Abstract
In lichens, each symbiotic partner cooperates for the survival of the symbiotic association. The protection of the susceptible photosynthetic apparatus is essential for both participants. The mycobiont and photobiont contribute to the protection against the damaging effect of excess light by various mechanisms. The present study investigated the effect of seasonality and microhabitat exposure on photoprotection and photoacclimation in the photo- and the mycobiont of six lichen species with different thallus morphology in inland dune system in the Kiskunság region (Hungary) with shaded, more humid and exposed, drier dune sides. High-Performance Liquid Chromatography, spectrophotometry, chlorophyll a fluorescence kinetic technique were used, and micrometeorological data were collected. The four years data series revealed that the north-east-facing side was characterized by higher relative humidity and lower light intensities compared to the south-west-facing drier and more exposed sides. The south-west facing side was exposed to direct illumination 3–4 hours longer in winter and 1–2 hours shorter in summer than the north-east facing side of the dune, influencing the metabolism of sun and shade populations of various species. Because rapid desiccation caused short active periods of lichens during bright and drier seasons and on exposed microhabitats, the rapid, non-regulated non-photochemical quenching mechanisms in the photobiont had a significant role in protecting the photosynthetic system in the hydrated state. In dehydrated conditions, thalli were mainly defended by the solar screening metabolites produced by the mycobiont and curling during desiccation (also caused by the mycobiont). Furthermore, the efficacy of light use (higher chlorophyll and carotenoid concentration) increased because of short hydrated periods. Still, a lower level of received irradiation was appropriate for photosynthesis in dry seasons and on sun exposed habitats. In humid seasons and microhabitats, more extended active periods lead to increased photosynthesis and production of solar radiation protectant fungal metabolites, allowing a lower level of photoprotection in the form of regulated non-photochemical quenching by the photobiont. Interspecific differences were more pronounced than the intraspecific ones among seasons and microhabitat types.
Highlights
Lichens are unique and complex ecosystems in which each symbiotic partner can contribute to the survival of extreme environmental conditions
A significant seasonal trend was detectable in the concentration of lichen secondary metabolites, plastid pigments and the proportion between ways of excitation energy absorbed by the antenna complex
Because the lichen can only produce solar radiation protectant substances in the hydrated state (Solhaug et al 2003), the decreased level of these metabolites in the dry summer may result from short active periods insufficient to provide required amounts of photosynthates for lichen secondary metabolites (LSMs) synthesis (Solhaug and Gauslaa 2004) in summer
Summary
Lichens are unique and complex ecosystems in which each symbiotic partner can contribute to the survival of extreme environmental conditions. The excitation energy absorbed by the antenna system may be used for photochemical charge separation in the reaction centres (RCs). When the electron transport chain is saturated, the excitation energy can be re-emitted as fluorescence or dissipated as heat. Both non-photochemical quenching via zeaxanthin (Demmig-Adams 1990; Färber et al 1997) and desiccation induced fluorescence quenching (Heber et al 2001, 2006; Kopecky et al 2005) are known in lichens. In the absence of effective thermal energy dissipation, the production of damaging reactive oxygen species (ROS) as by-products can cause irreversible damage in the PSII (Müller et al 2001; Krieger-Liszkay 2005)
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