Water status related photosynthesis and carbon isotope discrimination in species of the lichen genusPseudocyphellaria with green or blue-green photobionts and in photosymbiodemes

Summary Green lichens have been shown to attain positive net photosynthesis in the presence of water vapour while blue-green lichens require liquid water (Lange et al. 1986). This behaviour is confirmed not only for species with differing photobionts in the genusPseudocyphellaria but for green and blue-green photobionts in a single joined thallus (photosymbiodeme), with a single mycobiont, and also when adjacent as co-primary photobionts. The different response is therefore a property of the photobiont. The results are consistent with published photosynthesis/water content response curves. The minimum thallus water content for positive net photosynthesis appears to be much lower in green lichens (15% to 30%, related to dry weight) compared to blue-greens (85% to 100%). Since both types of lichen rehydrate to about 50% water content by water vapour uptake only green lichens will show positive net photosynthesis. It is proposed that the presence of sugar alcohols in green algae allow them to retain a liquid pool (concentrated solution) in their chloroplasts at low water potentials and even to reform it by water vapour uptake after being dried. The previously shown difference in δ¹³C values between blue-green and green lichens is also retained in a photosymbiodeme and must be photobiont determined. The wide range of δ¹³C values in lichens can be explained by a C₃ carboxylation system and the various effects of different limiting processes for photosynthetic CO₂ fixation. If carboxylation is rate limiting, there will be a strong discrimination of¹³CO₂, at high internal CO₂ partial pressure. The resulting very low δ¹³C values (-31 to-35‰) have been found only in green lichens which are able to photosynthesize at low thallus water content by equilibraiton with water vapour. When the liquid phase diffusion of CO₂ becomes more and more rate limiting and the internal CO₂ pressure decreases, the¹³C content of the photosynthates increases and less negative δ¹³C values results, as are found for blue-green lichens.     

Über den CO2-Gaswechsel einiger Flechten nach Wasserdampfaufnahme

Zusammenfassung Die Abhängigkeit des CO₂-Gaswechsels der Flechten Evernia divaricata, E. prunastri, Ramalina thrausta und R. farinacea vom Wassergehalt wurde untersucht. Es kann gezeigt werden, daß die im Zustand latenten Lebens befindlichen, ausgetrockneten Thalli aus nahezu wasserdampf-gesättigter Luft bis 70% ihres Trockengewichtes an Wasserdampf auf-nehmen. Diese Wasserdampfaufnahme reicht aus, um den CO₂-Gaswechsel zu reaktivieren. Die apparente CO₂-Aufnahme im Dampfdruck-gleichgewicht mit dem nahezu wasserdampfgesättigten Luftraum beträgt bei allen vier Arten etwa 90% der bei optimaler Einquellung mit tropfbarem Wasser erreichten Leistung. Reaktivierung des CO₂-Gaswechsels ist auch in nicht wasserdampfgesättigter Luft möglich, und der Kompensationspunkt des CO₂-Gaswechsels wird zwischen 80 und 85% rel. Feuchte (Sättigungsdefizit bei 10°C: 1,85-1,38 mm Hg) erreicht und überschritten.

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CO₂-exchange of some lichens after absorption of water vapour

Summary The relation between CO₂-exchange and water content of the lichens Evernia divaricata, E. prunastri, Ramalina thrausta and R. farinacea was investigated. The dry thalli absorb water vapour up to 70% of their dry weight. This uptake of water vapour is sufficient to reactivate the CO₂-exchange. In equilibrium with the vapour pressure of the nearly saturated air the apparent CO₂-uptake amounts to 90% of the value obtained after imbibition with liquid water. Even in unsaturated air the CO₂-exchange is reactivated and the compensation point is reached between 80 and 85% relative humidity (saturation deficit at 10°C: 1,85-1,38 mm Hg).

