Response of two Antarctic bryophytes to stratospheric ozone depletion

We report a study which measured changes to the radiative environment arising from stratospheric O₃ depletion at Rothera Point on the western Antarctic Peninsula (67°S, 68°W) and subsequent associations between these changes and the pigmentation and maximum quantum yield of photochemistry (F_v/F_m) of two Antarctic bryophytes, the liverwort Cephaloziella varians and the moss Sanionia uncinata. We found a strong relationship between O₃ column depth and the ratio of UV‐B to PAR irradiance (F_UV‐B/F_PAR) recorded at ground level. Weaker, but significant, associations were also found between O₃ column depth and noon irradiances and daily doses of unweighted and biologically effective UV‐B radiation received at ground level. Regression analyses indicated that F_UV‐B/F_PAR and daily dose of unweighted UV‐B were best predictors for concentrations of total carotenoids and UV‐B screening pigments extracted from bryophyte tissues. Concentrations of these pigments were loosely but significantly positively associated with O₃‐dependent irradiance parameters. HPLC analyses of carotenoids also suggested that both species increased the synthesis of neoxanthin during periods of O₃ depletion. Violaxanthin, lutein, zeaxanthin and b,bββ‐carotene concentrations were also apparently influenced by O₃ reduction, but not consistently across both bryophyte species. Concentrations of chlorophylls a and b were apparently unaffected by O₃ depletion. No direct associations between F_v/F_m and O₃‐dependent irradiance parameters were found. However stepwise multiple regression analyses suggested that the production of UV‐B screening pigments conferred protection from elevated F_UV‐B/F_PAR on F_v/F_m in both species and that carotenoids conferred protection on F_v/F_m in Sanionia. Our data suggest that changes to the radiative environment associated with stratospheric O₃ depletion influence the pigmentation of two Antarctic bryophytes, but that F_v/F_m is unaffected, at least in part because of rapid synthesis of protective pigments.     

DNA DAMAGE AND PHOTOSYNTHETIC PERFORMANCE IN THE ANTARCTIC TERRESTRIAL ALGA PRASIOLA CRISPA SSP. ANTARCTICA (CHLOROPHYTA) UNDER MANIPULATED UV-B RADIATION

The effect of reduced, natural ambient, and enhanced UV-B radiation (UVBR) on photosynthesis and DNA damage in the Antarctic terrestrial alga Prasiola crispa ssp. antarctica (Kützing) Knebel was investigated in two field experiments. Samples of P. crispa were collected underneath snow cover and exposed outside to reduced and natural UVBR in the austral spring. In a second experiment at the end of the austral summer, samples were exposed to ambient and enhanced UVBR. PSII efficiency, net photosynthetic rate (NP), dark respiration rate (DR), UV-absorbing pigments, and cyclobutyl pyrimidine dimer (CPD) formation were measured during the experiments. In October 1998, a spring midday maximum of 2.0 W·m ^{− 2} of UVBR did not significantly affect effective quantum yield (ΔF/F_m′), and a reduction in the ratio of variable to maximal fluorescence (F_v/F_m) in the late afternoon was transient. Exposure to natural ambient UVBR in October increased CPD values significantly. Midday maxima of UVBR during the experiments in October and January were comparable, but Setlow-DNA-weighted UVBR was more than 50% lower in January than in October. In January, 0.5 W·m ^{− 2} additional UVBR during 10 h did not have a negative effect on ΔF/F_m′. The reduction in F_v/F_m was not significant. NP and DR were not affected by supplementation of UVBR. Although photosynthetic activity remained largely unaffected by UVBR treatment, DNA damage was shown to be a sensitive parameter to monitor UVBR effects. Supplementation of additional UVBR did significantly enhance the amounts of CPD in exposed samples and repair took place overnight. It is concluded that PSII and whole-chain photosynthesis of P. crispa is well adapted to ambient and enhanced levels of UVBR but that CPD formation is more sensitive to UVBR than to photosynthesis.     

