Induction of Root Nodule Senescence by Combined Nitrogen in Pisum sativum L

Abstract

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

Respiration and the Energy Requirement for Nitrogen Fixation in Nodulated Pea Roots

“…Since appreciable nitrification is unlikely and no reduction of NH4+ is necessary, this similarity indicates that NO3-was reduced predominantly in the shoots. Reports from other studies with peas (4,17,24) support this idea and although some energy may be required for nitrate uptake and transport, it is likely that under the conditions used in the present studies little additional energy is required in the roots and nodules for nitrate utilization.…”

Root and Nodule Respiration in Relation to Acetylene Reduction in Intact Nodulated Peas

“…Tentatively, it must be concluded that under the conditions which were used, little NO3-or NO2-reduction occurred in the roots or nodules. Other results with peas (4,20,26) support the idea that with this concentration of NH4NO3 applied to already nodulated plants, high levels of nitrate reduction would be expected in the shoot relative to that in the roots and nodules.…”

Effect of Induced Nodule Senescence on Parameters Related to Dinitrogen Fixation, Bacteroid Size and Nucleic Acid Content

“…Although the decrease in acetylene-reducing activity in the dark has been attributed to the limited availability of energy and reductants from photosynthesis (Ching et al, 1975), the decrease caused by treatment with combined nitrogen has drawn varied explanations (Oghoghorie & Pate, 1971;Rigaud, 1976;Chen & Phillips, 1977;Rigaud & Puppo, 1977). Bisseling et al (1978) concluded that decreased nitrogenase activity in nodules of peas treated with NH4N03 resulted from a decreased leghaemoglobin content.…”

“…Ching et al (1975) demonstrated in soybean nodules that if photosynthesis became limiting, acetylene reduction, ATP and sucrose contents and the energy charge of the nodules sharply decreased. In a separate study, Chen & Phillips (1977) have shown that combined nitrogen (NH,Cl or KN03) also caused a sharp decrease in acetylene reduction in pea nodules. The reason for this effect is not known although various explanations have been suggested (Oghoghorie & Pate, 1971 ;Rigaud, 1976;Rigaud & Puppo, 1977).…”