Enhanced UV-B radiation under field conditions increases anthocyanin and reduces the risk of photoinhibition but does not affect growth in the carnivorous plant Pinguicula vulgaris

The effects of enhanced UV-B radiation were investigated in the carnivorous plant Pinguicula vulgaris in a field experiment performed in Abisko, North Sweden (68° 21' N, 18° 49' E, 380 m above sea level). Potted plants were exposed to either ambient or ambient plus supplemental UV-B radiation, simulating a 15% ozone depletion. No effect was observed on either the epicuticular (external) or cellular (internal) UV absorbing capacity of the leaves. However, the anthocyanin content was more than doubled by supplemental UV-B radiation. In laboratory experiments, the anthocyanin rich, UV-B treated leaves were less susceptible to a low temperature/high light photoinhibitory treatment, as judged by in vivo chlorophyll fluorescence measurements. Yet, this potential benefit did not considerably affect the growth of the plant in the field (leaf area and dry mass, reproductive dry mass, flowering frequency, senescence rates, dry mass of winter buds). However, there was a marginally significant increase in root dry mass and in the root to shoot ratio, which may underlie the significant increase in the nitrogen content of the leaves. We suggest that P. vulgaris is resistant against UV-B radiation damage and that the possible negative effects of additional UV-B radiation on the growth of these plants may have been effectively counterbalanced by the lower risk of photoinhibition, due to the concomitant increase in anthocyanins.     

Outdoor Studies on the Effects of Solar UV-B on bryophytes: Overview and Methodology

In this review all recent field studies on the effects of UV-B radiation on bryophytes are discussed. In most of the studies fluorescent UV-B tubes are used to expose the vegetation to enhanced levels of UV-B radiation to simulate stratospheric ozone depletion. Other studies use screens to filter the UV-B part of the solar spectrum, thereby comparing ambient levels of UV-B with reduced UV-B levels, or analyse effects of natural variations in UV-B arising from stratospheric ozone depletion. Nearly all studies show that mosses are well adapted to ambient levels of UV-B radiation since UV-B hardly affects growth parameters. In contrast with outdoor studies on higher plants, soluble UV-B absorbing compounds in bryophytes are typically not induced by enhanced levels of UV-B radiation. A few studies have demonstrated that UV-B radiation can influence plant morphology, photosynthetic capacity, photosynthetic pigments or levels of DNA damage. However, there is only a limited number of outdoor studies presented in the literature. More additional, especially long-term, experiments are needed to provide better data for statistical meta-analyses. A mini UV-B supplementation system is described, especially designed to study effects of UV-B radiation at remote field locations under harsh conditions, and which is therefore suited to perform long-term studies in the Arctic or Antarctic. The first results are presented from a long-term UV-B supplementation experiment at Signy Island in the Maritime Antarctic.     

The influence of enhanced UV-B radiation on the spring geophyte Pulmonaria officinalis

Pulmonaria officinalis is an understorey spring geophyte, which starts its vegetative period before full foliation of the tree storey. During its early growth phase it is exposed to full solar radiation, therefore the enhanced UV-B radiation could present a threat to this species. An outdoor experiment in which potted plants were exposed to below ambient, ambient, and above ambient (corresponding to 17% ozone reduction) UV-B radiation, was conducted in order to evaluate the radiation effects. The amount of photosynthetic pigments and photochemical efficiency of PSII were not affected, but the amount of UV-B absorbing compounds was lower in plants grown under reduced UV-B. This change was measurable after only fourteen days in reproductive shoots, while in the vegetative shoots, it was not detectable until after three months. The leaves of P. officinalis are variegated and the light green spots became less transparent to PAR under enhanced UV-B. The results reveal that under simulated 17% ozone depletion the harmful effects of UV-B on the measured parameters were negligible.     