Effects of ultraviolet-B radiation on leaf elongation, production and phenylpropanoid concentrations of Deschampsia antarctica and Colobanthus quitensis in Antarctica

Stratospheric ozone depletion by anthropogenic chlorofluorocarbons has lead to increases in ultraviolet‐B radiation (UV‐B; 280–320 nm) along the Antarctic Peninsula during the austral spring. We manipulated UV‐B levels around plants of Antarctic hair grass (Deschampsia antarctica; Poaceae) and Antarctic pearlwort (Colobanthus quitensis; Caryophyllaceae) for one field season near Palmer Station along the west coast of the Antarctic Peninsula. Treatments involved placing frames over naturally growing plants that either (1) held filters that absorbed most biologically effective radiation (UV‐B_BE; ‘reduced UV‐B’, 22% of ambient UV‐B_BE levels), (2) held filters that transmitted most UV‐B_BE (‘near‐ambient UV‐B’, 87% of ambient UV‐B_BE levels), or (3) lacked filters (‘ambient UV‐B’). Leaves on D. antarctica exposed to near‐ambient and ambient UV‐B were 16–17% shorter than those exposed to reduced UV‐B, and this was associated with shorter epidermal cells at the leaf base and tip. Leaves on C. quitensis exposed to near‐ambient and ambient UV‐B tended to be shorter (P=0.18) and epidermal cells at the leaf base tended to be smaller than those under reduced UV‐B (P<0.10). In order to further explain reductions in leaf length, we examined leaf concentrations of insoluble (cell‐wall bound) phenylpropanoids, since it has been proposed that wall‐bound phenylpropanoids such as ferulic acid may constrain cell expansion and leaf elongation. In both species, HPLC analysis revealed that ferulic and p‐coumaric acid were major components of both insoluble and soluble phenylpropanoids. Although there were no significant differences in concentrations between UV‐B treatments, concentrations of insoluble ferulic acid in D. antarctica tended to be higher under ambient and near‐ambient UV‐B than under reduced UV‐B (P=0.17). We also examined bulk‐leaf concentrations of soluble (methanol extractable) UV‐B‐absorbing compounds and found that concentrations were higher in plants exposed to near‐ambient and ambient UV‐B than in plants exposed to reduced UV‐B. We also assessed the UV‐B‐screening effectiveness of leaves that had developed on plants at the field site with a fiber‐optic microprobe. Leaf epidermal transmittance of 300‐nm UV‐B was 4.0 and 0.6% for D. antarctica and C. quitensis, respectively, which is low compared to grasses and herbaceous dicotyledonous plants found in more temperate climates. While the leaves of Antarctic vascular plants are relatively effective at screening UV‐B, levels of UV‐B in Antarctica are sufficient to reduce leaf epidermal cell size and leaf elongation in these species, although the mechanisms for these reductions remain unclear.     

Elevated UV-B radiation alters fluxes of methane and carbon dioxide in peatland microcosms

Increases in solar ultraviolet‐B radiation (UV‐B; 280–320 nm) reaching the earth have been estimated to continue until 2050s in the boreal and subarctic regions with an abundant peatland cover. Peatlands are significant sinks for carbon dioxide (CO₂) and sources for methane (CH₄). To assess whether the future increases in UV‐B could affect the fluxes of CO₂ and CH₄ in peatlands via an impact on vegetation, we exposed peatland microcosms to modulated 30% supplementation of erythemally weighted UV‐B at an outdoor facility for one growing season. The experimental design included appropriate controls for UV‐A and ambient radiation. The UV‐B caused a significant reduction in gross photosynthesis, net ecosystem CO₂ exchange, and CH₄ emission of the peatland microcosms. These changes in the carbon gas cycling can be partly explained by UV‐B‐induced morphological changes in Eriophorum vaginatum which acts as a conduit for CH₄. Leaf cross section and the percentage of CH₄‐conducting aerenchymatous tissue in E. vaginatum were significantly reduced by UV‐B. Methanol‐extractable UV‐B absorbing compounds decreased under both UV‐B and UV‐A in Sphagnum angustifolium, and tended to accumulate under UV‐B in S. papillosum. Membrane permeability to magnesium (Mg) and calcium (Ca) ions was higher in UV‐B exposed S. angustifolium. Amount of chlorophyll and carotenoid pigments was increased by UV‐A in S. magellanicum. The observed changes in Sphagnum mosses did not coincide with those in carbon gas fluxes but occurred at the time of the highest UV intensity in the mid summer. Our findings indicate that increasing UV‐B may have more substantial effects on gas exchange in peatlands than previously thought.     

