REGULATION OF ALDEHYDE OXIDASE AND NITRATE REDUCTASE IN ROOTS OF BARLEY (HORDEUM-VULGARE L.) BY NITROGEN-SOURCE AND SALINITY

The molybdenum cofactor (MoCo) is a component of aldehyde oxidase (AO EC 1.2.3.1), xanthine dehydrogenase (XDH EC 1.2.1.37) and nitrate redu ctase (NR, EC 1.6.6.1), The activity of AO, which catalyses the last s tep of the synthesis of abscisic acid (ABA), was studied in leaves and roots of barley (Hordeum vulgare L.) plants grown on nitrate or ammon ia with or without salinity. The activity of AO in roots was enhanced in plants grown with ammonium while nitrate-grown plants exhibited onl y traces. Root AO in barley was enhanced by salinity in the presence o f nitrate or ammonia in the nutrient medium while leaf AO was not sign ificantly affected by the nitrogen source or salinity of the medium. S alinity and ammonium decreased NR activity in roots while increasing t he overall MoCo content of the tissue. The highest level of AO in barl ey roots was observed in plants grown with ammonium and NaCl, treatmen ts that had only a marginal effect on leaf AO, ABA concentration in le aves of plants increased with salinity and ammonium.