The role of phosphorus in nitrogen fixation by young pea plants (Pisum sativum)

The influence of P on N2 fixation and dry matter production of young pea (P. sativum L. cv. Bodil) plants grown in a soil-sand mixture was investigated in growth cabinet experiments. Nodule dry weight, specific C2H2 reduction and P concentration in shoots responded to P addition before any growth response could be observed. The P concentration in nodules responded only slightly to P addition. A supply of P to P-deficient plants increased both the nodule dry weight, specific C2H2 reduction and P concentration in shoots relatively faster than it increased shoot dry weight and P concentration in nodules. Combined N applied to plants when N2 fixation had commenced, increased shoot dry weight only at the highest P levels. The smaller plant growth at the low P levels did not result from N deficiency. The reduced nodulation and N2 fixation in P-deficient plants were apparently caused by impaired shoot metabolism and not by a direct effect of P deficiency of the nodules.