Carbon isotope discrimination (Delta), growth analysis, water-use efficiency (WUE) and gas exchange characteristics were studied in rice plants (Oryza sativa L.) subjected to drought during different developmental stages. Drought caused major effects on growth, WUE, Delta and photosynthetic CO2 assimilation. Substantial differences in the Delta of the bulk biomass among different organs and in carbohydrates extracted from leaves and stems were observed. Possible influences of chemical composition, fractionation during translocation and seasonal changes in the ratio of intercellular and atmospheric partial pressures of CO2 On such differences in Delta are discussed. Stem carbohydrate Delta was correlated with relative growth rate, and, during early grain filling, was negatively correlated with WUE measured between flowering and early grain filling. Delta in leaf sugars was used to estimate mesophyll conductance (g(m)), the conductance to CO2 diffusion inside leaves, from the intercellular air spaces to the chloroplast. During ontogeny, g(m) showed a marked progressive decrease, evident in both droughted plants and fully irrigated controls. There was a positive correlation between the rate of CO2 assimilation and g(m). The analysis of Delta in leaf and stem carbohydrates is proposed as a useful indicator of growth, WUE and photosynthetic parameters relevant for yield of rice under drought-prone conditions.
