Yield Sensitivity to Heat Waves and High Night Temperatures During Fructification in Field‐Grown Soybean

The increasing frequency of heat waves (HW) and warm nights threatens soybean yield, particularly during reproductive stages. This study examined the impact of HW and high night temperature (HNT) during fructification on seed yield (SY), seed number (SN), and individual seed weight (SW) in field‐grown soybean and analysed the relationship between SY and its components with biomass accumulation (Biomass R4‐R7 ), cumulative intercepted photosynthetically active radiation, and radiation use efficiency (IPAR R4‐R7 and RUE R4‐R7 , respectively). Two field experiments were subjected to three thermal regimes during 7 or 10 d from full pod (R4 stage): (i) HNT from 19:00 to 07:00 h; (ii) HW from 10:00 to 16:00 h combined with HNT (HW × HNT); and (iii) ambient temperature as a control (AT), using shelters with transparent polyethylene films, with a delta in minimum and maximum temperature of ca. 3°C and 7°C (on average across experiments), respectively. HNT and HW × HNT significantly reduced SY in similar magnitude (~30–36%) compared to AT, mainly by decreasing SN, with no consistent impact on SW. Reductions in SN were explained by decreases of Biomass R4‐R7 ( R 2 = 0.76; p < 0.0001), which in turn was strongly associated with reductions of crop growth rate (CGR R4‐R7 , R 2 = 0.98, p < 0.0001). The CGR R4‐R7 decreased by 33% and 40% under HNT and HW × HNT compared to AT, primarily due to drops in RUE R4‐R7 . Heat load, a quantitative indicator of the heating intensity, emerged as a strong predictor for SY, SN, CGR R4‐R7 , and RUE R4‐R7 , explaining 54%, 70%, 79%, and 73% of their variability. No additive effects were observed for HW × HNT compared to HNT alone, suggesting a dominant effect of HNT over HW. Our results highlight the need to consider night‐time warming when evaluating soybean yield stability under climate change scenarios.