Non-photochemical quenching may contribute to the dominance of the pale mat-forming lichen Cladonia stellaris over the sympatric melanic Cetraria islandica.

The mat-forming fruticose lichens Cladonia stellaris and Cetraria islandica frequently co-occur on soils in sun-exposed boreal, subarctic, and alpine ecosystems. While the dominant reindeer lichen Cladonia lacks a cortex but produces the light-reflecting pale pigment usnic acid on its surface, the common but patchier Cetraria has a firm cortex sealed by the light-absorbing pigment melanin. By measuring reflectance spectra, high-light tolerance, photosynthetic responses, and chlorophyll fluorescence in sympatric populations of these lichens differing in fungal pigments, we aimed to study how they cope with high light while hydrated. Specimens of the two species tolerated high light equally well but with different protective mechanisms. The mycobiont of the melanic species efficiently absorbed excess light, consistent with a lower need for its photobiont to protect itself by non-photochemical quenching (NPQ). By contrast, usnic acid screened light at 450-700 nm by reflectance and absorbed shorter wavelengths. The ecorticate usnic species with less efficient fungal light screening exhibited a consistently lower light compensation point and higher CO2 uptake rates than the melanic lichen. In both species, steady state NPQ rapidly increased at increasing light with no signs of light saturation. To compensate for less internal shading causing light fluctuations with a larger amplitude, the usnic lichen photobiont adjusted to changing light by faster induction and faster relaxation of NPQ rapidly transforming excess excitation energy to less damaging heat. The high and flexible NPQ tracking fluctuations in solar radiation probably contributes to the strong dominance of the usnic mat-forming Cladonia in open lichen-dominated heaths.

Apparent electron transport rate - a non-invasive proxy of photosynthetic CO2 uptake in lichens.

MAIN CONCLUSION: During desiccation, both apparent electron transport rate (ETRapp) and photosynthetic CO2 uptake peak when external water has evaporated. External water, causing suprasaturation, weakens the strong correlation between ETRapp and CO2 uptake. Lichens are poikilohydric organisms passively regulated by ambient conditions. In theory, apparent electron transport rate (ETRapp), estimated by photosystem II yield measured in light (ΦPSII), is a proxy of photosynthetic CO2 uptake. Hydration level, however, is a complicating factor, particularly during suprasaturation that strongly reduces CO2 diffusion. Here, the cephalolichen Lobaria pulmonaria and two chlorolichens Parmelia sulcata and Xanthoria aureola were excessively hydrated before photosynthetic CO2 uptake and ΦPSII using imaging fluorescence tools were simultaneously measured while drying at 200 µmol photons m-2 s-1. CO2 uptake peaked when hydration had declined to a level equivalent to their respective internal water holding capacity (WHCinternal) i.e., the water per thallus area after blotting external water. CO2 uptake and ETRapp in all species were highly correlated at hydration levels below WHCinternal, but weaker at higher hydration (chlorolichens) or absent (cephalolichen). Yet, at a specimen level for the two chlorolichens, the correlation was strong during suprasaturation. The CO2 uptake-ETRapp relationship did not differ between measured species, but may vary between other lichens because the slope depends on cortical transmittance and fraction of electrons not used for CO2 uptake. For new lichen species, calibration of ETRapp against CO2 uptake is therefore necessary. At intrathalline scales, ΦPSII during drying initially increased along thallus margins before reaching maximum values in central portions when hydration approached WHCinternal. WHCinternal represents the optimal hydration level for lichen photosynthesis. In conclusion, ETRapp is an easily measured and reliable proxy of CO2 uptake in thalli without external water but overestimates photosynthesis during suprasaturation.

Different abscisic acid-deficient mutants show unique morphological and hydraulic responses to high air humidity.

