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Measurements of electrical potential, membrane resistance, and elongation rate have been carried out on the developing pollen tube of Oenothera drummondii.The plasmalemma potential was observed to be -138 millivolts +/- 19 (sd). Approximately 70% of this potential was apparently due to the operation of an electrogenic pump(s). KCN rapidly and reversibly depolarized the potential to about -40 millivolts. Lowering the temperature from 20 to 4 C brought about similar rapid depolarization. The effects of KCN and of temperature were in no degree additive. KCN had only a small effect on membrane resistance. In contrast, the latter was markedly increased by lowering the temperature.When the pollen tubes were submitted to cyclic temperature changes striking hysteresis effects were observed in the response of all three parameters, membrane potential, resistance, and growth rate. The hysteresis pattern for potential differed from that for resistance but resembled that for growth rate (measured simultaneously on the same pollen tube). The correlation coefficient between potential and growth rate was very high.The probable relevance to our results of the hysteresis phenomena associated with "clustering" and phase transition in lipids is pointed out. Attention is also drawn to the possible significance of the large difference between the electric potentials at the start of the cooling and heating paths, respectively.

Hysteresis in the Responses of Membrane Potential, Membrane Resistance, and Growth Rate to Cyclic Temperature Change