Effects of UV-B radiation and additional irrigation on the Mediterranean evergreen sclerophyll Ceratonia siliqua L. under field conditions

Seedlings of Ceratonia siliqua L. were grown for 1 year in the field under ambient or ambient plus supplemental UV-B radiation (corresponding to 15% ozone depletion over Patras) and received two levels of additional irrigation during the summer dry period. The experiment was started during February 1998 and two major samplings were performed, the first at the end of the dry period (September 1998) and the second at the end of the experiment (January 1999). Plants receiving additional irrigation showed significantly higher leaf number, plant height and chlorophyll content at the end of the summer, but these differences were abolished at the final harvest. Plants growing under enhanced UV-B radiation had significantly fewer leaves and less nitrogen content at the end of the dry period, but these effects were also abolished at the final harvest, during which significant UV-B induced increases in stem dry mass were observed. None of the other measured parameters (mean leaf area, leaf dry mass, leaf thickness, UV-B absorbing compounds, phenolics, tannins and photochemical efficiency of PSII) were affected by either treatment. Combined UV-B / water effects were not significant. We may conclude that although some minor responses to enhanced UV-B radiation were evident, C. siliqua is resistant against UV-B radiation damage at the level applied.     

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.     

The effects of enhanced UV-B radiation on physiological activity and growth of Norway spruce planted outdoors over 5 years

The responses of Norway spruce [Picea abies (L.) Karst.] to enhanced UV-B radiation during the 5-year treatment performed outdoors have been subjected to ecophysiological and growth analysis. The plants were exposed to UV-B radiation, simulating 17% ozone depletion. Ecophysiological parameters were monitored three times a year on three needle age classes, while growth was analysed at the end of each growth season. Spruce exhibited great variability in the amounts of photosynthetic pigments and methanol-soluble UV-B absorbing compounds, light use efficiency, photosynthesis and respiratory potential. The needle, branch and plant biomass production was not significantly affected during the 5-year treatment. The repeated-measures procedure comparing growth parameters through subsequent seasons, revealed a decrease of branch diameter under enhanced UV-B, which could be interpreted as a cumulative UV-B effect. The effects of UV-B radiation depended on needle development stage, interaction with environmental conditions and stresses. A reduced negative effect of UV-B radiation was observed during the prolonged drought in 2003, which was hypothesised as an alleviating effect. The tolerance of Norway spruce to elevated UV-B was to a large extent due to the high content of methanol-soluble UV-B absorbing compounds that was related neither to environmental conditions, including UV-B dose, nor to the developmental stage of the needles. The current year needles exhibited a tendency to increased production of UV-B absorbing compounds under elevated UV-B radiation. The outdoor study performed under variable environmental conditions showed great complexity of spruce response to enhanced UV-B.     

Antarctic terrestrial ecosystem and role of pigments in enhanced UV-B radiations

The terrestrial ecosystem of Antarctica are among the most extreme on earth, challenging the communities and making their existence difficult by rapidly increasing annual summer influx of solar ultraviolet radiations (UV-R), extremely cold conditions and lesser availability of nutrients. Spring time ozone depletion is due to release of chlorofluorocarbons in the earth atmosphere and is a serious cause of concern among environmentalists. Antarctic continent is mostly dominated by cryptogamic plants with limited distribution in different parts of the icy continent however; their distribution is mostly confined to Sub-Antarctic region. By the virtue of light requirement, cryptogams are exposed to extreme seasonal fluctuation in photosynthetically active radiation (PAR), and ultraviolet (UV) radiation which are closely associated with photosynthetic pigments in photoautotrophic organisms. Antarctic cryptogams cope up the stress imposed by UV radiation by the development of efficient systems for repairing damage by synthesis of screening compounds such as UV-B absorbing pigments and anthocyanin compounds. A major part of the UV absorbing compounds are appeared to be constitutive in lichens which are usnic acid, perlatolic acid and fumarphotocetraric acid which is particularly induced by UV-B. Secondary metabolites such as phenolics, atranorin, parietin and melanin also enhance the plant defense, by different molecular targets in specific solar irradiance and potential for increased antioxidative protection to UV induced vulnerability.     