High relative humidity (RH) perturbs plant growth, stomatal functioning and abscisic acid (ABA) homeostasis, but the role of ABA in this physiological regulation is equivocal. To determine the role(s) of ABA in plant responses to high RH, wild-type (WT) tomato and barley plants and their respective ABA-deficient mutants flacca and Az34 (which are mutated in the same locus of the ABA biosynthesis pathway) were grown in contrasting RHs (60% and 90%) to measure biomass partitioning, stomatal traits and water relations. Surprisingly, growth RH did not affect foliar ABA levels in either species. While Az34 showed similar stomatal size and density as WT plants, flacca had larger and more abundant stomata. High RH increased stomatal size in tomato, but decreased it in barley, and decreased stomatal density in tomato, but not in barley. Altered stomatal responses in ABA-deficient plants to high RH had little effect on tomato photosynthesis, but Az34 barley showed lower photosynthesis. ABA deficiency decreased relative shoot growth rate (RGRSHOOT ) in both species, yet this was counteracted by high RH increasing leaf water status in tomato, but not in barley. High RH increased RGRSHOOT in flacca, but not in WT tomatoes, while having no effect on RGRSHOOT in barley, but affecting barley net assimilation rate, leaf area ratio (LAR) and specific leaf area in an ABA-dependent manner. ABA-RH interaction affected leaf development in tomato only. Thus, different crop species show variable responses to both high RH and ABA deficiency, making it difficult to generalise on the role of ABA in growth regulation at contrasting RHs.

Effects of artificial ultraviolet B radiation on the macrophyte Lemna minor: a conceptual study for toxicity pathway characterization.

MAIN CONCLUSION: UVB radiation caused irradiance-dependent and target-specific responses in non-UVB acclimated Lemna minor. Conceptual toxicity pathways were developed to propose causal relationships between UVB-mediated effects at multiple levels of biological organisation. Macrophytes inhabit waterways around the world and are used in hydroponics or aquaponics for different purposes such as feed and wastewater treatment and are thus exposed to elevated levels of UVB from natural and artificial sources. Although high UVB levels are harmful to macrophytes, mechanistic understanding of irradiance-dependent effects and associated modes of action in non-UVB acclimated plants still remains low. The present study was conducted to characterise the irradiance-dependent mechanisms of UVB leading to growth inhibition in Lemna minor as an aquatic macrophyte model. The L. minor were continuously exposed to UVB (0.008-4.2 W m-2) and constant UVA (4 W m-2) and photosynthetically active radiation, PAR (80 µmol m-2 s-1) for 7 days. A suite of bioassays was deployed to assess effects on oxidative stress, photosynthesis, DNA damage, and transcription of antioxidant biosynthesis, DNA repair, programmed cell death, pigment metabolism and respiration. The results showed that UVB triggered both irradiance-dependent and target-specific effects at multiple levels of biological organization, whereas exposure to UVA alone did not cause any effects. Inhibition of photosystem II and induction of carotenoids were observed at 0.23 W m-2, whereas growth inhibition, excessive reactive oxygen species, lipid peroxidation, cyclobutane pyrimidine dimer formation, mitochondrial membrane potential reduction and chlorophyll depletion were observed at 0.5-1 W m-2. Relationships between responses at different levels of biological organization were used to establish a putative network of toxicity pathways to improve our understanding of UVB effects in aquatic macrophytes under continuous UVB exposures. Additional studies under natural illuminations were proposed to assess whether these putative toxicity pathways may also be relevant for more ecologically relevant exposure scenarios.

Photobiont-dependent humidity threshold for chlorolichen photosystem II activation.

