Chlorophyll concentrations and a/b ratios in mosses collected from exposed and shaded habitats in Kansas

Abstract

Chlorophyll concentrations and a/b ratios of 31 moss species in 16 families collected from various habitats in Kansas were lower than those of most higher vascular plants. Most of the mosses examined had higher chlorophyll concentrations and lower chlorophyll a/b ratios when collected from habitats with low solar irradiance, relative to mosses collected from habitats characterized by high irradiance. This was true for mosses collected from under a forest canopy compared with those collected from exposed sandstone outcrops, and for the same populations of mosses collected before and after forest canopy closure. Such changes in the characteristics of the light-harvesting pigment system appear to represent adaptations enhancing the efficiency of light capture.     

Determination of Prenylquinones in Green Photosynthetically Active Moss and Liver Moss Tissues

The prenylquinone composition of two species of mosses (Polytrichum formosum Hedw., Sphagnum acutifolium Ehrh.) and two species of liver mosses (Lunularia cruciata (L.) Dum., Pellia epiphylla (L.) Cord.) was determined and compared with the chlorophyll content and the photosynthetic activity of the intact moss and liver moss tissues.

• 1 Green moss and liver moss tissues possess in principle the same prenylquinone composition as higher plants with plastoquinone-9, α-tocopherol, α-tocoquinone and the phylloquinone K₁ as main components. On a chlorophyll basis the lipoquinone levels are lower than in higher plants. Differences among the individual mosses as well as within one species only occur in the quantitative levels of the chloroplast prenylquinones, but there are no differences between musci and liver mosses.
• 2 There are differences in the maximal fluorescence of liver mosses and mosses. The variable fluorescence in turn, which is a measure of in vivo photosynthetic activity, is very similar for all examined species of mosses and liver mosses (values from 0.7 to 1.0) but somewhat lower than in leaf pieces of higher plants. DCMU blocks the variable fluorescence and the concomitant oxygen evolution in all mosses and liver mosses.
• 3 From the lower prenylquinone levels and the low values for the variable fluorescence it is concluded that mosses and liver mosses exhibit on a chlorophyll basis fewer reaction centres and electron transport chains than chloroplasts of higher plants.

     

Effect of Magnesium on Chlorophyll-Protein Complexes

The effect of Mg²⁺ during the isolation of chlorophyll-protein complexes was studied in two moss species (Pleurozium schreberi and Ceratodon purpureus) and New Zealand spinach (Tetragonia expansa). When 2 mM MgCl₂ was included in all the extraction and separation phases, the proportions of chlorophyll-protein complex I. were very small in all plants studied. The withdrawal of Mg²⁺ considerably increased the proportions of CP I. The most pronounced increase in the chlorophyll present as CP I was found when Mg²⁺ was withdrawn from the gel, and this also increased the mobility of the CP II complex and free pigment zone. Exclusion of Mg²⁺ from the running buffer had very little effect. Although Mg²⁺ had little effect on the relative amount of chlorophyll in CP II, the withdrawal of Mg²⁺ from all the extraction and separation phases caused formation of polymers of CP II. In the mosses, the formation of polymers of CP II seemed to be more obvious in the species with large grana. Absence of Mg²⁺ from all the extraction and separation phases sometimes also produced a polymer of CP I.     

Photoinhibition as a control on photosynthesis and production of Sphagnum mosses

The effect of high light intensity on photosynthesis and growth of Sphagnum moss species from Alaskan arctic tundra was studied under field and laboratory conditions. Field experiments consisted of experimental shading of mosses at sites normally exposed to full ambient irradiance, and removal of the vascular plant canopy from above mosses in tundra water track habitats. Moss growth was then monitored in the experimental plots and in adjacent control areas for 50 days from late June to early August 1988. In shaded plots total moss growth was 2–3 times higher than that measured in control plots, while significant reductions in moss growth were found in canopy removal plots. The possibility that photoinhibition of photosynthesis might occur under high-light conditions and affect growth was studied under controlled laboratory conditions with mosses collected from the arctic study site, as well as from a temperate location in the Sierra Nevada, California. After 2 days of high-light treatment (800 mol photons m^–2 s^–1) in a controlled environmental chamber, moss photosynthetic capacity was significantly lowered in both arctic and temperate samples, and did not recover during the 14-day experimental period. The observed decrease in photosynthetic capacity was correlated (r ²=0.735, P<0.001) with a decrease in the ratio of variable to maximum chlorophyll fluorescence (F _v/F _m) in arctic and temperate mosses. This relationship indicates photoinhibition of photosynthesis in both arctic and temperate mosses at even moderately high light intensities. It is suggested that susceptibility to photoinhibition and failure to photoacclimate to higher light intensities in Sphagnum spp. may be related to low tissue nitrogen levels in these exclusively ombrotrophic plants. Photoinhibition of photosynthesis leading to lowered annual carbon gain in Sphagnum mosses may be an important factor affecting CO₂ flux at the ecosystem level, given the abundance of these plants in Alaskan tussock tundra.     

