On the Inside Photoprotection in Lichens

Lichens are often found growing on exposed rocks or trees, where they face high levels of irradiation while in the desiccated state. To grow and survive, lichens must resist photodamage while desiccated and resume photosynthesis soon after hydration. Since PSII is often a target of desiccation-induced damage, lichens must render PSII largely inactive and/or minimize the amount of solar radiation reaching it to maximize their endurance while desiccated and their recovery upon rehydration. PSII activity is often assessed in vivo by the measurement of variable chlorophyll (Chl) a fluorescence, which originates from active PSII reaction centers. A decrease in variable Chl a emission observed in desiccated lichens is likely associated with their phototolerance and may arise from multiple mechanisms. One mechanism involves structural changes that induce changes in light-scattering and ‘‘shading’’ properties. During desiccation the algae aggregate and change shape to limit exposure to light, while at the same time the lichen thallus curls to minimize the available surface area and reduce light absorbance. These mechanisms decrease the exposure of the photosynthetic apparatus of the photobiont to light. An alternative mechanism contributing to phototolerance is the dissipation of absorbed energy by the lichen’s photosynthetic apparatus. Veerman et al. (pp. 997–1005) have investigated the origins of fluorescence quenching in the poikilohydric lichen Parmelia sulcata (Fig. 1) by means of steady-state, low temperature, and timeresolved Chl fluorescence spectroscopy. They found the most dramatic target of quenching to be PSII, which produces negligible levels of fluorescence in desiccated lichens. They also report that fluorescence decay in desiccated lichens was dominated by a short-lifetime, longwavelength component energetically coupled to PSII. The long-wavelengthquenching species was responsible for most (about 80%) of the fluorescence quenching observed in desiccated lichens; the rest of the quenching was attributed to structural changes in the lichen thallus. Functional Sieve Element Protoplasts