INHERITANCE OF HEAT TOLERANCE DURING POD SET IN COWPEA

The productivity of cowpea [Vigna unguiculata (L.) Walp.] can be reduced substantially by hot weather during flowering. Inheritance and heritability of tolerance to heat during reproductive development were studied to develop efficient methods for breeding improved cultivars. Heat tolerant ('Prima' and TVu 4552) and heat sensitive (Bambey 23, 'Magnolia' and 7964) cowpea genotypes were chosen for studies of heat tolerance during pod set under hot long days. Parental genotypes, F1, F2 and backcross progenies were evaluated under field conditions in the Imperial Valley of California during the summers of 1987 and 1988, and heat-tolerant and heat-sensitive F2's were selected in 1987 and evaluated as F3 families in 1988. Average daily minimum and maximum air temperatures were 24 and 43-degrees-C, respectively, and the photoperiod decreased from 14 h 31 min to 13 h 20 min during flowering. Individual plants were scored for average number of pods per peduncle and plants with greater-than-or-equal-to 1.5 (the mean of the value for the most tolerant parent and the average value for the sensitive parents) were classified as heat tolerant. Analyses of pods per peduncle and proportions of tolerant plants for F1, F2, and backcross populations indicated that heat tolerance is conferred by a single dominant gene in both Prima and TVu 4552. Narrow-sense heritability obtained from variance components was 0.26. Comparison of pods per peduncle of F2 plants with their F3 families from crosses between heat-tolerant and heat-sensitive parents gave a realized heritability of 0.27. The low values for heritability were due to large environmental effects on pods per peduncle. Incorporating heat tolerance during pod set into other genetic backgrounds will require family selection in advanced generations to ensure that the trait is fixed, and, to partially overcome selection difficulties caused by environmentally induced variation.