cell types involves intercellular co-operation that requires modifications in regulation and transport to make C(4) photosynthesis work. Some examples of these modifications are discussed. Comparisons are made with the C(4)-type photosynthesis found in single-celled C(4)-type CO(2)-concentrating mechanisms, such as that found in the aquatic plant, Hydrilla verticillata and the single-celled C(4) system found in the terrestrial chenopod Borszczowia aralocaspica. The outcome of recent attempts to engineer C(4) enzymes into C(3) plants is discussed.

The photorespiratory pathway is described, together with the mutants which have been isolated for this metabolic pathway. Some of the key regulatory properties of the photorespiratory cycle are reviewed together with the regulatory interactions that might occur between photorespiration and other processes, such as the Benson-Calvin cycle and anaplerotic carbon flow into organic acids. These are discussed in relation to recent studies of the regulation and control of photorespiratory carbon and nitrogen metabolism in photorespiratory mutants, including possible mechanisms for the control of photosynthetic and photorespiratory carbon and nitrogen metabolism and of the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase in heterozygous barley mutants with reduced activities of glutamine synthetase or glutamate synthase.

We recently showed that maize (Zea mays L.) leaves contain appreciable amounts of phosphoenolpyruvate carboxykinase (PEPCK; R.P. Walker, R.M. Acheson, L.I. Técsi, R.C. Leegood [1997] Aust J Plant Physiol 24: 459-468). In the present study, we investigated the role of PEPCK in C 4 photosynthesis in maize. PEPCK activity and protein were enriched in extracts from bundle-sheath (BS) strands compared with whole-leaf extracts. Decarboxylation of [4-14 C]aspartate (Asp) by BS strands was dependent on the presence of 2-oxoglutarate and Mn 2؉ , was stimulated by ATP, was inhibited by the PEPCK-specific inhibitor 3-mercaptopicolinic acid, and was independent of illumination. The principal product of Asp metabolism was phosphoenolpyruvate, whereas pyruvate was a minor product. Decarboxylation of [4-14 C]malate was stimulated severalfold by Asp and 3-phosphoglycerate, was only slightly reduced in the absence of Mn 2؉ or in the presence of 3-mercaptopicolinic acid, and was light dependent. Our data show that decarboxylation of Asp and malate in BS cells of maize occurs via two different pathways: Whereas malate is mainly decarboxylated by NADPmalic enzyme, decarboxylation of Asp is dependent on the activity of PEPCK. C 4 plants have been classified as NADP-ME, NAD-ME, and PEPCK types, according to the major decarboxylase involved in the decarboxylation of C 4 acids in the BS cells Hatch et al., 1975). Maize (Zea mays) belongs to the NADP-ME subgroup of C 4 plants and possesses negligible activity of PEPCK, although suggested that some other NADP-ME species utilize PEPCK as an auxiliary decarboxylase. However, we showed previously that maize leaves contain large amounts of PEPCK (Walker et al., 1997), andFurumoto et al. (1999) identified a PEPCK gene from maize that is specifically expressed in BS cells. Other NADP-ME species such as Echinochloa colona, Echinochloa crus-galli, Digitaria sanguinalis, and Paspalum notatum also contain PEPCK, whereas PEPCK protein was not detectable in Sorghum bicolor, Saccharum officinarum, or Flaveria bidentis (Walker et al., 1997). The reason that the occurrence of PEPCK in these plants has been overlooked may be due to difficulties in measuring PEPCK activity (Walker et al., 1997) or to the lack of an antibody.PEPCK-type C 4 species mainly use Asp as the CO 2 donor and decarboxylate the oxaloacetate formed via the following reaction:In NADP-ME species such as maize, 14 CO 2 is initially incorporated into the C-4 position of malate and Asp (about 75% and 25%, respectively), and the C-4 of both is subsequently incorporated into other metabolites (Hatch, 1971; Morot-Gaudry and Farineau, 1978). Using isolated BS strands, Chapman and Hatch (1981) showed that BS cells of maize have a significant capacity to decarboxylate Asp, and they assumed that NADP-ME was responsible. In view of the occurrence of PEPCK in maize, the involvement of PEPCK in the decarboxylation of Asp appears more likely, because PEPCK can directly catalyze the decarboxylation of oxaloacetate formed from Asp without previous...