Temperature dependence of UV radiation effects in Arctic and temperate isolates of three red macrophytes

The temperature dependence of UV effects was studied for Arctic and temperate isolates of the red macrophytes Palmaria palmata, Coccotylus truncatus and Phycodrys rubens. The effects of daily repeated artificial ultraviolet B and A radiation (UVBR: 280–320?nm, UVAR: 320–400?nm) treatments were examined for all isolates at 6, 12 and 18?°C by measuring growth, optimal quantum yield of PSII (F_v/F_m) and cyclobutane-pyrimidine dimer (CPD) accumulation. Furthermore, possible ecotypic differences in UV sensitivity between Arctic and temperate isolates were evaluated. Large species-specific differences in UV sensitivity were observed for all parameters: the lower subtidal species C. truncatus and P. rubens were highly sensitive to the UV treatments, whereas P. palmata, which predominantly occurs in the upper subtidal zone, was not affected by these treatments. Only minor differences were found between Arctic and temperate isolates, suggesting that no differences in UV sensitivity have evolved in these species. Relative growth rates were temperature-dependent, whereas species-specific UV effects on growth rates were relatively independent of temperature. In contrast, the species-specific decrease in F_v/F_m and its subsequent recovery were temperature-dependent in all species. UV effects on F_v/F_m were lower at 12 and 18?°C compared with 6?°C. In addition, UV effects on F_v/F_m decreased in the course of the experiment at all temperatures, indicating acclimation to the UV treatments. CPDs accumulated during the experiment in both isolates of P. rubens, whereas CPD concentrations remained low for the other two species. CPD accumulation appeared to be independent of temperature. The results suggest that summer temperatures occurring in temperate regions facilitate repair of UV-induced damage and acclimation to UV radiation in these algae compared with Arctic temperatures. Because the differences in UV effects on F_v/F_m, growth and CPD accumulation were relatively small over a broad range of temperatures, it was concluded that the influence of temperature on UV effects is small in these species.     

The effects of long-term elevated UV-B on the growth and phenolics of field-grown silver birch (Betula pendula)

The effects of long‐term elevated UV‐B radiation on silver birch (Betula pendula Roth) seedlings were studied over three growing seasons in an outdoor experiment in Finland started 64 days after germination. One group of seedlings was exposed to a constant 50% increase in UV‐B_CIE radiation, which corresponds to 20–25% of ozone depletion; another group received a small increase in UV‐A radiation and a third (the control group) received ambient solar radiation. Changes in growth appeared during the third growing season; the stems of the UV‐B treated seedlings were thinner and their height tended to be shorter compared with that of the control seedlings. In contrast, there were no UV‐B effects on biomass, bud burst, bud dry weights, leaf area, rust frequency index or chlorophyll concentrations in any of the summers. During the three‐year study, the flavonols were significantly increased by the elevated UV‐B only in the first growing season. The responses varied greatly among individual compounds; the most induced were the quercetin glycosides, while the main flavonols, myricetins, were reduced by the UV‐A control treatment. In the second summer phenolic acids, such as 3,4′‐dihydroxypropiophenone‐3‐glucoside, neochlorogenic acid and 5‐coumarylquinic acid, were increased by the UV‐B treatment. In the third year, the constitutive concentrations of phenolics were not affected by the UV‐B treatment.     

Contribution of the UV component of natural sunlight to photoinhibition of photosynthesis in six species of subtidal brown and red seaweeds

Field‐collected specimens of three species of Laminaria and three species of subtidal red algae (Delesseria sanguinea, Plocamium cartilagineum and Phyllophora pseudoceranoides) were exposed to natural summer sunlight on Helgoland (southern North Sea) for up to 4 h at 15 °C. Dark‐adapted variable fluorescence (F_v : F_m) was measured immediately after these treatments, and following 6, 24 and 48 h of recovery in moderate irradiances of white light. The response of plants to the full spectrum of natural sunlight was compared with that to PAR alone, UV‐A + visible, UV‐A + UV‐B, or UV‐A alone. The F_v : F_m values of all species were reduced to minimal values after 4 h in all of these treatments, but those of the more resistant species (Laminaria spp. and P. pseudoceranoides) were higher after shorter exposures to UV radiation alone than to PAR with or without UV. The recovery of F_v : F_m in all species was also more rapid in the two treatments that contained UV radiation alone than in those that included PAR. These results suggest that it is the high irradiances of PAR in natural sunlight which are responsible for the photoinhibition of photosynthesis of subtidal seaweeds and that the current ambient irradiances of UV radiation (either UV‐B or UV‐A) in northern temperate latitudes would not contribute significantly to this photoinhibition.     

