Root-shoot interactions in mineral nutrition

In this paper four classes of co-operative root-shoot interations are addre ssed. (I) Nitrogen concentrations in the xylem sap originating from the roo t and in the phloem sap as exported from source leaves are much lower than those required for growth by apices and developing organs. Enrichment of xy lem sap N is achieved by xylem to xylem (X-X) transfer, by which reduced N, but not nitrate, is abstracted from the xylem of leaf traces and loaded in to xylem vessels serving the shoot apex. Nitrogen enrichment of phloem sap from source leaves is enacted by transfer of reduced N from xylem to phloem (X-P transfer). Quantitative data for the extent of the contribution of X- X and X-P transfer to the nutrition of young organs of Ricinus communis L. and for their change with time are presented. (II) Shoot and root cooperate in nitrate reduction and assimilation. The partitioning of this process be tween shoot and root is shifted towards the root under conditions of nitrat e- and K-deficiency and under salt stress, while P deficiency shifts nitrat e reduction almost totally to the shoot. All four changes in partitioning c an be attributed to the need for cation-anion balance during xylem transpor t and the change in electrical charge occurring with nitrate reduction. (II I) Even maintenance of the specificity of ion uptake by the root may - in a ddition to its need for energy - require a shoot-root interaction. This is shown to be needed in the case of the maintenance of K/Na selectivity under the highly adverse condition of salt stress and absence of K supply from t he soil. (IV) Hormonal root to shoot interactions are required in the whole plant for sensing mineral imbalances in the soil. This is shown and addres sed for conditions of salt stress and of P deficiency, both of which lead t o a strong ABA signalling from root to shoot but result in different patter ns of response in the shoot.