Importance of mineral nutrient cycling for salinity tolerance of plants

Abstract

This paper reports on two approaches to an understanding of the importance of phloem and xylem transport of minerals for salt tolerance, i.e. by quantitative assessment of mineral ion flows within the whole plant and by studying roots grown in the absence of external K⁺, both under conditions of external salinity. Partitioning and flows of mineral ions have been determined in Hordeum vulgare (salt-tolerant cereal), in Ricinus communis (moderately tolerant) and in Lupinus albus (sensitive). In Ricinus and Lupinus C and N flows were measured first, and ion flows were then determined on the basis of C flows and ion/C ratios in the phloem. In Hordeum K⁺ and Na⁺ flows were obtained without reference to C. Both methods involved collection and analyses of phloem and xylem sap and two consecutive harvests of plant organs for measuring increments of C, N and ions over the study period. The ions investigated were the K⁺ and the potentially harmful ions Na⁺ and Ch. K⁺ flows showed, as a common pattern for the three species, a major contribution of phloem transport with retranslocation from leaves and massive cycling through the root. The most salt-tolerant of these plants, Hordeum vulgare, proved to be most efficient in cycling and recovery of the essential nutrient K⁺. For Na⁺ the measurements revealed quite contrasting flow patterns for the three species. Hordeum showed high rates of xylem and very low phloem transport of Na⁺, resulting in an inclusion of Na⁺ as a ‘cheap osmoticum’ in mature and old leaves. In Ricinus Na⁺ was very efficiently excluded from leaf laminae, while in Lupinus Na⁺ exclusion was much less effective. In this species Na⁺ was loaded into the phloem and translocated also to younger organs. C1^- was not excluded to the same extent as Na⁺ in Ricinus. Studies of Ricinus grown with split roots and without external supply of K⁺ showed that phloem import of K⁺ is fully sufficient to meet the K⁺ demand of the root and maintain cytoplasmic K⁺ homoeostasis in root cells and K/Na selectivity even under severe salt stress. The importance is discussed of the specific flows and partitioning of Na⁺ and of the impact of potassium cycling and reutilization within the plant for salinity tolerance.