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

Abstract

Root NO3− absorption was examined under steady state conditions in the presence and absence of NH4+ using intact tomato plants (Lycopersicon esculentum cv. T-5). Plants grown under a low-salt regime showed much higher rates of NO3− absorption than plants grown under a high-salt regime, but the presence of NH4+ at concentrations less than 200 µM increased the capacity for net NO3− uptake for both the low- and high-salt conditions. Simultaneous changes in net NO3− , K+, and H+ exchanges were continuously monitored for 3 h prior to and up to 7 h following exposure to NH4+ . Upon first exposure to 50 or 100 µM NH4+, NO3− absorption remained constant; but during the subsequent 6 to 7 h, NO3− absorption continually increased. Net K+ absorption decreased iµMediately following its first exposure to NH4+ , but gradually recovered during the 7 h following first exposure. Changes in K+ absorption were not correlated with changes in NO3− absorption. Proton efflux gradually increased under NH4+ exposure and was significantly correlated with the observed increase in NO3− absorption. When roots absorbing NO3− were exposed to 5000 µM NH4+, NO3− absorption declined throughout the entire observation period.