OXYGEN-INDUCED RECOVERY FROM SHORT-TERM NITRATE INHIBITION OF N-2 FIXATION IN WHITE CLOVER PLANTS FROM SPACED AND DENSE STANDS

Nitrogenase (N(2)ase; EC 1.18.6.1) activity (H-2 evolution) and root respiration (CO2 evolution) were measured under either N-2:O-2 or Ar:O-2 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 N-2-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 pO(2) was increased from 20 to 80 kPa under N-2:O-2. A simple mono-exponential model, fitted to the nodule permeability response to pO(2), indicated NO3- induced changes in minimum and maximum nodule O-2-diffusion permeability. Peak H-2 production rates at 80 kPa O-2 and in Ar:O-2 were close to the pre-decline rates at 20 kPa O-2. At the end of the nitrate treatment, this O-2-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.