Plant and Soil 191: 67–76, 1997. 67 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Root elongation in saline solution related to calcium binding to root cell plasma membranes Uri Yermiyahu 1 , Shlomo Nir 2 , Gozal Ben-Hayyim 3 , Uzi Kafkafi 1 and Thomas B. Kinraide 45 1 Department of Field and Vegetable Crops, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76–100, Israel, 2 The Seagram Center for Soil and Water Sciences, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76–100, Israel, 3 Institute of Horticulture, The Volcani Center, Bet Dagan 50250, Israel and 4 Appalachian Soil and Water Conservation Research Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Beaver, WV, 25813–0400, USA. 5 Corresponding author Received 3 April 1996. Accepted in revised form 12 March 1997 Key words: calcium, plasma membrane, root elongation, salinity, sodium Abstract To gain a better understanding of the relations between root elongation and the amount of Ca 2 bound to the plasma membrane (PM), melon plants were grown in aerated solutions containing different concentrations of CaCl 2 with various concentrations of NaCl or mannitol. With increasing external concentrations of NaCl or mannitol, root elongation was suppressed. Addition of CaCl 2 to the external medium alleviated the inhibition of root elongation by high concentrations of Na , but not of mannitol. Root elongation in media containing high concentrations of NaCl was correlated with the computed amount of Ca 2 bound to the PM. A model describing relative root elongation (RRL) under salt stress was developed. This model takes into account the osmotic potential in the growing solution (based on the mannitol experiments) and the computed amount of Ca 2 bound to the PM. Calcium binding was calculated by applying a Gouy-Chapman-Stern sorption model using the same parameters deduced from studies on PM vesicles. This model combines electrostatic theory with competitive binding at the PM surface. The model for RRL allowed the computation of a critical value for the fraction of negative sites binding Ca 2 on the PM needed for nearly optimal (95%) root elongation. Any decrease below this critical value decreased the RRL. Root elongation of Honey Dew (salt-resistant cv.) was greater than that of Eshkolit Ha’Amaqim (salt-sensitive cv.) under NaCl stress. Nearly optimal root growth for Honey Dew and Eshkolit Ha’Amaqim occurred when 40% and 51% of total membrane charged sites were bound by Ca 2 , respectively. The effect of osmotic potential on the suppression of root elongation was the same for the two cultivars. To our knowledge, this report provides the first fully quantitative estimates of PM-bound Ca 2 relative to salt toxicity. Abbreviations: IPC – the number of Ca 2 atoms bound to each negatively charged binding site on the PM (i.e., to each intrinsic negative charge), K – intrinsic binding constant for ion i, OP – osmotic potential, PM – plasma membrane, RE – root elongation, RRL – relative root elongation, RRL – the RRL influenced by ionic effects, RRL – the RRL influenced by OP, 0 or – chemical activity of free ion i at the PM surface (0) or in the external rooting medium ( ), [i] 0 or [i] – concentration of free ion i at the PM surface (0) or in the external rooting medium ( ), – actual surface charge density, 0 – intrinsic surface charge density, 0 – PM surface electrical potential. FAX No.: +13042562921. E-mail: tkinraide@asrr.arsusda.gov Introduction Salinity decreases plant growth and yield to various degrees depending on plant species, salinity level and