Phosphorus fertilisation differentially affects morpho-physiological traits and yield in soybean exposed to water stress

Background and aims Soybean plants frequently encounter challenges such as phosphorus (P) deficiency and water stress in many regions. However, the mechanisms governing low P and water stress tolerance in soybeans at different growth stages remain unclear. This study investigates the effect of P availability and water stress on soybean growth, morpho-physiological traits, and seed yield. Methods We conducted experiments using the soybean genotype (PI 561271) grown under two P levels (10 or 60 mg P kg(-1) dry soil) with three watering conditions: well-watered (WW), early water stress (EWS) and terminal water stress (TWS). Plant assessments occurred at the vegetative, flowering, seed formation, and maturity stages. Results Water stress decreased leaf area, shoot and root dry weights, root length, photosynthetic rate, stomatal conductance, total carboxylates, water use, water use efficiency, shoot, root and seed P contents, seed nitrogen contents, and seed yield compared to WW at the corresponding P level and growth stage. P60 significantly increased all parameters except P use efficiency (PUE) under all water treatments. EWS plants supplemented with P60 exhibited the highest root dry weight, root length and total carboxylates. Notably, flowering and seed formation stages had the highest carboxylate proportions (oxalic, malic, malonic and citric acids) under WW and EWS at P60. P10 had significantly higher PUE than P60, irrespective of water treatments during flowering and seed formation, while the opposite trend occurred at the maturity stage. Seed protein content significantly varied between P treatments regardless of the water stress. Conclusions Our findings underscore the adverse impacts of combined low P and water stress on soybean growth, morpho-physiological traits, seed yield and protein content. Moreover, increased P availability alleviated the adverse effects of water stress, highlighting the importance of adequate P fertilisation for soybean resilience to water stress.