Genotype × environment interaction in carbon isotope discrimination and seed yield in cowpea (Vigna unguiculata L. Walp.)

Summary

Twenty locally-adapted but genetically diverse cowpea genotypes were grown in 17 replicated trials in a range of sites and seasons in the drylands of eastern Kenya, and the discrimination against the heavy isotope of carbon (with a mass of 13) (Δ) was determined in grain and straw samples. Most genotypes gave similar, low grain yields in the environments which subjected the plants to drought stress, but high yielding and low yielding genotypes were clearly distinct in the less stressful environments. The converse pattern was found for Δ: the genotypes were all very similar in the non-stressful (high-Δ) environments, but in the stressful environments, low-Δ (drought susceptible) and high-Δ (drought avoiding) genotypes were clearly distinct. This pattern was clearest for straw Δ, but was also apparent for grain Δ. The early-maturing genotypes, which escaped terminal drought, were generally those with the most stable and highest Δ, but in the case of straw Δ there were statistically significant deviations from this relationship. It is concluded that Δ provides a reliable measure of the specific response of cowpea genotypes to drought, which may be of value in breeding programs provided that carbon isotope discrimination can be measured reasonably economically.