Oxygen‐induced recovery from short‐term nitrate inhibition of N2 fixation in white clover plants from spaced and dense stands

Nitrogenase (N2ase; EC 1.18.6.1) activity (H2 evolution) and root respiration (CO2 evolution) were measured under either N2:O2 or Ar:O2 gas mixtures in intact nodulated roots from white clover (Trifolium repens L.) plants grown either as spaced or as dense stands. The short‐term nitrate (5 mM) inhibition of N2‐fixation was promoted by competition for light between clover shoots, which reduced CO2 net assimilation rate. Oxygen‐diffusion permeability of the nodule declined during nitrate treatment but after nitrate removal from the liquid medium its recovery parallelled that of nitrogenase activity. Rhizosphere pO2 was increased from 20 to 80 kPa under N2:O2. A simple mono‐exponential model, fitted to the nodule permeability response to pO2, indicated NO−3 induced changes in minimum and maximum nodule O2‐diffusion permeability. Peak H2 production rates at 80 kPa O2 and in Ar:O2 were close to the pre‐decline rates at 20 kPa O2. At the end of the nitrate treatment, this O2‐induced recovery in nitrogenase activity reached 71 and 82%; for clover plants from spaced and dense stands, respectively. The respective roles of oxygen diffusion and phloem supply for the short‐term inhibition of nitrogenase activity in nitrate‐treated clovers are discussed.