Photosynthetic Responses of Nitrate-Fed and Nitrogen-Fixing Soybeans to Progressive Water Stress

ABSTRACT

The effect of nitrogen (N) source (nitrogen fixation or nitrate assimilation) and progressive water stress on pigment content, carbon assimilation and changes in the activity of certain photosynthetic (Rubisco and phosphoenol pyruvate carboxilase) and photorespiratory enzymes (glutamate synthetase and glycolate oxidase) during vegetative development of soybean plants was studied. Glycolate oxidase declined by 13% in nitrogen-fixing plants under water deficit, and increased in nitrate-fed ones. Nodulated plants were less sensitive to drought than nitrate-fed individuals; although as general growth was inhibited under drought stress in both experimental models. Results support the importance of nitrogen source in soybean responses to water stress. Difference in sensitivity of nitrate-fed and nitrogen-fixing plants towards water stress seems to be related not to nitrogen assimilation process itself, but to complex interactions with photorespiratory flux and stomatal conductance.

Keywords:

• carbon assimilation
• different nitrogen sources
• Glycine max L.
• nitrate assimilation
• nitrogen fixation
• nitrogen nutrition
• water stress

ACKNOWLEDGMENT

Authors would like to thank to Prof. Losanka Popova for her valuable advises and co-operation.