Photochemical and non-photochemical energy dissipation in response to 5-aminolaevulinic acid-induced photosensitization of green leaves of wheat ( Triticum aestivum L.) and lettuce ( Lactuca sativa L.)

Abstract

To obtain a clearer understanding of the photosensitization process, we have investigated the effect of photosensitization on the photochemical and non-photochemical energy dissipation in green leaves of wheat ( Triticum aestivum L.) and lettuce ( Lactuca sativa L.), pretreated with 5-aminolaevulinic acid (ALA) for 2–24 h in the dark, using chlorophyll (Chl) fluorescence quenching analysis and measurement of the influence on CO 2 uptake and xanthophyll cycle pigments. In response to dark pretreatment, leaves accumulated high levels of protochlorophyllide (PChlide) and non-metabolized ALA. The dark pretreatment had no effect on the intrinsic photochemical efficiency of photosystem II (PS II). Although quantitative differences exist between wheat and lettuce, exposure to actinic light caused significant effects with a similar overall response pattern in both plant species. Changes in excitation energy dissipation were obtained already by a very low photon flux density of 85 μmol photons m −2 s −1 within a few minutes CO 2 uptake was almost completely suppressed in photosensitized leaves, but they were able to build up the ΔpH necessary to drive de-epoxidation of violaxanthin. Fluorescence quenching analysis revealed that a progressive increase in non-radiative energy dissipation (measured as the non-photochemical quenching of Chl fluorescence, q N) was paralled by a stronger reduction in the primary quinone electron acceptor of PS II (Q A) with increasing time of ALA pretreatment in the dark. In the early stages of photosensitization, high-energy state quenching mainly contributed to q N. In the later stages, q N was dominated by a photoinhibitory component together with the photodegradation of xanthophyll cycle pigments, in particular antheraxanthin and zeaxanthin. The latter phenomenon appeared to be promoted in response to a highly increased reduction state of Q A. If ALA was simultaneously applied to leaves with 4,6-dioxoheptanoic acid, which acts as a competitive inhibitor of the enzyme ALA dehydratase, photosensitization disappeared when PChlide synthesis was completely inhibited, but was enhanced when inhibition was incomplete. Electron paramagnetic resonance studies using the spin trapping technique revealed a specific ALA-mediated formation of hydroxyl and carbon-centred radicals in response to actinic light exposure of chloroplasts. On the basis of these findings, a possible role of ALA is proposed: ALA enhances the photosensitizing effect(s) triggered by PChlide.