Temperature Effects on Root Nodule Activity and Nitrogen Release in Some Sub-Tropical and Temperate Legumes

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We examined the effect of temperature on dinitrogen fixation and exudation of N compounds from the root systems of soybean (Glycine max (L.) Merr. cv. Kurosengoku), sesbania (Sesbania cannabina), and chinese mild vetch (Astragalus sinicus L.) under water culture conditions. The shoots of the plants were exposed to day/night temperature regimes of 15/10, 25/20, and 35/30℃ with constant root temperatures of 10, 20, and 30℃, respectively for seven consecutive days. Root nodule activity per plant determined by the acetylene reduction assay was highest at 25/20℃ for all the legumes. Soybean and sesbania root nodule activities were higher at 35/30℃ than 15/10℃, but the reverse was true for chinese mild vetch. The release of nitrogenous compounds by the root system was largest at 35℃, for all the legumes. Ammonia, phosphoserine, and phosphoethanolamine were consistently detected in all the excreted solutions of the legumes regardless of temperature treatments. The amount of total amino acids released accounted for between 13-15% of the total amount of N released by soybean and 3% in both sesbania and chinese milk vetch root systems. The amount of total N released expressed as percentage of the N amount fixed by soybean and sesbania ranged between 0.5-4.2%.
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