The fast-growing leguminous tree Acacia mangium Willd, was grown for a
t least 22 wk in aerated solution culture either under N-2-fixing cond
itions or with 2 mmol urea per plant per wk. Inorganic phosphorus was
supplied at between 1 and 100 mu mol P-i per plant per wk: the latter
was determined to be the optimum P supply for growth. The external P r
equirement for growth and the efficiency of utilization of internal P
were similar for both N sources. However, shoot growth and the concent
rations of N and P in leaves were decreased by P deficiency to a sligh
tly higher extent in N-2-fixing plants than in urea-fed plants. Even t
hough P deficiency limited nodule growth more drastically than it did
shoot growth, the fraction of P allocated to both nodules and nodule n
itrogenase 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 t
hat, once nodules are fully functional, the efficiency of utilization
of internal P in N-2-fixing A. mangium is high compared with that of o
ther N-2-fixing legumes.