Seasonal Variation in Frost Tolerance and Sugar Content of Two Plagiomnium Species

Abstract

Changes offrost resistance and the contents of sucrose, glucose, and fructose of shoots ofPlagiomnium 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 -15C, hardiness increased pro- gressively during the fall and reached temperatures below -35C in winter. In both species, acqui- sition 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 lessfrost-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.