Impact of changes in natural ultraviolet radiation on pigment composition, physiological and morphological characteristics of the Antarctic moss, Grimmia antarctici

The impact of ambient ultraviolet (UV)‐B radiation on the endemic bryophyte, Grimmia antarctici, was studied over 14 months in East Antarctica. Over recent decades, Antarctic plants have been exposed to the largest relative increase in UV‐B exposure as a result of ozone depletion. We investigated the effect of reduced UV and visible radiation on the pigment concentrations, surface reflectance and physiological and morphological parameters of this moss. Plexiglass screens were used to provide both reduced UV levels (77%) and a 50% decrease in total radiation. The screen combinations were used to separate UV photoprotective from visible photoprotective strategies, because these bryophytes are growing in relatively high light environments compared with many mosses. G. antarctici was affected negatively by ambient levels of UV radiation. Chlorophyll content was significantly lower in plants grown under near‐ambient UV, while the relative proportions of photoprotective carotenoids, especially β‐carotene and zeaxanthin, increased. However, no evidence for the accumulation of UV‐B‐absorbing pigments in response to UV radiation was observed. Although photosynthetic rates were not affected, there was evidence of UV effects on morphology. Plants that were shaded showed fewer treatment responses and these were similar to the natural variation observed between moss growing on exposed microtopographical ridges and in more sheltered valleys within the turf. Given that other Antarctic bryophytes possess UV‐B‐absorbing pigments which should offer better protection under ambient UV‐B radiation, these findings suggest that G. antarctici may be disadvantaged in some settings under a climate with continuing high levels of springtime UV‐B radiation.     

The influence of UV-B radiation on light-dependent photosynthetic performance in <Emphasis Type="Italic">Sanionia uncinata </Emphasis>(Hedw.) Loeske in Antarctica

Photosynthetic activity of the moss Sanionia uncinata (Hedw.) Loeske was investigated on Léonie Island (67°35'S, 68°20'W, Antarctic Peninsula) in response to short-term changes of UV-B radiation. The UV-environment of natural mat formations dominated by S. uncinata was altered using filter screens. Two filter experiments were conducted in the Antarctic summers 1998 and 1999. A third filter experiment was conducted during springtime ozone depletion in October 1998. Photosynthetic activity of S. uncinata was mainly determined by photosynthetically active photon flux density (PPFD). Light response of relative electron transport rate through photosystem II (rel ETR=jF/Fm'2PPFD) remained unaffected by ambient summer levels of UV-B radiation. The same was found for net photosynthesis and dark respiration. In October 1998, S. uncinata was mainly metabolically inactive due to low temperatures. No significant levels of DNA-damage measured as cyclobutane pyrimidine dimers (CPDs) were induced by ambient summer levels of UV-B. Artificially enhanced UV-B radiation supplying a Setlow-DNA-dose of 8.7 kJ m^ф day^у UV-B led to formation of 7Dž CPD (10⁶ nucleotides)^-1. It is concluded that current ambient summer levels of UV-B radiation do not affect photosynthetic activity in S. uncinata.     

Carbon dioxide balance of a fen ecosystem in northern Finland under elevated UV-B radiation

The effect of elevated UV‐B radiation on CO₂ exchange of a natural flark fen was studied in open‐field conditions during 2003–2005. The experimental site was located in Sodankylä in northern Finland (67°22′N, 26°38′E, 179 m a.s.l.). Altogether 30 study plots, each 120 cm × 120 cm in size, were randomly distributed between three treatments (n=10): ambient control, UV‐A control and UV‐B treatment. The UV‐B‐treated plots were exposed to elevated UV‐B radiation level for three growing seasons. The instantaneous net ecosystem CO₂ exchange (NEE) and dark respiration (R_TOT) were measured during the growing season using a closed chamber method. The wintertime CO₂ emissions were estimated using a gradient technique by analyzing the CO₂ concentration in the snow pack. In addition to the instantaneous CO₂ exchange, the seasonal CO₂ balances during the growing seasons were modeled using environmental data measured at the site. In general, the instantaneous NEE at light saturation was slightly higher in the UV‐B treatment compared with the ambient control, but the gross photosynthesis was unaffected by the exposure. The R_TOT was significantly lower under elevated UV‐B in the third study year. The modeled seasonal (June–September) CO₂ balance varied between the years depending on the ground water level and temperature conditions. During the driest year, the seasonal CO₂ balance was negative (net release of CO₂) in the ambient control and the UV‐B treatment was CO₂ neutral. During the third year, the seasonal CO₂ uptake was 43±36 g CO₂‐C m⁻² in the ambient control and 79±45 g CO₂‐C m⁻² in the UV‐B treatment. The results suggest that the long‐term exposure to high UV‐B radiation levels may slightly increase the CO₂ accumulation to fens resulting from a decrease in microbial activity in peat. However, it is unlikely that the predicted development of the level of UV‐B radiation would significantly affect the CO₂ balance of fen ecosystems in future.     

