Turions of aquatic plants are primarily adapted to overwintering in the hypoxic or anoxic conditions found in the sediments at the bottom of water courses. Here, the temperature is above freezing. Some turions, however, overwinter above the surface on wet substrates where they are subject to drying out and frost (Adamec, 1999a, Adamec, 1999b and Adamec, 2008a). Therefore, on the basis of their dormant state and a low water content, a certain degree of turion frost resistance, conditioned by frost hardening, can be expected –