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

Abstract

Adaptation to excessive light is one of the requirements of survival in an alpine environment particularly for poikilohydric organisms which in contrast to the leaves of higher plants tolerate full dehydration. Changes in modulated 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 montanum L., Gentiana lutea L. and Pisum sativum L., all collected at altitudes 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 higher plants. It increased on rehydration in the lichens and the moss, but decreased in the higher plants. Light-induced charge separation in photosystem II was indicated by pulse-induced fluorescence increases only in dried leaves, not in the dry moss and dry lichens. Strong illumination caused photodamage in the dried leaves, but not in the dry moss and dry lichens. Light-dependent increases in 820-nm absorption revealed formation of potential quenchers of chlorophyll fluorescence in all dehydrated plants, but energy transfer 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 because far more electrons became available after actinic illumination for the reduction of photo-oxidized P700 than were available in the pool of electron carriers between photosystems II and I. In the moss Grimmia, but not in the lichens or in leaves, light-dependent quenching of chlorophyll fluorescence was extensive even under nitrogen, indicating anaerobic thylakoid acidification by persistent cyclic electron transport. In the absence of actinic illumination, acidification by ca. 8% CO2 in air quenched the initial chlorophyll fluorescence yield Fo only in the hydrated moss and the lichens, not in leaves of the higher plants. Under the same conditions, 8% CO2 reduced the maximal fluorescence yield Fm strongly in the poikilohydric organisms, but only weakly or not at all in leaves. The data indicate the existence of deactivation pathways which enable poikilohydric organisms to avoid photodamage not only in the hydrated but also in the dehydrated state. In the hydrated state, strong nonphotochemical quenching of chlorophyll fluorescence indicated highly sensitive responses to excess light which facilitated the harmless dissipation of absorbed excitation energy into heat. Protonation-dependent fluorescence quenching by cyclic electron transport, P700 oxidation and, possibly, excitation transfer between the photosystems were effectively combined to produce phototolerance.

Ideal Osmotic Spaces for Chlorobionts or Cyanobionts Are Differentially Realized by Lichenized Fungi

“…PSII fluorescence quenching in response to desiccation has been reported previously for chlorolichens (Sigfridsson and Öquist, 1980;Lange et al, 2001) and desiccation-tolerant bryophytes (Heber et al, 2000). Our data confirm that this phenomenon applies to desiccation-tolerant cyanolichens, chlorolichens, and green algae, which is consistent with the report by Lange et al (1989).…”

Photoinhibition of photosynthesis in Antarctic lichen Usnea antarctica. I. Light intensity- and light duration-dependent changes in functioning of photosystem II

“…Last studies (e.g. Heber et al 2000Heber et al , 2006Heber et al , 2007, however, indicated that there is another effective quencher in photosynthetic apparatus of PS II of desiccating lichens. It is not related to zeaxanthin formation and independent on light.…”

Photoprotection in the Lichen Parmelia sulcata: The Origins of Desiccation-Induced Fluorescence Quenching

“…Upon exposure to water, an immediate increase in fluorescence, back to F O , is observed, followed by a resumption of normal PSII activity as indicated by a return of F V upon exposure to saturating light flashes (Heber et al, 2000;Green et al, 2002). This ability to resume photosynthesis almost immediately after hydration of desiccated lichens is an adaptation to their particular cycles of wetting and dehydration.…”