The effect of temperature on the fluorescence kinetics of spinach chloroplasts

Abstract

1. 1. The fluorescence kinetics of spinach chloroplasts were studied over the temperature range +40° to −196°. At temperatures above −5°, the ratio of the steady-state level of fluorescence ( F ∞ to the initial level ( F 0 was determined by the intensity of the excitation light. At −5°, F ∞ F 0 was independent of excitation intensity and F ∞ was equal to the fluorescence yield in the presence of dithionite ( F reduced). In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, there was a rapid rise in the fluorescence to the steady-state level. The fluorescence rise ( F ∞- F 0) in untreated chloroplasts was dependent on pH with maxima at pH 6.1 and pH 7.2; F 0 was independent of pH. 2. 2. The biphasic kinetics, observed at 20° were reproducible provided the chloroplasts remained in the dark for a few min between excitations, but at −5° the initial biphasic kinetic curve was replaced by an exponential curve at subsequent excitations. Illumination of the chloroplasts at −5° by far-red light restored the chloroplasts to their initial state, and a biphasic kinetic curve was then obtained at the second excitation. An action spectrum for the far-red restoration showed a peak at 707 nm. The rate of the far-red response declined gradually at temperatures below −5°, and it was completely inhibited at −30°. 3. 3. Over the temperature range −40° to −196° exponential fluorescence kinetics were obtained, but only at the first excitation. Subsequent excitations gave an immediate rise to the F ∞ level. The rate of the initial exponential rise in fluorescence declined with decreasing temperature. 4. 4. The results are discussed in terms of a sequential series of carriers (E,Q,P) on the reducing side of Photosystem II. Analysis of the kinetic curves obtained at −5° indicated a P/Q ratio of 6, and 1 Q per 140 chlorophylls. E is present to a smaller extent than Q. The quantum efficiency for the reduction of Q by Photosystem II declined with temperature between −5° and −196°. To explain the temperature responses, it is postulated that oxygen interacts at 3 sites with the electron transport chain of chloroplasts.