Prolonging the hydration and active metabolism from light periods into nights substantially enhances lichen growth

This study investigates how hydration during light and dark periods influences growth in two epiphytic old forest lichens, the green algal Lobaria pulmonaria and the cyanobacterial L. scrobiculata. The lichens were cultivated in growth chambers for 14 days (200 μmol m^?1 s^?2; 12 h photoperiod) at four temperature regimes (25/20 °C, 21/16 °C, 13/8 °C, and 6/1 °C; day/night temperatures) and two hydration regimes (12 h day-time hydration; 12 h day-time + 12 h night-time hydration). Growth was highly dynamic, showing that short-term growth experiments in growth cabinets have a high, but largely unexplored potential in functional lichen studies. The highest measured growth rates were not far from the maximal dry matter gain estimated from published net photosynthetic CO₂ uptake data. For the entire data set, photobiont type, temperature, hydration regime and specific thallus mass accounted for 46.6 % of the variation in relative growth rate (RGR). Both species showed substantially higher relative growth rates based on both biomass (RGR) and thallus area (RT_AGR) when they were hydrated day and night compared to hydration in light only. Chronic photoinhibition was substantial in thalli hydrated only during the day time and kept at the highest and lowest temperature regimes, resulting in exponential increases in RGR with increasing maximal PSII efficiency (F _v/F _m) in both species. However, the depression in F _v/F _m was stronger for the cyanolichen than for the cephalolichen at extreme temperatures. The growth-stimulating effect of night-time hydration suggests that nocturnal metabolic activity improves recovery of photoinhibition and/or enhances the conversion rate of photosynthates into thallus extension.     

Lichen palatability depends on investments in herbivore defence

Lichens are well-suited organisms for experimental herbivory studies because their secondary compounds, assumed to deter grazing, can be non-destructively extracted. Thalli of 17 lichen species from various habitats were cut in two equal parts; compounds were extracted from one part by acetone, the other served as a control. These two pieces were offered as a paired choice to the generalist herbivore snail Cepaea hortensis. Control thalli of all lichens were consumed at a low rate regardless of their investments in acetone-extractable lichen compounds; naturally compound-deficient lichen species were not preferred compared to those with high contents. However, for extracted thalli, there was a highly significant positive correlation between rate of consumption and the extracted compound contents. These data imply that herbivore defence has evolved in different directions in different lichens. Studied members of Parmeliaceae, common in oligotrophic habitats, have high contents of carbon-rich acetone-soluble compounds; these lichens became highly palatable to snails subsequent to acetone rinsing. Extracted lichen compounds were applied to pieces of filter paper and fed to snails. Extracts from members of the Parmeliaceae significantly deterred feeding on paper. Such data suggest that generalist herbivores may have shaped evolution in the widespread and highly diverse Parmeliaceae towards high investments in lichen compounds. On the other hand, lichens belonging to the Physciaceae and Teloschistales, common in nutrient-enriched habitats, are deficient in, or have low concentrations of, lichen compounds. Such lichens did not become more palatable after acetone rinsing. The orange anthraquinone compound parietin, restricted to the Teloschistales, and which has previously been found to protect against excess light, did not deter grazing.     

The lichens <Emphasis Type="Italic">Xanthoria elegans</Emphasis> and <Emphasis Type="Italic">Cetraria islandica</Emphasis> maintain a high protection against UV-B radiation in Arctic habitats

This study reports UV screening pigments in the upper cortices of two widespread lichens collected in three sun-exposed locations along a latitudinal gradient from the Arctic lowland to alpine sites of the Central European Alps. Populations from the Alps receive 3–5 times higher UV-B irradiance than their Arctic counterparts from Svalbard because of latitudinal and altitudinal gradients in UV-B irradiance. In Cetraria islandica, the screening capacity of melanin in the upper cortices was assessed by direct measurements of cortical transmittance (250–1,000 nm). A comparison of cortical transmittances in brown sun-exposed and pale shade-adapted forest C. islandica thalli showed that fungal melanins strongly absorb both UV-B and photosynthetically active radiation (PAR). For Xanthoria elegans cortical UV-B absorbing pigments, mainly the orange parietin, were extracted and quantified. Field experiments with extracted, parietin-deficient X. elegans thalli cultivated under various filters showed that UV-B was essential for the induction of parietin synthesis. The parietin resynthesis in these parietin-deficient samples increased with decreasing latitude of their location in which they had been sampled, which may imply that the synthesis of pigments is habitat specific. However, no latitudinal gradient in cortical screening capacity was detected for any of the two species investigated in the field. This implies that Arctic populations maintain a high level of screening pigments in spite of low ambient UV-B, and that the studied lichen species presumably may tolerate an increase in UV-B radiation due to the predicted thinning of the ozone layer over polar areas     