Acclimation of a pleurocarpous moss Pleurozium schreberi (Britt.) Mitt. to enhanced ultraviolet radiation in situ

We investigated the responses of ultraviolet (UV)‐absorbing compounds, chlorophylls a and b, carotenoids and the growth responses of the pleurocarpous moss Pleurozium schreberi (Britt.) Mitt. to enhanced UV radiation in situ. The moss was exposed to a 52% elevation above the ambient level of erythemally weighted UV‐B radiation, simulating an approximate 20% reduction in the ozone column, in a dry pine forest in Sodankylä, Finland (67 °22′N, 26 °38′E), under arrays of lamps filtered with cellulose diacetate, which transmitted both UV‐B and UV‐A radiation. The moss was also exposed to elevated UV‐A radiation under control arrays of lamps filtered with Melinex polyester and to ambient radiation under arrays with no lamps in them. Effects of enhanced UV radiation on P. schreberi were recorded during the first 3 years of exposure. Enhanced UV‐B radiation did not affect the segment height growth of the moss. The annual dry mass after the second growing season was higher in the UV‐A control than in the other treatments, and dry mass decreased significantly during the third treatment year in both UV treatments compared with the ambient. The specific leaf area of the UV‐B‐treated mosses was significantly higher than the ambient control mosses during the first 2 years. An increase of UV‐absorbing compounds was found in the mosses under enhanced UV‐B radiation compared with the UV‐A control mosses during the first year. Even though the treatment effect on UV‐absorbing compounds was transient, the concentrations of these compounds correlated with the amount of UV‐A and UV‐B radiation received under the elevated UV‐B treatment. A correlation with the irradiation of previous days and preceding month of the sampling day was found. A seasonal reduction occurred in the amount of UV‐absorbing compounds from the beginning of the summer to late summer. The amount of photosynthetic pigments correlated with the amount of photosynthetically active radiation. The moss P. schreberi was thus found to tolerate increasing UV‐B radiation. Our data indicate that P. schreberi tolerates a 52% increase in erythemally weighted UV‐B radiation above ambient, responding during the first few years of exposure by increasing UV‐absorbing compounds and specific leaf area, and decreasing annual dry mass, and then acclimating to its altered radiation environment.     

Functional acclimation to solar UV-B radiation in Gunnera magellanica, a native plant species of southernmost Patagonia

The ecosystems of Tierra del Fuego (in southern Patagonia, Argentina) are seasonally exposed to elevated levels of ultraviolet‐B radiation (UV‐B: 280–315 nm), due to the passage of the ‘ozone hole’ over this region. In the experiments reported in this article the effects of solar UV‐B and UV‐A (315–400 nm) on two UV‐B defence‐related processes: the accumulation of protective UV‐absorbing compounds and DNA repair, were tested. It was found that the accumulation of UV‐absorbing sunscreens in Gunnera magellanica leaves was not affected by plant exposure to ambient UV radiation. Photorepair was the predominant mechanism of cyclobutane‐pyrimidine dimer (CPD) removal in G. magellanica. Plants exposed to solar UV had higher CPD repair capacity under optimal conditions of temperature (25 °C) than plants grown under attenuated UV. There was no measurable repair at 8 °C. The rates of CPD repair in G. magellanica plants were modest in comparison with other species and, under equivalent conditions, were about 50% lower than the repair rates of Arabidopsis thaliana (Ler ecotype). Collectively our results suggest that the susceptibility of G. magellanica plants to current ambient levels of solar UV‐B in southern Patagonia may be related to a low DNA repair capacity.     

Effect of UV‐B radiation on the content of UV‐B absorbing compounds and photosynthetic parameters in Parmotrema austrosinense from two contrasting habitats

• We studied the resistance of Parmotrema austrosinense to UV‐B stress. We focused on the effects of a high dose UV‐B radiation on the content of chlorophylls, carotenoids and UV‐B screening compounds.
• Photosynthetic parameters were measured by chlorophyll fluorescence (potential and effective quantum yields, photochemical and non‐photochemical quenching) and evaluated in control and UV‐B‐treated lichens. Lichens from two different locations in Cordoba, Argentina, were selected: (i) high altitude and dry plots at (Los Gigantes) and (ii) lowland high salinity plots (Salinas Grandes).
• UV‐B treatment led to a decrease in the content of photosynthetic pigments and UV‐B screens (absorbance decrease in 220–350 nm) in the samples from Salinas Grandes, while in Los Gigantes samples, an increase in UV‐B screen content was observed. Chlorophyll fluorescence parameters showed a UV‐B‐induced decline in F_V/F_M, Φ_PSII and qP indicating limitation of primary photosynthetic processes in photosystem II (PSII) of symbiotic alga, more pronounced in Salinas Grandes samples. Protective mechanism of PSII were activated by the UV‐B treatment to a higher extent in samples from Salinas Grandes (NPQ 0.48) than in Los Gigantes samples (NPQ 0.26).
• We concluded that site‐related characteristics, and in particular different UV‐B radiation regimen, had a strong effect on resistance of the photosynthetic apparatus of P. austrosinense to UV‐B radiation.

