Investigations of ion absorption during NH4+ exposure I. Relationship between H+ efflux and NO3- absorption

Abstract

Root NO 3 - , absorption was examined under steady-state conditions in the presence and absence of NH 4 + using intact tomato plants (Lycopersicon esculentum cv. T-5). Plants grown under a low-salt regime showed much higher rates of NO 3 - , absorption than plants grown under a high-salt regime, but the presence of NH 4 + at concentrations less than 200 μM increased the capacity for net NO 3 - , uptake for both the low- and high-salt conditions. Simultaneous changes in net NO 3 - , K + , and H + exchanges were continuously monitored for 3 h prior to and up to 7 h following exposure to NH 4 + . Upon first exposure to 50 or 100 μM NH 4 + , NO 3 - , absorption remained constant; but during the subsequent 6 to 7h, NO 3 - , absorption continually increased. Net K + absorption decreased immediately following its first exposure to NH 4 + , but gradually recovered during the 7 h following first exposure. Changes in K + absorption were not correlated with changes in NO 3 - absorption. Proton efflux gradually increased under NH 4 + exposure and was significantly correlated with the observed increase in NO 3 - absorption. When roots absorbing NO 3 - were exposed to 5000 μM NH 4 + , NO 3 - absorption declined throughout the entire observation period.