Photoprotection of reaction centres in photosynthetic organisms: mechanisms of thermal energy dissipation in desiccated thalli of the lichen Lobaria pulmonaria.

*The photobionts of lichens have previously been shown to reversibly inactivate their photosystem II (PSII) upon desiccation, presumably as a photoprotective mechanism. The mechanism and the consequences of this process have been investigated in the green algal lichen Lobaria pulmonaria. *Lichen thalli were collected from a shaded and a sun-exposed site.

Conservation and dissipation of light energy as complementary processes: homoiohydric and poikilohydric autotrophs.

The relationship between photosynthetic energy conservation and thermal dissipation of light energy is considered, with emphasis on organisms which tolerate full desiccation without suffering photo-oxidative damage in strong light. As soon as water becomes available to dry poikilohydric organisms, they resume photosynthetic water oxidation. Only excess light is then thermally dissipated in mosses and chlorolichens by a mechanism depending on the protonation of a thylakoid protein and availability of zeaxanthin.

Nocturnal respiration of lichens in their natural habitat is not affected by preceding diurnal net photosynthesis.

Dark respiration (DR) of lichens is reported to be higher in species with a high photosynthetic potential (suggesting a metabolic maintenance cost effect) and also, often in laboratory studies, transiently after photosynthesis (suggesting a substrate-driven effect). We investigated the occurrence of the latter, the effect of diurnal net photosynthesis (NP) on subsequent nocturnal DR, under natural temperate climate conditions in the chlorolichens Lecanora muralis and Cladonia convoluta and the cyanolichen Collema cristatum. Data sets totalling 15 months, 106 and 113 days, respectively, were obtained from automatic cuvettes that continually measured CO2 exchange and ambient conditions at 30 min intervals.

Lichens show that fungi can acclimate their respiration to seasonal changes in temperature.

Five species of lichens, the majority members of a soil-crust community ( Cladonia convoluta, Diploschistes muscorum, Fulgensia fulgens, Lecanora muralis, Squamarina lentigera) showed seasonal changes of temperature sensitivity of their dark respiration (DR) to such an extent that several substantially met the definition of full acclimation, i.e. near identical DR under different nocturnal temperature conditions during the course of the year.

High thallus water content severely limits photosynthetic carbon gain of central European epilithic lichens under natural conditions.

Experiments under controlled conditions have shown that net photosynthesis (NP) of many lichens is depressed when their thalli are highly hydrated. In this study we characterise the light and water content (WC) dependency of CO exchange for selected epilithic lichens in the laboratory and match this against samples monitored in their natural habitat by a novel, fully automatic cuvette. Laboratory measurements showed that, at a photosynthetic photon flux density (PPFD) of 1500 μmol m s, NP of the epilithic foliose lichen Xanthoria calcicola was reduced by about 85% (compared to NP at optimal water content) when the thallus was suprasaturated (maximal hydration was defined as WC after spraying, submerging and subsequent removal of adhering water droplets by shaking).

Moisture content and CO exchange of lichens : I. Influence of temperature on moisture-dependent net photosynthesis and dark respiration in Ramalina maciformis.

Net photosynthesis (10 klx light intensity, 150 μE m s PAR) and dark respiration of the lichen Ramalina maciformis at different temperatures are measured in relation to thallus water content. Both first increase with increasing hydration. Dark respiration then remains constant with increased water content until thallus saturation.

Frerea indica, a stem succulent CAM plant with deciduous C leaves.

During the winter in greenhouse culture, Frerea indica(Asclepiadaceae) is a leafless stem succulent resembling the other members of the Stapelieae subfamily. However, in spring it produces leaves which persist during the summer period. CO exchange measurements were carried out with Frerea indica at its different seasonal states of development.