Svenning, M. M., R0snes, K. and Junttila, O. 1997. Frost tolerance and biochemical changes during hardening and dehardening in contrasting white clover populations. -Physioi. Plant. 101:31-37.Successful winter survival of perennial plants, like white clover, is dependent on proper timing of both hardening and dehardening. The purpose of this study was to investigate the regulation of these processes in two cultivars (AberCrest and AberHerald) and two Norwegian ecotypes (Ssrheim collected at 58°46'N lat. and Bod0 at 67°20'N lat.) of white clover {Trifolium repens L.). For hardening and dehardening, plants were exposed to controlled temperature conditions and frost hardiness of stolons was tested by programmed freezing at the rate of 3°C per hour. In addition, stolons were analysed for starch, soluble sugars and soluble amino acids. Cultivars Aber-Crest and AberHerald, selected for growth at low temperature and winter hardiness in the United Kingdom, were significantly less hardy than the Norwegian populations. After six weeks of hardening (2 weeks at 6°C and 4 weeks at 0.5°C), estimated LT50 values were -13.8, -13.0, -17.8 and -20.3°C for AberCrest, AberHerald, Sserheim and Bod0, respectively. The rate of dehardening increased with increasing temperature. At low temperature (6°C), the northern ecotype from Bod0 was more resistant to dehardening than AberHerald. However, at 18°C the absolute rate of dehardening (°C day"') was twice as high in Bod0 as in AberHerald plants. Stolon elongation during dehardening was initiated at lower temperatures in AberHerald than in plants of the Bod0 ecotype. The content of total soluble sugars, sucrose and the amino acids proline and arginine were significantly higher in hardy plants of Bod0 than in those of Aber-Herald. Sucrose levels decreased during dehardening and correlations between sucrose content and LT50 during this process were statistically highly significant for both Bod0 and AberHerald. The least hardy populations of white clover were characterized by thick stolons, long internodes and large leaves.

Successful winter survival of perennial plants, like white clover, is dependent on proper timing of both hardening and dehardening. The purpose of this study was to investigate the regulation of these processes in two cultivars (AberCrest and AberHerald) and two Norwegian ecotypes (Særheim collected at 58°46′N lat. and Bodø at 67°20′N lat.) of white clover (Trifolium repens L.). For hardening and dehardening, plants were exposed to controlled temperature conditions and frost hardiness of stolons was tested by programmed freezing at the rate of 3°C per hour. In addition, stolons were analysed for starch, soluble sugars and soluble amino acids. Cultivars AberCrest and AberHerald, selected for growth at low temperature and winter hardiness in the United Kingdom, were significantly less hardy than the Norwegian populations. After six weeks of hardening (2 weeks at 6°C and 4 weeks at 0.5°C), estimated LT50 values were −13.8, −13.0, −17.8 and −20.3°C for AberCrest, AberHerald, Saerheim and Bodø, respectively. The rate of dehardening increased with increasing temperature. At low temperature (6°C), the northern ecotype from Bodø was more resistant to dehardening than AberHerald. However, at 18°C the absolute rate of dehardening (°C day−1) was twice as high in Bodø as in AberHerald plants. Stolon elongation during dehardening was initiated at lower temperatures in AberHerald than in plants of the Bodø ecotype. The content of total soluble sugars, sucrose and the amino acids proline and arginine were significantly higher in hardy plants of Bodø than in those of AberHerald. Sucrose levels decreased during dehardening and correlations between sucrose content and LT50 during this process were statistically highly significant for both Bodø and AberHerald. The least hardy populations of white clover were characterized by thick stolons, long internodes and large leaves.

Development of cold tolerance in white clover (fi;Jo~ium repens L.) in relation to carbohydrate and free amino acid concent. Accepted February 5, 1993. Acta Agric. Scand., Sect. B, Soil and Plant Sci. 43: 151-155, 1993. 0 Acta Agric. Scand. 1993.Development of cold tolerance in white clover (fiifoliioir repelis L.) genotypes originating from various parts of Norway and grown outdoors at Tromsu, latitude 69" 39'N, has been studied. Stolon samples for an artificial freezing test and for chemical analysis were collected three times during the autumn. Generally, the most northern genotypes were more cold tolerant than the southern ones. Cold tolerance increased significantly from September to November in all genotypes. Content of dry matter, soluble carbohydrates, sucrose and proline in stolons increased during the hardening process. These changes were similar in both northern and southern genotypes of white clover.

Effects of mineral nitrogen (2, 4, 6 and 8 mM NH NO ) and nodulation with Rhizobium on frost hardiness in seedlings of white clover (Trifolium repens) have been studied. Seedlings of a population from Bodø (67°N lat.) were grown in Leonard jars under controlled conditions in a phytotron. For induction of frost hardening, plants were first exposed to 12 h photoperiod conditions for 2 weeks at 18°C, then for 2 weeks at 6°C and finally for 2 weeks at 0.5°C. Frost hardiness after treatments at 6 and 0.5°C was significantly enhanced by increasing nitrogen supply and was positively correlated with total nitrogen content of the stolons. Frost hardiness of nodulated plants correlated to the tissue nitrogen concentration. Content of soluble proteins in stolons decreased during hardening at 6°C but did not change during treatment at 0.5°C. There were minor changes in total amount of free amino acids during hardening. Both absolute and relative amounts of proline and arginine increased, and those of asparagine decreased during hardening. Absolute amounts of all free amino acids increased with increasing nitrogen supply, but the changes during hardening were similar in all treatments. There was a significant increase in the content of soluble carbohydrates during hardening. However, this increase was inversely related to nitrogen supply.

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