Phototolerance of lichens, mosses and higher plants in an alpine environment: analysis of photoreactions

Adaptation to excessive light is one of the requirements of survival in an alpine environment particularly for poikilohydric organisms which in contra st to the leaves of higher plants tolerate full dehydration. Changes in mod ulated chlorophyll fluorescence and 820-nm absorption were investigated in the lichens Xanthoria elegans (Link) Th. Fr. and Rhizocarpon geographicum ( L.) DC, in the moss Grimmia alpestris Limpr. and the higher plants Geum mon tanum L., Gentiana lutea L. and Pisum sativum L., all collected at altitude s higher than 2000 m above sea level. In the dehydrated state, chlorophyll fluorescence was very low in the lichens and the moss, but high in the high er plants. It increased on rehydration in the lichens and the moss, but dec reased in the higher plants. Light-induced charge separation in photosystem II was indicated by pulse-induced fluorescence increases only in dried lea ves, not in the dry moss and dry lichens. Strong illumination caused photod amage in the dried leaves, but not in the dry moss and dry lichens. Light-d ependent increases in 820-nm absorption revealed formation of potential que nchers of chlorophyll fluorescence in all dehydrated plants, but energy tra nsfer to quenchers decreased chlorophyll fluorescence only in the moss and the lichens, not in the higher plants. In hydrated systems, coupled cyclic electron transport is suggested to occur concurrently with linear electron transport under strong actinic illumination particularly in the lichens bec ause far more electrons became available after actinic illumination for the reduction of photo-oxidized P700 than were available in the pool of electr on carriers between photosystems II and I. In the moss Grimmia, but not in the lichens or in leaves, light-dependent quenching of chlorophyll fluoresc ence was extensive even under nitrogen, indicating anaerobic thylakoid acid ification by persistent cyclic electron transport. In the absence of actini c illumination, acidification by ca. 8% CO2 in air quenched the initial chl orophyll fluorescence yield F-o only in the hydrated moss and the lichens, not in leaves of the higher plants. Under the same conditions, 8% CO2 reduc ed the maximal fluorescence yield F-m strongly in the poikilohydric organis ms, but only weakly or not at all in leaves. The data indicate the existenc e of deactivation pathways which enable poikilohydric organisms to avoid ph otodamage not only in the hydrated but also in the dehydrated state. In the hydrated state, strong nonphotochemical quenching of chlorophyll fluoresce nce indicated highly sensitive responses to excess light which facilitated the harmless dissipation of absorbed excitation energy into heat. Protonati on-dependent fluorescence quenching by cyclic electron transport, P700 oxid ation and, possibly, excitation transfer between the photosystems were comb ined to produce phototolerance.