MAIN CONCLUSION: Photobiont type influences the relative humidity threshold at which photosystem II activates in green algal lichens. Water vapor uptake alone can activate photosynthesis in lichens with green algal photobionts. However, the minimum relative humidity needed for activation is insufficiently known. The objective of this study was to quantify the humidity threshold for photosystem II (PSII) activation in a range of chlorolichen species associated with photobionts from Trebouxiaceae, Coccomyxaceae and Trentepohliaceae. These lichens exhibit distribution, habitat and substrate patterns that are likely coupled to their efficiency in utilizing water vapor at lower levels of relative humidity (RH) for photosynthesis. Using chlorophyll fluorescence imaging during water uptake from humid air of 25 species of chlorolichens representing the above photobiont groups, we monitored PSII activation within controlled chambers with constant RH at five levels ranging from 75.6 to 95.4%. The results demonstrate clear photobiont-specific activation patterns: the trentepohlioid lichens activated PSII at significantly lower RH (75.6%) than trebouxioid (81.7%) and coccomyxoid (92.0%) lichens. These responses are consistent with a preference for warm and sheltered habitats for trentepohlioid lichens, with cool and moist habitats for the coccomyxoid lichens, and with a more widespread occurrence of the trebouxioid lichens. Within each photobiont group, lichen species exposed to marine aerosols in their source habitats seemed to be activated at lower RH than lichens sampled from inland sites. High osmolyte concentration may therefore play a role in lowering a photobiont's activation threshold. We conclude that photobiont type influences water vapor-driven photosynthetic activation of lichens, thereby shaping the ecological niches in which they occur.

Why chartreuse? The pigment vulpinic acid screens blue light in the lichen Letharia vulpina.

MAIN CONCLUSION: Chlorophyll fluorescence, infrared gas exchange and photoinhibition data consistently show that vulpinic acid in L. vulpina functions as a strong blue light screening compound. The cortical lichen compounds, parietin, atranorin, usnic acid and melanins are known to screen photosynthetically active radiation (PAR), thereby protecting the underlying photobionts. The role of the toxic UV-/blue light-absorbing vulpinic acid in lichen cortices is poorly documented. By comparing controls with acetone-rinsed Letharia vulpina thalli (75% reduced vulpinic acid concentration), we aimed to test PAR screening by vulpinic acid. We exposed such thalli to blue, green and red irradiance, respectively, and recorded light quality-specific light saturation curves of CO2 uptake, quantum yields of CO2 uptake (QYCO2) and effective quantum yields of PSII (ΦPSII). We also quantified light quality-dependent photoinhibition after 4-h exposure to 400 µmol photons m-2 s-1. In controls, the greatest high light-induced reductions in CO2 uptake and ΦPSII, as well as the strongest photoinhibition [lowered maximal quantum yield of PSII (Fv/Fm)], occurred in red light, followed by green, and was low in blue light. Removal of vulpinic acid significantly exacerbated photoinhibition, reduced ΦPSII, and increased QYCO2 in blue light. By contrast, acetone rinsing had no or weak effects in green and red lights. Comparing control with acetone-rinsed thalli, blue light screening was estimated at 69% using ΦPSII data and 49% using QYCO2. To compensate for the 25% residual vulpinic acid left after rinsing, we repeated the screening estimation by comparing responses in blue and red lights. This resulted in 88% screening using ΦPSII data and 77% using QYCO2. The consistent responses in all photosynthetic parameters support the hypothesis that vulpinic acid functions as a blue light screen in L. vulpina.

Comparative sensitivity to gamma radiation at the organismal, cell and DNA level in young plants of Norway spruce, Scots pine and Arabidopsis thaliana.

