Genotypic Variation in Carbon Isotope Discrimination and Transpiration Efficiency in Wheat. Leaf Gas Exchange and Whole Plant Studies

The relationship between carbon isotope discrimination, A, measured in plant dry matter and the ratio of intercellular to atmospheric partial pressures of C02, pi/p,, in leaves was examined in two glass-house experiments using 14 wheat genotypes selected on the basis of variation in A of dry matter. Genotypic variation in A was similar in both experiments, with an average range of 1.8 x 10W3. Variation in p,/p, was significant but the range in pi/pa was relatively small, averaging 0.075. In both experiments, A measured in dry matter and pi /p, measured in flag leaves were positively correlated. Variation among genotypes in pi/pa was attributed, approximately equally, to variation in leaf con-ductance and in photosynthetic capacity. The relationship between plant transpiration efficiency, W * (the amount of above-ground dry matter produced per unit water transpired) and A was also examined. There was a negative correlation between W * and A under well watered conditions and under gradually increasing terminal water stress. The relationship between W * of stressed plants and A measured in well watered plants was also negative. These results indicate that genotypic variation in A measured in dry matter should provide