Heat tolerance in cowpea: effect of timing and duration of heat stress

Reproductive processes and pod yield in cowpea (Vigna unguiculata (L.) Walp), an important crop grown in semi-arid sub-Saharan Africa, are adversely affected by high temperature. Genotypic differences in heat tolerance have been identified under hot, long-days, but it was not known if this tolerance is also exhibited in hot, short-day environments typical of sub-Saharan Africa. The objectives of the work reported here were to determine whether heat tolerance identified under hot, long-days was expressed at the same stages of development under hot, short-days, and whether responsiveness to temperature was additive and quantitative. A heat-tolerant (Prima) and heat-susceptible (IT84S-2246) cultivar of cowpea were grown in controlled environments under short-days (12 h day(-1)), initially at 30 degrees C/24 degrees C (Mod-T), and then transferred at 0, 10, 20, 30 and 40 days after emergence (DAE) to 36 degrees C/27 degrees C (High-T), where they remained for 5, 10 or 20 days duration before returning to Mod-T. Control plants remained at Mod-T or High-T for 50 days, when the first pods were mature and the experiment was terminated. There were significant effects of duration (D) and timing (T) (P < 0.001), and interactions between D x T (P < 0.001), Tx genotype (G) (P < 0.01) and D x Tx G (P < 0.05) on pod weight plant(-1). Prima was significantly (P < 0.001) more tolerant to high temperature during flowering than IT84S-2246, confirming that heat tolerance was expressed under hot, short days. The greater heat tolerance of Prima was associated with an ability to maintain peduncle and flower production at High-T, and with greater podset. The sensitive period in IT84S-2246 started at floral bud initiation (15-20 DAE), and effects of High-T thereafter were additive and quantitative.