Quenching of chlorophyll fluorescence and photochemical activity of chloroplasts at low temperature.

Fluorescence kinetics and emission spectra of pea and spinach chloroplasts were studied between 294 and 4.2 K. In the presence of MgCl2 the fluorescence-induction curves were sigmoidal between 294 and 180 K but they lost their inflection points at lower temperature. In the absence as well as in the presence of Mg2+ ions, analysis of the kinetics of the area over the induction curve revealed two different components, indicating the existence of two different types of reaction centres at all temperatures. The 'rate constants' for these centres were nearly independent of temperature between 294 and 200 K, but showed a sharp decrease on further cooling. Emission spectra at low temperature showed the previously observed bands at 685, 695 and 735 nm. All three bands showed a considerable increase on cooling between 180 and 4.2 K, but with different temperature-dependence. The amplitude of the 695 band became constant below about 50 K, whereas the 685 emission increased markedly in this region. The relative proportion of the socalled variable fluorescence was almost the same at 685 and 695 nm, both at 80 and at 5 K. The data are discussed in terms of changes of energy-transfer rates on cooling.