Yermiyahu, U., Dept. of Field and Vegetable Crops, Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot 76-100, Israel
Nir, S., Seagram Ctr. for Soil and Water Sci., Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot 76-100, Israel
Ben-Hayyim, G., Institute of Horticulture, Volcani Center, Bet Dagan 50250, Israel
Kafkafi, U., Dept. of Field and Vegetable Crops, Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot 76-100, Israel
Kinraide, T.B., Appalachian Soil Water Conserv. R., Agricultural Research Service, U.S. Department of Agriculture, Beaver, WV 25813-0400, United States

To gain a better understanding of the relations between root elongation and the amount of Ca2+ bound to the plasma membrane (PM), melon plants were grown in aerated solutions containing different concentrations of CaCl2 with various concentrations of NaCl or mannitol. With increasing external concentrations of NaCl or mannitol, root elongation was suppressed. Addition of CaCl2 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 Ca2+ 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 Ca2+ 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 Ca2+ 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 Ca2+, 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 Ca2+ relative to salt toxicity.