LOW PH ACCELERATES LIGHT-INDUCED DAMAGE OF PHOTOSYSTEM-II BY ENHANCING THE PROBABILITY OF THE DONOR-SIDE MECHANISM OF PHOTOINHIBITION

The effect of low pH on the process of photoinhibition was studied in isolated spinach thylakoids and PS II core complexes. Both the rate of photoinhibition of oxygen evolution and of D1 protein loss are substa ntially accelerated at pH 4.5 as compared to pH 7.0. Lowering the pH a lso affects the light-induced cleavage pattern of the D1 protein: at p H 6.0-7.0, the characteristic C-terminal fragments are of 8-10 kDa, wh ereas below pH 4.5, 23-24 kDa C-terminal products are accumulated. In addition, the predominant active oxygen species at pH 6.0-7.0 is singl et oxygen, but at low pH, it is replaced by hydroxyl radicals. Rapid D 1 protein loss, which is accompanied by 23-24 kDa C-terminal fragments and hydroxyl radical production is characteristic of donor-side-induc ed photoinhibition. Thus, our results indicate that low pH conditions enhance light-induced damage to PS II function and protein structure b y facilitating the donor-side mechanism of photoinhibition. The releva nce of this effect to in vivo photoinhibition is discussed.