不同微生境中水光温变化对毛尖紫萼藓叶绿素荧光特性的影响

毛尖紫萼藓（Grimmia pilifera）多生长在裸岩表面且具有多种微生境,其生长过程受到水分、光照和温度等环境因素的交互影响,但其光合生理特征如何响应这种变化的环境条件尚不清楚。开展原位（荫蔽和向阳裸岩2种微生境）和室内模拟实验,分析了不同水分（模拟降水量和降水频次）、光照、温度及其复合梯度处理对毛尖紫萼藓叶绿素荧光特性的影响。原位实验结果显示荫蔽生境原位生长毛尖紫萼藓光化学效率显著高于向阳生境。在室内相同培养条件下,脱水过程中来自荫蔽生境植株有效光合效率保持时间比向阳生境明显缩短,表现出较低的脱水耐受性。水-光-温复合模拟实验显示,降水频次和光-温变化对毛尖紫萼藓光化学效率均有极显著影响且存在一定的交互作用,而降水量的影响相对较弱;弱光低温及1次/（2 d）的降水频率条件下毛尖紫萼藓具有最高的光合活性。总体来看,荫蔽生境、弱光低温条件及中等频次降水有利于毛尖紫萼藓生长,但向阳生境毛尖紫萼藓则具有更强的环境耐受性。     

Photoprotective pigment as an adaptive strategy in the antarctic moss Ceratodon purpureus

Summary Variation in leaf pigmentation from green to ginger is observed for Ceratodon purpureus (Hedw.) Brid. in Antarctica. Electron microscopy of ginger and green leaves reveals less thylakoid stacking, a response to greater light exposure, in the ginger leaves. In extremely exposed sites C. purpureus has low chlorophyll a/b ratios which correlate with decreased 77K chlorophyll fluorescence, indicating damage to chlorophyll a. Pigment analysis of ginger moss shows that even when the chlorophyll a/b ratio has not decreased the pigment composition differs from green moss. The increase in anthocyanin and decrease in chlorophyll concentrations largely account for the visual change from green to ginger. The ratio of total carotenoid to chlorophyll varies from 0.35 in green moss to 0.55 in the ginger moss, with violaxanthin increased preferentially. Since these changes in pigmentation are consistent with photoprotection and they are linked to light dependent variations in chloroplast structure, it appears that photoprotective pigments are a useful adaptation for the bright Antarctic environment.     

Effect of Changed Environmental Conditions on Glycolipids of the Mosses Pleurozium Schreberi and Ceratodon purpureus

The effect of changed environmental conditions on the content of glycolipids and component fatty acids was studied in the moss species Pleurozium schreberi and Ceratodon purpureus. The mosses were collected from their natural habitats when frozen and covered by snow. After one week's exposure to rhythmic light (150 μE m^?2 s^?1, 12 h 17°C) no changes were observed in the absolute amount of fatty acids in either mono- (MGDG) or diglycosyl diglyceride (DGDG) fractions. Some changes were recorded in the content of individual fatty acids, however. The long chain, polunsaturated fatty acids (mainly 20:4ω6 and 20:5ω3 in P. schreberi and in addition 16:3ω3 and 18:3ω3 in C. purpureus) tended to decrease and the shorter chain, more saturated ones increased correspondingly. Under continuous light conditions (17°C) the total amount of fatty acids decreased in both MGDG and DGDG fractions, more significantly at 150 than at 75 μE m^?2 s^?1. This was due to the accelerated degradation and/or decreased synthesis of polyunsaturated fatty acids, which in this case was not totally compensated by the increase in shorter chain, more saturated ones.     