MAIN CONCLUSION: Persistent DNA damage in gamma-exposed Norway spruce, Scots pine and Arabidopsis thaliana, but persistent adverse effects at the organismal and cellular level in the conifers only. Gamma radiation emitted from natural and anthropogenic sources may have strong negative impact on plants, especially at high dose rates. Although previous studies implied different sensitivity among species, information from comparative studies under standardized conditions is scarce. In this study, sensitivity to gamma radiation was compared in young seedlings of the conifers Scots pine and Norway spruce and the herbaceous Arabidopsis thaliana by exposure to 60Co gamma dose rates of 1-540 mGy h-1 for 144 h, as well as 360 h for A. thaliana. Consistent with slightly less prominent shoot apical meristem, in the conifers growth was significantly inhibited with increasing dose rate ≥ 40 mGy h-1. Post-irradiation, the conifers showed dose-rate-dependent inhibition of needle and root development consistent with increasingly disorganized apical meristems with increasing dose rate, visible damage and mortality after exposure to ≥ 40 mGy h-1. Regardless of gamma duration, A. thaliana showed no visible or histological damage or mortality, only delayed lateral root development after ≥ 100 mGy h-1 and slightly, but transiently delayed post-irradiation reproductive development after ≥ 400 mGy h-1. In all species dose-rate-dependent DNA damage occurred following ≥ 1-10 mGy h-1 and was still at a similar level at day 44 post-irradiation. In conclusion, the persistent DNA damage (possible genomic instability) following gamma exposure in all species may suggest that DNA repair is not necessarily mobilized more extensively in A. thaliana than in Norway spruce and Scots pine, and the far higher sensitivity at the organismal and cellular level in the conifers indicates lower tolerance to DNA damage than in A. thaliana.

Elevated air humidity increases UV mediated leaf and DNA damage in pea (Pisum sativum) due to reduced flavonoid content and antioxidant power.

Growth in high relative air humidity (RH, >85%) affects plant morphology and causes diminished response to stomatal closing signals. Many greenhouses are prone to high RH conditions, which may negatively affect production and post-harvest quality. UV radiation induces stomatal closure in several species, and facilitates disease control. We hypothesised that UV exposure may trigger stomatal closure in pea plants (Pisum sativum) grown in high RH, thereby restoring stomatal function. The effects of UV exposure were tested on plants grown in moderate (60%) or high (90%) RH. UV exposure occurred at night, according to a disease control protocol. Lower stomatal conductance rates were found in UV-exposed plants, though UV exposure did not improve the rate of response to closing stimuli or desiccation tolerance. UV-exposed plants showed leaf curling, chlorosis, necrosis, and DNA damage measured by the presence of cyclobutane pyrimidine dimers (CPD), all of which were significantly greater in high RH plants. These plants also had lower total flavonoid content than moderate RH plants, and UV-exposed plants had less than controls. Plants exposed to UV had a higher content of cuticular layer uronic compounds than control plants. However, high RH plants had a higher relative amount of cuticular waxes, but decreased proteins and uronic compounds. Plants grown in high RH had reduced foliar antioxidant power compared to moderate RH. These results indicate that high RH plants were more susceptible to UV-induced damage than moderate RH plants due to reduced flavonoid content and oxidative stress defence.

No evidence of a protective or cumulative negative effect of UV-B on growth inhibition induced by gamma radiation in Scots pine (Pinus sylvestris) seedlings.

Exposure to ambient UV-B radiation may prime protective responses towards various stressors in plants, though information about interactive effects of UV-B and gamma radiation is scarce. Here, we aimed to test whether UV-B exposure could prime acclimatisation mechanisms contributing to tolerance to low-moderate gamma radiation levels in Scots pine seedlings, and concurrently whether simultaneous UV-B and gamma exposure may have an additive adverse effect on seedlings that had previously not encountered either of these stressors. Responses to simultaneous UV-B (0.35 W m-2) and gamma radiation (10.2-125 mGy h-1) for 6 days with or without UV-B pre-exposure (0.35 W m-2, 4 days) were studied across various levels of organisation, as compared to effects of either radiation type. In contrast to UV-B, and regardless of UV-B presence, gamma radiation at ≥42.9 mGy h-1 caused increased formation of reactive oxygen species and reduced shoot length, and reduced root length at 125 mGy h-1. In all experiments there was a gamma dose rate-dependent increase in DNA damage at ≥10.8 mGy h-1, generally with additional UV-B-induced damage. Gamma-induced growth inhibition and gamma- and UV-B-induced DNA damage were still visible 44 days post-irradiation, even at 20.7 mGy h-1, probably due to genomic instability, but this was reversed after 8 months. In conclusion, there was no evidence of a protective effect of UV-B on gamma-induced growth inhibition and DNA damage in Scots pine, and no additive adverse effect of gamma and UV-B radiation on growth in spite of the additional UV-B-induced DNA damage.

