Photosynthetic light‐response curves in soybean: Impact of nitrogen availability and water deficit during seed filling

Understanding how photosynthesis responds to environmental conditions is essential, as it underpins crop growth and yield. However, yield gains through photosynthesis depend on water and nitrogen availability interactions. The aim of this study was to evaluate how water deficit and nitrogen availability affect photosynthetic light‐response curve parameters in field‐grown soybean during seed filling. Experiments combined two water levels (irrigated and water deficit during seed filling), two nitrogen levels (unfertilized and fertilized with 600 kg N ha −1 , N 600 ) and two genotypes contrasting in seed protein concentration. Water deficit significantly reduced the maximum light‐saturated CO 2 assimilation rate (A max ), which declined linearly during seed filling. N 600 increased leaf chlorophyll content (estimated indirectly through SPAD values) and mitigated drought effects on A max at the onset of seed filling but did not delay senescence in both genotypes. Adequate nitrogen supply can enhance photosynthetic performance during key reproductive stages, even under drought, but cannot fully compensate for water deficit. Field results show that A max reduction during seed filling effectively characterizes leaf senescence, highlighting this parameter as a target trait for crop modelling, breeding and management under climate change scenarios.