INHIBITION OF NITROGENASE ACTIVITY AND NODULE OXYGEN PERMEABILITY BY WATER-DEFICIT

Short-term effects of water deficit on nitrogenase activity were inves tigated with hydroponically grown soybean plants (Glycine max L. Merr. cv. Biloxi) by adding polyethylene glycol (PEG) to the hydroponic sol ution and measuring nitrogenase activity, nodule respiration, and perm eability to oxygen diffusion (P-o). These experiments showed a rapid d ecrease in acetylene reduction activity (ARA) and nodule respiration. A consequence of the decreased respiration rate was that P-o calculate d by Pick's Law also decreased. However, these results following PEG t reatment were in direct conflict with a previous report of stability i n P-o determined by using an alternative technique. To resolve this co nflict, an hypothesis describing a sequence of responses to the initia l PEG treatment is presented, An important finding of this study was t hat the response to water deficit induced by PEG occurred in two stage s, The first stage of decreased nodule activity was O-2-limited and co uld be reversed by exposing the nodules to elevated pO(2). The second stage which developed after 24 h of exposure to PEG resulted in substa ntial loss in nodule activity and this activity could not be recovered with increased pO(2). Severe water deficit treatments disrupt nodule activity to such a degree that O-2 is no longer the major limitation.