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 substantially accelerated at pH 4.5 as compared to pH 7.0. Lowering the pH also affects the light-induced cleavage pattern of the D1 protein: at pH 6.0-7.0, the characteristic C-terminal fragments are of 8-10 kDa, whereas 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 singlet oxygen, but at low pH, it is replaced by hydroxyl radicals. Rapid D1 protein loss, which is accompanied by 23-24 kDa C-terminal fragments and hydroxyl radical production is characteristic of donor-side-induced photoinhibition. Thus, our results indicate that low pH conditions enhance light-induced damage to PS II function and protein structure by facilitating the donor-side mechanism of photoinhibition. The relevance of this effect to in vivo photoinhibition is discussed.