Consistency of genotypic ranking for carbon isotope discrimination by cowpea grown in tropical and subtropical zones

Abstract

Abstract Measurements of plant composition of stable carbon isotopes (13C/12C) can be used to estimate the extent that plants discriminate (Δ) against the heavier carbon isotope. Theoretical and experimental studies have shown that Δ can be negatively correlated with transpiration efficiency (W = seasonal total biomass production/seasonal transpiration) in C3 plants. Consequently, selection for low Δ (and high W) may be useful in breeding C3 plants for some water-limited environments. Development of effective breeding methods depends upon the extent of genotype × environment interaction (G × E) as it influences the consistency of genotypic ranking across environments. Sets of cowpea cultivars and lines were grown under contrasting water supply regimes in subtropical zones in Riverside, California, and Lubbock, Texas, and in tropical zones in Senegal. Leaf carbon isotope composition was measured and significant genotypic differences in Δ were observed in most trials with little G × E for trials within these regions. Genotypic rankings were quite consistent between wetter and drier environments and different years within these regions with significant correlation coefficients for genotypic means in most cases. However, when comparing Δ values between the two subtropical zones (California versus Texas), correlation coefficients for genotypic means only were moderate and not significant. Comparisons of Δ values between the tropical zone (Senegal) and the two subtropical zones indicated no consistency in genotypic ranking, and correlation coefficients for genotypic comparisons were very small. Apparently, G × E for Δ would not necessarily constrain cowpea breeding programs which aim at developing improved cultivars for specific target production regions, such as semiarid tropical Senegal. However, cowpea performance, with respect to Δ and W, would not be transferable to radically different production zones where attainment of high W may require different sets of genes.