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Carbon isotope discrimination (Av) was analyzed in leaf starch and soluble sugars, which represent most of the recently fixed carbon. Plants of three C3 species (Populus nigra L. x P. deltoides Marsh., Gossypium hirsutum L. and Phaseolus vulgaris L.) were kept in the dark for 24 hours to decrease contents of starch and sugar in leaves. Then gas exchange measurements were made with constant conditions for 8 hours, and subsequently starch and soluble suprs were extracted for analysis of carbon isotope composition. The ratio of intercellular, pi, and atmospheric, p., partial pressures of C02, was calculated from gas exchange measurements, integrated over time and weighted by assimilation rate, for comparison with the carbon isotope ratios in soluble sugars and starch. Carbon isotope discrimination in soluble sugars correlated strongly (r = 0.93) with pdp. in all species, as did A in leaf starch (r = 0.84). Starch was found to contain significantly more 13C than soluble sugar, and possible explanations are discussed. The strong correlation found between A and p/p. suggests that carbon isotope analysis in leaf starch and soluble sugars may be used for monitoring, indirectly, the average of pip, weighted by C02 assimilation rate, over a day. Because p,/p. has a negative correlation with transpiration efficiency (mol C02/ mol H20) of isolated plants, A in starch and sugars may be used to predict differences in this efficiency. This new method may be useful in ecophysiological studies and in selection for improved transpiration efficiency in breeding programs for C3 species.During photosynthetic CO2 fixation, plants discriminate against the naturally occurring stable isotope 13C. In C3 species, fractionation ofcarbon in plant material is caused by the primary carboxylating enzyme, ribulose-1,5-bisphosphate carboxylase, which discriminates against 13C (24) and by diffusion from the atmosphere to the sites of CO2 fixation (12,23,28). Farquhar et al. (12) developed a model which predicts a linear relationship between A2 and the ratio of intercellular (pi) to atmospheric (pa) partial pressure of CO2 for C3 plants.

We analyzed antioxidative defenses, photosynthesis, and pigments (especially xanthophyll-cycle components) in two wheat (Triticum durum Desf.) cultivars, Adamello and Ofanto, during dehydration and rehydration to determine the difference in their sensitivities to drought and to elucidate the role of different protective mechanisms against oxidative stress. Drought caused a more pronounced inhibition in growth and photosynthetic rates in the more sensitive cv Adamello compared with the relatively tolerant cv Ofanto. During dehydration the glutathione content decreased in both wheat cultivars, but only cv Adamello showed a significant increase in glutathione reductase and hydrogen peroxideglutathione peroxidase activities. The activation states of two sulfhydryl-containing chloroplast enzymes, NADP ؉ -dependent glyceraldehyde-3-phosphate dehydrogenase and fructose-1,6-bisphosphatase, were maintained at control levels during dehydration and rehydration in both cultivars. This indicates that the defense systems involved are efficient in the protection of sulfhydryl groups against oxidation. Drought did not cause significant effects on lipid peroxidation. Upon dehydration, a decline in chlorophyll a, lutein, neoxanthin, and ␤-carotene contents, and an increase in the pool of de-epoxidized xanthophyll-cycle components (i.e. zeaxanthin and antheraxanthin), were evident only in cv Adamello. Accordingly, after exposure to drought, cv Adamello showed a larger reduction in the actual photosystem II photochemical efficiency and a higher increase in nonradiative energy dissipation than cv Ofanto. Although differences in zeaxanthin content were not sufficient to explain the difference in drought tolerance between the two cultivars, zeaxanthin formation may be relevant in avoiding irreversible damage to photosystem II in the more sensitive cultivar.

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The effects of salinity on growth, stomatal conductance, photosynthetic capacity, and carbon isotope discrimination (A) of Gossypium hirsutum L. and Phaseolus vulgaris L. were evaluated. Plants were grown at different NaCI concentrations from 10 days old until mature reproductive structures were formed. Plant growth and leaf area development were strongly reduced by salinity, in both cotton and bean. Stomatal conductance also was reduced by salinity. The A always declined with increasing external salinity concentration, indicating that stomatal limitation of photosynthesis was increased. In cotton plant dry matter, A correlated with the ratio of intercellular to atmospheric CO2 partial pressures (pulpa) calculated by gas exchange. This correlation was not clear in bean plants, although A showed a more pronounced salt induced decline in bean than in cotton. Possible effects of heterogeneity of stomatal aperture and consequent overestimation of pi as determined from gas exchange could explain these results. Significant differences of A between leaf and seed material were observed in cotton and bean. This suggests different pattems of carbon allocation between leaves and seeds. The photon yield of 02 evolution determined at rate-limiting photosynthetic photon flux density was insensitive to salinity in both species analyzed. The light-and C02-saturated rate of CO2 uptake and 02 evolution showed a salt induced decline in both species. Possible explanations of this observation are discussed.02 hypersensitivity was observed in salt stressed cotton plants.These results clearly demonstrate that the effect of salinity on assimilation rate was mostly due to the reduction of stomatal conductance, and that calculation of pu may be overestimated in salt stressed plants, because of heterogeneity of stomatal aperture over the leaf surface.Salinity causes large effects on higher plants, both halophytes and non-halophytes. In the latter, growth rate is generally reduced by salinity even at low salt concentration. However, within non-halophytes there is still large variability among species, ranging from extremely sensitive to tolerant species overlapping with halophytes (18). The nutrition, stomatal behavior, photosynthetic efficiency, carbon allocation, and utilization (17,18,20).The rate of photosynthetic CO2 assimilation is generally reduced by salinity. This reduction is partly due to a reduced stomatal conductance (7, 12, 23) and consequent restriction of the availability of CO2 for carboxylation.Nonstomatal inhibition of photosynthesis, caused by direct effects of NaCl on photosynthetic apparatus independent of stomatal closure, has also been reported for several plant species, both halophytes and non-halophytes (1,23,24). This inhibition of photosynthetic capacity has been attributed to a reduced efficiency of RuBP2 carboxylase when RuBP is in limiting supply (24), to a reduction of RuBP regeneration capacity (1, 24), or to the sensitivity of PSII to NaCl (2).Recently, it has been argued (14, 26) that some ofthe...