HEAT INJURY DURING EARLY FLORAL BUD DEVELOPMENT IN COWPEA

Heat stress during the vegetative and early reproductive stages induces floral bud abortion in cowpea [Vigna unguiculata (L.) Walp.] and several other crop species growing under long days, and can substantially reduce productivity. The objective of this study was to determine whether there is a specific developmental stage when heat stress causes abortion of floral buds. Cowpea plants were grown in four reach-in growth chambers under either metal halide-based (MH) or fluorescent-based (FL) lighting systems with 14-h photoperiod and thermoperiod. Heat injury to floral bud development was quantified as floral bud suppression; the percentage of peduncles on the first five reproductive nodes that did not produce at least one flower. In all cases, floral buds were initiated on these nodes. Light quality influenced plant responses, but similar responses to temperature were obtained when red/far red ratios were 1.3 for the MH system and 1.6 for the FL system. High night temperature (30-degrees-C) caused complete suppression of floral buds, whereas development was normal at a night temperature of 20-degrees-C. Transfer and heat-pulse experiments demonstrated that plants were not sensitive to high night temperature at a particular stage of development but that the duration of heat experience may be critical for the suppression of floral bud development. Two weeks or more of consecutive or interrupted hot nights during the first 4 wk after germination caused complete suppression of the development of the first five floral buds on the main stem. Mechanisms for the effects of heat stress on floral bud development are discussed.