Rapid Dinitrogen Fixation During Soybean Pod Fill Enhances Net Photosynthetic Output and Seed Yield: A New Perspective
Nitrogen is frequently the limiting nutrient in plant growth and reproduction. Nevertheless, it is widely reported that the seed yield of the nodulated soybean [Glycine max (L.) Merr.] grown on fertile soil is not N-limited. This paper, based on new experimental evidence obtained under carefully controlled laboratory conditions, seeks to review and reevaluate this conclusion. To test this claim, inoculated soybean plants were grown hydroponically in the presence of suboptimal urea or with an excess of either nitrate or nitrate plus urea. Acetylene reduction activities (Le., N2 fixation rates) of plants grown on suboptimal urea were approximately tenfold greater than those of plants grown on excess nitrate. At maturity, plants that had fixed N2 at a rapid rate during pod fill had a greater biomass (i.e., a greater net photosynthetic output), a higher N content, and produced larger seeds than uninoculated or poorly nodulated plants grown on excess nitrate. Likewise, published reports show that plant biomass, seed N, and mean seed weight (i.e., g/lOO seeds) of nodulated plants grown in the field without added fertilizer N were significantly greater than those of nonnodulated plants grown on adjacent field plots and given a modest level of fertilizer N. Calculations show that mean seed weight of both hydroponically grown and fieldgrown plants correlated significantly (r ≥ 0.93) with seed yield. Because of this strong positive correlation and because rapid N2 fixation during pod fill significantly increases mean seed weight without lowering the number of seeds, it is concluded that elevated N2 fixation by field grown plants would increase soybean yield significantly. An increase in both total plant N and net photosynthetic output is thought to promote this increase in seed yield.
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