WATER STATUS EFFECT ON DINITROGEN FIXATION AND PHOTOSYNTHESIS IN SOYBEAN

Water deficiency brings about a marked limitation in soybean (Glycine max. L. Merr.) yield by impairing photosynthesis and symbiotic N2 fixation. The objective of this study was to determine the photosynthetic and N2-fixation response to short- and long-duration water status variations in leaves and nodules. Plants were grown in pots in a greenhouse. Carbon dioxide exchange rate was measured by gas analysis and N2 fixation by the acetylene reduction method. Leaf water status was determined with a pressure bomb and nodule water potential with a psychrometer. Dinitrogen fixation decreased steadily throughout the water deficiency period whereas photosynthesis first decreased only slightly, and then dropped dramatically. After a severe water stress, partial recovery was slower for N2 fixation than for photosynthesis. The N2-fixation response appeared to be directly related to a reduction in nodule mass, which affected nodule structural constituents after a severe stress. During the water deficiency period, the water status of the nodules responded only partially to water status variations within the plant during the day. Dinitrogen fixation depended more on the water status of the nodules than on the changes in nodule mass. There was a higher dependence of N2 fixation on nodule water content than on nodule water potential, particularly for values < - 1.2 MPa. The N2-fixation activity maintained during the severe phase of water stress, when photosynthesis was zero, resulted from the relative independence of the nodules on the daily water variations within the plant and from the strong binding of the nodule water.