Root nodule senescence induced by nitrate and ammonium in Pisum sativum L. was defined by determining nitrogenase activity and leghemoglobin content with the acetylene reduction and pyridine hemochrome assays. Root systems supplied with 100 mM KNO3 or 100 mM NH4CI exhibited a decrease in nitrogenase activity foUlowed by a decline in leghemoglobin content. Increasing the CO2 concentration from 0.000320 atm to 0.00120 atm had no effect on the time course of root nodule senescence when 20 mM KNO3 was supplied to the roots; in vitro nitrate reductase activity was detected in leaves and roots, but not bacteroids. Nitrate appeared in leaves, roots, and the nodule cytosol fraction but not bacteroids when 20 mM KNO3 was supplied to roots. When nitrate entered through the shoots, however, no root nodule senescence was observed, and no nitrate was detected in root or nodule cytosol fractions although nitrate and nitrate reductase were found in leaves. The results suggest that nitrate does not induce root nodule senescence through competition between nitrate reductase and nitrogenase for products of photosynthesis.Field-grown soybeans reportedly obtain only 25% of their nitrogen from N2 (6). The remaining nitrogen is drawn from soil reserves of this element. Understanding the manner in which nitrate and ammonium decrease biological N2 reduction in legume root nodules is important for devising strategies for maximizing N2 fixation.One method for studying interactions between combined nitrogen and N2 fixation is to supply excess nitrate or ammonium to legumes and examine the effect on root nodule functioning. Morphological effects of supplying NH4NO3 to root nodules have been described (2), but the physiological basis of this response has not been elucidated. Small and Leonard (12) Nitrogen treatments were started 28 days after planting the peas. Each 15-cm diameter pot containing five plants was treated with 500 ml of the appropriate nitrogen-containing solution in experiments utilizing root-supplied nitrogen. When nitrate was supplied to the shoot, the apical meristem was removed, and the entire shoot was dipped into a 1-liter solution containing 3 g KNO and 0.5 ml Triton X-100 for 4 hr. Control plants were treated with a solution containing only Triton X-100.In vivo nitrogenase activity of intact pea plants was measured with the acetylene reduction assay (3). Pea nodules were extracted with 0
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