     

Spatial variation in carbon isotope discrimination across the thalli of several lichen species

Variations in stable carbon isotope discrimination (δ) were investigated across the thalli of several lichen species possessing different photobiont associations. Lichens containing (i) green algae (phycobiont), (ii) green algae in association with cyanobacteria confined in cephalodia, or (iii) cyanobacteria (cyanobiont) as the photobiont partner were studied. Carbon isotope discrimination was analysed in different thallus sections, which varied in distance from the margin and in age. The marginal thallus region is considered to be youngest, while the central region is thought to be oldest. This analysis showed a clear variation in δ across the thallus related to distance from the growing margin. In most of the species examined, the highest δ values were found in marginal regions (younger), while the central and basal regions (older) showed significantly lower δ. To investigate the effects of the historical increase in atmospheric CO₂ concentration and the concurrent decrease in the ¹³C content of atmospheric CO₂ on the δ of lichens, experiments were carried out on herbarium samples of Lobaria pulmonaria collected from the mid 19th Century to 1953. The results obtained showed a pattern of variation of δ consistent with that of freshly collected samples; δ decreased substantially with increasing distance from the thallus margin, irrespective of the collection date. Moreover, no consistent variation of discrimination was found among different collection dates. These results demonstrate that the observed variation in δ is caused by age-related changes in the physiological behaviour of different thallus sections, and that the past 150 years of increasing CO₂ concentration have not had significant effects on A in L. pulmonaria. Photosynthesis measurements, chlorophyll analysis and observations using optical microscopy, performed on freshly collected lichens, showed significant changes in morphological and physiological characteristics across the thallus. Particularly, remarkable variations in thickness were found across the thallus. These anatomical changes may be responsible for the variation in δ, through variations in CO2 transfer resistance and, consequently, in CO₂ availability across the thallus. The lack of age-dependent variation in δ in cyanobiont lichens is possibly attributable to the operation of a CO₂-coneentrating mechanism and, therefore, to a more constant CO₂ environment across the thallus in this lichen group.     

CO2-Gaswechsel von Moosen nach Wasserdampfaufnahme aus dem Luftraum

Summary The CO₂-gas exchange of dry mosses which were exposed to air of high water vapour content has been followed. Some moss species behave as do lichens and aerophilic green algae: they are able to take up enough water vapour to make a rather high photosynthetic activity possible. Other species lack this ability. They need liquid water for reactivation of photosynthesis, as do poikilohydric ferns and phanerogams. In this respect too the mosses are located between the real thallophytes and the cormophytes. From this point of view they are useful objects for studying the relationships between water vapour reactivation, morphological organisation and ecological capability.     

Water relations and CO2 exchange of the terrestrial lichen Teloschistes capensis in the Namib fog desert: Measurements during two seasons in the field and under controlled conditions

