Growth, proline accumulation, and ion content in sodium chloride-stressed callus of date palm

Growth and physiological responses of date palm, Phoenix dactylifera L. cv. Barhee, callus to salinity stress were examined. Callus induced from shoot tips of offshoots was cultured on Murashige and Skoog medium supplemented with NaCl at concentrations ranging from 0 to 225 mM, in consecutive increments of 25 mill. Data obtained after 6 wk of exposure to salt have shown a significant increase in callus proliferation in response to 25 mM NaCl, the lowest level tested, beyond which callus weight decreased. At 125 mM NaCl and higher, callus growth was nearly completely inhibited. Physiological studies on callus exposed to salt stress have shown an increase in proline accumulation in response to increased salinity. Proline accumulation was correlated to callus growth inhibition. Furthermore, increasing the concentration of NaCl in the culture medium generally resulted in a steady increase in Na+ and reduction in K+ concentrations. However, at 25 mM NaCl, the only level at which callus growth was significantly enhanced, an increase in K+ content was noted in comparison to the NaCl-free control. In response to increasing external NaCl level, the Na+/K+ ratio increased. The Na+/K+ ratio was positively correlated to proline accumulation and hence callus growth inhibition. This study provides an understanding of the response of date palm callus to salinity, which is important for future studies aimed at developing strategies for selecting and characterizing somaclonal variants tolerant to salt stress.