Ionic Interactions of Petiole and Lamina During the Life of a Leaf of Castor Bean (Ricinus communisL.) Under Moderately Saline Conditions

Ion (K+, Na+, Mg2+, Ca2+ and Cl∼) flows and partitioning in the petiole and lamina of leaf 6 of castor bean {Ricinus communis L.) plants growing in the presence of a mean of 71 mol m−3 NaCl were described by an empirical modelling technique. This combined data on changes in ion contents of petiole and lamina, ion: carbon molar ratios of phloem bleeding sap and pressure-induced xylem exudates of the leaf with previously described information on the economies of C and N in identical leaf material. Data were expressed as daily exchanges of ions in xylem and phloem, or depicted as models of ion balance and transport activity of petiole and lamina during four consecutive phases of leaf life. The early import phase was characterized by high intake of K and Mg through phloem, and of Ca mainly through xylem, but only limited intake of Na and Cl. The next phase up to full leaf expansion showed similar relative differences in xylem intake between ions and the onset of rapid phloem export from the lamina of K and Mg, some export of Na and Cl but scarcely any of Ca. The next mature phase, marked by maximal photosynthesis and transpiration by the leaf, showed high xylem intake of all ions in xylem. This was more than matched by phloem export of Mg and K, but by only fractional re-export of Na and Cl and again very limited cycling through the leaf of Ca. The final pre-senescence phase exhibited similar behaviour, but with generally greater contribution to phloem transport from mobilization of ion reserves of the lamina. The petiole retained particularly large amounts of Na and Cl in its early growth, thereby protecting the lamina from excessive entry of salt, but these petiolar pools, together with those or other nutrient ions, were later partially mobilized to the lamina via the xylem stream. Data were discussed in relation to the relatively high salt tolerance exhibited by the species.