Nodule formation and development in soybeans (Glycine max L.) in response to phosphorus supply in solution culture

Phosphorus (P) deficiency is a main factor limiting legume production in most agricultural soils due to its necessity for growth and nitrogen fixation. The effect of phosphorus supply on nodule development and function in soybean (Glycine max (L.) Merr. cv. Heinong 35) was studied in nutrient solution. Plants were inoculated with Bradyrhizobium japonicum and were grown for 35 days in glasshouse at 25/15 °C day and night. While increasing P supply increased concentrations of P and N in shoots and roots, P concentration in nodules was not significantly affected by external P supply, and the amount of N fixed per unit nodule biomass decreased with increasing P supply. Nitrogen content in the shoots correlated well with P content (r = 0.92**). At inoculation level of 102 cells mL-1, P supply did not affect nodule number. At inoculation levels of 103.5 and 105 cells mL-1, increasing P supply increased both nodule number and size. Irrespective of inoculation level, increasing P supply increased nodule biomass relative to biomass of host plants. It is suggested that P deficiency specifically inhibited nodule development and thus total N2 fixation.