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The nature of subthreshold changes in excitable plasma membranes has been investigated in stem parenchyma cells of Cucurbita pepo L. during action-potential generation induced by gradual cooling (from 23 to 10 ° C). The character of the subthreshold depolarization of excitable cells is shown to be mainly defined by a decrease in the activity of the plasma-membrane electrogenie pump (H(+)-ATPase). In its turn, the pump activity is controlled by thermal changes in the structure of the membrane lipid matrix. Based on the results obtained, a sequence of subthreshold changes has been suggested in which thermally induced structural rearrangements of membrane lipids play the role of trigger.

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Macroelectrodes and electrophysiological setup were used in experiments with stems of 15-day-old pumpkin ( Cucurbita pepo L.) seedlings for computer-assisted data recording. It is shown that local bioelectric responses induced by graded local chilling are similar to the receptor potentials of animals. These responses increase gradually with stimulus strength and trigger the action potential generation when a temperature threshold is attained. The excitation threshold of cells in seedling stems displays the phenomenon of accommodation. Parameters of local bioelectric responses induced by intermittent cooling can undergo changes similar to sensitization-habituation patterns. The results indicate that local electrical responses may be involved in early stages of cooling perception in cells of higher plants devoid of locomotive functions.Abbreviations : AP-action potential; LBER-local bioelectric response; RP-receptor potential; ∆ T -temperature shift sufficient for local generation of action potential.

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