Cation and Chloride Partitioning through Xylem and Phloem within the Whole Plant ofRicinus communisL. under Conditions of Salt Stress

Uptake and partitioning through the xylem and phloem of K+, Na+, Mg2+ , Ca2+ and Cl− were studied over a 9 d interval during late vegetative growth of castor bean (Ricinus communis L.) plants exposed to a mean salinity stress of 128 mol m−3 NaCl. Empirically based models of flow and utilization of each ion within the whole plant were constructed using information on ion increments of plant parts, molar ratios of ions to carbon in phloem sap sampled from petioles and stem internodes and previously derived information on carbon flow between plants parts in xylem and phloem in identical plant material. Salient features of the plant budget for K+ were prominent deposition in leaves, high mobility of K+ in phloem, high rates of cycling through leaves and downward translocation of K+ providing the root with a large excess of K+ . Corresponding data for Na+ showed marked retention in the root, lateral uptake from xylem by hypocotyl, stem internodes and petioles leading to low intake by young leaf laminae and substantial cycling from older leaves back to the root. The partitioning of the anionic component of NaCl salinity, Cl−, contrasted to that of Na+ in that it was not substantially retained in the root, but deposited more or less uniformly in stem, petiole and leaf lamina tissues. The flow pattern for Mg2+ showed relatively even deposition through the plant but some preferential uptake by young leaves, generally lesser export than import by leaf laminae, and a return flow of Mg2+ from shoot to root considerably less than the recorded increment of the root. Ca2+ partitioning contrasted with that of the other ions in showing extremely poor phloem mobility, leading to progressive preferential accumulation in leaf laminae and negligible cycling of the element through leaves or root. Features of the response of Ricinus to salinity shown in the present study were discussed with data from similar modelling studies on white lupin (Lupinus albus L.) and barley (Hordeum vulgare L.)