RELATIONSHIP OF NITROGENASE ACTIVITY TO PLANT WATER-STRESS IN FIELD-GROWN SOYBEANS

Soybeans are often subjected to periods of water deficits during ontogeny. Water stress can produce lower leaf water potentials, stomatal closure, decreased photosynthesis and ultimately result in reduced yields. Although the responses of many physiological processes to water deficits have been extensively studied, the effects on N2 fixation, an important process for leguminous plants, have received little attention in field experiments. An experiment to relate the effects of reduced soil moisture on the nitrogenase activity of soybean nodules to changes in leaf water status, stomatal activity, nodule water content and apparent canopy C exchange during an imposed drying cycle was conducted. Field-grown soybeans were subjected to a 19-day drying cycle during early seed fill. At chosen intervals, midday measurements of selected physiological processes were made in stressed and well-watered control plants. Leaf water potential, leaf conductance to water vapor, nodule moisture content, apparent C exchange and nitrogenase activity declined as the duration of the stress period increased. All physiological processes, except apparent canopy C exchange rate, returned to control levels on adequate rewatering. The incomplete recovery of apparent C exchange was attributed to decreased leaf area index resulting from leaf senescence during the stress period. Decreases in nitrogenase activity apparently are primarily caused by nodule dehydration and tissue damage and not directly related to short-term changes in photosyntheses. Nitrogenase activity apparently declined similarly with other physiological processes, however, the obsrvations did not conclusively define the precise relationships of nitrogenase activity to the other physiological processes.