Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/1997
Publication Date: N/A
Citation: Interpretive Summary: Soybean is important in crop rotation schemes employed by growers because it does not require the application to the soil of nitrogen fertilizer. Soybean plants obtain their nitrogen from the atmosphere in a process called dinitrogen fixation. It has recently been discovered by scientists at the Agricultural Research Service in Gainesville, FL, that the dinitrogen fixation process is particularly sensitive to soil drying. Early in the drying cycle the dinitrogen fixation process begins to decrease. In other studies of plant stress responses, it has been observed that the compound gamma-aminobutyric acid accumulates upon the imposition of stress. This research showed that gamma-aminobutyric acid also accumulated in soybean with drying stress, particularly in the nodules where dinitrogen fixation occurs. In addition, the activity of the enzyme responsible for the synthesis of gamma-aminobutryic acid was studied for possible changes in response to soil drying. The activity of the enzyme itself was stable under the stress conditions.
Technical Abstract: Nitrogen fixation and nodule permeability to O2 diffusion are decreased by drought stress. It was hypothesized that decreased O2 availability in soybean (Glycine max L. Merr.) nodules as a result of drought stimulates glutamate decarboxylase (GAD) activity [EC 220.127.116.11]. Consequently, this could result in gamma-aminobutyric acid (GABA) accumulation. The amino acid composition of xylem sap was determined in plants subjected to soil water deficits. While 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 nutrient solution. The exposure of soybean nodules to hypoxia for 6 h enhanced the GABA concentration by six-fold, but there was no change in GABA concentration in response to the PEG treatment. No major changes in the in vitro GAD activity were measured in nodule cytosol or bacteroids. The present data do not support the hypothesis that decreased nodule O2 permeability and a resulting O2 deprivation inside nodules may stimulate in vitro GAD activity and thus GABA accumulation.