Heritable chilling tolerance improvement in rice through somaclonal variation and cell line selection

Embryo-derived calli of four rice varieties (Oryza sativa L.) were submitted to different continuous or discontinuous periods of chilling stress at 4 degrees C, with a total length varying from 2 to 6 weeks. Other calli were cultivated during different times without cold stress. The reduction of plant regeneration percentages induced by low temperature was more pronounced in the more cold-sensitive varieties. Regenerated plants (RO) and their descendants in R1, R2 and R3 generations were cold-screened together with control plants. A mass selection was applied to the control plants during three successive generations. In all varieties, significantly higher chilling survival rates were obtained in R3 with in vitro grown plants than with control plants. Higher plant survival rates were obtained with the more chilling-sensitive varieties when a short discontinuous chilling treatment or no treatment had been applied, with the intermediate variety with short or intermediate treatments, and with the more cold-tolerant variety with longer, continuous treatments. The relative importance of pre-existing versus in vitro-induced variation and of epigenetic versus heritable variation, along with the significance of such cold tolerance improvement for breeding purposes are discussed.