Partitioning of K+, Na+, Mg++ , and Ca++ through Xylem and Phloem to Component Organs of Nodulated White Lupin under Mild Salinity

Abstract

Summary Rates of cation (K + , Na + , Mg ++ , Ca ++ ) transport via xylem and phloem and of cation exchange among component organs of effectively nodulated white lupin ( Lupinus albus L. cv. Ultra) were determined during an 8-day period at flowering under mild salinity (10 mol m −3 NaCl). Carbon, nitrogen and mineral cations were analyzed in roots, leaflets, stem, and petiole tissue of each of four strata of leaves of the shoot, in root bleeding (xylem) sap and in phloem saps obtained from stems and petioles of the corresponding strata. Respiration of roots and respiration and photosynthesis of stem segments and leaves of each stratum of the shoot were measured. Carbon and nitrogen flows within the shoot and between root and shoot were then estimated as in earlier studies (Pate et al., 1979 b). Using these flow profiles, ratios of each cation to carbon in transport fluids and increments of ions in plant parts during the study period and patterns of partitioning of individual cations were estimated. The data revealed high rates of transport in phloem and xylem within the shoot for K + , and to a lesser extent also for Mg ++ and Na + . Substantial rates of cycling of K + and Mg ++ between shoot and root were observed. K + was translocated preferentially towards young, growing organs, but Na + mainly deposited in stems (partly in exchange for K + ) and transported preferentially to the root. Acropetal translocation of K + in phloem occurred preferentially towards the inflorescence, in xylem principally towards lateral branches. The flow model for K + suggested progressive enrichment of the upward moving xylem stream with K + through mobilization of K + from mature stem tissues, combined with selective phloem to xylem transfer of K + in the stem vasculature. Translocation of Na + to the root was partly due to preferential xylem to phloem transfer within stem tissue. Basipetal transport of Na + was particularly evident when the external supply of NaCl was removed. Phloem mobility of Na + is discussed in relation to the relatively limited salt tolerance of white lupin.