Ecology of Nitrogen Fixation in Soils and Rhizospheres V. Oxygen Limited Growth and N-Metabolism in Wheat Roots

Abstract

Summary The effects of reduced oxygen concentration on root growth and activities of enzymes of N-metabolism of wheat ( Triticum aestivum var. Kolibri) have been studied, for low O 2 tensions are required for N 2 fixation by microaerophilic bacteria (e.g. Azospirillum ) associated with root systems of grasses. In hydrocultures with oxygen concentrations in the range of 0.2 to 1 mg O 2 · 1 −1 compared to aerated cultures (8-9 mg O 2 · 1 −1 ) root growth was reduced from 10 mg fresh weight · day −1 · plant −1 to one tenth 15 to 30 d after sowing. Specific activity of NADH and NADPH dependent glutamate dehydrogenase (E.C. 1.4.1.2 and 1.4.1.4) is reduced by 50 % in the cultures with low oxygen concentrations 20 to 30 days after sowing, whereas specific activity of aspartate aminotransferase (E.C. 2.6.1.1) and alanine amino transferase (E.C. 2.6.1.2) is enhanced by a factor of two to three. Specific activity of glutamine synthetase is almost unaffected. Specific activity of glutamate dehydrogenase is lowest in the root tips, medium in young root hair zone and highest in the old root hair zone, glutamine synthetase activity is reverse in the three zones with differences by a factor of 3-5; aspartate aminotransferase is similarly active in the three zones. Nitrate concentration used (100 μM) for cultivation of the wheat plants was tested with Azospirillum brasilense in pure culture on agar surfaces exposed to air at the same pH (5.8), used for cultivation of the wheat plants. Activity over a 14 day period (peak activity 70 nmol C 2 H 4 · mg protein −1 · h−1) was not affected, however 1 mM and 5 mM nitrate added reduced the total activity to 50 % and 10 % respectively.