     

Responses to solar ultraviolet-B radiation in a shrub-dominated natural ecosystem of Tierra del Fuego (southern Argentina)

A study was made of the effects of solar ultraviolet‐B radiation (UV‐B) on the growth of the dominant plant species of a shrub‐dominated ecosystem in Tierra del Fuego. This part of southern Argentina can be under the direct influence of the Antarctic ‘ozone hole’ during the austral spring and lingering ozone‐depleted air during the summer. The plant community is dominated by an evergreen shrub (Chiliotrichum diffusum) with an herbaceous layer of Gunnera magellanica and Blechnum penna‐marina in the interspaces between the shrubs. Inspections of ozone trends indicate that the springtime and summertime ozone column over Tierra del Fuego has decreased by 10–13% from 1978/9 to 1998/9. In a set of well‐replicated field plots, solar UV‐B was reduced to approximately 15–20% of the ambient UV‐B using plastic films. Polyester films were used to attenuate UV‐B radiation and UV‐transparent films (～90% UV‐B transmission) were used as control. Treatments were imposed during the growing season beginning in 1996 and continued for three complete growing seasons. Stem elongation of the shrub C. diffusum was not affected by UV‐B attenuation in any of the three seasons studied. However, frond length of B. penna‐marina under attenuated UV‐B was significantly greater than that under near‐ambient UV‐B in all three seasons. Attenuation of solar UV‐B also promoted the expansion of G. magellanica leaves in two of the growing seasons. Differences between treatments in leaf or frond length in B. penna‐marina and G. magellanica did not exceed 12%. Another significant effect of UV‐B attenuation was a promotion of insect herbivory in G. magellanica, with a 25–75% increase in the leaf area consumed. Changes in plant phenology or relative species cover were not detected within the time frame of this study. The results suggest that the increase in UV‐B radiation associated with the erosion of the ozone layer might be affecting the functioning of this ecosystem to some degree, particularly by inhibiting the growth of some plant species and by altering plant–insect interactions.     

The effects of altered levels of UV-B radiation on an Antarctic grass and lichen

We report a long-term experiment on the photosynthetic response of natural vegetation of Deschampsia antarctica (Poaceae) and Turgidosculum complicatulum (Lichenes) to altered UV-B levels on Léonie Island, Antarctica.UV-B above the vegetation was reduced by filter screens during two seasons. Half of the screens were transparent to UV-A and UV-B (ambient treatment) or absorbing UV-B and part of the UV-A (below-ambient treatment). Half of the wedge- shaped filters had side walls leading to an enhancement of the daily mean temperature in summer by 2–4 °C, simulating rising mean air temperature on the Antarctic Peninsula. The other half of the filters were without side walls resulting in close-to-ambient temperature underneath. Plots without filters served as controls.UV-B supplementation of an extra 1.3 kJ UV-B_BE was achieved using UV-mini-lamp systems during 15 days in the second season.We found no evidence that altered incident UV-B levels and temperature had an effect on maximum photosystem II efficiency (F _v/F _m) and effective photosystem II efficiency (F/F _m) in both species. UV-B reduction did not influence contents of chlorophyll, carotenoids and methanol-soluble UV absorbing compounds in D. antarctica.Flowering shoot length of D. antarctica was not affected by UV-B reduction. Temperature enhancement tended to result in longer inflorescence axes. Results of two austral summer seasons of UV- reduction in natural stands of D. antarctica and T. complicatulum suggest that current ambient levels of UV-B do not have a direct effect on the photosynthetic performance and pigment contents of these species. Cumulative effects on growth have not been recorded after two years but can not be excluded on a longer term.     

