Comparison of selection for either leaf-electrolyte-leakage or pod set in enhancing heat tolerance and grain yield of cowpea

Cowpea is a warm-season adapted grain legume, however, moderately high night temperatures reduce pod set of most cultivars. Breeding for heat resistance, i.e. enhanced grain production under hot field conditions, has been achieved by selecting for reproductive-stage heat tolerance. This selection is not efficient because heritability of heat tolerance during pod set is low. We tested whether selection for slow leaf-electrolyte-leakage under heat stress may be useful in breeding heat-resistant cowpea cultivars. Reciprocal crosses were made between a heat-sensitive cultivar and a breeding line that has greater pod set and slower leaf-electrolyte-leakage in hot conditions and similar genetic background. Divergent selection for leaf-electrolyte-leakage under heat stress was imposed on 120 F-2 plants and again in the F-3 generation. Lines were advanced two generations by single-seed descent, producing 16 lines with slow and 16 lines with fast leaf-electrolyte-leakage. Another population was selected out of 600 F2 plants based on either abundant flower production and pod set or minimal flower production in a very hot, long-day field environment. Lines were advanced three generations by single-seed descent, producing 66 lines selected for heat tolerance during reproductive development and 40 heat-sensitive lines. The two populations were evaluated in F-6 and F-7 generations in three hot environments and a moderate-temperature environment. Selected F-8 lines were evaluated in a very hot field environment. Leaf-electrolyte-leakage had low to intermediate realized heritabilities indicating selection can be effective but will require several cycles to identify lines with leaf-electrolyte-leakage as slow as the slow parent. Slow leaf-electrolyte-leakage selections had high pod set, and high pod-set selections had slow leaf-electrolyte-leakage indicating that these traits are closely associated. Strong negative genetic correlations were observed between leaf-electrolyte-leakage and pod set in hot long-day environments. Both selection schemes enhanced pod set, harvest index and grain yield in hot long-day environments but not in a hot short-day glasshouse or a moderate-temperature long-day field environment. Leaf-electrolyte-leakage measured in three contrasting environments was negatively correlated with grain yield of cowpea grown in a very hot field environment. Under very hot conditions, grain yield was positively correlated with pod production, pod set and harvest index, and negatively correlated with vegetative shoot dry weight and plant height. Heat-resistant cowpea cultivars may be more efficiently bred by combining selection for slow leaf-electrolyte-leakage under heat stress in the fall and winter, using plants grown in moderate-temperature glasshouses, with selection for abundant flower production and pod set in the summer, using plants grown under very hot long-day conditions. (C) 2003 Elsevier B.V. All rights reserved.