Ultraviolet-B radiation effects on the Mediterranean ruderal Dittrichia viscosa

Young seedlings of Dittrichia viscosa L. (syn. Inula viscosa (L.) Aiton) (Asteraceae) were extensively treated with artificial rain in order to remove the water soluble component of their epicuticular UV-B absorbing compounds. As a result, 75% of the epicuticular absorbing capacity at 300 nm was lost. The seedlings were subsequently grown in a naturaly lit glasshouse for 80 days under 0.06, 6.41 and 10.14 kJ m^-2 day ^-1 biologicaly effective UV-B radiation doses. The initial, pre-rain values of the water soluble, epicuticular UV-B absorbing potential was restored in about three weeks. During this transient period the plants were exposed to the enhanced UV-B radiation doses with part of their UV-B radiation screen removed. Although a trend for increased accumulation of epicuticular UV-B absorbing capacity was observed with increasing UV-B radiation doses, the allelopathic potential of the epicuticular material remained unchanged. Internal (cellular) UV-B absorbing compounds and chlorophylls were unaffected, but total carotenoids were increased, indicating a possible protective role against UV-B radiation damage. Leaf, stem and root dry mass were the same under all treatments but UV-B radiation caused a reduction in the dry mass invested per unit leaf area with a concomitant increase in leaf area. The importance of this UV-B radiation induced selective allocation of photosynthate to the production of assimilative surfaces is discussed.     

Effects of UV-B radiation on a charophycean alga,Chara aspera

The charophycean algal species Chara aspera was exposed for 73 days to three levels of UV-B radiation (weighted according to Caldwell's generalized plant action spectrum): 1.9 kJ m^–2 day^–1 (`no UV-B'), 6.4 KJ m<sup>–2</sup> day<sup>–1</sup> (ambient) and 10.5 kJ m<sup>–2</sup> day<sup>–1</sup> (enhanced UV-B), the latter level simulating 30% ozone reduction in The Netherlands.Charophycean algae are mainly freshwater organisms and are thought to be the algae most closely related to higher land plants. Therefore we expected that responses of charophycean algae to UV-B radiation might be more related to those observed in the higher land plants than those of other `lower' algal groups.Under elevated UV-B radiation algal length was reduced. There was no induction of UV absorbing compounds under enhanced UV-B. This might relate to a sensitive response to UV-B radiation. The charophycean algae show similar adaptations to UV-B radiation as terrestrial plants, while not having UV-screens as occur in many angiosperms. Vegetative reproduction (bulbils) increased in the presence of UV-B radiation, while generative reproduction (antheridia and oogonia) decreased.     

UV-B has larger negative impacts on invasive populations of Triadica sebifera but ozone impacts do not vary

Aims Abiotic stresses may interact with each other to determine impacts on plants so that their combined impact is less than or more than additive. Increasing UV-B radiation and surface ozone (O 3) are two major components of global change that may have such interactive impacts. Moreover, invasive and native populations of plants may respond differently to stresses as they can vary in primary and secondary metabolism.Methods Here, we conducted a factorial field experiment with open-top chambers assigned to an ozone treatment (ambient, 100 ppb, or 150 ppb) and UV-B treatment (ambient or increased 20%). We grew seedlings of native and invasive populations of Triadica sebifera in these chambers for one growing season.Important findings Invasive plants grew faster than native plants in ambient UV-B but they did not differ significantly in elevated UV-B. Litter production of invasive plants was especially sensitive to UV-B in a way that increased with UV-B for native plants but decreased for invasive plants which may be important for nutrient cycling. In ambient UV-B, total mass decreased as ozone increased. Total mass was lower with elevated UV-B but there was no additional impact of increasing ozone. Leaf area did not decrease with UV-B so SLA and LAR were lowest at ambient ozone levels. These results suggest that the effects of ozone will depend on UV-B conditions perhaps due to changes in foliar traits. The traits that allow invasive populations of plants to be successful invaders may make them especially sensitive to UV-B which may reduce their success in future climatic conditions.     

