The effect of potassium deficiency on growth and N2-fixation in Trifolium repens

The effects of potassium (K) deficiency on growth, N₂-fixation and photosynthesis in white clover ( Trifolium repens L.) were investigated using natural occurring gas fluxes on the nodules in real time of plants under three contrasting relative addition rates of K causing mild K deficiency, or following abrupt withdrawal of the K supply causing strong K deficiency of less than 0.65% in dry matter. A steady-state below-optimum K supply rate led to an increase in CO₂-fixation per unit leaf surface area as well as per plant leaf surface. However, nitrogenase activity per unit root weight and per unit nodule weight was maintained, as was the efficiency with which electrons were allocated to the reduction of N₂ in the nodules. Abrupt K removals stimulated nodule growth strongly without delay, but as K concentrations decreased in the plant tissue a significant decline in nitrogenase activity per unit root weight as well as per unit nodule mass occurred. Further, the rate of photosynthesis per unit leaf area was unaffected, while the CO₂ acquisition for the plant as a whole increased due to an expansion of total leaf area whereas the leaf area per unit leaf weight was unaffected. The ratio between CO₂-fixation and N₂-fixation increased, although not statistically significant, under short-term K deprivation as well as under long-term low K supply indicating a downregulation of nodule activity following morphological and growth adjustments. This downregulation took place despite a partly substitution of the K by Na. It is concluded that N₂-fixation does not limit the growth of K-deprived clover plants. K deprivation induces changes in the relative growth of roots, nodules, and shoots rather than changes in N and/or carbon uptake rates per unit mass or area of these organs.