Nitrate Uptake by Wheat as Influenced by Ammonium and Other Cations 1

Abstract

The influence of ammonium on nitrate uptake was studied using 14-day-old nitrogen-depleted wheat seedlings and seedlings of the same age which had received an adequate supply of nitrate nitrogen. With the nitrogen depleted seedlings, nitrate absorption from NH4NO3 was less than from other nitrate salts. The difference was clearly evident by 3 hours and became progressively larger with time. Nitrate uptake from all salts exhibited a slower rate during the first 3 hours than subsequently. Ammonium uptake was more rapid during the first 3 hours than in the subsequent 9 hours, following which the rate again increased. This pattern was similar even when ammonium uptake was depressed by high temperatures or prior exposure of the seedlings to nitrate. It was suggested that ammonium, and to some extent the high acidity adjacent to cellular boundary membranes resulting from ammonium uptake in excess of nitrate uptake, resulted in alterations in membrane permeability thereby restricting capacity for nitrate absorption.