Sustained Trans-Thylakoid Proton Gradient Induced by Light Stress at Low Temperatures? Possible Effects on the Xanthophyll Cycle
Under conditions of excessive photon absorption by green plants, enhanced thermal energy dissipation is induced in the antennae of PS II. This photoprotective process is manifested by non-photochemical ‘energy-dependent’ quenching of Chl a fluorescence. The quenching is known to be controlled by the trans-thylakoid H+ gradient and facilitated by zeaxanthin (Z) and possibly antheraxanthin (A) formed from violaxanthin (V) via the xanthophyll cycle (1, 2). A component of non-photochemical quenching that persists for many minutes to hours in low light or darkness denotes ‘photoinhibition’ of PS II and is stimulated by light stress at physiologically low temperatures. It has been discusssed that the sustained quenching might be based on persistent thylakoid ATPase activity, which would maintain a transmembrane ΔpH by ATP hydrolysis (2). The low pH in the thylakoid lumen may keep the V de-epoxidase of the xanthophyll cycle in the active state, and thus prevent net epoxidation of Z and A. It has been suggested that this effect is responsible for maintaining high levels of Z and A associated with fluorescence quenching in leaves of overwintering plants (3). We tested this hypothesis by investigating photoinhibition and recovery of PS II in relation to xanthophyll cycle activity in a chilling-sensitive (Cucurbita maxima) and chilling-tolerant (Spinacia oleracea) species. Effects of the uncoupler nigericin (Nig) on Chl fluorescence of leaf sections and isolated thylakoids served to indicate a sustained ΔpH.
Key wordsepoxidation fluorescence photoinhibition photosystem 2 zeaxanthin
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