Nitrogenase and nitrate reductase activities in young Alnus glutinosa, relationship and effect of light/dark treatments

Relations between in vivo nitrogenase (N 2 -ase ; EC 1.18.6.1) and nitrate reductase (NR ; EC 1.6.6.1) activities were studied in young nodulated Alnus glutinosa (L.) Gaertn. Positive correlations linked N 2 -ase activity, constitutive (non-inducible by nitrate) leaf NR activity and plant growth. Light/dark treatments applied to the whole shoot or, separately, to the upper and lower part of shoot led to the finding that (a) the constitutive leaf NR activity depended on direct illumination of the leaf and did not appear subordinate to the N 2 -ase activity ; (b) the N 2 -ase activity was much more efficiently supported by the illumination of the upper (young) leaves than by that of the lower (mature) leaves ; (c) the maintenance of nitrate-inducible root NR activity required leaf illumination. The variations of both N 2 -ase and root NR activities in response to light/dark treatments emphasize the importance of the photosynthesis for the reduction of dinitrogen and nitrate in nodulated roots of Alnus glutinosa.