UV triggers the synthesis of the widely distributed secondary lichen compound usnic acid

Synthesis of the cortical dibenzofuran derivative usnic acid and the medullary depsidone salazinic acid was studied in Xanthoparmelia stenophylla thalli from which the compounds had been removed by acetone rinsing prior to a 21-day field experiment with UV absorbing and transmitting screens. Natural levels of ultraviolet radiation clearly induced the re-synthesis of usnic acid. The re-synthesis was boosted by the addition of ribitol, the carbohydrate delivered from the Trebouxia photobiont to the mycobiont. Salazinic acid was also weakly induced by UV. Re-synthesis was relatively low, up to 2.5 and 3.1% of start values for usnic and salazinic acid, respectively. However, given that the natural content of both compounds was high, constituting 12% of thallus dry weight, the absolute amounts of lichen compounds re-synthesised were not so small. We also studied the extractability of nine extracellular lichen compounds in three species X. stenophylla, Hypogymnia tubulosa, and Vulpicida pinastri, and found two distinct fractions of cortical compounds, one major that was completely extractable from living lichens and one minor that was only extractable with grinding. Medullary compounds were completely extracted without grinding. These findings did not influence the relative differences between treatments in our experiment, but may be of importance for future assessments of, e.g., quantitative studies of extracellular lichen compounds.     

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.     

Irradiance prior to and during desiccation improves the tolerance to excess irradiance in the desiccated state of the old forest lichen Lobaria pulmonaria

Hydrated thalli of the lichen Lobaria pulmonaria were either preconditioned to dim irradiance (DI, 5 μmol m⁻² s⁻¹) or medium irradiance (MI, 200 μmol m⁻² s⁻¹) for 6 h. After this 6 h period, the thalli were allowed to desiccate under the two respective irradiances. Thereafter, these dry lichens were exposed to high irradiance (HI, 1 000 μmol m⁻² s⁻¹) for 60 h. After this HI treatment, the maximal photochemical quantum yield (F_V/F_M) and the de-epoxidation state of xanthophyll cycle pigments (DEPS) were highest in thalli preconditioned to MI. Hence irradiance in the last hydrated period before sampling is significant for the physiological state of lichens. A standardized irradiance pre-treatment before start of experiments is recommended.     

Adverse Effects of Epiphytic Crustose Lichens upon Stem Photosynthesis and Chlorophyll of Populus tremula L.

Dry cork layer (phellem) in stems of Populus tremula transmitted 35–55 percent of incident irradiation, depending upon moisture content. A cover of crustose Lecanora lichens reduced transmission through phellem to 10 percent or less of incident irradiation. The bark contains photosynthetically active cells. Apparent quantum yield for photosynthetic O₂-evolution was 0.017 in bark covered with dry Lecanora compared with 0.070 in naked bark. The capacity for gross photosynthesis in high light (1090 μmol photons m^?2 s^?1) was reduced by 50 percent in Lecanora-covered bark. Lecanora did not reduce the ratio between variable and maximal chlorophyll a fluorescence (F_v/F_m). Chlorophyll content per unit area was similar in leaves and naked bark of Populus tremula. The chlorophyll content in the bark decreased with increasing chlorophyll content in Lecanora. Chlorophyll a/b ratio was 2.5 in the bark compared with 4.0 in leaves and in Lecanora, and the ratio decreased down the stems. The a/b ratio was 2.3 in Lecanora covered bark compared with 2.6 in naked bark. The changes in bark photosynthesis below a Lecanora crust were probably due to acclimation of bark photosynthesis to shade, since the lichen acids in the measured lichens neither suppressed photosynthetic O₂-evolution nor changed the F_v/F_m in bark disks.     

