Photosystem II photochemical efficiency, zeaxanthin and antioxidant contents in the poikilohydric Ramonda serbica during dehydration and rehydration

Changes in photochemical efficiency, non-radiative energy dissipation (NRD) , de-epoxidation state of xanthophyll cycle components (DPS) and contents o f the antioxidants ascorbic acid and glutathione were studied in leaves of the poikilohydric Ramonda serbica Pane. (Gesneriaceae) during cycles of deh ydration and subsequent rehydration. In drying leaves, the intrinsic effici ency of PS II photochemistry and the photon yield of PS II electron transpo rt showed strong progressive decreases. Simultaneously, the fraction of exc itation energy dissipated as heat in the PS II antenna increased markedly. The energy-dependent component of non-photochemical quenching (NPQ) showed an increase in dehydrating leaves down to relative water contents (RWC) val ues near 30%. Further decreases in RWC below these values caused a decrease in NPQ. Accordingly, DPS showed a similar behaviour, with a sharp increase and a subsequent decrease at very low RWC, although the maximum DPS was re ached at slightly lower RWC than that for the maximum NPQ. The pools of red uced ascorbate and glutathione increased strongly when the RWC values fell below 40% and remained high in fully dehydrated leaves. When plants were re -watered photosynthetic efficiency, NRD, DPS and antioxidant contents recov ered their initial control values. However, during rehydration, the zeaxant hin content showed a transient increase, as did NPQ, indicating an increasi ng demand for non-radiative dissipation. On the other hand, the contents of reduced ascorbate and reduced glutathione decreased but were still relativ ely high in the initial phase of rehydration, when the rate of photosynthet ic electron transport, proton pumping and NRD were still relatively low. Th ese results indicate that several photoprotective mechanisms are operating in R. serbica. Protection from photo-oxidation and photoinhibition appears to be achieved by coordinated contributions by ascorbate, glutathione and z eaxanthin-mediated NPQ. This variety of photoprotective mechanisms may be e sse nti al for conferring desiccation-tolerance.