CARBON-ISOTOPE DISCRIMINATION AND GAS-EXCHANGE OF COWPEA ACCESSIONS AND HYBRIDS

Carbon isotope composition may be useful for selection in plant breedi ng. Theory predicts a negative association between C-13 discrimination (A) and the ratio of CO2 assimilation (A) to leaf diffusive conductan ce (g) in C3 plants. In cowpea [Vigna unguiculata (L.) Walp.] previous studies showed that the association between DELTA and gas exchange ef ficiency (A/g) agreed with theory for drought-induced effects, but not for genotypic effects when gas exchange was measured in the afternoon . Field studies were conducted for 2 yr to evaluate the association be tween DELTA and A/g measured during different time periods within the day. Four cowpea genotypes were grown either on weekly irrigation or s tored soil moisture in 1 yr, and under weekly irrigation, together wit h two hybrids between low and high DELTA parents, in the second year. Cultivars CB46 and Prima had higher leaf DELTA than genotypes UCR 237A and TVx 309-1G with different irrigation treatments, sampling dates a nd years. The hybrids had values of DELTA and gas exchange close to th eir midparent means. A negative correlation was observed between DELTA and A/g for both drought-induced and genotypic effects. Genotypic mea n values of DELTA were more strongly correlated with A/g when measured early in the morning than in the afternoon. Both A and leaf conductan ce to water vapor (g(h)) were significantly higher early in the mornin g and then decreased progressively towards the end of the day. Early m orning measurements were most effective in detecting genotypic differe nces in A/g, and they were associated with genotypic differences in DE LTA, as expected, based on theory, but with some unexplained variation . Low DELTA was associated with high photosynthetic rate for TVx 309-1 G, but low stomatal conductance for UCR 237A.