CONSISTENCY OF GENOTYPIC RANKING FOR CARBON-ISOTOPE DISCRIMINATION BYCOWPEA GROWN IN TROPICAL AND SUBTROPICAL ZONES

Measurements of plant composition of stable carbon isotopes (C-13/C-12 ) can be used to estimate the extent that plants discriminate (DELTA) against the heavier carbon isotope. Theoretical and experimental studi es have shown that DELTA can be negatively correlated with transpirati on efficiency (W = seasonal total biomass production/seasonal transpir ation) in C3 Plants. Consequently, selection for low DELTA (and high W ) may be useful in breeding C3 plants for some water-limited environme nts. Development of effective breeding methods depends upon the extent of genotype X environment interaction (G X E) as it influences the co nsistency of genotypic ranking across environments. Sets of cowpea cul tivars and lines were grown under contrasting water supply regimes in subtropical zones in Riverside, California, and Lubnock, Texas, and in tropical zones in Senegal. Leaf carbon isotope composition was measur ed and significant genotypic differences in DELTA were observed in mos t trials with little G X E for trials within these regions. Genotypic rankings were quite consistent between wetter and drier environments a nd different years within these regions with significant correlation c oefficients for genotypic means in most cases. However, when comparing DELTA values between the two subtropical zones (California versus Tex as), correlation coefficients for genotypic means only were moderate a nd not significant. Comparisons of DELTA values between the tropical z one (Senegal) and the two subtropical zones indicated no consistency i n genotypic ranking, and correlation coefficients for genotypic compar isons were very small. Apparently, G X E for DELTA would not necessari ly constrain cowpea breeding programs which aim at developing improved cultivars for specific target production regions, such as semiarid tr opical Senegal. However, cowpea performance, with respect to DELTA and W, would not be transferable to radically different production zones where attainment of high W may require different sets of genes.