External potassium supply is not required or root growth in saline conditions: experiments with Ricinus communis L. grown in a reciprocal split-root system

• Published in: Journal of experimental botany 1988-09, Vol.39 (206), p.1149-1167
• Main Authors: Jeschke, W.D, Wolf, O
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biological and medical sciences; Cytoplasm; Deficiencies. Phytotoxicity of elements. Salinity; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; growth; ion transport; nitrate; Nitrates; Phloem; phloem transport; Plant physiology and development; Plant roots; Plants; Potassium; Ricinus communis; root (split-root); Root growth; Root tips; roots; Salt tolerance; sodium chloride; Soil-plant relationships. Soil fertility. Fertilization. Amendments; Water and solutes. Absorption, translocation and permeability; Xylem
• ISSN: 0022-0957; 1460-2431
• DOI: 10.1093/jxb/39.9.1149

Ricinus communis L. (castor bean) plants were grown in the absence (control) and in the presence of 100 mol m-3 NaCl 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 no K+ (minus K/plus N-roots') was substantially more vigorous than under the reverse conditions ('plus K/minus N-roots'). 100 mol m-3 NaCl inhibited growth of 'minus K/plus N-roots' 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 Cmin (1.0 to 1.4mmol 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.