O. Faurie et Jf. Soussana, OXYGEN-INDUCED RECOVERY FROM SHORT-TERM NITRATE INHIBITION OF N-2 FIXATION IN WHITE CLOVER PLANTS FROM SPACED AND DENSE STANDS, Physiologia Plantarum, 89(3), 1993, pp. 467-475
Nitrogenase (N(2)ase; EC 1.18.6.1) activity (H-2 evolution) and root r
espiration (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 compet
ition for light between clover shoots, which reduced CO2 net assimilat
ion 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-exponenti
al model, fitted to the nodule permeability response to pO(2), indicat
ed NO3- induced changes in minimum and maximum nodule O-2-diffusion pe
rmeability. 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 nitra
te treatment, this O-2-induced recovery in nitrogenase activity reache
d 71 and 82% for clover plants from spaced and dense stands, respectiv
ely. The respective roles of oxygen diffusion and phloem supply for th
e short-term inhibition of nitrogenase activity in nitrate-treated clo
vers are discussed.