Grain Protein Response to Phosphorus Nutrition of Wheat

Efficacy of N fertilizer in increasing wheat (Triticum aestivum L.) grain protein is well recognized, but effects of P nutrition are unclear. Phosphorus nutrition of wheat in relationship to grain protein was investigated to ascertain differences among cultivars and interactions of P with N and K. Five hard winter wheat genotypes that differed in grain protein concentration were grown at two Kansas locations on Smolan Pachic Agriustolls and Parsons Mollic Albaqualfs soils with three P levels (0, 56, 112 kg P ha⁻¹) as superphosphate and three K levels (0, 112,224 kg ha⁻¹) as KCL to elucidate effects of P nutrition. Hydroponic studies with 0.005, 0.01, 0.05, 0.1, and 0.5 mM NaH₂PO₄, 5, 10, and 15 mM NaNO₃, and 0.05, 0.5, and 5.0 mM KCI determined effects and interactions of P on nitrate reductase activity and soluble protein in seedling leaves. Other hydroponic studies determined effect of 0.01, 0.1, and 1.0 mM NaH₂³²PO₄ on grain yield, protein concentration, and ³²P distribution. Field application of 56 and 112 kg P ha⁻¹ increased mean grain yield 150 and 314 kg ha⁻¹, respectively, over both locations and decreased grain protein 0.2 and 0.4% , respectively, at one location, an effect attributed to dilution of grain protein. Nitrate reductase activity of hydroponically grown seedlings was highest with 0.1 mM P and lowest with 0.005 and 0.5 mM P, whereas soluble protein was highest at the lowest P level. In plants grown to maturity in hydroponic cultures, 0.1 and 1.0 mM P increased plant tillers, vegetative N and P concentrations, grain yield, and grain protein and P concentrations. Time of application of ³²P did not influence grain ³²P concentration. We concluded that P fertilizer had no intrinsically detrimental effect on N metabolism and grain protein in wheat, but that it might indirectly affect grain protein concentration by influencing yield.