Short- and long-term freezing effects in a coastal (Lobaria virens) versus a widespread lichen (L. pulmonaria).

Lichens are considered freezing tolerant, although few species have been tested. Growth, a robust measure of fitness integrating processes in all partners of a lichen thallus, has not yet been used as a viability measure after freezing. We compared relative growth rates (RGR) after freezing with short-term viability measures of photo- and mycobiont functions in the coastal Lobaria virens and the widespread L. pulmonaria to test the hypothesis that low temperature shapes the coastal distribution of L. virens. Hydrated thalli from sympatric populations were subjected to freezing at -10, -20 and -40 °C for 5 h. The rate of cooling and subsequent warming was 5 °C h-1. Short-term viability measures of photobiont (maximal photosystem II efficiency, effective PSII yield) and mycobiont viability (conductivity index), as well as subsequent RGR, were assessed. The exotherms showed that L. virens froze at -3 °C; L. pulmonaria, at -4 °C. Freezing significantly impaired short-term viability measures of both photo- and mycobiont, particularly in the coastal species. Lobaria pulmonaria grew 2.1 times faster than L. virens, but the short-term damage after one freezing event did not affect the long-term RGR in any species. Thereby, short-term responses were impaired by freezing, long-term responses were not. While the lacking RGR-responses to freezing suggest that freezing tolerance does not shape the coastal distribution of L. virens, the significant reported adverse short-term effects in L. virens may be aggravated by repeated freezing-thawing cycles in cold winters. In such a perspective, repeated freezing may eventually lead to reduced long-term fitness in L. virens.

Soluble carbohydrates and relative growth rates in chloro-, cyano- and cephalolichens: effects of temperature and nocturnal hydration.

This growth chamber experiment evaluates how temperature and humidity regimes shape soluble carbohydrate pools and growth rates in lichens with different photobionts. We assessed soluble carbohydrates, relative growth rates (RGRs) and relative thallus area growth rates (RTA GRs) in Parmelia sulcata (chlorolichen), Peltigera canina (cyanolichen) and Peltigera aphthosa (cephalolichen) cultivated for 14 d (150 µmol m(-2) s(-1) ; 12-h photoperiod) at four day : night temperatures (28 : 23°C, 20 : 15°C, 13 : 8°C, 6 : 1°C) and two hydration regimes (hydration during the day, dry at night; hydration day : night). The major carbohydrates were mannitol (cephalolichen), glucose (cyanolichen) and arabitol (chlorolichen). Mannitol occurred in all species. During cultivation, total carbohydrate pools decreased in cephalo-/cyanolichens, but increased in the chlorolichen. Carbohydrates varied less than growth with temperature and humidity. All lichens grew rapidly, particularly at 13 : 8°C. RGRs and RTA GRs were significantly higher in lichens hydrated for 24 h than for 12 h. Strong photoinhibition occurred in cephalo- and cyanolichens kept in cool dry nights, resulting in positive relationships between RGR and dark-adapted photosystem II (PSII) efficiency (Fv /Fm ). RGR increased significantly with the photobiont-specific carbohydrate pools within all species. Average RGR peaked in the chlorolichen lowest in total and photobiont carbohydrates. Nocturnal hydration improved recovery from photoinhibition and/or enhanced conversion rates of photosynthates into growth.

Unequal allocation of excitation energy between photosystem II and I reduces cyanolichen photosynthesis in blue light.

