Enhanced nitrogen fixation increases net photosynthectic output and seed yield of hydroponically grown soybean
1988
Imsande, J.
Ten to 20% of the net photosynthetic output of a tropical grain legume may be consumed by the nodulation-nitrogen-fixation process. If plant growth activities during the reproductive phase were limited by photosynthetic output, enhanced nitrogen fixation would seemingly lower total plant mass and seed yield. To test this possibility, soybean [Glycine max (L.) Merr.] plants were grown hydroponically on nutrient medium supplemented with minimal urea or with an excess of either nitrate or nitrate plus urea. Acetylene reduction activities (i.e. nitrogen fixation rates) and transpiration rates were measured twice weekly on these plants through pod fill. Of the plants inoculated, those grown on minimal urea revealed significantly greater acetylene reduction activities and transpiration rates. At maturity, plants that had fixed nitrogen at a rapid rate during pod fill had a significantly greater seed size, total plant mass (i.e. net photosynthetic rate) and nitrogen content than uninoculated or poorly nodulated plants grown on an excess of nitrate. It is concluded, therefore, that a rapid rate of nitrogen fixation during pod fill enhances both transpiration and net photosynthetic output. The increased availability of usable nitrogen (i.e. ureides), coupled with enhanced transpiration and photosynthetic output, significantly increases total plant mass and seed yield. Thus, enhanced nitrogen fixation seems to be an inexpensive means of increasing seed yield of soybean and perhaps of other tropical grain legumes.
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