Abstract Lichens have been shown to optimize their photosynthetic apparatus in response to seasonal changes in light and temperature. However, there have been few investigations on seasonal variations in the biochemical mechanisms used by lichen photobionts to protect themselves against the potentially harmful effects of high light. In our earlier work, carried out during the summer months, we showed that Afromontane lichens collected from shaded habitats display higher values of non-photochemical quenching (NPQ) and lower maximum relative electron transport rates (rETR MAX ) than thalli collected from sunny habitats. Here we employ chlorophyll fluorescence to show that, in contrast, winter collections of the Afromontane lichens Ramalina celastri and Usnea undulata from a diversity of sites display no significant differences in NPQ and rETR MAX between sun and shade forms. Results suggest that in the winter, shade forms downregulate their NPQ because loss of leaves causes less fluctuation in light levels, but retain similar values of rETR MAX to those of summer collections. Conversely, during the winter, thalli from exposed microhabitats downregulate their rETR MAX , presumably to reflect reduced possibilities for carbon fixation, and retain generally low values of NPQ. All winter collections display similar, high values of cyclic electron flow (CEF) and pseudocyclic electron flow (PCEF), indicating strong photoprotection of photosystem I. Taken together, the results suggest that photobionts can flexibly adjust their photosynthetic apparatus to seasonal variations in light availability.
Beckett et al. (Thu,) studied this question.