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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.

A negative correlation between water‐use efficiency (W), defined as the ratio of moles of carbon in the plant to moles of water transpired, and carbon isotope discrimination (Δ) was established for barley in pot experiments using 12 cultivars. The correlation was strong in two independent experiments in four different controlled environment where ambient temperature and vapour pressure deficit were varied and plants were either well‐watered or given limited amounts of water. Variation among cultivars was found in both Δ and W and rankings of both parameters, according to cultivar, were similar in different environments. Limiting water usually increased water‐use efficiency of plants. Total dry matter can be substituted for moles of carbon when calculating water‐use efficiency but the correlation between W and Δ were calculated using the carbon content of dry matter. There were differences varied significantly among cultivars. Despite these differences, correlations were also large between whole plant W and Δ of any of the plant parts. The amount of dry matter partitioned into reproductive growth varied genetically, as did the effect of stress on the partitioning. Growth, W and Δ of barley were compared with theory derived from gas exchange properties and with other literature. The effect on W of variation in vapour pressure deficit in these experiments was removed by multiplying W by vapour pressure deficit to derive the parameter, k(Pa mol C/mol H2O). This allowed comparisons among experiments with different vapour pressure deficits. The mean k for these barley cultivars was similar to that calculated by others for grasses. However, variation was found, and, in contrast with previous work which treats k as a species constant, we conclude that there is promise in selecting for increased k.