Simultaneous observation of nitrogen uptake, transport and assimilation by rice and wheat young plants fed ¹⁵N-nitrate

Abstract

Physiochemical processes operating in nitrate assimilation by young whole plants of rice and wheat were studied after transferring them from 0.07 mM to 3 or 15 mM nitrate medium. Uptake, xylem exudation and metabolism of nitrogen were analyzed by the ¹⁵N tracer method to distinguish previously and newly uptaken nitrogen. In rice plants at an early stage (0–2 h after transfer), nitrate uptake, assimilation and xylem transport of newly absorbed nitrogen were more active in medium containing 15 mM than 3 mM nitrate. However, at a later stage (10–12 h), amino acid synthesis in roots from newly uptaken nitrogen and xylem transport of the amino acids decreased in the plant treated with 15 mM nitrate, though transport of nitrate nitrogen and amino-nitrogen originating from previously uptaken nitrogen actively continued. In wheat plants at a late stage, both uptake and xylem transport of newly absorbed nitrogen in plants treated with 15 mM became lower than in those with 3 mM nitrate. In the plants treated with 15 mM nitrate, xylem transport of newly absorbed nitrate nitrogen decreased more markedly than that of amino-nitrogen derived from newly absorbed nitrate, whereas transport of previously uptaken nitrate actively continued till late stage in the plants treated with both 3 and 15 mM. Nitrogen uptake, transport and metabolism in roots and shoots were correlated in the whole plant, and the two plant species responded differently to the increase of medium nitrate.