Photosynthesis was compared in two cyanobacterial lichens (Lobaria hallii and Peltigera praetextata) and two green algal lichens (Lobaria pulmonaria and Peltigera leucophlebia) exposed to red, green or blue light. Cyanolichens had substantially lower photosynthetic CO(2) uptake and O(2) evolution than the green algal lichens in blue light, but slightly higher photosynthesis in red and green light. The effective quantum yield of photosystem (PS) II (Φ(PSII)) decreased with increasing red and green light for all species, but in blue light this response occurred in green algal lichens only. Cyanolichen Φ(PSII) increased with increasing blue light at low irradiances, but decreased at stronger exposures. However, after adding red light the efficiency of blue light for photosynthetic O(2) evolution increased by 2.4 times. Because phycobilisomes associated with PSII have a low blue light absorption, our results are consistent with blue light absorption mainly by Chl in PSI. Thereby, unequal allocation of excitation energy between PSII and PSI results in low cyanolichen photosynthesis under blue light. This is new knowledge in the science of lichenology with important implications for e.g. the reliability of using Chl fluorometers with blue light for cyanolichens.

Sunscreening fungal pigments influence the vertical gradient of pendulous lichens in boreal forest canopies.

Pendulous lichens dominate canopies of boreal forests, with dark Bryoria species in the upper canopy vs. light Alectoria and Usnea species in lower canopy. These genera offer important ecosystem services such as winter forage for reindeer and caribou. The mechanism behind this niche separation is poorly understood. We tested the hypothesis that species-specific sunscreening fungal pigments protect underlying symbiotic algae differently against high light, and thus shape the vertical canopy gradient of epiphytes. Three pale species with the reflecting pigment usnic acid (Alectoria sarmentosa, Usnea dasypoga, U. longissima) and three with dark, absorbing melanins (Bryoria capillaris, B. fremontii, B. fuscescens) were compared. We subjected the lichens to desiccation stress with and without light, and assessed their performance with chlorophyll fluorescence. Desiccation alone only affected U. longissima. By contrast, light in combination with desiccation caused photoinhibitory damage in all species. Usnic lichens were significantly more susceptible to light during desiccation than melanic ones. Thus, melanin is a more efficient light-screening pigment than usnic acid. Thereby, the vertical gradient of pendulous lichens in forest canopies is consistent with a shift in type and functioning of sunscreening pigments, from high-light-tolerant Bryoria in the upper to susceptible Alectoria and Usnea in the lower canopy.

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 (RTAGR) 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.

A high proportion of blue light increases the photosynthesis capacity and leaf formation rate of Rosa × hybrida but does not affect time to flower opening.

Alterations in light quality affect plant morphogenesis and photosynthetic responses but the effects vary significantly between species. Roses exhibit an irradiance-dependent flowering control but knowledge on light quality responses is scarce. In this study we analyzed, the responses in morphology, photosynthesis and flowering of Rosa × hybrida to different blue (B) light proportions provided by light-emitting diodes (LED, high B 20%) and high pressure sodium (HPS, low B 5%) lamps. There was a strong morphological and growth effect of the light sources but no significant difference in total dry matter production and flowering. HPS-grown plants had significantly higher leaf area and plant height, yet a higher dry weight proportion was allocated to leaves than stems under LED. LED plants showed 20% higher photosynthetic capacity (Amax ) and higher levels of soluble carbohydrates. The increase in Amax correlated with an increase in leaf mass per unit leaf area, higher stomata conductance and CO2 exchange, total chlorophyll (Chl) content per area and Chl a/b ratio. LED-grown leaves also displayed a more sun-type leaf anatomy with more and longer palisade cells and a higher stomata frequency. Although floral initiation occurred at a higher leaf number in LED, the time to open flowers was the same under both light conditions. Thereby the study shows that a higher portion of B light is efficient in increasing photosynthesis performance per unit leaf area, enhancing growth and morphological changes in roses but does not affect the total Dry Matter (DM) production or time to open flower.

