Effect of proline on nitrogenase activity in symbiosomes from root nodules of soybean (Glycine max L.) subjected to drought stress

Experiments were carried out to investigate if drought stress affects the ability of bacteroids from soybean (Glycine max L.) root nodules to utilize proline and malate to support nitrogenase activity. The bacteroids were isolated in sub-ambient oxygen and nitrogenase activity was measured by acetylene reduction. Nitrogenase activity supported by proline was 8-fold higher in bacteroids from drought-stressed nodules than in bacteroids from control nodules. In contrast to the results with proline there was no significant response to drought stress in the rate of bacteroid nitrogenase activity supported by malate. The effect of drought stress on transport of proline and malate across the symbiosome membrane was investigated by incubation of symbiosomes isolated in sub-ambient oxygen with radioactive tracers. Drought stress tended to increase the rate of proline uptake relative to a minor decrease in malate uptake into symbiosomes in response to drought. There was no indication of a saturable camer in the symbiosome membrane for either substrate at concentrations in the range 0.1-2 mM. The rate of malate uptake into symbiosomes was twice as high as the rate of proline uptake at all substrate levels tested. The protein composition of the symbiosome membrane was altered in response to drought stress and these changes may relate .to the permeability of the symbiosome membrane.