Heat resistance and energy budget in different Scandinavian plants

• 1 Alpine vascular plants seem to use other strategies in surviving a cold environment than reducing the reflectance in level leaves. Pubescence in alpine plants has small effect upon total reflectance, but may increase the amount of photosynthetic active radiation within the sheltered canopy. Alpine cushion plants like Silene acaulis, Diapensia lapponica and Loiseleuria procumbens maximize the absorptance of radiant energy with minimum heat losses, probably as an effect of the dense canopy structure. The young inflorescences of Eriophorum vaginatum were found to be extremely efficient absorbators, while the reflectance in Salix catkins was close to that of green leaves.
• 2 In lichens, a great variation both in visible (400–700 nm) and infrared (700–1400 nm) reflectance was found: (A) The Pseudephebe pubescence group consists of species with very low reflectance at all measured wavelengths. The species are chionophobous, probably because of the high absorptance which makes growth possible during the cold season. (B) The Sticta sylvatica group, characterized by very low visible reflectance and very high infrared reflectance, is well adapted to shade. (C) The Cetraria nivalis group consists of fruticose species with high reflectance both in the visible and the near infrared. The intense visible reflectance probably makes net photosynthesis possible in well protected layers of the canopy. (D) The Nephroma arcticum group with spectral properties resembling green leaves in vascular plants. (E) The Haematomma ventosum group and the Parmelia perlata group with spectral properties intermediate between group C and D.
• 3 A modified method determining lethal temperatures and energies of activation in the process leading to death during a heat shock, is described. The two parameters are rather species specific in many of the 118 Scandinavian plants investigated. The lethal temperatures completely overlap the values in hotter parts of the world. However, habitat specific lethal temperatures were found; low values in wet- or shade-growing species and high values in dry-growing species. In Picea abies lethal temperatures and energies of activation showed pronounced, but diverging, year cycles in 12 ecotypes from different parts of Europe. Only negligible differences between the ecotypes exist, and cycles are probably photoperiodically determined.
• 4 Heat hardening can be achieved quickly, both in an active and dormant stage, by increasing the temperature. A linear correlation between hardening temperature, both in the optimal and supraoptimal temperature range, and hardening capability was found. In most species, but especially in cold adapted species, the hardening capability at supraoptimal temperatures decreases with increasing cultivation temperature.
• 5 Diffusion resistances with open (r_s) and closed stomata (r_c) are measured on excised leaf samples in 72 species. A positive correlation between r_s and r_c was found. r_c ranges from 2.2-62.5 s cm⁻¹ in mesophytes and from 19-425 s cm⁻¹ in xerophytes, the highest values were found in succulents. Some of the alpine species had extremely low r_c, falling within the r_s-range. Some habitat specific differences in r_c were found, but the relatively few significant differences in r_s between different habitats indicate that a lot of different drought avoidance mechanisms exist. The greatest variation in r_c between different species was found in periodically dry habitats, though a few species like Epilobium alsinifolium (r_c= 62.5 s cm⁻¹) growing in constantly wet habitats had remarkably high r_c.
• 6 In Saussurea alpina leaf size increases with improved moisture conditions. Calculations of leaf temperatures with closed stomata and somewhat extreme meteorological conditions showed that the mean leaf size in the wettest part of the transect was below, but very close to the size giving lethal leaf temperatures. In Rubus chamaemorus leaf size increases with increasing artificial shading. The leaves growing in sunexposed sites will be only 0.5°C below the lethal limit when the stomata are closed. All the shade-leaves would exceed the lethal limit if the screen was removed and closing of stomata occurred. The northern distribution of this species is probably due to its low ability to avoid heat stress.
• 7 In Silene acaulis heat damage was observed under natural conditions at an air temperature of only 21°C. Leaf temperatures about 20°C above air temperature was often found in prostrate alpine vascular plants during sunny periods. The highest overtemperature (25.5°C) was observed in the broad leaves of Rubus chamaemorus. A comparison with maximum leaf temperatures measured in different parts of the world revealed rather uniform maximum leaf temperatures in spite of very contrasting air temperatures. Thus, vascular plants seem to control the leaf temperatures to a great extent by means of morphological modifications.
• 8 Leaf temperatures in a hot and dry period were calculated and compared with the heat resistance in 69 Scandinavian, mainly alpine, plants. In 14 wet growing species the lethal limits were exceeded if closing of the stomata occurred. In the remaining species calculated temperatures in single leaves never exceeded the lethal limit. Most of these species have leaves densely crowded in cushions or prostrate rosettes. Hence they get warmer than indicated by the calculated temperatures in single leaves, and will probably be heated close to the lethal limit. A highly significant correlation between lethal temperatures and cuticular diffusion resistances was found, probably illustrating the importance of transpirational cooling during a hot period. A combination of cuticular diffusion resistances and lethal temperatures segregates the species better in natural groups than only one parameter alone.
• 9 Factors involved in limiting the downward distribution of alpine plants are discussed. Some species avoid lowlands since they are drought sensitive (low cuticular diffusion resistance), others, mainly cushion plants with low heat exchange capacity, are probably overheated in lowlands.