Although the coastal zone of the Central Namib Desert (Namibia) has negligible rainfall, frequent fog, dew and high air humidity support a luxurious lichen flora. Large areas of soil crust communities are dominated by the multibranched, fruticose Teloschistes capensis interspersed by a (still indeterminable) Ramalina species. In earlier communications, based on field measurements in autumn, we began the analysis of functional mechanisms that allow these lichens to exist under the special conditions of a fog desert. We have extended this work by monitoring lichen CO₂ exchange and water relations in spring and by experiments under controlled conditions.In both seasons, nocturnal hydration, by fog and/or dew, activated dark respiration of the lichens which was followed, after sunrise, by a short period of positive net photosynthesis (NP) that continued until metabolic inactivation occurred from desiccation. Dry thalli of T. capensis were able to reactivate NP through water vapour uptake alone, beginning at an air relative humidity of 82%, i.e. at a water potential of −26.3 MPa; the moisture compensation point during desiccation was at 13% thallus water content (WC, dry weight related). Optimal WC for photosynthesis was around 100%, and both species showed a large and extended suprasaturation depression of CO₂ assimilation. Light response showed “sun-plant” characteristics with saturation >1000 μmol m⁻² s⁻¹ photosynthetically active photon flux density (PPFD). However, due to rapid desiccation, the combination of light saturation with optimal WC very rarely occurred under field conditions. Light compensation point after sunrise was highly dependent on actual WC: at low hydration, it amounted to only ca. 10 μmol m⁻² s⁻¹ PPFD so that even the smallest levels of hydration could be used for carbon gain before desiccation took place again. This phenomenon was probably due to a hydration gradient in the thallus branches during transient moistening so that the outer photobiont layer was favoured in contrast to the internal mycobiont which remained dry longer and did not contribute respiratory CO₂ loss. Fully hydrated thalli had light compensation points around 50 μmol m⁻² s⁻¹ PPFD. Extended desiccation of 1–3 days had no impact on the magnitude and recovery of photosynthesis but, imposed desiccation of 10 days reduced NP in lab and field experiments and caused an extended period of recovery. “Resaturation respiration” was not detected in the field data, although it was present after experimental moistening of dry thalli.In spring, the higher fog frequency and intensity increased maximal nocturnal WC, maximal attained NP as well as integrated daily carbon income (ΣNP) compared to the autumn measurements. NP_max and ΣNP depended on maximal nocturnal WC with a saturation-type response. In terms of carbon gain both species seem to be optimally adapted to nocturnal moistening up to 160% WC and were not able to make use of higher degrees of hydration, a feature that might well influence their habitat selection.Maximal daily carbon-related ΣNP for T. capensis was 4.6 mg_C (g_C)⁻¹ day⁻¹. A rough estimate of the annual (projected) area-related carbon balance (photosynthetic income minus respiratory losses) based on published fog and dew frequencies and personal observations was 15–34 mg_C m⁻² yr⁻¹.     

Photosynthetic Depression at High Thallus Water Contents in Lichens: Concurrent Use of Gas Exchange and Fluorescence Techniques with a Cyanobacterial and a Green Algal Peltigera Species

Abstract: Lichens, being poikilohydric, have varying thallus water contents (WC) and show a complex interaction between net photosynthesis (NP) and WC. NP can be depressed at low WC (desiccation effects) and, in some species, also at high WC. In the latter case the depression is normally ascribed to increased CO₂ diffusion resistances through water blockage. Recently, an earlier explanation, that the depression at high WC is due to recycling of CO₂ from increased dark respiration processes (DR), has been given renewed prominence. The two explanations were distinguished by the concurrent use of gas exchange and chlorophyll fluorescence techniques to investigate NP: WC relationships in the lichens Peltigera leucophlebia (green algal) and P. neckeri (cyanobacterial). Both species had a distinct optimal WC for NP with depressed values at low and high WC. The maximal quantum yield for both CO₂ fixation (initial slope of light response curves of NP) and photosystem II (fluorescence signals of dark-adapted thalli) was depressed only at low WC and remained high at optimal and greater WC. In contrast, the relative electron transport rate (ETR, derived from fluorescence signals of thalli in the light) tracked NP and was depressed at low and high WC. The depression of both NP and ETR at high WC (not that at low WC) could be prevented by using elevated external CO₂ concentrations. A single, linear relationship was found between all values of gross photosynthesis (NP + DR) and ETR regardless of external CO₂ concentration or WC. Our results show that, for these lichens, the depression in NP at high WC is a real fall in photosynthetic rate of the photobionts and is not due to recycling of CO₂. The removal of the depression in NP and ETR at high WC by using elevated external CO₂ levels allows us to conclude that an additional CO₂ diffusion resistance is present.     

Temperate rainforest lichens in New Zealand: high thallus water content can severely limit photosynthetic CO2 exchange

