Correlation between the carbon isotope discrimination in leaf starch and sugars of C₃ plants and the ratio of intercellular and atmospheric partial pressures of carbon dioxide

Carbon isotope discrimination (Δ) 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 CO₂, 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 p₁p_a. in all species, as did Δ in leaf starch (r = 0.84). Starch was found to contain significantly more¹³C than soluble sugar, and possible explanations are discussed. The strong correlation found between Δand p₁/p_a. suggests that carbon isotope analysis in leaf starch and soluble sugars may be used for monitoring, indirectly, the average of pip, weighted by CO2 assimilation rate, over a day. Because p₁/p_a. has a negative correlation with transpiration efficiency (mol CO₂/ mol H₂O) 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.