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Effect of Irradiance on Development of Apparent Nitrogen Fixation and Photosynthesis in Soybean ¹

Department of Agronomy and Range Science, University of California, Davis, California 95616

Soybeans grown with 2 millimolar NO₃^–, which optimized apparent N₂ fixation by Rhizobium symbionts, showed significantly different rates of apparent photosynthesis and C₂H₂ reduction during seedling development at two irradiances. Those physiological processes were lower for several weeks in plants grown at 1,500 microeinsteins per meter² per second than in those exposed to 700 microeinsteins per meter² per second. The irradiance-induced retardation was evident in short-term rates of apparent photosynthesis and N₂ fixation, as well as in measures of dry matter and total N accumulation. In spite of their previously inhibited development, plants grown at 1,500 microeinsteins per meter² per second were indistinguishable by day 28 from those exposed to 700 microeinsteins per meter² per second in terms of whole-shoot CO₂-exchange rate; by day 35 they were identical in terms of whole-plant C₂H₂-reduction rate. On day 38 there was no significant difference in dry weight or N content between treatments. Shifting plants between irradiance treatments on day 21 showed that the higher irradiance also had a short-term inhibitory effect on C₂H₂ reduction. The fact that 16 millimolar NO₃^– prevented the continuous exposure to 1,500 microeinsteins per meter² per second from inhibiting apparent photosynthesis suggested that seedlings grown on 2 millimolar NO₃^– with Rhizobium were N-limited. Although rates of apparent photosynthesis were similar by day 28, the additional week required to produce equal rates of apparent N₂ fixation between irradiance treatments showed that physiological adaptations of shoots, as well as photosynthesis per se, can affect root nodule activity.

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