CO₂ exchange rate in relation to thallus water content (WC, % of dry weight) was determined for 22 species of lichens, mainly members of the genera Pseudocyphellaria and Sticta, from a temperate rainforest, Urewere National Park, New Zealand. All data were obtained in the field, either using a standard technique in which the lichens were initially wetted (soaked or sprayed, then shaken) and allowed to slowly dry, or from periodic measurements on samples that were continuously exposed in their natural habitat. A wide range of WC was found, with species varying from 357 to 3360% for maximal WC in the field, and from 86 to 1300% for optimal WC for photosynthesis. Maximal WC for lichens, wetted by the standard technique, were almost always much less than the field maxima, due to the presence of water on the thalli. The relationships between CO₂ exchange rate and WC could be divided into four response types based on the presence, and degree, of depression of photosynthesis at high WC. Type A lichens showed no depression, and Type B only a little at maximal WC. Type C had a very large depression and, at the highest WC, CO₂ release could occur even in the light. Photosynthetic depression commenced soon after optimal WC was reached. Type D lichens showed a similar depression but the response curve had an inflection so that net photosynthesis was low but almost constant, and never negative, at higher WC. There was little apparent relationship between lichen genus or photobiont type and the response type. It was shown that high WC does limit photosynthetic CO₂ uptake under natural conditions. Lichens, taken directly from the field and allowed to dry under controlled conditions, had net photosynthesis rates that were initially strongly inhibited but rose to an optimum, before declining at low WC. The limiting effects of high WC were clearly shown when, under similar light conditions, severe photosynthetic depression followed a brief, midday, rain storm. Over the whole measuring period the lichens were rarely at their optimal WC for photosynthesis, being mostly too wet or, occasionally, too dry. Photosynthetic performance by the lichens exposed in the field was similar to that expected from the relationship between the photosynthetic rate and WC established by the standard procedure.     

Photosynthetic CO2-use efficiency in lichens and their isolated photobionts: the possible role of a CO2-concentrating mechanism

The CO₂ dependence of net CO₂ assimilation was examined in a number of green algal and cyanobacterial lichens with the aim of screening for the algal/cyanobacterial CO₂-concentrating mechanism (CCM) in these symbiotic organisms. For the lichens Peltigera aphthosa (L.) Willd., P. canina (L.) Willd. and P. neopolydactyla (Gyeln.) Gyeln., the photosynthetic performance was also compared between intact thalli and their respective photobionts, the green alga Coccomyxa PA, isolated from Peltigera aphthosa and the cyanobacterium Nostoc PC, isolated from Peltigera canina. More direct evidence for the operation of a CCM was obtained by monitoring the effects of the carbonic-anhydrase inhibitors acetazolamide and ethoxyzolamide on the photosynthetic CO₂use efficiency of the photobionts. The results strongly indicate the operation of a CCM in all cyanobacterial lichens investigated and in cultured cells of Nostoc PC, similar to that described for free-living species of cyanobacteria. The green algal lichens were divided into two groups, one with a low and the other with a higher CO₂-use efficiency, indicative of the absence of a CCM in the former. The absence of a CCM in the low-affinity lichens was related to the photobiont, because free-living cells of Coccomyxa PA also apparently lacked a CCM. As a result of the postulated CCM, cyanobacterial Peltigera lichens have higher rates of net photosynthesis at normal CO₂ compared with Peltigera aphthosa. It is proposed that this increased photosynthetic capacity may result in a higher production potential, provided that photosynthesis is limited by CO₂ under natural conditions.     

Moisture content and CO2 exchange of lichens. II. Depression of net photosynthesis in Ramalina maciformis at high water content is caused by increased thallus carbon dioxide diffusion resistance

Summary Thalli of Ramalina maciformis were moistened to their maximal water holding capacity, thus, simulating actual conditions following a heavy rainfall. Time courses of net photosynthesis at 17° C and 750 E m^-2 s^-1 light intensity (PAR) were obtained during drying of the thalli. At ambient CO₂ concentrations from 200 to 1,000 ppm, CO₂ uptake of the moist lichens was depressed at high water content. After a certain water loss, net photosynthesis increased to a maximal value and decreased again with further drying of the thalli. The degree of initial depression of photosynthesis decreased with increasing ambient CO₂ concentration, and it was fully absent at 1,600 ppm ambient CO₂. Under these conditions of CO₂ saturation, net photosynthesis remained constant at maximum for many hours and decreased only when substantial amounts of water had been lost. We conclude that the carboxylation capacity of the lichen is not affected by high contents of liquid water. Therefore, the depression of CO₂ uptake of the water saturated lichen at lower (e.g. natural) ambient CO₂ must be due exclusively to increased resistance to CO₂ diffusion from the external air to the sites of carboxylation.     

