Stolon characteristics related to winter survival in white clover

Changes in the stolon carbohydrate contents (water-soluble and total non-structural) of four white clover populations were measured at the beginning and end of winter. Three of the populations were of Swiss origin - two of these were from high-altitude pastures, and the other from a valley location. Grasslands Huia was measured as a control variety. Levels of both types of carbohydrate declined during the winter in all populations. The Swiss populations from high altitudes contained the highest levels of both carbohydrate types. An artificial freezing test was carried out on stolon segments collected from a range of clover populations (including the two high-altitude Swiss clovers) growing in field plots in January, and values of ‘lethal dose 50 1(LD 50) were calculated. The two Swiss populations had the lowest LD 50 values, indicating a greater intrinsic tolerance to freezing in those plants than in the other populations (of lowland temperate origin). One of the Swiss populations, Ac3785, was grown under two temperature regimes and short daylength to determine whether its tolerance to freezing increased with time. Significant increases in tolerance occurred after 8 days at 2 °C, but plants grown at 8 °C showed no change in tolerance even after 30 days. The results suggest that (i) stolon carbohydrate content is an important factor in the overwintering of white clover, (ii) there is a considerable amount of genetic variation in cold tolerance within white clover, (iii) artificial freezing tests can provide a method of predicting the survival of clover stolons during winter and (iv) the cold hardiness of white clover increases with time spent at temperatures near 0 °C under short daylengths.