MORPHO‐FUNCTIONAL PATTERNS OF PHOTOSYNTHESIS IN THE SOUTH PACIFIC KELP LESSONIA NIGRESCENS: EFFECTS OF UV RADIATION ON 14C FIXATION AND PRIMARY PHOTOCHEMICAL REACTIONS1

The morpho‐functional patterns of photosynthesis, measured as ¹⁴C‐fixation and chl fluorescence of PSII, also as affected by different doses of UV radiation in the laboratory were examined in the South Pacific kelp Lessonia nigrescens Bory of the coast of Valdivia, Chile (40°S). The results indicated the existence of longitudinal thallus profiles in physiological performance. In general, blades exhibited higher rates of carbon fixation and pigmentation as compared with stipes and holdfasts. Light‐independent ¹⁴C fixation (LICF) was high in meristematic zones of the blades (3.5 μmol ¹⁴C·g^?1 fresh weight [FW]·h^?1), representing 2%–16% (percentage ratio) of the photosynthetic ¹⁴C fixation (20 μmol ¹⁴C·g^?1 FW·h^?1). Exposures to UV radiation indicated that biologically effective UV‐B doses (BED_{photoinhibition300}) of 200–400 kJ·m^?2 (corresponding to current daily doses measured in Valdivia on cloudless summer days) inhibit photosynthetic ¹⁴C fixation of blades by 90%, while LICF was reduced by 70%. The percentage ratio of LICF to photosynthetic ¹⁴C fixation increased under UV exposure to 45%. Primary light reactions measured as maximum quantum yield (F_v/F_m) and electron transport rate (ETR) indicated a higher UV susceptibility of blades as compared with stipes and holdfasts: after a 48 h exposure to UV‐B, the decrease in the blades was close to 30%, while in the stipes and holdfasts it was <20%. The existence of translocation of labeled carbon along the blades suggests that growth at the meristem may be powered by nonphotosynthetic processes. A possible functional role of LIFC, such as during reduction of photosynthetic carbon fixation due to enhanced UV radiation, is discussed. These results in general support the idea that the UV‐related responses in Lessonia are integrated in the suite of morpho‐functional adaptations of the alga.     

Photosynthesis of two Arctic macroalgae under different ambient radiation levels and their sensitivity to enhanced UV radiation

The change in optimal quantum efficiency (F _v/F _m) of the Arctic species Laminaria saccharina and Palmaria palmata was investigated in a long-term experiment in situ under different radiation levels during the summer of 1997 in the Kongsfjord (Ny-Ålesund, Spitsbergen, Norway, 78°55.5′N, 11°56.0′E). Whole plants were incubated in an open box system made of UV-transparent Perspex and exposed to solar radiation (λ>295?nm), solar radiation excluding UVB (λ?>?320?nm) and solar radiation excluding UVA?+ UVB (λ?>?400?nm). Increasing radiation levels were simulated by transplantation of the pre-adapted algae from their growth depth at 2?m to a water depth of 1?m. Sensitivity to artificially increased UV radiation was determined by exposure of algae from the three treatments to 6?h of strong UV radiation. P. palmata was relatively insensitive to increasing UV radiation and recovered very fast and almost completely in 2?h. Even plants pre-cultured in ambient radiation levels excluding UVA?+?UVB or UVB only showed no photoinhibition after exposure to extra UV radiation in the laboratory. L. saccharina was, in comparison to P. palmata, more sensitive and showed photoinhibition under solar radiation and solar minus UVB radiation after transplantation from 2 to 1?m water depth. However, after 3?weeks at 1?m depth, F _v/F _m of L. saccharina was equal in all treatments and restored to the original values at the start of the experiment. Sensitivity to extra UV radiation in the laboratory increased in time, although recovery was also fast and occurred within 20?h.     

Growth in elevated CO2 protects photosynthesis against high-temperature damage

We present evidence that plant growth at elevated atmospheric CO₂ increases the high‐temperature tolerance of photosynthesis in a wide variety of plant species under both greenhouse and field conditions. We grew plants at ambient CO₂ (~ 360 μ mol mol ^{? 1}) and elevated CO₂ (550–1000 μ mol mol ^{? 1}) in three separate growth facilities, including the Nevada Desert Free‐Air Carbon Dioxide Enrichment (FACE) facility. Excised leaves from both the ambient and elevated CO₂ treatments were exposed to temperatures ranging from 28 to 48 °C. In more than half the species examined (4 of 7, 3 of 5, and 3 of 5 species in the three facilities), leaves from elevated CO₂‐grown plants maintained PSII efficiency (F_v/F_m) to significantly higher temperatures than ambient‐grown leaves. This enhanced PSII thermotolerance was found in both woody and herbaceous species and in both monocots and dicots. Detailed experiments conducted with Cucumis sativus showed that the greater F_v/F_m in elevated versus ambient CO₂‐grown leaves following heat stress was due to both a higher F_m and a lower F_o, and that F_v/F_m differences between elevated and ambient CO₂‐grown leaves persisted for at least 20 h following heat shock. Cucumis sativus leaves from elevated CO₂‐grown plants had a critical temperature for the rapid rise in F_o that averaged 2·9 °C higher than leaves from ambient CO₂‐grown plants, and maintained a higher maximal rate of net CO₂ assimilation following heat shock. Given that photosynthesis is considered to be the physiological process most sensitive to high‐temperature damage and that rising atmospheric CO₂ content will drive temperature increases in many already stressful environments, this CO₂‐induced increase in plant high‐temperature tolerance may have a substantial impact on both the productivity and distribution of many plant species in the 21st century.     