     

Possible functional roles of cortical depsides and medullary depsidones in the foliose lichen Hypogymnia physodes

Secondary compounds were quantified in 75 thalli of the foliose lichen Hypogymnia physodes collected in habitats along a natural shade-sun gradient from dark spruce forests to sun-exposed sea cliffs. The irradiance in all the 75 lichen sites was estimated from hemispherical photographs. The content of lichen compounds per thallus area correlated positively with irradiance level (r²=0.73), and the mean concentration increased from 6.7% in the spruce forest to 14.4% on sea cliffs. The medullary depsidones, physodic, physodalic and protocetraric acids, constituted >95% of the total pool of extractable secondary compounds, the cortical depsides, atranorin and chloratranorin, represented <5%. Both cortical compounds correlated well with direct and with diffuse radiation, whereas the three medullary compounds correlated better with diffuse than with direct radiation. Mentioned trends are consistent with a solar radiation screening hypothesis of both groups of these colourless compounds occuring as tiny crystals outside fungal hyphae. However, the UV-B protective hypothesis of the compounds was further tested in a lab experiment. Unnaturally high UV-B doses were required to significantly reduce the PSII efficiency (F_V/F_M) of symbiotic algae. Removal of the major pool of secondary compounds with acetone did not increase photobiont susceptibility to UV-B. Therefore, the main functional role of the UV-B absorbing secondary compounds in H. physodes is hardly UV-B screening. Other roles such as PAR-screening and defence against herbivores and pathogenic organisms are discussed.     

High-light damage in air-dry thalli of the old forest lichen Lobaria pulmonaria-interactions of irradiance, exposure duration and high temperature

High-light damage in air-dry thalli of Lobaria pulmonaria were measured in the laboratory as reductions in maximal PSII efficiency (Fv/Fm) after a 48 h recovery in a hydrated state at low light to account for permanent damage. Thalli treated with the lowest light dose (90 mol photons m^-2) recovered normal Fv/Fm-values with increasing irradiances (400-700 nm) up to 1000 mol photons m^-2 s^-1. Doubling this dose lowered the threshold level for damage from 1000 to 320 mol photons m^-2 s^-1, and reduced Fv/Fm at 1000 mol photons m^-2 s^-1 by more than 50%. A second doubling of the dose to 360 mol photons m^-2 caused damage at 200 mol photons m^-2 s^-1, and a nearly complete cessation of PSII efficiency occurred at 1000 mol photons m^-2 s^-1. No reciprocity of irradiance and duration of illumination for PSII function was found. The measured time-dependent decrease in Fv/Fm was remarkably similar for the naturally coupled, but artificially separated, light and temperature factors. Therefore, the damage of high light on desiccated L. pulmonaria seemed to be an additive effect of high irradiance and high temperatures. Air-dry thalli were highly heat susceptible, being affected already at temperatures around 40C. Logging operations in forests are likely to raise the solar radiation at remaining lichen sites to destructive levels.Keywords: Lichens, high-light damage, heat stress, poikilohydric organisms, reciprocity.