Screening Compounds Vary Along Spatial and Temporal Light Gradients

Variation in the concentration of colored pigments such as parietin and melanins can be seen with the naked eye. They are most clearly seen in the desiccated state, in which the longest and strongest sun exposures take place. For example, the melanin content in Lobaria pulmonaria varied around a Quercus stem with darkly brown thalli on the southern side and pale gray melanin-deficient thalli on the northern side (Gauslaa and Solhaug 2001). The browning of the upper cortex strongly increased after transplanting pale, shade-adapted thalli from forests to exposed clear-cut areas

(Gauslaa et al. 2006). Xanthoria parietina thalli on exposed roofs and sea cliffs had a stronger orange color due to much higher parietin content than those growing on trees (Hill and Woolhouse 1966). There was a linear relationship between the openness of the habitat and the parietin content, ranking from pale shade-adapted low-parietin thalli of X. parietina in closed forests to orange parietin-rich and highlight exposed thalli on sea cliffs (Gauslaa and Ustvedt 2003). In H. physodes, the concentration of the cortical compounds atranorin and the closely related chloroatranorin increased with the openness of their natural growing sites. In Cladonia subtenuis, the usnic acid showed a clear increase in concentration with increasing sunlight (Rundel 1969), and Nephroma arcticum thalli from a closed spruce forest contained much less usnic acid than thalli from an open subalpine birch forest or from an open alpine heath (McEvoy et al. 2007a).

Cortical compounds do not only vary on spatial scales. In X. parietina, the parietin content varied seasonally with twice as high contents per thallus area and per weight in midsummer than in winter (Gauslaa and McEvoy 2005). During the period from December to May, the rise in solar radiation was faster than the rise in parietin content. Although parietin screens visible light, the screening efficiency did not fully compensate for increased light levels during spring, leading to a decrease in maximal quantum yields of PSII (Fv/Fm). However, from May to June the parietin content and Fv/Fm increased simultaneously. This is consistent with the hypothesis inferring faster parietin synthesis in periods with higher metabolism due to higher temperatures. Thereby, the photobionts experienced less excess irradiance energy resulting in reduced level of photoinhibition (Vrablikova et al. 2006). Similar seasonal variations in usnic acid concentration occurred in the alpine lichen Flavocetraria nivalis (Bjerke et al. 2005).

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