Heat tolerance of plants related to cell membrane thermostability is commonly estimated via the measurement of ion leakage from plant segments after defined heat treatment. To compare heat tolerance of various plants, it is crucial to select suitable heating conditions. This selection is time-consuming and optimizing the conditions for all investigated plants may even be impossible. Another problem of the method is its tendency to overestimate basal heat tolerance. Here we present an improved ion leakage method, which does not suffer from these drawbacks. It is based on gradual heating of plant segments in a water bath or algal suspensions from room temperature up to 70-75°C. The electrical conductivity of the bath/suspension, which is measured continuously during heating, abruptly increases at a certain temperature TCOND (within 55-70°C). The TCOND value can be taken as a measure of cell membrane thermostability, representing the heat tolerance of plants/organisms. Higher TCOND corresponds to higher heat tolerance (basal or acquired) connected to higher thermostability of the cell membrane, as evidenced by the common ion leakage method. The new method also enables determination of the thermostability of photochemical reactions in photosynthetic samples via the simultaneous measurement of Chl fluorescence.

Department of Biophysics Centre of the Region Haná for Biotechnological and Agricultural Research Faculty of Science Palacký University 783 71 Olomouc Czech Republic

Department of Mathematical Analysis and Applications of Mathematics Faculty of Science Palacký University 771 46 Olomouc Czech Republic

Laboratory of Ecophysiological Plant Physiology Global Change Research Institute Czech Academy of Sciences 603 00 Brno Czech Republic

Photon Systems Instruments Ltd 664 24 Drásov Czech Republic

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