Hardening by partial dehydration and ABA increase desiccation tolerance in the cyanobacterial lichen Peltigera polydactylon

* BACKGROUND AND AIMS: The ability of partial dehydration and abscisic acid pretreatments to increase desiccation tolerance in the cyanobacterial lichen Peltigera polydactylon was tested. * METHODS: Net photosynthesis and respiration were measured using infrared gas analysis during a drying and rehydration cycle. At the same time, the efficiency of photosystem two was measured using chlorophyll fluorescence, and the concentrations of chlorophyll a were spectrophotometrically assayed. Heat production was also measured during a shorter drying and rehydration cycle using differential dark microcalorimetry. * KEY RESULTS: Pretreating lichens by dehydrating them to a relative water content of approx. 0.65 for 3 d, followed by storing thalli hydrated for 1 d in the light, significantly improved their ability to recover net photosynthesis during rehydration after desiccation for 15 but not 30 d. Abscisic acid pretreatment could substitute for partial dehydration. The improved rates of photosynthesis during the rehydration of pretreated material were not accompanied by preservation of photosystem two activity or chlorophyll a concentrations compared with untreated lichens. Partial dehydration and ABA pretreatments appeared to have little direct effect on the desiccation tolerance of the mycobiont, because the bursts of respiration and heat production that occurred during rehydration were similar in control and pretreated lichens. * CONCLUSIONS: Results indicate that the photobiont of P. polydactylon possesses inducible tolerance mechanisms that reduce desiccation-induced damage to carbon fixation, and will therefore improve the supply of carbohydrates to the whole thallus following stress. In this lichen, ABA is involved in signal transduction pathways that increase tolerance of the photobiont.     

Further evidence that activation of net photosynthesis by dry cyanobacterial lichens requires liquid water

Abstract: In contrast to green algal lichens, cyanobacterial species of different families, growth forms and habitats proved to be unable to attain positive net CO₂ assimilation when the dry thalli were treated with air of high relative humidity; they needed liquid water for the reactivation of their photosynthetic apparatus. Identical behaviour is shown by all of the 47 lichen species with cyanobacterial photobionts, from six different genera, studied so far. This suggests a widely distributed, if not general, characteristic of cyanobacterial lichens. The difference in performance between both groups of photobionts was maintained when the lichen thallus was macerated. Furthermore, cultures of Chroococcidiopsis were unable to make use of water vapour hydration for positive net photosynthesis, and were similar in this respect to some free-living aerophilic cyanohacteria tested earlier. Possible physiological implications as well as ecological consequences for water-relation-dependent habitat selection of green-algal and cyanobacterial lichens are discussed.     

Curling during desiccation protects the foliose lichen Lobaria pulmonaria against photoinhibition

This study aims to assess the photoprotective potential of desiccation-induced curling in the light-susceptible old forest lichen Lobaria pulmonaria by using chlorophyll fluorescence imaging. Naturally curled thalli showed less photoinhibition-induced limitations in primary processes of photosynthesis than artificially flattened specimens during exposures to 450 μmol m⁻² s⁻¹ in the laboratory after both 12- (medium dose treatment) and 62-h duration (high dose treatment). Thallus areas shaded by curled lobes during light exposure showed unchanged values of measured chlorophyll fluorescence parameters (F _V/F _M, Φ_{PS II}), whereas non-shaded parts of curled thalli, as well as the mean for the entire flattened thalli, showed photoinhibitory limitation after light treatments. Furthermore, the chlorophyll fluorescence imaging showed that the typical small-scale reticulated ridges on the upper side of L. pulmonaria caused a spatial, small-scale reduction in damage due to minor shading. Severe dry-state photoinhibition readily occurred in flattened and light-treated L. pulmonaria, although the mechanisms for such damage in a desiccated and inactive stage are not well known. Natural curling is one strategy to reduce the chance for serious photoinhibition in desiccated L. pulmonaria thalli during high light exposures.     

