Changes of the antenna of photosystem I induced by short-term heating

Changes in low-temperature fluorescence spectra of pea chloroplasts induced by the short-term heating were studied. Excitation spectra of the long-wavelength fluorescence were studied as well. Heating was carried out at 45°C for 5 min in the darkness or in the presence of white light sourced with intensities of 260 or 1400 μmol/m² s. All variants of heating decreased the intensity of the long-wavelength fluorescence band. The integral of the excitation spectrum decreased after the exposure to heating in the darkness and increased after the exposure to heating in the presence of light. The observed changes in most intensive components — 726, 729 and 731 nm — of the long-wavelength fluorescence band, induced by various modes of heating, were similar. The changes in the fourth intensive component at 735 nm were different. Twenty-five components were found in the fine structure of the excitation spectrum of the long-wavelength fluorescence. Positions of most of peaks corresponded to the absorption peaks of Lhca proteins. Heat-induced changes in the excitation spectrum in the regions corresponding to the absorption of chl b and short-wavelength forms of chl a have been shown to correlate with changes in the intensities of the 726-, 729-, and 731-nm components of the long-wavelength fluorescence. This allows one to assign them to the emission of the outer antenna of Photosystem I. Changes in the intensity of the component at 735 nm correlated only with changes in excitation spectrum in the long-wavelength region that corresponded to the absorption of the long-wave-length forms of chlorophyll a. Therefore, the 735-nm component could be assigned to the emission of the Photosystem I inner antenna. Analysis of the changes induced by heating in the emission and excitation spectra of fluorescence revealed changes in the energy transfer in the outer and the inner antennas of Photosystem I. Heating in the darkness lowered the energy transfer in the outer and in the inner antennas. Both modes of heating in the presence of light increased the energy transfer in the outer antenna. For the inner antenna, presence of the light promotes an efficient of energy transfer at the levels close to the control one. It is proposed that illumination during heating exposure causes a specific state of the antenna complex in Photosystem I that provides an increase in funneling of the energy toward the reaction centers.