Ecophysiological analysis of moss-dominated biological soil crusts and their separate components from the Succulent Karoo, South Africa

Biological soil crusts, formed by an association of soil particles with cyanobacteria, lichens, mosses, fungi and bacteria in varying proportions, live in or directly on top of the uppermost soil layer. To evaluate their role in the global carbon cycle, gas exchange measurements were conducted under controlled conditions. Moss-dominated soil crusts were first analyzed as moss tufts on soil, then the mosses were removed and the soil was analyzed separately to obtain the physiological response of both soil and individual moss stems. Net photosynthetic response of moss stems and complete crusts was decreased by insufficient and excess amounts of water, resulting in optimum curves with similar ranges of optimum water content. Light saturation of both sample types occurred at high irradiance, but moss stems reached light compensation and saturation points at lower values. Optimum temperatures of moss stems ranged between 22 and 27°C, whereas complete crusts reached similar net photosynthesis between 7 and 27°C. Under optimum conditions, moss stems reached higher net photosynthesis (4.0 vs. 2.8 μmol m^?2 s^?1) and lower dark respiration rates (?0.9 vs. ?2.4 μmol m^?2 s^?1). Respiration rates of soil without moss stems were high (up to ?2.0 μmol m^?2 s^?1) causing by far lower absolute values of NP/DR ratios of soil crusts as compared to moss stems. In carbon balances, it therefore has to be clearly distinguished between measurements of soil crust components versus complete crusts. High rates of soil respiration may be caused by leaching of mosses, creating high-nutrient microsites that favor microorganism growth.     

Mosses and the struggle for light in a nitrogen-polluted world

The impact of reduced light conditions as an indirect effect of nitrogen (N) deposition was determined on three mosses in a montane ecosystem, where sedge and grass cover increase due to N enrichment. Additionally, in the greenhouse we established the importance of low light to moss growth as an indirect N deposition effect relative to the direct toxic effects of N. The amount of light reaching the moss layer was strongly and negatively related to graminoid abundance. Mosses showed differing sensitivities to reduced light in the field. Racomitrium lanuginosum biomass was found to be highest under high-light conditions, Polytrichum alpinum at intermediate light levels, whilst that of Dicranum fuscescens was unrelated to light availability. Moreover, Racomitrium biomass decreased with increasing amounts of graminoid litter, whereas the other species were little affected. All three mosses responded differently to the combination of elevated N (20 vs 10 kg N ha^–1 year^–1) and reduced light (60 and 80% reduction) in the greenhouse. Racomitrium growth was strongly influenced by both light reduction and elevated N, in combination reducing shoot biomass up to 76%. There was a tendency for Dicranum growth to be modestly reduced by elevated N when shaded, causing up to 19% growth reduction. Polytrichum growth was not influenced by elevated N but was reduced up to 40% by shading. We conclude that competition for light, induced by vascular plants, can strongly influence moss performance even in unproductive low biomass ecosystems. The effects of reduced light arising from N pollution can be as important to mosses as direct toxicity from N deposition. Yet, different sensitivities of mosses to both toxic and shading effects of elevated N prevent generalisation and can lead to competitive species replacement within moss communities. This study demonstrates the importance of understanding moss-vascular plant interactions to allow interpretation and prediction of ecosystem responses to anthropogenic drivers such as atmospheric N deposition or climate change.     

Functional organization and plasticity of the photosynthetic unit of the cyanobacterium Anacystis nidulans

Anacystis nidulans grown under high and low light, 100 and 10 μE m^?2 s^?1, respectively, was analyzed with respect to chlorophyll/P700, phycobiliproteins/P700, chlorophyll/cell, and oxygen evolution parameters. The photosynthetic unit sizes of this cyanobacterium, measured as the ratio of total chromophores (chlorophyll and bilin) to P700, were shown to be similar to those of higher plants and green algae. High light grown cells possessed a photosynthetic unit consisting of a core of 157 ± 6 chlorophyll a molecules per P700 associated with a light harvesting system of 95 ± 3.5 biliprotein chromophores. Low light grown cells had substantially more biliprotein chromophores per P700 (125 ± 3.1) than high light cells, but showed no significant difference in the numbers of chlorophyll a molecules per P700 (149 ± 4). Analyses of aqueous biliprotein extracts indicate that low light grown cells produce proportionately more phycocyanin relative to allophycocyanin than high light cells. Calculations of the molecular weight of biliproteins per P700 suggest that there is less than one phycobilisome per reaction center I under both growth conditions. Differences in chlorophyll/cell ratios and oxygen evolution characteristics were also observed. High light cells contain 6.3 × 10^?12 mg chlorophyll cell^?1, while low light grown cells contain 12.8 × 10^?12 mg chlorophyll cell^?1. Photosynthetic oxygen evolution rate vs. light intensity curves indicate that high light grown cells reach maximal levels of oxygen evolution at higher light intensity than low light grown cells. Maximal rates of oxygen evolution were 16.6 μmol oxygen min^?1 (mg chlorophyll)^?1 for high and 8.4 μmol oxygen min^?1 (mg chlorophyll)^?1 for low light cells. Maximal oxygen evolution rates per cell were equivalent for both cell types, although the amount of P700 per cell was lower in high light cells. High light grown cells are therefore capable of producing more oxygen per reaction center I than low light grown cells.     

