Evidence that P deficiency induces N feedback regulation of symbiotic N 2 fixation in white clover ( Trifolium repens L.)

Abstract

Trifolium repens L. was grown to test the following hypotheses: when P is deficient (i) N 2 fixation decreases as a result of the plant's adaptation to the low N demand, regulated by an N feedback mechanism, and (ii) the decrease in the photosynthetic capacity of the leaves does not limit N 2 fixation. Severe P deficiency prevented nodulation or stopped nodule growth when the P deficiency occurred after the plants had formed nodules. At low P, the proportion of whole‐plant‐N derived from symbiotic N 2 fixation decreased, whereas specific N 2 fixation increased and compensated partially for poor nodulation. Leaf photosynthesis was reduced under P deficiency due to low Vc,max and Jmax . Poor growth or poor performance of the nodules was not due to C limitation, because (i) the improved photosynthetic performance at elevated p CO 2 had no effect on the growth and functioning of the nodules, (ii) starch accumulated in the leaves, particularly under elevated p CO 2 , and (iii) the concentration of WSC in the nodules was highest under P deficiency. Under severe P deficiency, the concentrations of whole‐plant‐N and leaf‐N were the highest, indicating that the assimilation of N exceeded the amount of N required by the plant for growth. This was clearly demonstrated by a strong increase in asparagine concentrations in the roots and nodules under low P supply. This indicates that nodulation and the proportion of N derived from symbiotic N 2 fixation are down‐regulated by an N feedback mechanism.