Water Use Efficiency and Nutrient Uptake of Soybean Grown in P-Deficient Soil Under Water Deficit.

Drought affects growth and causes yield reductions, especially when drought stress occurs at reproductive stages.  The objective of this research was to investigate the effects of P application on water use efficiency and nutrient uptake of two cultivars of soybean [Glycine max (L.) Merr.] grown under water deficit at R1 and R3 stages.  The two cultivars were Chiangmai 60 (CM 60), a traditional Thai cultivar, and KKU 74, an improved cultivar developed at Khon Kaen University in Thailand.  The plants grew under greenhouse conditions in pots of soil (Morrill loam) that was unfertilized (6.5 ppm P in the soil) or was fertilized with triple superphosphate (40 ppm P added to the soil).  Three watering regimes were imposed:  well-watered throughout the growing season of 101 days (control); water deficit beginning at the R1 stage; and water deficit beginning at the R3 stage.  The results showed that P application increased plant height, stomatal resistance, water use efficiency, nutrient uptake, and shoot and root dry weights of both cultivars.  Water deficit treatments generally reduced plant height, water use efficiency, nutrient uptake, and shoot and root dry weights of both cultivars.  Water use efficiency of the soybeans was increased by 73% when they received P fertilizer.  Plants grown with water deficit at the R1 stage had the greatest reduction in plant height, water use efficiency, nutrient uptake, and shoot and root dry weights.  Under water deficit and P fertilization, KKU 74 had a higher plant height, stomatal resistance, water use efficiency, nutrient uptake, and shoot and root dry weights compared to CM 60.  These results indicated that KKU 74 was better adapted to drought than CM 60.  This study showed that application of P fertilizer increased water use efficiency and nutrient uptake, which mitigated drought stress at reproductive stages.