CARBOHYDRATE CONTENT OF EUCALYPTUS-GUNNII LEAVES ALONG AN ANNUAL CYCLE IN THE FIELD AND DURING INDUCED-FROST HARDENING IN CONTROLLED CONDITIONS

The annual changes in frost hardiness were studied for three Eucalyptu s gunnii genotypes. Frost resistance evaluated on leaf discs by the el ectrolyte leakage method reached a maximum in the coldest period and a minimum in summer demonstrating winter frost hardening. Genotype 634 exhibited a higher intrinsic resistance than the other genotypes both in the hardened and in the nonhardened stages. Plants of this genotype were also frost acclimated in controlled conditions by a progressive decrease of culture temperature (25 to 0 degrees C) but the degree of hardening appeared to be lower in these conditions. The carbohydrate p atterns in leaves varied with acclimation. In controlled conditions th e leaves of genotype 634 exhibited a rise in sucrose, fructose and raf finose concentration up to a temperature of 10 to 7 degrees C which su bsequently decreased. In natural conditions a comparison of the three genotypes allowed us to correlate the higher intrinsic resistance of g enotype 634 to a higher soluble sugar content. During acclimation fruc tose and raffinose changes were also correlated to an increase in cold resistance even though the kinetics of these changes differed in cont rolled and natural conditions. The starch content was very low in the various genotypes in the different conditions but oligosaccharides suc h as stachyose and possibly verbascose were detected. The results poin t out the relationships occurring between increased frost resistance a nd changes in fructose and raffinose concentration in E. gunnii leaves .