SEASONAL-VARIATION IN FROST TOLERANCE AND SUGAR CONTENT OF 2 PLAGIOMNIUM SPECIES

Changes of frost resistance and the contents of sucrose, glucose, and fructose of shoots of Plagiomnium affine (Funck) Kop. and P. undulatum (Hedw.) Kop. were determined in the course of the year. Frost tolerance was defined as the minimum freezing temperature at which a 2 hr. exposure caused 50% irreversible inactivation of the net photosynthetic capacity relative to unfrozen plants. While tolerance limits in summer were around -10 to -15-degrees-C, hardiness increased progressively during the fall and reached temperatures below -35-degrees-C in winter. In both species, acquisition of frost resistance was not correlated with an increase of the sugar content of the shoots, although winter material exhibited a considerably higher sucrose content than less frost-hardy plants. Comparison of net photosynthetic capacity at supraoptimal temperatures revealed a significantly higher thermostability of the shoots of both species in winter relative to summer material. The results indicate that an accumulation of sucrose may contribute to frost hardiness of Plagiomnium shoots, but seasonal variations in frost tolerance cannot solely be attributed to alterations in the sugar concentration of the tissue.