Probing the events of photoinhibition by altering electron‐transport activity and light‐harvesting capacity in chloroplast thylakoids

Abstract. The effect of photoinhibition on the activity of photosystem II (PSII) in spinach chloroplasts was investigated. Direct light‐induced absorbance change measurements at 320 nm (Δ A 320 ) provided a measure of the PSII charge separation reaction and revealed that photoinhibition prevented the stable photoreduction of the primary quinone acceptor Q A . Sensitivity to photoinhibition was substantially enhanced by treatment of thylakoids with NH 2 OH which extracts manganese from the H 2 O‐splitting enzyme and prevents electron donation to the reaction centre. Incubation with 3‐(3,4,‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) during light exposure did not affect the extent of photoinhibitory damage. The chlorophyll (Chl) b ‐less chlorina (2 mutant of barley displayed a significantly smaller light‐harvesting antenna size of PSII (about 20% of that in wild type chloroplasts) and, simultaneously, a lower sensitivity to photoinhibition. These observations suggest that photoinhibition depends on the amount of light absorbed by PSII and that the process of photoinhibition is accelerated when electron donation to the reaction centre is prevented. It is postulated that the probability of photoinhibition is greater when excitation energy is trapped by P680 + , the oxidized form of the PSII reaction centre. The results are discussed in terms of the D1/D2 heterodimer which contains the functional PSII components P680, pheophytin, Q A and Q B .