Responses of cowpea (Vigna unguiculata (L.) Walp.) in the field to high night air temperature during flowering. II. Plant responses

Abstract

The agronomic responses of a heat-sensitive (CB5) and two heat-tolerant (TVu4552 and Prima) cowpea strains to different levels of night air temperature during flowering were evaluated. An experimental field system at Riverside, California, was used to subject large plots of the contrasting genotypes to different controlled levels of warmed air at night. Differential thermostats were used to raise air temperatures in nighttime enclosures to simulate differential tropical and subtropical locations where cowpea are commercially grown. In 1982, CB5 and TVu4552 were subjected to five treatments in which average daily minimum temperature ( T min ) was 15.0, 16.5, 20.0, 23.0, and 26.5°C for the 4-week period of flowering and pod-fill. In 1983, CB5 and Prima were subjected to four treatments in which T min was 16.5, 22.0, 24.0, and 25.5°C for the 2-week period when most flowering occurred. During the day, plots were not enclosed and average daily maximum temperature during the treatment period was 33.5°C in 1982 and 34.5°C in 1983. In both years, percent flower abscission of CB5 was significantly and substantially increased by all higher night air temperature treatments. Percent flower abscission of TVu4552 and Prima was only significantly increased when mean T min was 24.0°C or higher. TVu4552 and Prima also exhibited lower absolute levels of flower abscission, more pods per plant, and smaller relative decreases in pods per plant than CB5 with higher night air temperatures during flowering. Increase in flower abscission was the primary factor responsible for reductions in the number of pods per plant in all three genotypes. Reduced pot setting ability was the major cause of reductions in grain yield under higher night air temperatures, while reductions in seed weight and the number of seeds per pod were small. It should be noted that during 1983, plants were only subjected to higher night temperatures during the 2 weeks of flowering and plants experienced the same environmental conditions during a major part of the period when seeds were growing. TVu4552 and Prima exhibited less reduction in seed yield from warm air at night in relation to ambient controls, than did CB5 which was strongly affected by high night air temperature during flowering.