Soybean Plants Affected by Phosphorus and Water Deficiency.

Three experiments were carried out to determine if the effect of water stress on soybean (Glycine max (L.) Merr.) changes with soil P availability, and whether soil water content affects plant P nutrition. Soils used had three levels of P availability (PI: 4,0 , PII: 6,0 y PIII: 10,0 mg kg-1). Treatments included two levels of P supply (HP: fertilized with 96 kg ha-1 of P, and LP: no fertilized) and two levels of water supply (Irrigated: soils kept at field capacity, and Non irrigated: plants were irrigated to field capacity after leaf water potential reached -2.7 MPa). Several water deficient cycles were applied from flowering stage (R2) to beginning of seed-filling (R5). In general, most of the effects of phosphorus and water stress on soybean growth were additive, so, effects of water stress were similar at every P level. Fertilized plants increased their vegetative development: increased shoot growth (10,5%), leaf area (22,1%), P and N concentration (29,2 and 18,8%, respectively), seed size (27,4%) and yield (29,8%). Water stress accelerated seed maturity, reduced shoot growth, leaf area, P and N concentration. Water deficiency caused a large decrease in seed yield but seed weight was not so much affected, showing that soybean response to the dry cycle was mainly due to a reduction in seed number but not seed weight. Soybeans had a positive response to P availability in the three soils, with and without irrigation, suggesting that phosphorus addition would be justified even when a dry growing season is likely to occur.