REPRODUCTIVE RESPONSES OF COWPEA TO HIGH-TEMPERATURE DURING DIFFERENT NIGHT PERIODS

High night temperatures cause excessive floral abscission in cowpea (Vigna unguiculata (L.) Walp.), while similar or even higher temperatures during the day do not impair reproduction. The objectives of this study were to determine if high temperatures during the early part of the night have different effects on cowpea pod set than high temperatures during the late part of the night, and if distinct events of pollen development are associated with the diurnal sensitivity to heat. Two genotypes of cowpea were grown in controlled environments under a 12-h photoperiod and subjected to high temperatures (33-degrees-C) during the day and either the first or second 6-h period of the night. Temperatures were optimal (24-degrees-C) during the other 6 of the night. All plants experienced the same heat units. Floral buds were sampled at different stages of development and the developing pollen were microscopically examined. For the two genotypes, high temperatures during the late-night period resulted in much lower pod set (7 and 20%) and pollen viability (2 and 35%) than high temperatures during the early-night period (51 and 76%, and 65 and 69%, respectively). No diurnal synchrony in pollen development was observed during the late-night period when developing pollen was the most sensitive to high night temperatures. No changes in floral morphology were observed that could impede pollen transfer from the anthers to the stigma and be responsible for the low pod set. We hypothesize that the results can be explained by the existence of a physiological process that is sensitive to heat and under circadian regulation.