The phosphorus requirement of N2 -fixing and urea-fed Acacia mangium

The fast-growing leguminous tree Acacia mangium Willd, was grown for at least 22 wk in aerated solution culture either under N2 -fixing conditions or with 2 mmol urea per plant per wk. Inorganic phosphorus was supplied at between 1 and 100 μmol P(1) per plant per wk: the latter was determined to be the optimum P supply for growth. The external P requirement for growth and the efficiency of utilization of internal P were similar for both N sources. However, shoot growth and the concentrations of N and P in leaves were decreased by P deficiency to a slightly higher extent in N2 -fixing plants than in urea-fed plants. Even though P deficiency limited nodule growth more drastically than it did shoot growth, the fraction of P allocated to both nodules and nodule nitrogenase activity (acetylene reduction) increased with P deficiency. It is concluded that this increase in nitrogenase activity reflects a higher N requirement per unit nodule mass in P-deficient plants and that, once nodules are fully functional the efficiency of utilization of internal P in N2 -fixing A. mangium is high compared with that of other N2 -fixing legumes.