Evidence that P deficiency induces N feedback regulation of symbiotic N2 fixation in white clover (Trifolium repens L.)

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, 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, fixation decreased, whereas specific N, fixation increased and compensated partially for poor nodulation, Leaf photosynthesis was reduced under P deficiency due to low V-c,V-max and J(max). Poor growth or poor performance of the nodules was not due to C limitation, because (i) the improved photosynthetic performance at elevated p(CO2) had no effect on the growth and functioning of the nodules, (ii) starch accumulated in the leaves, particularly under elevated P-CO2, 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, fixation are down-regulated by an N feedback mechanism.