Cyanolichens can have both cyanobacteria and green algae in a common layer as major contributors to photosynthesis

Background and Aims

Cyanolichens are usually stated to be bipartite (mycobiont plus cyanobacterial photobiont). Analyses revealed green algal carbohydrates in supposedly cyanobacterial lichens (in the genera Pseudocyphellaria, Sticta and Peltigera). Investigations were carried out to determine if both cyanobacteria and green algae were present in these lichens and, if so, what were their roles.

Methods

The types of photobiont present were determined by light and fluorescence microscopy. Small carbohydrates were analysed to detect the presence of green algal metabolites. Thalli were treated with selected strengths of Zn²⁺ solutions that stop cyanobacterial but not green algal photosynthesis. CO₂ exchange was measured before and after treatment to determine the contribution of each photobiont to total thallus photosynthesis. Heterocyst frequencies were determined to clarify whether the cyanobacteria were modified for increased nitrogen fixation (high heterocyst frequencies) or were normal, vegetative cells.

Key Results

Several cyanobacterial lichens had green algae present in the photosynthetic layer of the thallus. The presence of the green algal transfer carbohydrate (ribitol) and the incomplete inhibition of thallus photosynthesis upon treatment with Zn²⁺ solutions showed that both photobionts contributed to the photosynthesis of the lichen thallus. Low heterocyst frequencies showed that, despite the presence of adjacent green algae, the cyanobacteria were not altered to increase nitrogen fixation.

Conclusions

These cyanobacterial lichens are a tripartite lichen symbiont combination in which the mycobiont has two primarily photosynthetic photobionts, ‘co-primary photobionts’, a cyanobacterium (dominant) and a green alga. This demonstrates high flexibility in photobiont choice by the mycobiont in the Peltigerales. Overall thallus appearance does not change whether one or two photobionts are present in the cyanobacterial thallus. This suggests that, if there is a photobiont effect on thallus structure, it is not specific to one or the other photobiont.     

Mollusc grazing limits growth and early development of the old forest lichen Lobaria pulmonaria in broadleaved deciduous forests

This study aims: (1) to quantify mollusc grazing on juvenile and mature thalli of the foliose epiphytic lichen Lobaria pulmonaria, and (2) to test the hypothesis inferring a herbivore defensive role of lichen depsidones in forests with indigenous populations of lichen-feeding molluscs. Lichens were transplanted in shaded and less shaded positions in each of two calcareous broadleaved deciduous forests, one poor in lichens, one with a rich Lobarion community. Preventing the access of molluscs significantly reduced the loss of juvenile L. pulmonaria, particularly in the naturally lichen-poor forest. Molluscs also severely grazed mature thalli in the lichen-poor forest, especially thalli placed under the more shading canopies. Furthermore, reducing the natural concentration of depsidones by pre-rinsing with acetone increased subsequent grazing significantly, showing that lichen depsidones function as herbivore defence in natural habitats. Our results suggest that mollusc grazing may play important roles in shaping the epiphytic vegetation in calcareous deciduous forests, and that recently established juvenile L. pulmonaria thalli seem to be particularly vulnerable.     

Seasonal acclimation in the epiphytic lichen Parmelia sulcata is influenced by change in photobiont population density

