Phosphorus Deficiency Affects Memory-Mediated Recovery From Recurrent Water Stress in Drought-Sensitive Soybean

Phosphorus (P) deficiency and water deficit are major constraints to soybean yield worldwide. While their individual impacts are well established, little is known about how P deficiency modulates soybean recovery from recurrent water stress. This study evaluated the effects of P deficiency on the recovery capacity of two soybean cultivars, contrasting in drought sensitivity, during the grain-filling stage. Plants were grown under either high P availability or P deficiency and subjected to different irrigation regimes: well-watered (WW), severe water deficit at R5 (WS-R5), and moderate deficit at V5 followed by severe deficit at R5 (WS-V5 + R5). The experiment followed a randomized complete block design in a 2 × 3 factorial scheme. Under water stress, P deficiency delayed stomatal resistance, extending photosynthetic decline in both cultivars. However, recovery of photosynthetic rate and stomatal conductance was faster under P deficiency than under high P supply. In the sensitive cultivar, P deficiency enhanced memory-mediated recovery of photosynthesis only after two stress cycles, with compensatory increases in mesophyll conductance, decreasing mesophyll limitations and favoring recovery. In contrast, the tolerant cultivar showed stable photosynthetic responses regardless of P level, with similar recovery in light saturation and photorespiration. Grain composition was affected by P deficiency in both cultivars, with lower protein concentration and increased oil content, particularly of unsaturated fatty acids. These results indicate that P deficiency alters physiological adjustments in soybean genotypes sensitive to water deficit, influencing their capacity to recover from recurrent drought stress and affecting grain quality.