Interactive effects of Ca²⁺ and NaCl salinity on the ionic relations and proline accumulation in the primary root tip of Sorghum bicolor

The effects of supplemental Ca²⁺ supply and NaCl salinity on the ionic relations and levels of proline and other amino acids in the primary root of Sorghum bicolor (cv. Hegari) seedlings were investigated. Two days of exposure to 150 mM NaCl resulted in a 50-fold increase in the proline level in the 0-10 mm root tips of seedlings supplied with 5.0 mM Ca²⁺, but only a 4-fold increase in seedlings with 0.5 mM Ca²⁺. In contrast to the high levels of proline in the root tip, proline accumulation was only modest in the expanded tissues of the root. The enhancement of proline accumulation in the root tip of salinized seedlings with the Ca²⁺ supplement may be related to their more favorable tissue K to Na ratio. Thus, elevated Ca²⁺ may mitigate the NaCl-induced inhibition of S. bicolor root growth via the maintenance of net K to Na selectiv-ity and the enhancement of proline accumulation in the root tip.