Multi-trait phenotyping under reproductive-stage heat stress reveals heritable reproductive and yield indicators of tolerance in common bean (Phaseolus vulgaris L.)
Abstract
Common bean (Phaseolus vulgaris L.) exhibits significant susceptibility to heat stress during its reproductive phase. However, the physiological and reproductive traits underlying heat tolerance remain inadequately characterised across diverse bean germplasm. In this study, 56 accessions were assessed under controlled heat stress conditions (32/22 °C day/night) and optimal conditions (25/15 °C) during the flowering-to-podding stage over a seven-day period across two consecutive growing seasons (2023 and 2024). Seventeen traits encompassing phenological, growth, physiological, reproductive, and yield categories were evaluated. Heat stress resulted in severe and consistent yield reductions across both seasons, with the extent of these reductions varying significantly between the tolerant and sensitive accessions. Single seed weight decreased by approximately 8% in tolerant accessions compared to approximately 34% in sensitive accessions, whereas total seed weight decreased by approximately 50% and 71%, respectively, with differences that were highly reproducible across both growing seasons. Pollen viability, pollen germination, stigma receptivity, and ovule viability were the most pronounced differentiators between the groups. Pollen viability decreased by 55–59% in tolerant accessions compared to 64–72% in sensitive lines, and pollen germination decreased by 59–65% and 71–79% across both years, respectively. In addition, tolerant accessions exhibited substantially lower electrolyte leakage and smaller declines in stomatal conductance, chlorophyll content, and photosynthetic efficiency. Two-way ANOVA revealed significant effects of accession, treatment, and their interaction for the key reproductive and yield traits. Regression analysis identified total seed weight ratio (R² = 0.876 and 0.801) and single seed weight ratio (R² = 0.522 and 0.523) as the traits most strongly associated with heat tolerance, whilst pollen viability (R² = 0.449 and 0.488), pollen germination (R² = 0.428 and 0.460), ovule viability (R² = 0.438 and 0.387), and stigma receptivity (R² = 0.428 and 0.366) showed moderate associations. Cluster and principal component analyses consistently identified IC-14912, EC-106613, IC-545783, and IC-16906 as heat-tolerant and IC-545792, EC-100098, EC-121013, and IC-362075 as heat-sensitive in both seasons. The identified heat-tolerant accessions represent priority genetic resources for heat tolerance breeding programmes in common bean.