Overwintering temperatures affect freezing temperatures of turions of aquatic plants

The effect of different overwintering temperatures (2.5 ± 1 °C in a refrigerator or outdoor natural overwintering on wet topsoil with weak frosts) on the freezing temperature and survival rate of turions of 10 aquatic plant species with different ecological traits (free-floating habit or bottom rooting) was studied using mini thermocouples. Dormant, non-hardened turions of 9 species exhibited freezing within a narrow temperature range of −7.0 to −10.2 °C, while Hydrocharis morsus-ranae froze at −3.6 °C. The survival rate of the turions after the measurements was, however, very low (0–38%). In several species, the freezing temperature of turions at the beginning of germination was not significantly different (at p < 0.05) from the dormant ones. The mean freezing temperature of outdoor hardened turions of 6 species was within a very narrow range of −2.8 to −3.3 °C and was thus significantly higher by 4–7 °C (p < 0.0002) than that for the non-hardened turions. It is assumed that the freezing temperatures indicate freezing of the extracellular water. The hardened turions of all 7 species were able to survive mild winter frosts under the topsoil conditions at a rate of 76–100%. These characteristics suggest that the turions of aquatic species can be hardened by weak frosts and that their frost hardiness is based on the shift from frost avoidance in non-hardened turions to frost tolerance.