CO₂ gas exchange, radial growth, chlorophyll (Chl) content and photobiont density of an epiphytic population of Parmelia sulcata were monitored every 2 months during 1 year in a temperate deciduous forest of Central Italy, to verify possible seasonal variations. Light response curves of south-exposed thalli, built up in the laboratory at 6 and 27°C at optimal thallus hydration, showed that CO₂ gas exchange changed significantly during the year, with a maximum for gross photosynthesis in December at both temperatures. Photoinhibition phenomena occurred in early spring, immediately before tree leaves sprouted. The principal component analysis of CO₂ gas exchange parameters clearly separated the months with from the months without tree canopy cover. Radial growth, measured on marginal lobes of north- and south-exposed thalli, was the highest in December, and the lowest in April. Photobiont density, measured in lobes of south- and north-exposed thalli with a sedimentation chamber, also changed during the year: the number of photobionts was highest in June and December, and lowest in April, although no significant change in cell size and Chl content per cell was evident throughout the year. South-exposed thalli had slightly, but constantly higher photobiont density both on a weight and an area basis. The acclimation of lichen photosynthesis and Chl content to seasonal temperature and light changes should partially be re-visited on the basis of the significant variation in photobiont population density. This phenomenon still awaits, however, a satisfactory explanation, although it is probably related to the seasonal change in nutrient availability.     

THE EFFECT OF AGRONOMIC PHOTOSYSTEM-II HERBICIDES ON LICHENS

The sensitivity ofHypogymnia physodes,Lobaria pulmonariaandPeltigera aphthosaH. physodesto six photosystem II herbicides and to DBMIB was tested in the laboratory by chlorophyll flouresence and oxygen-exchange measurements. in addition, experiments with freshly isolated photobiont cells fromH. physodesandL. pulmonariawere performed. Generally, the lichens were most sensitive to the urea herbicides diuron and isoproturon, whereas the triazines atrazine, terbuthylazine, and simazine and the triazinone metamitron wre less inhibitory. Among the three lichen species invesigated,H. physodeswas the most sensitive to the urea herbicides. For the other agents, no signifiant differences between lichen species could be found. The highest pI₅₀values obtained from dose response curves were around 6.5 for isolated photobionts, but most values for lichen thalli were in the range 5-6. Thus, there is no particular sensitivity of green algal lichen photobionts to photosytem II herbicides as compared to other algae, higher plant chloroplasts or protoplasts. In nature, we observed recovery from (damaging) treatment with 10⁻⁵mol diuron 1⁻¹forH. physodeswithin weeks. Therefore, damage to lichens fromt he use of photosystem-II herbicides in agriculture is probably only of very local occurence.     

Effect of high light on the efficiency of photochemical energy conversion in a variety of lichen species with green and blue-green phycobionts

Exposure to high light induced a quantitatively similar decrease in the rate of photosynthesis at limiting photon flux density (PFD) and of photosystem II (PSII) photochemical efficiency, F_V/F_M, in both green and blue-green algal lichens which were fully hydrated. Such depressions in the efficiency of photochemical energy conversion were generally reversible in green algal lichens but rather sustained in blue-green algal lichens. This greater susceptibility of blue-green algal lichens to sustained photoinhibition was not related to differences in the capacity to utilize light in photosynthesis, since the light-and CO₂-saturated rates of photosynthetic O₂ evolution were similar in the two groups. These reductions of PSII photochemical efficiency were, however, largely prevented in lichen thalli which were fully desiccated prior to exposure to high PFD. Thalli of green algal lichens which were allowed to desiccate during the exposure to high light exhibited similar recovery kinetics to those which were kept fully hydrated, whereas bluegreen algal lichens which became desiccated during a similar exposure exhibited greatly accelerated recovery compared to those which were kept fully hydrated. Thus, green algal lichens were able to recover from exposure to excessive PFDs when thalli were in either the hydrated or desiccated state during such an exposure, whereas in blue-green algal lichens the decrease in photochemical efficiency was reversible in thalli illuminated in the desiccated state but rather sustained subsequent to illumination of thalli in the hydrated state.Abbreviations and Symbols F_o yield of instantaneous fluorescence - F_M maximum yield of fluorescence induced by pulses of saturating light - F_V variable yield of fluorescence - PFD photon flux density (400–700 nm) - PSII photosystem II This work was supported by the Deutsche Forschungsgeneinschaft (Forscherguppe Ökophysiologic and Sonderforschungsbereich 251 of the University of Würzburg) and the Fonds der Chemischen Industrie. W.W.A. gratefully acknowledges the support of a fellowship from the Alexander von Humboldt Foundation. We thank Professor T.G.A. Green for identifying and supplying all of the New Zealand lichen material and Professor F.-C. Czygan for advice concerning the chlorophyll analyses which were performed by Johanna Leisner.     

