ACCUMULATION OF GAMMA-AMINOBUTYRIC-ACID IN NODULATED SOYBEAN IN RESPONSE TO DROUGHT STRESS

Nitrogen fixation and nodule permeability to O-2 diffusion are decreas ed by drought stress. Since gamma-aminobutyric acid (GABA) synthesis i s rapidly stimulated by a variety of stress conditions including hypox ia, it was hypothesized that decreased O-2 availability in nodules sti mulates glutamate decarboxylase (GAD) activity (EC 4.1.1.15), thereby resulting in GABA accumulation. First, the amino acid composition of x ylem sap was determined in plants subjected to soil water deficits. Wh ile the xylem sap concentration of several amino acids increased when the plant was subjected to a water deficit, the greatest increase was in GABA. GABA accumulation was examined in response to stress induced by hypoxia or the addition of polyethylene glycol (PEG) to the nutrien t solution. The exposure of soybean nodules to hypoxia for 6 h enhance d the GABA concentration by 6-fold, but there was no change in GABA co ncentration in response to the PEG treatment. No major changes in the in vitro GAD activity were measured in nodule cytosol or bacteroids. T he present data do not support the hypothesis that decreased nodule O- 2 permeability and a resulting O-2 deprivation inside nodules may stim ulate in vitro GAD activity and thus GABA accumulation. However, the d ata could indicate a possible effect of hypoxia and drought stress on the in vivo activity of GAD.