Flowering of contrasting cowpea (Vigna unguiculata (L.) walp.) genotypes under different temperatures and photoperiods

Abstract

Abstract The practical objective of this research was to develop a field screening technique that reliably detects genotypes which flower early and are insensitive to photoperiod (day-neutral). In pursuing this objective, the number of days to the first appearance of floral buds and open flowers was studied in two field environments and in controlled environments with eight contrasting cowpea (Vigna unguiculata (L.) Walp.) genotypes. The summer field environments had decreasing long days (15-13 h) and either high (Riverside, CA) or extremely high (Imperial Valley, CA) temperatures. The controlled environments included two daylengths (short, 11.5 h, or long, 15 h) and different day/night temperatures (30/20°C, 30/27°C, 30°C for 15 h/27°C, 36°C for 11.5 h/27°C) in eight different combinations. The flowering responses observed in the controlled environments were consistent with the responses observed in the field environments. Environmental control of flowering of four of the genotypes consisted of a strong photoperiod × temperature interaction. The number of days to first flower was not influenced by daylength at moderate temperatures but was delayed by long days at high temperatures, and with the combination of long days and very hot environments flowers were not produced. When flowering was delayed, the flowers first appeared on later nodes. Delays in flowering were mainly due to delays in first appearance of floral buds, except at extremely high temperatures where bud development was inhibited as well. Extending the daylength with low intensity light did not influence the number of days to first appearance of floral buds but it slowed down bud development. This indicated that bud initiation and bud development may be regulated differently by environment and that regulation of bud development may involve a classical photoperiod response. The other four genotypes were day-neutral and days to first flower exhibited an optimum response to temperature, with earliest flowering occurring at intermediate temperatures. For the same genotypes in different environments, floral buds first appeared at the same node and the first flowers were produced on the following node. A screening procedure is proposed for detecting cowpea genotypes that are early and day-neutral. This simply consists of screening under hot, long-day field conditions, such as occur in Imperial Valley, California, in the summer. Cowpea genotypes that flower early in this environment would probably flower early, compared with other cowpeas, in most cowpea production environments in the world.