External Potassium Supply is not Required for Root Growth in Saline Conditions: Experiments withRicinus communisL. Grown in a Reciprocal Split-Root System

Ricinus communis L. (castor bean) plants were grown in the absence (control) and in the presence of 100molm−3NaCl with a reciprocal split-root system, in which K+ was supplied to one and NO3− to the other part of the root system. In these plants shoot and, to a lesser extent, total root growth were inhibited compared to plants with non-split roots. Without and with NaCl, growth of roots receiving NO3− but noK+ (‘minus K/plus N-roots’) was substantially more vigorous than under the reverse conditions (‘plus K/minus N-roots1).100 mol m−3 NaCl inhibited growth of minus K/plus N-roots1 to the same extent as that of non-split roots, indicating that externally supplied K+ was not required for root growth under saline conditions. In growth media without added K+ the root depleted the external low K + levels resulting from chemicals down to a minimum value Cmln (1.0 to 1.4 mmol m−3); in the presence of 100 mol m−3 NaCl, Cmin, was higher (10–18 mmol m−3) and resulted from an initial net loss of K +. Cmin, was pH-dependent The distribution of K+, Na+ and Mg2+ along the root was measured. In meristematic root tissues, K + concentrations were scarcely affected by external K+ or by NaCl, where Na + concentrations were low, but somewhat elevated at low external K+ and/or high NaCl. In differentiated, vacuolated tissues K + concentrations were low and Na+ concentrations high, if K + was not supplied externally and/or NaCl was present. The longitudinal distribution of ions within the root was used to estimate cytoplasmic and vacuolar ion concentrations. These data showed a narrow homoeostasis of cytoplasmic K+ concentrations (100–140 mol m−3) independent of external K + supply even in the presence of 100 mol m −3 NaCl. Cytoplasmic Na + concentrations were maintained at remarkably low levels. Hence, external K+ concentrations above Cmin, were not required for maintaining K/Na selectivity, i.e. for controlling Na+ entry. The results are discussed with regard to mechanisms of K/Na selectivity and to the importance of phloem import of K+ for salt tolerance of roots and for cytoplasmic K+ homoeostasis.