Effects of nitrogen supply and irradiance on growth and nitrogen status in the moss Dicranum majus from differently polluted areas

Abstract

A growth experiment was undertaken to study the effects of nitrogen supply and irradiance on growth and nitrogen status in the moss Dicranum majus Sm. from two areas receiving different amounts of atmospheric nitrogen deposition. Intact samples of D. majus carpets were taken from two Picea abies forests, one located in southern Norway (high-N site) and the other in central Norway (low-N site). The moss carpets were grown for 120 days at three irradiance levels (PPFD: 20,40 or 80 μmol m^?2 s^?l) and sprayed daily with equal amounts of a nutrient solution containing 30, 180 or 330 μM nitrogen as NO₃ - and NH₄ +. Concentrations and total amounts of nitrogen, soluble proteins and chlorophyll were highest in moss plants from the high-N site, both at the start and the end of the experiment. The elongation growth was highest at the lowest irradiance level. As total biomass production did not differ between nitrogen and light treatments, moss growth was presumably limited by other factors, even at the lowest supply rates. Concentrations and total amounts of nitrogen increased with increasing nitrogen supply in moss plants from both sites. Accumulated nitrogen was partly stored as protein and chlorophyll. Recycling of nitrogen from old to young tissues is discussed as a possible explanation for the rather low nitrogen demand in D. majus and the persistently higher nitrogen contents in moss plants from the high-N site.     

Photosynthetic response to dynamic changes of light and air humidity in two moss species from the Tibetan Plateau

Bryophytes are widely distributed in alpine meadow and wetlands on the Tibetan Plateau, where light intensity and air moisture are highly variable in time. To address how bryophytes respond to their light and moisture environments, we examined dynamic photosynthesis in two moss species, Distichium inclinatum common in meadow, and Encalypta alpine frequently found in wetland. Photosynthetic induction response was faster in the two moss species than in most vascular species. In both species the 90% induction time after a sudden light increase from 50 to 600 μmol m⁻² s⁻¹ was within 3 min. The induction was faster in mosses experiencing a period of weak light than in those in total darkness. E. alpina, the wetland species, showed more rapid induction response and shorter post-illumination CO₂ fixation to sunflecks than D. inclinatum, the meadow species. Photosynthetic rate (A _max) under saturated light in the two species increased linearly with increasing air relative humidity (RH). The meadow species D. inclinatum showed higher A _max under low RHs, but exhibited lower A _max under high RHs in comparison with the wetland moss E. alpina. Moreover, the quantum efficiency increased linearly with increasing RH, indicating that air humidity plays a critical role in photochemical activities in the alpine mosses. The study suggests that there are acclimations in dynamic photosynthesis in response to light and humidity, and the acclimations would benefit a high leaf carbon gain in the two alpine moss species in their common habitats.     

Green light drives photosynthesis in mosses

Temperate forests are characterised by variable light quality (i.e. spectral composition of light) at or near the forest floor. These understory environments have a high concentration of green light, as red and blue light are preferentially absorbed by upper canopy leaves. Understory species may be well-adapted for using green light to drive photosynthesis. Angiosperms have been shown to use green light for photosynthesis, but this ability has not been demonstrated in shade-dwelling bryophytes. In this study, net photosynthetic rate (P_N) of three temperate understory species of moss (Dichodontium pellucidum (Hedw.) Schimp., Leucobryum albidum (Brid. ex P.Beauv) Lindb. and Amblystegium serpens (Hedw.) Schimp.) was measured under green, red?+?blue, and red?+?blue?+?green light to assess green light use efficiency. All three species were capable of photosynthesising beyond their respiratory demands using solely green light, with higher green light use efficiency measured in plants collected from areas with greater canopy cover, suggesting growth in a green light concentrated environment increases green light use efficiency. Each species was also collected from sites differing in their degree of canopy cover and grown under three light treatments (high light, low light, and green light). Photosynthetic efficiency (chlorophyll fluorescence), tissue nitrogen and carbon isotope concentrations were assessed after a short growth period. Growth conditions had little effect on leaf chemistry and monochromatic green light did not significantly degrade photosynthetic efficiency. This study provides the first evidence to date of positive net ‘green light photosynthesis’ in mosses.     

