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

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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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