Effect of enhanced UV-B radiation on pollen quantity,quality, and seed yield in Brassica rapa (Brassicaceae)

We conducted three experiments to examine the influence of ultraviolet-B radiation (UV-B; 280–320 nm) exposure on reproduction in Brassica rapa (Brassicaceae). Plants were grown in a greenhouse under three biologically effective UV-B levels that simulated either an ambient stratospheric ozone level (control), 16% (“low enhanced”), or 32% (“high enhanced”) ozone depletion levels at Morgantown, WV, USA in mid-March. In the first experiment, we examined whether UV-B level during plant growth influenced in vivo pollen production and viability, and flower production. Pollen production and viability per flower were reduced by ≈50% under both enhanced UV-B levels relative to ambient controls. While plants under high-enhanced UV-B produced over 40% more flowers than plants under the two lower UV-B treatments, whole-plant production of viable pollen was reduced under high-enhanced UV-B to 17% of that of ambient controls. Whole-plant production of viable pollen was reduced under low-enhanced UV-B to 34% of ambient controls. In the second experiment, we collected pollen from plants under the three UV-B levels and examined whether source-plant UV-B exposure influenced in vitro pollen germination and viability. Pollen from plants under both enhanced-UV-B treatments had initially lower germination and viability than pollen from the ambient level. After in vitro exposure to the high-enhanced UV-B levels for 6 h, viability of the pollen from plants grown under ambient UV-B was reduced from 65 to 18%. In contrast, viability of the pollen from plants grown under both enhanced UV-B treatments was reduced to a much lesser extent: only from ≈43 to 22%. Thus, ambient source-plant pollen was more sensitive to enhanced UV-B exposure. In the third experiment, we used pollen collected from source plants under the three UV-B levels to fertilize plants growing under ambient-UV-B levels, and assessed subsequent seed production and germination. Seed abortion rates were higher in plants pollinated with pollen from the enhanced UV-B treatments, than from ambient UV-B. Despite this, seed yield (number and mass) per plant was similar, regardless of the UV-B exposure of their pollen source. Our findings demonstrate that enhanced UV-B levels associated with springtime ozone depletion events have the capacity to substantially reduce viable pollen production, and could ultimately reduce reproductive success of B. rapa.     

Effects of UV-B radiation on soybean yield and seed quality: a 6-year field study

Two soybean [ Glycine max (L.) Merr.] cultivars, Essex and Williams, were grown in the field for 6 consecutive seasons under ambient and supplemental levels of ultravio-Set-B radiation to determine the potential for alterations in yield or seed quality with a reduction in the stratospheric ozone column. The supplemental UV-B fluences simulated a 16 or 25% ozone depletion. The data presented here represent the first field experiment conducted over multiple seasons which assesses the effects of increased UV-B radiation on seed yield. Overall, the cultivar Essex was found to be sensitive to UV-B radiation (yield reductions of 20%) while the cultivar Williams was tolerant. However, the effectiveness of UV-B radiation in altering yield was strongly influenced by the seasonal microclimate, and the 2 cultivars responded differently to these changing factors. Yield was reduced most in Essex during seasons in which water availability was high and was reduced in Williams only when water was severely limiting. The results of these experiments demonstrate the necessity for multiple-year experiments and the need to increase our understanding of the interaction between UV-B radiation and other environmental stresses in order to assess the potential consequences of stratospheric ozone depletion.     

Principal component analysis of intraspecific responses of tartary buckwheat to UV-B radiation under field conditions

Fifteen populations of tartary buckwheat (Fagopyrum tataricum Gaertn.) occurring in habitats with different natural UV-B levels were sampled, and the plants were exposed to enhanced UV-B radiation under field conditions simulating 25% depletion of the stratospheric ozone layer. The experimental design was a 2 × 15 factorial, with two levels of UV-B radiation (ambient and enhanced UV-B radiation) and plants from 15 populations. The responses of plants in growth, morphology, productivity and in the composition of photosynthetic pigments were measured. The results demonstrated that there were significant differences among populations in responses to UV-B radiation: some populations exhibited a positive effect while others were negatively affected. The UV-B effects on plant traits were correlated with the constitutive values. A principal component analysis (PCA) was used to evaluate the overall sensitivity of responses to UV-B radiation. Our results suggest that the sensitivity of plants to UV-B radiation is not only associated with the ambient UV-B level in natural habitats but also with the relative growth rate and other factors.     