Physiological ecology of desert biocrust moss following 10 years exposure to elevated CO₂: evidence for enhanced photosynthetic thermotolerance

In arid regions, biomes particularly responsive to climate change, mosses play an important biogeochemical role as key components of biocrusts. Using the biocrust moss Syntrichia caninervis collected from the Nevada Desert Free Air CO₂ Enrichment Facility, we examined the physiological effects of 10 years of exposure to elevated CO₂, and the effect of high temperature events on the photosynthetic performance of moss grown in CO₂‐enriched air. Moss exposed to elevated CO₂ exhibited a 46% decrease in chlorophyll, a 20% increase in carbon and no difference in either nitrogen content or photosynthetic performance. However, when subjected to high temperatures (35–40°C), mosses from the elevated CO₂ environment showed higher photosynthetic performance and photosystem II (PSII) efficiency compared to those grown in ambient conditions, potentially reflective of a shift in nitrogen allocation to components that offer a higher resistance of PSII to heat stress. This result suggests that mosses may respond to climate change in markedly different ways than vascular plants, and observed CO₂‐induced photosynthetic thermotolerance in S. caninervis will likely have consequences for future desert biogeochemistry.     

Changes of Photosystem Ⅱ Electron Transport in the ChlorophyⅡ-deficient Oilseed Rape Mutant Studied by ChlorophyⅡ Fluorescence and Thermoluminescence

The photosystem Ⅱ （PSII） complex of photosynthetic membranes comprises a number of chlorophyll-binding proteins that are important to the electron flow. Here we report that the chlorophyll b-deficient mutant has decreased the amount of light-harvesting complexes with an increased amount of some core polypeptldes of PSII, including CP43 and CP47. By means of chlorophyll fluorescence and thermolumlnescence, we found that the ratio of Fv/Fm, qP and electron transport rate in the chlorophyll b-deficient mutant was higher compared to the wild type. In the chlorophyll lPdeflclent mutant, the decay of the primary electron acceptor quinones （QA-） reoxidation was decreased, measured by the fluorescence. Furthermore, the thermoluminescence studies in the chlorophyll bdeficient mutant showed that the B band （S2/S3QB-） decreased slightly and shifted up towards higher temperatures. In the presence of dlchlorophenyl-dlmethylurea, which is inhibited in the electron flow to the second electron acceptor quinines （QB） at the PSll acceptor side, the maximum of the Q band （S2QA-） was decreased slightly and shifted down to lower temperatures, compared to the wild type. Thus, the electron flow within PSll of the chlorophyⅡ b-deficient mutant was down-regulated and characterized by faster oxidation of the primary electron acceptor quinine QA-via forward electron flow and slower reduction of the oxidation S states.     

Physiological effects of five months exposure to low concentrations of O3 and NH3 on Douglas fir (Pseudotsuga menziesii)

Three years old seedlings of Douglas fir (Pseudotsuga menziesii) were exposed lo filtered air, O₃ (day and night concentrations of 78 and 30 μgm^?3: respectively). NH₃ (54 μg m^?3) and to a mixture of NH₃+O₃ (day and night concentrations of 49 + 83 and 49 + 44 μg m^?3 respectively), for 5 months in fumigation chambers. Both gas exchange and chlorophyll fluorescence were measured on shoots which had sprouted at the beginning of the exposure period. After 4. 8, 10 and 20 weeks of exposure, light response curves of electron transport rate (J) were determined, in which J was deduced from chlorophyll fluorescence. Net CO₂ assimiialion was measured at maximum light intensity of 560) μmol m^?2 S^?1 (P_n.560). After 8 and 10 weeks of exposure also light response curves of CO₂ assimilation were assessed. Shoots exposed to O₃ showed a reduction in net CO₂ assimilation as compared to the control shoots during the entire exposure period. The reduction was related lo a lower chlorophyll content and a lower electron transport rate, whereas no effect on quantum yield efficiency (qy) was observed. In contrast, shoots exposed to NH₃ showed a positive effect on photosynthesis. Shoots exposed to NH₃. + O₃ showed a rapid increase in P_n.560, in the period between 4 and 8 weeks to a level equal of that of the NH₃-treatment. After this period a decline in P_n.560 was observed. After 10 weeks of exposure shoots exposed to O₃ showed an increased transpiration rate in the dark as compared to the control shoots. In addition, water use efficiency (WUE) declined as a result of an increase in leaf conductance. Both observations indicate that the stomatal apparatus was affected by O₃. A high transpiration rate in the dark was also found for shoots esposed to NHX. However, shoots exposed to NH₃+ O₃ showed neither an effect on WUE, nor an effect on transpiration rate in the dark. The possibility that NH₃ delayed the O₃ induced effects on photosynthesis and stomatal conductance is discussed.     

