Heat tolerance of contrasting cowpea lines in short and long days

Abstract

High-temperature damage to reproductive processes of cowpea ( Vigna unguiculata L. Walp.) occurs when minimum night air temperatures are greater than 20°C. The extent of damage is strongly influenced by photoperiod. Genes conferring heat tolerance under hot long-day environments have been identified in cowpea, and breeding lines with this trait have been developed. Genetic variability within cowpea germplasm for heat tolerance has not been assessed in short days. The objectives of this study were to determine if the heat-tolerant accessions and breeding lines developed from them under hot long-day conditions also exhibit heat tolerance under hot short-day conditions. In addition, we evaluated the heat tolerance of 48 lines that do not flower in hot long-day conditions because of sensitivity to heat or photoperiod. Three short-day experiments and one long-day experiment were conducted in greenhouses with average daily minimum and maximum air temperatures of 27 and 36°C, respectively. Simultaneously, moderate-temperature short-day experiments were conducted in nearby greenhouses with average daily minimum and maximum air temperatures of 18 and 32°C, respectively. Days to first flower, pods per peduncle, total number of pods, and grain yield were determined in all experiments. Individual seed weight and seeds per pod were determined for two of the three hot short-day and moderate-temperature short-day experiments. Heat-tolerant lines that had been selected under hot long-day conditions had larger than average grain yield (i.e. heat tolerance) under hot short days. Several lines bred for high grain yield under hot tropical conditions had heat tolerance under short days but not under long days, whereas many cultivars developed under cooler tropical conditions did not exhibit heat tolerance. In hot short-day environments, grain yield was positively correlated with pods per peduncle and number of pods per plant, indicating the importance of these traits for conferring heat tolerance in these lines. Most lines experienced substantial heat-induced reductions in number of seeds per pod, even though there were no strong indications of carbohydrate source limitations. Two of the heat-tolerant lines, B89-600 and TN88-63, did not exhibit heat-induced reductions in number of seeds per pod in hot short days, however, and represent additional sources of heat tolerance that would further enhance grain yield in hot production zones.