Cycling of Amino-Nitrogen and other Nutrients between Shoots and Roots in Cereals—A Possible Mechanism Integrating Shoot and Root in the Regulation of Nutrient Uptake

Abstract

A split root system was used to investigate the cycling of nitrogen between shoots and roots in young wheat and rye plants. ¹⁵N-nitrate was supplied to one part of the root system for various periods, at the end of which these roots were excised. Xylem sap was then collected from the other roots which had not been supplied directly with ¹⁵N-nitrate. ¹⁵N detected in the xylem sap indicated cycling of nitrogen between shoots and roots. Calculations showed that over 60% of the amino-N flux in the xylem was cycling. Thus nitrate assimilation in the root could account for only a minor part of amino-N in the xylem sap. The specific activity of ¹⁵N in the total N of xylem sap was higher than in the total N of roots and shoots through which it had cycled. This is because exchange between amino-N in the transport pools and bulk tissue N is limited. It is proposed that there is, in effect, a single regulatory pool of amino-N, common to shoots and roots, and that this pool may be a key element in the control of N uptake at the level of the whole plant. The likely energy costs of cycling and implications for the partitioning of N between shoots and roots are discussed. In further investigations the cycling of ⁴²K-potassium and ³²S-sulphur was demonstrated.