Seasonal partitioning of growth into biomass and area expansion in a cephalolichen and a cyanolichen of the old forest genus Lobaria.

Growth in two old forest lichens was studied to evaluate how temporal (seasonal) and spatial (aspect-wise) partitioning of biomass and area growth respond to seasonal changes in light and climate. We monitored relative growth rates during annual courses in the cephalolichen Lobaria pulmonaria and the cyanolichen Lobaria scrobiculata transplanted in boreal clear-cut to five fixed aspects in winter, spring, summer, and autumn. For each annual set, growth was quantified in January-March, April-June, July-September and October-December. Mean biomass and area increased in all seasons, but growth was highest in July-September. Mass growth did not follow area increment during a year. As a result, mass per area (specific thallus mass (STM)) declined (L. scrobiculata) or stayed constant (L. pulmonaria) in the dark, humid October-December season, whereas it strongly increased in the dry, sunny April-June season. Aspect influenced growth in species-specific ways. Seasonality in biomass growth mainly followed light availability, whereas area growth was strongest during humid seasons. The substantial STM changes across seasons, species, and aspects can be explained as passive responses to seasonal climate. However, as STM, according to the literature, is a driver of water storage, recorded changes probably improve fitness by prolonging hydration in places or during times with high evaporative demands.

The paradox of higher light tolerance during desiccation in rare old forest cyanolichens than in more widespread co-occurring chloro- and cephalolichens.

Desiccation tolerance was quantified in four cyanolichens (Lobaria hallii, Lobaria retigera, Lobaria scrobiculata, Pseudocyphellaria anomala), one cephalolichen (Lobaria pulmonaria) and one chlorolichen (Platismatia glauca) from xeric and mesic, open and closed North American boreal forests. These sympatric epiphytes were exposed to 0%, 33%, 55% and 75% relative humidity with or without medium light (200 µmol m⁻² s⁻¹) for 7 d. Permanent and temporary photoinhibitory damage was recorded as viability measures. All species tolerated well the drying in darkness, but L. hallii and L. retigera, associated with a very humid climate, showed minor damage at the hardest drying (silica gel). Simultaneous exposure to medium light severely aggravated the drying damage at all relative humidity levels. Combined drying-light exposure was particularly devastating for the widespread chloro- and cephalolichens, whereas cyanolichens, including rare old forest species, were fairly resistant. The ability to recover after combined drying-light stress (this study) correlated positively with increasing species-specific water holding capacities (from the literature). Cyanolichens, depending on liquid water and large internal water storage, probably require strong drying-light resistance to handle long periods between hydration events, whereas chlorolichens can regularly maintain their photosynthetic apparatus during frequent and rapid activation by humid air on clear mornings.

Melanisation in Boreal Lichens Is Accompanied by Variable Changes in Non-Photochemical Quenching.

Lichens often grow in microhabitats where they absorb more light than they can use for fixing carbon, and this excess energy can cause the formation of harmful reactive oxygen species (ROS). Lichen mycobionts can reduce ROS formation by synthesizing light-screening pigments such as melanins in the upper cortex, while the photobionts can dissipate excess energy radiationlessly using non-photochemical quenching (NPQ). An inherent problem with using fluorimetry techniques to compare NPQ in pale and melanised thalli is that NPQ is normally measured through a variously pigmented upper cortex. Here we used a dissection technique to remove the lower cortices and medullas of Lobaria pulmonaria and Crocodia aurata and then measure NPQ from the underside of the thallus. Results confirmed that NPQ can be satisfactorily assessed with a standard fluorimeter by taking measurement from above using intact thalli. However, photobionts from the bottom of the photobiont layer tend to have slightly lower rates of PSII activity and lower NPQ than those at the top, i.e., display mild "shade" characteristics. Analysis of pale and melanised thalli of other species indicates that NPQ in melanised thalli can be higher, similar or lower than pale thalli, probably depending on the light history of the microhabitat and presence of other tolerance mechanisms.

