Nitrate assimilation by decapitated corn root systems: effects of ammonium during induction

Abstract

Development of the accelerated phase of nitrate uptake by darkgrown decapitated corn seedlings upon first exposure to nitrate (1.0 mM, pH 6, 30°C) was restricted by the presence of ambient ammonium (0.5 mM (NH 4) 2SO 4). Upon transfer to ammonium-free solutions, the increase in the nitrate uptake rate paralleled the increase in seedlings not exposed to ammonium, but did not, within 6 h, completely recover to yield the rate occuring in the absence of ammonium throughout. In contrast, the restriction in potassium uptake was completely eliminated within 2 h following transfer. Presence of ammonium decreased reduction of [ 15N]nitrate and in vitro nitrate reductase activity (NRA) of the root tissue, but the decrease in [ 15N]nitrate reduction was not sufficient to account for the decrease in uptake. The data are interpreted as indicating that ammonium exerted a detrimental effect on the formation of the nitrate uptake system.