Temperature dependence of chlorophyll α fluorescence in lettuce and spinach chloroplasts at sub-zero temperatures1

Abstract

The temperature dependence of chlorophyll α fluorescence was measured in spinach and lettuce chloroplasts at sub-zero temperatures in the presence of 50% ethylene glycol. In the presence of 5 mM Mg²⁺, a fluorescence maximum appeared at −31°C in both the spinach and lettuce chloroplasts, while in the presence of only 5 mM Na⁺ as cations the maximum shifted to −20°C in the spinach chloroplasts and to −11°C in the lettuce chloroplasts. Since the occurrence of a maximum in the temperature versus fluorescence curve is an indication for the transition of the physical phase of thylakoid membrane lipids between the liquid crystalline and the phase-separation state (16, 18), these findings suggest that the (major) phase transition of membrane lipids occurs at these low temperatures in chloroplasts of higher plants and also that the phase transition temperature is markedly lowered by the presence of divalent cations. Ethylene glycol at a concentration of 50% had almost no effect on the temperature dependence of chlorophyll α fluorescence in a lamellar membrane preparation of Anabaena variabilis. In a water suspension of dimyristoylphosphatidylcholine, the addition of ethylene glycol to 50% did not alter the characteristic feature of the temperature dependence of fluorescence of 1-anilinonaphthalene-8-sulfonate. These findings suggest that 50% ethylene glycol does not affect the temperature of the transition of the physical phase of membrane lipids.