Neotyphodium lolii, a Fungal Leaf Endophyte, Reduces Fertility ofLolium perenneExposed to Elevated UV-B Radiation

Plants ofLolium perenne, grown with and without the balansoidfungal leaf endophyteNeotyphodium lolii, were exposed to threeultraviolet radiation treatments at an outdoor facility in theUK for 172 d. Plants were exposed to either (a) a 30% elevationabove the ambient erythemally-weighted level of UV-B (280–315nm) radiation under banks of cellulose diacetate filtered fluorescentlamps that also produce UV-A (315–400 nm) radiation (UV-B+A);(b) elevated UV-A radiation alone under banks of polyester filteredlamps; or (c) ambient levels of solar radiation under banksof unenergized lamps. The fertility of plants grown withN. loliiwassignificantly reduced by the elevated UV-B+A exposure. After172 d, these plants produced 70% fewer spikes, 75% fewer seeds,71% lower total weight of seed and 78% fewer seeds per g d.wt of plant tissue than plants colonized byN. loliiwhich wereexposed to ambient radiation. There was no discernible effectof elevated UV-B+A exposure on the fertility of endophyte-freeplants. Plants irradiated with UV-B+A developed 14% thickerleaves than those exposed to ambient radiation. Those whichwere irradiated with elevated UV-A alone produced seeds thatwere 20% heavier than plants exposed to ambient levels of radiation.Plants grown withN. loliihad 7% thicker leaves, 4% thicker stembases and 7% fewer tillers than those grown without it. Thefresh mass of tillers of plants grown withN. loliiwas 11% greaterthan those of endophyte-free plants, owing to their higher moisturecontents. These results suggest that the fertility ofL. perennecolonizedbyN. loliiin the natural environment could be deleteriouslyaffected by elevated fluxes of UV-B radiation associated withstratospheric ozone depletion and that this may affect the populationdynamics of the species.Copyright 1998 Annals of Botany Company Fungal leaf endophyte,Neotyphodium lolii, perennial ryegrass (Lolium perenne), stratospheric ozone depletion, UV-B radiation.     

Morphological and physiological responses of bean plants to supplemental UV radiation in a Mediterranean climate

During the last few decades many experiments have been performed to evaluate the responses of plants to enhanced solar UV-B radiation (280–320 nm) that may occur because of stratospheric ozone depletion; most of them were performed in controlled environment conditions where plants were exposed to low photosynthetically active radiation (PAR) levels and high UV-B irradiance. Since environmental radiative regimes can play a role in the response of plants to UV-B enhancement, it appears doubtful whether it is valid to extrapolate the results from these experiments to plants grown in natural conditions. The objective of this work was to evaluate the effects on physiology and morphology of a bean (Phaseolus vulgaris L.) cultivar Nano Bobis, exposed to supplemental UV radiation in the open-air. UV-B radiation was supplied by fluorescent lamps to simulate a 20% stratospheric ozone reduction. Three groups of plants were grown: control (no supplemental UV), UV-A treatment (supplementation in the UV-A band) and UV-B treatment (supplemental UV-B and UV-A radiation). Each group was replicated three times. After 33 days of treatment plants grown under UV-B treatment had lower biomass, leaf area and reduced leaf elongation compared to UV-A treatment. No significant differences were detected in photosynthetic parameters, photosynthetic pigments and UV-B absorbing compounds among the three groups of plants. However, plants exposed to UV-A treatment showed a sort of 'stimulation' of their growth when compared to the control. The results of this experiment showed that plants may be sensitive to UV-A radiation, thus it is difficult to evaluate the specific effects of UV-B (280–320 nm) radiation from fluorescent lamps and it is important to choose the appropriate control. Environmental conditions strongly affect plant response to UV radiation so further field studies are necessary to assess the interaction between UV-B exposure and meteorological variability.     

