NITRATE REDUCTION, NITRITE REDUCTION AND AMMONIUM ASSIMILATION IN BARLEY ROOTS IN RESPONSE TO ANOXIA

Nitrate reduction, nitrate reductase (NR)-activity and activation stat e, nitrite reduction and ammonium assimilation were measured in barley roots (Hordeum vulgare cv. Gaulois) in response to anoxia, uncouplers or cellular acidification. Ammonium and nitrite accumulation were fol lowed in root segments and in the suspending buffer solution hushed wi th air or nitrogen, in the presence of methionine sulfoximine (MSX). C atabolic ammonium formation was subtracted from nitrate-dependent ammo nium formation for a correct estimation of nitrite reduction rates in vivo. In anoxic roots, nitrite was accumulated and partly released. Th is was due to both, a stimulation of nitrate reduction and a partial i nhibition of nitrite reduction. Ammonium assimilation rates were also impaired by anoxia. Due to nitrite and ammonium leakage, anoxic roots lost a major proportion of their primary nitrate assimilation products . In aerobic roots, excess nitrite production could be also induced by treatment with uncoupler (CCCP) or by tissue acidification (propionic acid at pH 4.8). The same conditions caused an activation of (NADH)-n itrate reductase (NR, EC 1.6.6.1), a partial inhibition of nitrite red uction and of ammonium assimilation. Increased nitrite production and activation of NR were prevented by cantharidin, a protein phosphatase inhibitor. Our data indicate that nitrite release by anoxic roots was a consequence of both, an activation of NR by protein dephosphorylatio n and an inhibition of nitrite reduction. Both events may be causally related to cellular acidification under anoxia.