Actual and potential nitrogen fixation in pea and field bean as affected by combined nitrogen

Abstract

Actual nitrogen fixation of pea and field-bean plants, grown in soil in the open air, was determined as the acetylene reduction of nodulated roots. During the major part of the vegetative growth of these plants, actual nitrogen fixation was equal to the potential maximum nitrogenase activity of the bacteroids present in the nodules. This means that increase of the actual nitrogen fixation could be achieved only if the potential nitrogenase activity of the bacteroids would be enhanced or if more nodules would be present. During the generative growth phase, the potential nitrogenase activity of the bacteroids was not entirely utilized irrespective of the shoot mass of the host plant or the nitrogen-fixing capacity of the Rhizobium microsymbiont.The addition of nitrate to nodulated plants sharply reduced actual nitrogen fixation but did not affect potential nitrogen fixation within 10 days. The nitrate effect was temporarily eliminated by treatment of the root nodules with benzyladenine, a synthetic plant-growth regulator with a photosynthate-attracting action. It is concluded that incomplete utilization of the nitrogenase present in the bacteroids is caused by an inadequate carbohydrate supply of the nodules upon supply of the leguminous plant with nitrate.In root systems of pea plants growing in symbiosis with R. leguminosarum strains of poor nitrogen-fixing capacity, the N-limited growth led to an accumulation of carbohydrates. Upon the supply of such plants with nitrate, the carbohydrate level was decreased concomitantly to an increase in alternative (i.e. cyanide-resistant) respiration. Low rates of alternative respiration were found when nitrate was added to pea plants inoculated with highly effective strains, as the N-limitation of the plants was less severe. The carbohydrates respired in the nodules amounted to 6.3 mg C/mg N fixed with a moderately effective strain but only 3.1 mg C/mg N fixed with a highly effective strain.Some rhizobial strains possess a hydrogenase that is capable of recirculating part of the hydrogen evolved in air by nitrogenase simultaneously with nitrogen fixation. However, the energy gain by hydrogen oxidation was very low, viz.0.6-4.3% Of the costs of nitrogen fixation. As hydrogen oxidation did not delay nodule senescence or increase nitrogenase utilization, the presence of hydrogenase in rhizobial strains seems to be an unimportant factor in determining the nitrogen-fixing capacity of the symbiosis.Strains of R. leguminosarum with a low nitrogen-fixing capacity showed a distinctly different nodule formation pattern on primary and lateral roots of pea plants as compared with highly effective strains. The estimates of the yield of fixed nitrogen, derived from the acetylene-reduction method, of plants inoculated with rhizobia] strains of different nitrogen-fixing capacity, equally deteriorated during the growth period. When rates of hydrogen production in air were subtracted from the acetylene reduction rates, the estimates of nitrogen fixation were severely biased in favour of a hydrogenase-containing strain.Nitrogen fixation of pea plants infected with a highly effective strain of R. leguminosarum was decreased by the supply of combined nitrogen to a greater extent than that of plants with a moderately effective strain. In plants inoculated with a strain of poor nitrogen-fixing capacity, nitrogen fixation per plant was even stimulated by a low dose of combined nitrogen. This increase was probably due to an enhanced photosynthetic capacity which counteracted the adverse effect of combined nitrogen. Seed yields were increased by nitrate dressings, regardless of the rhizobia] strain. Seed yields of plants inoculated with moderately effective strains were slightly higher than those of highly effective strains at high levels of combined nitrogen, owing to higher nitrate uptake, higher nitrogen fixation and increased photosynthesis.<p/