Enhanced UV-B radiation alleviates the adverse effects of summer drought in two Mediterranean pines under field conditions

The effects of enhanced UV-B (290-320 nm) radiation on two native Mediterranean pines (Pinus pinea L., Pinus halepensis Mill.) were recorded during a one-year field study. Plants received ambient or ambient plus supplemental UV-B radiation (simulating a 15% stratospheric ozone depletion over Patras. Greece, 38.3°N. 29.1°E) and only natural precipitation, i.e. they were simultaneously exposed to other natural stresses. particularly water stress during summer. Supplemental UV-B irradiation started in early February, 1993 and up to late June, no effects were observed on growth and photochemical efficiency of photosystem II, as measured by chlorophy II fluorescence induction. Water stress during the summer was manifested in the control plants as a decline in the ratio of variable to maximum fluorescence (F_v/F_m), the apparent photon yield for oxygen evolution (φ_l) and the photosynthetic capacity at 5% CO₂ (P_m). In addition, a partial needle loss was evident. Under supplemental UV-B radiation, however, the decreases in F_v/F_m, φ_i, and P_m. as well as needle losses were significantly less. Soon after the first heavy autumn rains. photosynthetic parameters in both control and UV-B treated plants recovered to similar values. but the transient summer superiority of UV-B irradiated plants resulted in a significant increase in their dry weight measured at plant harvest. during late January. 1994. Plant height. UV-B absorbing compounds, photosynthetic pigments and relative water content measured at late spring. late summer and at plant harvest, were not significantly affected by supplemental UV-B radiation. The results indicate that enhanced UV-B radiation may be beneficial for Mediterranean pines through a partial alleviation of the adverse effects of summer drought.     

Non-invasive measurements of leaf epidermal transmittance of UV radiation using chlorophyll fluorescence: field and laboratory studies

Ratios of chlorophyll fluorescence induced by ultraviolet (UV) and bluegreen (BG) radiation [F(UV)/F(BG)] were determined with a Xe‐PAM fluorometer to test the utility of this technique as a means of non‐intrusively assessing changes in the pigmentation and optical properties of leaves exposed to varying UV exposures under laboratory and field conditions. For plants of Vicia faba and Brassica campestris, grown under controlled‐environmental conditions, F(UV‐B)/F(BG) was negatively correlated with whole‐leaf UV‐B‐absorbing pigment concentrations. Fluorescence ratios of V. faba were similar to, and positively correlated with (r²=0.77 [UV‐B]; 0.85 [UV‐A]), direct measurements of epidermal transmittance made with an integrating sphere. Leaves of 2 of 4 cultivars of field‐grown Glycine max exposed to near‐ambient solar UV‐B at a mid‐latitude site (Buenos Aires, Argentina, 34° S) showed significantly lower abaxial F(UV‐B)/F(BG) values (i.e., lower UV‐B epidermal transmittance) than those exposed to attenuated UV‐B, but solar UV‐B reduction had a minimal effect on F(UV‐B)/F(BG) in plants growing at a high‐latitude site (Tierra del Fuego, Argentina, 55° S). Similarly, the exotic Taraxacum officinale did not show significant changes in F(UV‐B)/F(BG) when exposed to very high supplemental UV‐B (biologically effective UV‐B=14–15 kJ m^?2 day^?1) in the field in Tierra del Fuego, whereas a native species, Gunnera magellanica, showed significant increases in F(UV‐B)/F(BG) relative to those receiving ambient UV‐B. These anomalous fluorescence changes were associated with increases in BG‐absorbing pigments (anthocyanins), but not UV‐B‐absorbing pigments. These results indicate that non‐invasive estimates of epidermal transmittance of UV radiation using chlorophyll fluorescence can detect changes in pigmentation and leaf optical properties induced by UV‐B radiation under both field and laboratory conditions. However, this technique may be of limited utility in cold environments where UV and low temperatures can stimulate the production of BG‐absorbing pigments that interfere with these indirect measurements of UV‐transmittance.