EFFECT OF AMMONIUM ON THE REGULATION OF NITRATE AND NITRITE TRANSPORT-SYSTEMS IN ROOTS OF INTACT BARLEY (HORDEUM-VULGARE L) SEEDLINGS

The effect of NH4+ on the regulation of NO3- and NO2- transport system s in roots of intact barley (Hordeum vulgare L.) seedlings grown in NO 3- or NO2- was studied. Ammonium partially inhibited ''induction'' of both transport systems. The inhibition was less severe in NO2--fed tha n in NO3--fed seedlings, presumably due to lower uptake of NP4+ in the presence of NO2-. In seedlings pretreated with NH4+ subsequent: ''ind uction'' was inhibited only when NH4+ was also present during ''induct ion'', even though pretreated roots accumulated high levels of NH4+. T his indicates that inhibition may be regulated by NH4+ concentration i n the cytoplasm rather than its total accumulation in roots. L-Methion ine sulfoximine did not relieve the inhibition by NH4+, suggesting tha t inhibition is caused by NH4+ itself rather than by its assimilation product(s). Ammonium inhibited subsequent expression of NO3- transport activity similarly in roots grown In 0.1, 1.0, or 10 mM NO3- for 24 h (steady-state phase) or 4 d (decline phase), indicating that it has a direct, rather than general feedback effect. ''Induction'' of the NO3 - transport system was about twice as sensitive to NH4+ as compared to the NO2- transport system. This may relate To higher turnover rates o f membrane-associated NO3--transport proteins.