Abstract: Somaclonal tolerant cell line and its regenerants were obtained from suspension culture cells of nucellar calli in Citrus sinensis (L.) Osbeck cv. Jincheng via gamma ray mutagenesis and in vitro selection with concentrated hydroxyproline, which were cold tolerant at 1.4 ℃ and 2.4 ℃ more than the suspension cells of wild types respectively. The capability of enhanced cold tolerance in the variant plantlet L3-1 was genetically stable based on a three-year evaluation. There might be slight differences between their total DNA from leaves of L3-1 and the wild type according to the polymorphism between their total DNA RAPD patterns. Except proline accumulation either at the suspension cell or at the plantlet levels, antioxygenases SOD and CAT activities were stimulated much higher and MDA contents much lower than the wild type after cold acclimation at the suspension cell levels, which indicated that physiological mechanism on the enhanced cold tolerance of the variant was very complex. Not only proline metabolism, but antioxygenases and some others were also involved in its enhancement process on cold hardiness.

Key words: Citrus sinensis, Somaclonal variant, Cold tolerance, Genetic stability, Antioxygenase