Carbon Isotope Discrimination Varies Genetically in C(4) Species

Carbon-isotope discrimination (Δ) is used to distinguish between different photosynthetic pathways. It has also been shown that variation in Δ occurs among varieties of C(3) species, but not as yet, in C(4) species. We now report that Δ also varies among genotypes of sorghum (Sorghum bicolor Moench), a C(4) species. The discrimination in leaves of field-grown plants of 12 diverse genotypes of sorghum was measured and compared with their grain yields. Discrimination varied significantly among genotypes, and there was a significant negative correlation between grain yield and Δ. The variation in Δ may be caused by genetic differences in either leakiness of the bundle-sheath cells or by differences in the ratio of assimilation rate to stomatal conductance. At the leaf level, the former should be related to light-use efficiency of carbon fixation and the latter should be related to transpiration efficiency. Both could relate to the yield of the crop