Genotypic Variation and Classification of Cowpea for Reproductive Responses to High Temperature under Long Photoperiods

Hot weather can substantially reduce pod production by cowpea [Vigna unguiculata (L.) Walp.]. The objectives were to characterize and classify cowpea based on their responses to heat during reproductive development. Diverse cowpea accessions were grown over six summers in Imperial Valley, CA under hot temperatures (mean daily max/min 41/24 °C) and long days (14 h, 51 min decreasing to 12 h, 18 min). Selected genotypes were grown at Riverside, CA, under more optimal temperatures (35/17 °C) and the same daylengths. At Imperial Valley, genotypic variation was observed for duration to first macroscopic floral bud and extent of floral bud abortion, peduncle elongation, flower production, and podding. At Riverside, similar genotypic variation occurred for days to first floral bud, but subsequent reproductive development was normal. Controlled environment studies in growth chambers showed that the detrimental effects induced in sensitive genotypes by high temperature were enhanced by long photoperiods. Based on these differing reproductive responses, the accessions were classified into eight groups. Heat-tolerant accessions in Group I exhibited normal peduncle elongation, early flowering, and produced many flowers and pods. Accessions in Groups II and III performed similarly but set few pods (Group II) or no pods (Group III). Accessions in Groups IV through VII differed in duration to first floral bud, with Group VII being the latest and exhibiting substantial bud abortion, suppressed peduncle elongation, and no flowers. Group VIII accessions produced no visible floral buds. Cowpea developed in different regions and for specific seasons within a region fell into discrete classification groups, indicating that the system has a genetic and ecological basis. This classification system is useful for characterizing the types of heat responses present in cowpea genotypes and for selecting appropriate combinations of parents for breeding heat-tolerant cultivars for different production zones and seasons. This research was partially supported by the Bean-Cowpea CRSP, USAID grant no. MSU-AID-DAN-1310-G-SS-6008-00 and the Blackeye Varietal Council of California. The opinios and recommendations are those of the authors and not necessarily those of USAID.