Analysis of temperature‐responsive yield indices in wheat ( Triticum aestivum L.) under varied sowing conditions

Wheat ( Triticum aestivum L.), a major global staple, suffers yield and quality losses due to high temperature stress. This study evaluated 60 spring × winter derivative lines, including five checks, for yield stability under early, timely, and late sowing during 2019‐2020 and 2020‐2021 at Punjab Agricultural University, Ludhiana, India. Thirteen yield‑based stress indices (stress tolerance [TOL], stress tolerance index, stress susceptibility percentage index, yield index, yield stability index [YSI], relative stress index, mean productivity, geometric mean productivity, harmonic mean, mean yield (relative) performance [MYP], percent yield reduction [PYR], reduction rate of 1000 kernel weight, and heat susceptibility index) were calculated from grain yield across environments. Hierarchical clustering (Ward's method, Euclidean distance) produced three clusters, with final merging distances of 15.19 for timely versus early (TE) and 13.78 for timely versus late (TL), indicating clear divergence among environments. Under timely sowing, lines showed better tolerance (mean TOL: −166.32) and stability (mean YSI: 1.19), with some yield gains (mean PYR: −19.10%). Late sowing caused higher susceptibility (mean TOL: 265.39) and marked yield loss (mean PYR: 29.33%). Although temperature differed among sowing dates, maximum temperature during grain filling stayed below terminal heat thresholds, so responses reflect sowing‑time‑associated temperature variations rather than true heat stress. Across indices and multivariate analyses, lines EWS 78, EWS 252, EWS 431, EWS 421, EWS 439, and EWS 282 combined higher mean yield with favorable tolerance in both TE and TL, whereas EWS 1, EWS 139, EWS 93, EWS 92, and EWS 94 were consistently temperature‑sensitive. Principal coordinate analysis confirmed strong shifts between TE and TL but highlighted a small subset of stable derivatives as promising donors for breeding wheat adapted to variable sowing windows and associated thermal regimes.