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A NaCi-tolerant cell lne which was selected from ovular callus of 'Shamouti' orange (Citrus sinensis L. Osbeck) proved to be a true cell lne variant. This conclusion is based on the following observations. (a) Cells which have been removed from the selection pressure for at least four passages retain the same NaCI tolerance as do cells which are kept constantly on 0.2 molar NaCI. (b) Na+ and Cl uptake are considerably lower in salt-tolerant cells (R-10) than in salt-sensitive cells (L-5) (2,3,25,28,30). Selection has commonly been practiced following exposure to salt, drought, cold, and herbicides. Of these agriculturally useful traits, resistance or tolerance to salt is becoming increasingly important among crops of agricultural value grown in arid zones. Selection of cell lines which exhibit tolerance to salt stress has been reported before (6,7,11,21)

1- to 8-week-old ovules and nucelli from three Citrus cultivars-Shamouti and Valencia (Citrus sinensis) oranges and Marsh Seedless (C. paradisi) grapefruit-were cultured in vitro. No embryo differentiation was observed in the explants prior to culture. The Shamouti ovules had degenerated and were apparently unfertilized. Embryoids formed on Murashige and Tucker nutrient medium supplemented with 500 mg/l malt extract. Whole plants developed on the same basal medium supplemented with kinetin and indole-3-acetic acid (IAA), coconut milk or gibberellic acid (GA3). A higher kinetin/IAA ratio or the addition of coconut milk favoured stem elongation more than root formation while a lower kinetin/IAA ratio favoured root formation and inhibited stem elongation. The addition of GA3 to the basal medium stimulated rooting and stem elongation. These results can be of aid in mutation research, allowing irradiation at stages prior to embryonic development.

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