Short-term thermal acclimation and heat tolerance of gametophytes of mosses.

The ability of gametophytes of two Bryidae, Atrichum undulatum (Hedw.) P. Beauv. and Polytrichum formosum Hedw., to rapidly acquire thermotolerance was investigated by measuring chlorophyll a fluorescence and electrolyte leakage. Short-term acclimation of turgid shoots to elevated sublethal temperatures resulted in a small but significant increase in the heat stability of the photosynthetic apparatus and of cellular membranes by around 1 K, indicating that the heat hardening capacity of hydrated mosses is very low. While thermal adaptation occurred within few hours, dehardening required several days. The pattern of rapid thermal hardening and dehardening of turgid mosses resembles that of flowering plants. However, as opposed to the low potential for short-term thermal acclimation of hydrated gametophytes, a dramatic rise in heat resistance occurred with decline of the water content of the poikilohydric shoots, which was achieved either by equilibrating of the thalli at different relative humidities or by incubation in sugar solutions of various concentrations. The lower the water potential of the tissue the higher the heat tolerance of the shoots. The data show that in contrast to homoiohydric higher plants, in desiccation-resistant mosses changes in the water status of the thalli play the dominant role in short-term variations of thermal tolerance rather than specific heat hardening and dehardening reactions.