Photosynthetic organisms have evolved a great variety of mechanisms to optimize their use of sunlight. Some of the clearest examples of adaptations can be seen by comparing photosynthesis in different species and in different individuals of the same species that grow under high and low light levels. While the adaptations of sun and shade higher plants have been relatively well studied, much less information is available on the photobionts of lichenized Ascomycetes. An important adaptation that can protect photosynthetic organisms from the potentially harmful effects of excess light is non-photochemical quenching (NPQ); NPQ can dissipate unused light energy as heat. Here we used chlorophyll fluorescence to compare the induction and relaxation of NPQ and the induction of electron transport (rETR) in collections of the same lichen species from exposed and from more shaded locations. All species have trebouxioid photobionts and normally grow in more exposed microhabitats but can also be readily collected from more shaded locations. Shade forms display generally higher NPQ, presumably to protect lichens from occasional rapid increases in light that occur during sunflecks. Furthermore, the NPQ of shade forms relaxes quickly when light levels are reduced, presumably to ensure efficient photosynthesis after a sunfleck has passed. The maximal relative electron transport rate is lower in shade than sun collections, probably reflecting a downregulation of photosynthetic capacity to reduce energy costs. We also compared collections of pale and melanized thalli from three species of shade lichens with Symbiochloris as their photobiont. Interestingly, NPQ in melanized thalli from slightly more exposed microhabitats induced and relaxed in a way that resembled shade rather than sun forms of the trebouxioid lichens. This might suggest that in some locations melanization induced during a temporary period of high light may be excessive and could potentially reduce photosynthesis later in the growing season. Taken together, the results suggest that lichen photobionts can flexibly adjust the amount and type of NPQ, and their levels of rETR in response to light availability.

Non-photochemical quenching (NPQ) plays an important role in protecting photosynthetic organisms from photoinhibition by dissipating excess light energy as heat. However, excess NPQ can greatly reduce the quantum yield of photosynthesis at lower light levels. Recently, there has been considerable interest in understanding how plants balance NPQ to ensure optimal productivity in environments in which light levels are rapidly changing. In the present study, chlorophyll fluorescence was used to study the induction and relaxation of non-photochemical quenching (NPQ) in the dark and the induction of photosynthesis in ten species of lichens, five sampled from exposed and five sampled from shaded habitats. Here we show that the main difference between sun and shade lichens is the rate at which NPQ relaxes in the dark, rather than the speed that photosynthesis starts upon illumination. During the first two minutes in the dark, NPQ values in the five sun species declined only by an average of 2%, while by contrast, in shade species the average decline was 40%. For lichens growing in microhabitats where light levels are rapidly changing, rapid relaxation of NPQ may enable their photobionts to use the available light most efficiently.