Changes in antenna of photosystem II induced by short-term heating

Changes in antenna of photosystem II, induced by short-term heating, were studied using characteristics of a short-wavelength band in low-temperature fluorescence spectra (77 K) of pea chloroplasts. Heating for 5 min was carried out at 25 and 45°C in the darkness or in the presence of white light with intensity of 260 or 1,400 μmol/m²s. Most modes of thermal treating induced a decrease in integral intensity of the band and an increase of its half-width. The changes were more prominent at high-temperature heating. The second derivative of the contour of a short-wavelength band showed its three components around 680, 685, and 693 nm, the first of which belongs to emission of the outer antenna of Photosystem II, and the other two to its inner antenna. As the fourth derivative shows, high-temperature heating in the presence of light evokes an appearance of some additional components in a short-wavelength region (654, 658, 661, 666, 672, and 675 nm) as well as of two additional components, 682 and 689 nm, in the region of 685-nm peak. Two subcomponents, 692 and 694 nm, can be detected in the 693-nm component. The results are discussed on the basis of the data concerning energy levels and pathways of energy transfer in pigment–protein complexes of the outer and the inner antennas of photosystem II. It is assumed that a protective role of low light relates to inducing of an essential disarrangement in the outer and the inner antennas and of a subsequent decrease in energy funneling to reaction centers, which, in turn, lowers the extent of photoinhibition.