Bryophytes as food: comparative consumption and utilization of mosses by a generalist insect herbivore

Compared with angiosperms, bryophytes are seldom fed upon by insects, despite being commonly used for shelter. Bryophytes are assumed to be unpalatable, and three classes of mechanisms have been suggested as possible barriers to bryophagy: chemical defenses, low digestibility, and low nutrient content. However, very few studies have tested these hypotheses. The present study examines pre‐ and post‐ingestive defenses of mosses. The acceptability and quality of four species of moss –Bryum argenteum Hedw. (Bryales: Bryaceae), Climacium americanum Brid. (Leucodontales: Climaciaceae), Leucobryum glaucum (Hedw.) (Dicranales: Leucobryaceae), and Sphagnum warnstorfii Russ. (Sphagnales: Sphagnaceae) – were compared with two control diets using the generalist caterpillar, Trichoplusia ni Hübner (Lepidoptera: Noctuidae: Plusiinae). In no‐choice trials, caterpillars consumed much less of any of the mosses than lettuce or wheat germ. The only moss consumed in sufficient quantities to evaluate post‐ingestive responses was C. americanum. Digestibility, assimilation, and overall utilization efficiency of C. americanum did not differ from that of lettuce, although C. americanum and lettuce were both less digestible than artificial diet. Choice assays using leaf discs showed that ethanol extract of L. glaucum, the least consumed moss, was deterrent, implying that chemical defenses play a major role in deterring feeding on L. glaucum. This study suggested that pre‐ingestive mechanisms are more important than post‐ingestive mechanisms in discouraging herbivory on mosses, and offered evidence that mosses are not simply nutrient poor.     

Freeze avoidance: a dehydrating moss gathers no ice

Using cryo‐SEM with EDX fundamental structural and mechanical properties of the moss Ceratodon purpureus (Hedw.) Brid. were studied in relation to tolerance of freezing temperatures. In contrast to more complex plants, no ice accumulated within the moss during the freezing event. External ice induced desiccation with the response being a function of cell type; water‐filled hydroid cells cavitated and were embolized at ?4 °C while parenchyma cells of the inner cortex exhibited cytorrhysis, decreasing to ～20% of their original volume at a nadir temperature of ?20 °C. Chlorophyll fluorescence showed that these winter acclimated mosses displayed no evidence of damage after thawing from ?20 °C while GCMS showed that sugar concentrations were not sufficient to confer this level of freezing tolerance. In addition, differential scanning calorimetry showed internal ice nucleation occurred in hydrated moss at ～?12 °C while desiccated moss showed no evidence of freezing with lowering of nadir temperature to ?20 °C. Therefore the rapid dehydration of the moss provides an elegantly simple solution to the problem of freezing; remove that which freezes.     

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 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 F_o 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 F_m 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. Received: 10 December 1999 / Accepted: 13 April 2000     

Photoinhibition in the Antarctic moss Grimmia antarctici Card when exposed to cycles of freezing and thawing

Freezing and thawing of the endemic moss species Grimmia antarctici Card, caused photoinhibition. When snow cover was removed from moss in the field, resulting in exposure to fluctuating temperatures and light conditions, photoinhibition, measured as a reduction in the ratio of variable to maximum chlorophyll a fluorescence (F_v/F_m), was observed. The extent of photoinhibition was highly variable and appeared to be reversible during periods of warmer temperatures. A series of controlled laboratory studies found that the light conditions that prevail between freezing and thawing events influenced the recovery from photoinhibition observed during freezing and thawing, with low light conditions facilitating the greatest rates of recovery. After four cycles of freezing and thawing, recovery from photoinhibition in hydrated moss was achieved within 12 h of transfer to 5°C and 15 μmol quanta m^?2 s^?1. These results favour the hypothesis that photoinhibition observed during freezing represents a protective process involving the down-regulation of photo-system II when photosynthetic carbon assimilation is limited by low temperatures.