Inhibition of Nodule Development in Soybean by Nitrate or Reduced Nitrogen

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Abstract
Imsande, J. 1986. Inhibition of nodule development in soybean by nitrate or reduced nitrogen.—J. exp. Bot. 37: 348–355. Nodulation of hydroponically grown soybean plants [Glycine max (L.) Merr.] is inhibited by continuous growth in the presence of 4· mol m−3 KNO3 The presence of 4·0 mol m−3 ‘starter nitrate’ for 3-6 d during nodule development, however, subsequently stimulates nodule dry weight accumulation and nitrogenase activity. These stimulations occur even though 4· mol m−3 nitrate temporarily delays nodule development, i.e. the late steps of nodule development are reversibly inhibited by a short-term exposure to 4·0 mol m−3 nitrate. On the other hand, treatment with 4·0 mol m−3 nitrate in excess of 14 d significantly reduces nodule dry weight Thus, extended growth in the presence of 4·0 mol m−3 KNO3 seems to block both early and late steps of nodule development. Nodulation of hydroponically grown soybeans is also inhibited by continuous growth in the presence of 2·0 mol m−3 (NH4)2SO4 This inhibition is not caused by acidity of the growth medium. On the other hand, nodule development 6 d after inoculation with Rhizoblum japonicum is not delayed by a 7-d exposure to 2·0 mol m−3 (NH4)2SO4 but is partially inhibited by a prolonged exposure to (NH4)2SO4 Because repression of nodulation by 4·0 mol m−3 KNO3 is more severe than that by 2·0 mol m−3 (NH4)2SO4 and because ammonium taken up by the soybean plant is not actively oxidized to nitrate, it is suggested that there are at least two mechanisms by which nitrate utilization represses nodule formation in soybean.