THE ROLE OF THE STEM IN THE PARTITIONING OF NA+ AND K+ IN SALT-TREATED BARLEY

Hordeum vulgare cv. California Mariout was grown for 50 d in sand culture at 100 mol m-3 NaCl. Xylem sap was collected through incisions at the base of individual leaves along the stem axis by applying pressure to the root system. K+ concentrations in the xylem sap reaching individual leaves increased towards the apex, while concentrations of Na+, NO3-, and Cl- declined. Phloem exudate was obtained by collecting into Li2EDTA from the base of excised leaves. K/Na ratios of phloem exudates increased from older to younger leaves. K/Na ratios in xylem sap and phloem exudate were combined with changes in ion content between two harvests (38 and 45 d after germination) and the direction of phloem export from individual leaves, to construct an empirical model of K+ and Na+ net flows within the xylem and phloem of the whole plant. This model indicates that in old leaves, phloem export of K+ greatly exceeded xylem import. In contrast, Na+ export was small compared to import and Na+ once imported was retained within the leaf. The direction of export strongly depended on leaf age. Old, basal leaves preferentially supplied the root, and most of the K+ retranslocated to the roots was transferred to the xylem and subsequently became available to the shoot. Upper leaves exported to the apex. Young organs were supplied by xylem and phloem, with the xylem preferentially delivering Na+, and the phloem most of the K+. For the young ear, which was still covered by the sheath of the flag leaf, our calculation predicts phloem import of ions to such an extent that the surplus must have been removed by an outward flow in the xylem. Within the culm, indications for specific transfers of K+ and Na+ between xylem and phloem and release or absorption of these ions by the tissue were obtained. The sum of these processes in stem internodes and leaves led to a non-uniform distribution of Na+ and K+ within the shoot, Na+ being retained in old leaves and basal stem internodes, and K+ being available for growth and expansion of young tissues.