Genotypic and Drought‐Induced Differences in Carbon Isotope Discrimination and Gas Exchange of Cowpea

Abstract

Carbon isotope composition may be useful for selection in plant breeding. Theory predicts that 13C discrimination (Δ) by leaves can be associated with the ratio of photosynthesis (as indicated by CO2 assimilation rate, A) to leaf conductance (g) to diffusion of water vapor or CO2. In earlier studies with a cowpea (Vigna unguiculata [L.] Walp.) mutant and parent, drought‐induced differences in Δ were associated with expected differences in A/g based on theory, whereas genotypic differences in Δ were not. Studies were conducted for 2 yr with a broader range of genotypes to determine whether these conclusions are generally valid. Eighteen cowpea accessions were grown under weekly irrigation or stored soil moisture in four randomized split blocks at Riverside, CA, in 1987. Twelve of these cowpea accessions were studied under similar conditions in 1988. Gas exchange measurements were made of A and leaf conductance to water vapor (gh), and leaves were sampled for Δ. Significant drought‐induced reduction in Δ of 1.6 × 10−3 was observed and an increase in A/g of 42% which is similar to the value (36%) predicted based upon the change in Δ. The drought‐induced increase in A/g was due to substantial decreases in gh, which more than compensated for a 59% reduction in A. Genotypic differences in Δ of up to 2 × 10−3 were observed which were predicted to be associated with 43% higher A/g; however, genotypic differences in A/g were small, and genotypic means of A/g and Δ were not associated as expected based on theory. Genotypic differences in Δ were more consistent than differences in A/g, A, orgh, and should be easier to select in breeding, but the physiological basis for the genotypic differences in Δ has not been elucidated for cowpea.