Trifolium repens L. was grown to test the following hypotheses: when P is deficient (i) N2 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 N2 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 N2 fixation decreased, whereas specific N2 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 pCO2 had no effect on the growth and functioning of the nodules, (ii) starch accumulated in the leaves, particularly under elevated pCO2, 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 N2 fixation are down-regulated by an N feedback mechanism.
