Ion Concentration and Carbohydrate Status of the Elongating Leaf Tissue 4Hordeum vulgare Growing at High External NaCl: I. RELATIONSHIP BETWEEN SOLUTE CONCENTRATION AND GROWTH
Seedlings of a salt tolerant barley variety, cv ‘Beecher’, were grown for 5 d between ‘0’ mM and 180 mM external NaCl (NaClcxt). The rate of leaf elongation and shoot dry weight decreased as the NaCl increased from ‘ 0’ –180 mM. Cl− and (Na+ + K+) concentrations accounted for about 50% of the osmotic pressure of the elongating tissues at all NaClext. The increase in (Na+ + K+) concentration with increasing NaClext was attributable to decreases in the elongation rate and not to increases in net ion uptake. However, there were large increases in Cl− uptake between ‘0’ mM and 40 mM NaCl, and even above 40 mM NaCl not more than two-thirds of the increases in Cl− concentration of the elongating leaf tissues could be attributed to reduced elongation. Thus, the rate of net Cl− uptake increased appreciably with increasing NaClext Nevertheless, the overall results indicate that most of the osmotic adjustment of elongating leaf tissues was probably a result of reduced growth. These data, together with those presented in the following paper, are consistent with the hypothesis that this salt tolerant variety of Hordeum vulgare is reduced in growth at high NaCl by water deficit in the elongating leaf tissues. Following an increase in NaClext, the rates of Cl− and Na+ uptake into the elongating zone were about 2–4 times the rates into the mature leaves, indicating that a substantial proportion of ions entering the elongating tissue was derived from the phloem rather than the xylem. This raises the possibility that the water deficit in the elongating tissue could be due to an insufficient supply of ions by the phloem. Nevertheless, the evidence indicates that the water deficit in the elongating tissue is due to saturation of ion uptake by individual cells, rather than to a limited input to the tissue as a whole.