Ultraviolet B modulates gamma radiation-induced stress responses in Lemna minor at multiple levels of biological organisation.

Elevated levels of ionizing and non-ionizing radiation may co-occur and pose cumulative hazards to biota. However, the combined effects and underlying toxicity mechanisms of different types of radiation in aquatic plants remain poorly understood. The present study aims to demonstrate how different combined toxicity prediction approaches can collectively characterise how chronic (7 days) exposure to ultraviolet B (UVB) radiation (0.5 W m-2) modulates gamma (γ) radiation (14.9, 19.5, 43.6 mGy h-1) induced stress responses in the macrophyte Lemna minor. A suite of bioassays was applied to quantify stress responses at multiple levels of biological organisation. The combined effects (no-enhancement, additivity, synergism, antagonism) were determined by two-way analysis of variance (2 W-ANOVA) and a modified Independent Action (IA) model. The toxicological responses and the potential causality between stressors were further visualised by a network of toxicity pathways. The results showed that γ-radiation or UVB alone induced oxidative stress and programmed cell death (PCD) as well as impaired oxidative phosphorylation (OXPHOS) and photosystem II (PSII) activity in L. minor. γ-radiation also activated antioxidant responses, DNA damage repair and chlorophyll metabolism, and inhibited growth at higher dose rates (≥20 mGy h-1). When co-exposed, UVB predominantly caused non-interaction (no-enhancement or additive) effects on γ-radiation-induced antioxidant gene expression, energy quenching in PSII and growth for all dose rates, whereas antagonistic effects were observed for lipid peroxidation, OXPHOS, PCD, oxidative stress, chlorophyll metabolism and genes involved in DNA damage responses. Synergistic effects were observed for changes in photochemical quenching and non-photochemical quenching, and up-regulation of antioxidant enzyme genes (GST) at one or more dose rates, while synergistic reproductive inhibition occurred at all three γ-radiation dose rates. The present study provides mechanistic knowledge, quantitative understanding and novel analytical strategies to decipher combined effects across levels of biological organisation, which should facilitate future cumulative hazard assessments of multiple stressors.

Light screening in lichen cortices can be quantified by chlorophyll fluorescence techniques for both reflecting and absorbing pigments.

Lichens, representing mutualistic symbioses between photobionts and mycobionts, often accumulate high concentrations of secondary compounds synthesized by the fungal partner. Light screening is one function for cortical compounds being deposited as crystals outside fungal hyphae. These compounds can non-destructively be extracted by 100% acetone from air-dry living thalli. Extraction of atranorin from Physcia aipolia changed the lichen colour from pale grey to green in the hydrated state, whereas acetone-rinsed and control thalli were all pale grey when dry. Removal of parietin from Xanthoria parietina changed the colour of desiccated thalli from orange to grey. Colour changes were quantified by reflectance measurements. By a new chlorophyll fluorescence method, screening was assessed as the decrease in incident irradiance (PAR) necessary to reach identical effective quantum yields of PSII (Phi(PSII)) in acetone-rinsed and control thalli. Thereby, we estimated a screening efficiency due to cortical atranorin crystals at 61, 38, and 40% of blue, green and red light, respectively, whereas parietin screened 81, 27 and 1% of these wavelength ranges. Removal of atranorin caused similar levels of increased photoinhibition for P. aipolia in blue, green and red light, whereas parietin-deficient thalli of X. parietina exhibited increased photoinhibition with decreasing wavelengths. Atranorin possibly prevents water from entering the spaces between the hyphae in the cortex. The air-filled cavities with white atranorin crystals reflect excess light, whereas the yellow compound parietin absorbs excess light. Thereby, both atranorin and parietin play significant photoprotective roles for symbiotic green algae, but with compound-specific screening mechanisms.