Heat Tolerance in Cowpea (Vigna unguiculata [L.] Walp.) Is Associated With Coordinated Canopy Cooling and Reproductive Stability Under Field Conditions

Abstract

Elevated temperature during the reproductive phase severely constrains cowpea (Vigna unguiculata (L.) Walp.) productivity; however, the physiological and reproductive traits associated with heat tolerance under field conditions remain inadequately understood. In this study, 30 cowpea genotypes were evaluated across two locations and two sowing environments to impose terminal heat stress under field conditions. Heat stress significantly reduced canopy greenness (~20%), chlorophyll content (~26%), leaf area (~30%), net photosynthetic rate (~60%), pollen viability (15%–30%) and grain yield (~11%). Results revealed that maintenance of canopy function, reflected by higher normalized difference vegetation index (NDVI) and SPAD values together with lower canopy temperature, was associated with improved grain yield under heat stress. Physiological measurements conducted at the Baramati location further indicated that genotypes maintaining higher photosynthetic activity and pollen viability exhibited greater reproductive resilience under heat stress. Integrated multi-trait analyses, including the Multi-trait Genotype–Ideotype Distance Index (MGIDI) and Multi-trait Stability Index (MTSI), consistently identified EC240920, IC488270 and IC488085 as promising genotypes combining favourable physiological and agronomic traits. Overall, the results suggest that heat tolerance in cowpea is associated with coordinated canopy cooling and reproductive stability under field conditions. Although the physiological characterization was based on a single cropping season, the identified genotypes are consistent with previous multi-year field evaluations and represent promising genetic resources for breeding programs targeting high-temperature environments. Nevertheless, further multi-year physiological validation will strengthen confirmation of their stability under diverse environments.