The influence of phosphorus deficiency on growth and nitrogen fixation of white clover plants

The effects of P deficiency on growth, N(2)-fixation and photosynthesis in white clover (Trifolium repens L.) plants were investigated using three contrasting relative addition rates of P, or following abrupt withdrawal of the P supply. Responses to a constant below-optimum P supply rate consisted of a decline in N(2)-fixation per unit root weight and a small reduction in the efficiency with which electrons were allocated to the reduction of N(2) in nodules. Abrupt removal of P arrested nodule growth and caused a substantial decline in nitrogenase activity per unit root weight, but not per unit nodule mass. Similarly, the rate of photosynthesis per unit leaf area was unaffected by abrupt P removal, whereas CO(2) acquisition for the plant as a whole decreased due to a decline in total leaf area, leaf area per unit leaf weight and utilization of incoming radiation. These changes followed the decline in tissue P concentrations. The ratio between CO(2)-fixation and N(2)-fixation was maintained under short-term P deprivation but increased under long-term low P supply, indicating a regulatory inhibition of nodule activity following morphological and growth adjustments. It is concluded that N(2)-fixation did not limit the growth of clover plants experiencing P deficiency. A low P status induced changes in the relative growth of roots, nodules and shoots rather than changes in N and/or C uptake rates per unit mass or area of these organs.