It has been well established that plants are capable to protect themselves against the damage of excess light intensities, but the physical mechanisms by the aid of which the energy utilization is down-regulated are not fully understood. Most molecular mechanisms require a feedback via redox changes or the transmembrane pH gradient, which are coupled to the photosynthetic electron and proton transport. These type of mechanisms, although they have been shown to play an important role, themselves (due their saturation) would not permit a physiological response that would be proportional to the extent of overexcitation. This seminar will deal with the exeprimental results showing that LHCII(-containing) lamellar aggregates (granal thylakoids, and purified LHCII) are capable of undergoing light-induced reversible structural changes, the rates of which are apparently non-saturable and are linearly proportional to the excess light-intensity (Barzda et al. 1996, Biochemistry 35:8981; Garab et al, 1998, Budapest Congress). The physical mechanism of these structural changes and of the associated changes in the yield of fluorescence/energy dissipation, and the possible physiological importance will be discussed.

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