Abscisic acid and polyamine contents in the lichens Pseudevernia furfuracea and Ramalina farinacea

The levels of endogenous abscisic acid and total polyamine contents (putrescine and spermidine) were studied in two lichen species differing in desiccation tolerance. ABA and polyamine contents showed significant differences between air-dried and water-treated thalli of Pseudevernia furfuracea and Ramalina farinacea. The levels of putrescine (put) and spermidine (spd) in water-treated P. furfuracea thalli were found higher than in dry samples. The values for spd were almost threefold and for put almost 1.4-fold higher than in airdried samples. The levels of put and spd in water-treated R. farinacea were lower than in the air-dried samples. The lowest values for spd and put were almost 33-fold and 2.8-fold less, respectively. ABA contents of water-treated P. furfuracea and R. farinacea increased by 2.65% and 25.5% when compared with air-dried thallus. This text was submitted by the authors in English.     

Water vapor uptake and photosynthesis of lichens: performance differences in species with green and blue-green algae as phycobionts

Summary Dry lichen thalli were enclosed in gas exchange chambers and treated with an air stream of high relative humidity (96.5 to near 100%) until water potential equilibrium was reached with the surrounding air (i.e., no further increase of weight through water vapor uptake). They were then sprayed with liquid water. The treatment took place in the dark and was interrupted by short periods of light. CO₂ exchange during light and dark respiration was monitored continuously. With no exception water uptake in all of the lichen species with green algae as phycobionts lead to reactivation of the photosynthetic metabolism. Further-more, high rates of CO₂ assimilation were attained without the application of liquid water. To date 73 species with different types of Chlorophyceae phycobionts have been tested in this and other studies. In contrast, hydration through high air humidity alone failed to stimulate positive net photosynthesis in any of the lichens with blue-green algae (Cyanobacteria). These required liquid water for CO₂ assimilation. So far 33 species have been investigated, and all have behaved similarly. These have included gelatinous as well as heteromerous species, most with Nostoc phycobionts but in addition some with three other Cyanophyceae phycobionts. The same phycobiont performance differences existed even within the same genus (e.g. Lobaria, Peltigera) between species pairs containing green or blue-green phycobionts respectively. Free living algae also seem to behave in a similar manner. Carbon isotope ratios of the lichen thalli suggest that a definite ecological difference exists in water status-dependent photosynthesis of species with green and blue-green phycobionts. The underlying biochemical or biophysical mechanisms are not yet understood. Apparently, a fundamental difference in the structure of the two groups of algae is involved.     

Photosynthetic activity and proteomic analysis highlights the utilization of atmospheric CO2 by Ulva prolifera (Chlorophyta) for rapid growth

Free‐floating Ulva prolifera is one of the causative species of green tides. When green tides occur, massive mats of floating U. prolifera thalli accumulate rapidly in surface waters with daily growth rates as high as 56%. The upper thalli of the mats experience environmental changes such as the change in carbon source, high salinity, and desiccation. In this study, the photosynthetic performances of PSI and PSII in U. prolifera thalli exposed to different atmospheric carbon dioxide (CO₂) levels were measured. Changes in photosynthesis within salinity treatments and dehydration under different CO₂ concentrations were also analyzed. The results showed that PSII activity was enhanced as CO₂ increased, suggesting that CO₂ assimilation was enhanced and U. prolifera thalli can utilize CO₂ in the atmosphere directly, even when under moderate stress. In addition, changes in the proteome of U. prolifera in response to salt stress were investigated. Stress‐tolerance proteins appeared to have an important role in the response to salinity stress, whereas the abundance of proteins related to metabolism showed no significant change under low salinity treatments. These findings may be one of the main reasons for the extremely high growth rate of free‐floating U. prolifera when green tides occur.