Consequences of depletion of stratospheric ozone for terrestrial Antarctic ecosystems: the response of Deschampsia antarctica to enhanced UV-B radiation in a controlled environment

Mini UV lamps were installed over antarctic plants at Léonie Island, Antarctic peninsula, and shoot length measurements of Deschampsia antarctica were performed during the austral summer January–February 1999.We studied the response of the antarctic hairgrass, Deschampsia antarctica to enhanced UV-B. In a climate room experiment we exposed tillers of Deschampsia antarctica, collected at Léonie Island, Antarctic peninsula, to ambient and enhanced levels of UV-B radiation. In this climate room experiment with 0, 2.5 and 5 kJ m^–2 day^–1 UV-B_BE treatments we observed that length growth of shoots at 2.5 and 5 kJ m^–2 day^–1 UV-B_BE was markedly reduced compared to 0 kJ m^–2 day^–1 UV-B_BE. In addition, there was an increased number of shoots and increased leaf thickness with enhanced UV-B. The Relative Growth Rate (RGR) was not affected by UV-B, possibly because reduced shoot length growth by enhanced UV-B was compensated by increased tillering. Light response curves of net leaf photosynthesis of plants exposed to 5 kJ m^–2 day^–1 UV-B_BE did not differ from those exposed to 0 kJ m^–2 day^–1 UV-B_BE. The content of UV-B absorbing compounds of plants exposed to increasing UV-B did not significantly change.Mini UV-B lamp systems were installed in the field, to expose the terrestrial antarctic vegetation at Léonie Island to enhanced solar UV-B. In that study, the increment of shoot length of tagged plants of Deschampsia antarctica during the January-February 1999 at Léonie Island, was recorded and compared to shoot length growth under controlled conditions.The consequences of enhanced UV-B radiation as a result of ozone depletion for the terrestrial antarctic ecosytems are discussed.     

Enhanced UV-B radiation,artificial wounding and leaf chemical defensive potential in Phlomis fruticosa L.

The combined effects of additional UV-B radiation and artificial wounding on leaf phenolics were studied in a short term field experiment with the drought semi-deciduous Mediterranean shrub Phlomis fruticosa L. The seedlings were grown under ambient or ambient plus supplemental UV-B radiation (biologically equivalent to a 15% ozone depletion over Patras, 38.3° N, 29.1° E) for 7 months before wounding. Unexpectedly, supplemental UV-B radiation decreased leaf phenolics. Subsequently, wounding was effected by removing leaf discs from some of the plants, while the rest remained intact and served as controls. Wounding significantly increased phenolics of the wounded leaves and the increase was more pronounced under supplemental UV-B radiation. In addition, wounding had a significant positive effect on the phenolics of the opposite, intact leaf, but only under additional UV-B radiation. We conclude that UV-B radiation, wounding and their combination may affect the chemical defensive potential of Phlomis fruticosa. In addition, increased levels of phenolics after herbivore attack under field conditions may afford extra protection against enhanced UV-B radiation levels.     

UV-B radiation causes early ripening and reduction in size of fruits in two lines of tomato (Lycopersicon esculentum Mill.)

An open-air experiment was performed in Pistoia (Italy) to investigate the possible protective role played by different contents of UV-B absorbing compounds to realistic UV-B supplementation and to study its effect on plant fruit production. A mutant line and its normal counterpart of Lycopersicon esculentum Mill, which differ in the content of UV-B absorbing compounds, were used. Additional UV-B radiation in the field was supplied to simulate a 20% stratospheric ozone depletion. Two groups of plants were grown: ‘control’, where plants received only natural solar UV-B radiation, and ‘UV-B’ treatment, where plants were grown under supplemental UV-B. The results of the experiment showed that the content of UV-B absorbing compounds of treated plants did not differ from that of the control in both lines. This indicates that natural sunlight, in Mediterranean areas, is saturating for synthesis of these compounds also in plants with normal content of UV-B absorbing compounds. Consequently, plants are not able to produce significant additional amounts of them, in response to a realistic UV-B supplementation, in order to protect the plant from additional UV-B radiation. No different responses to the UV-B supplementation were found between the two lines. The most significant UV-B effect was an earlier reddening of fruits in comparison with the ‘control’ accompanied by a reduction in the size of mature fruits. No significant effects of UV-B treatment were observed in biomass accumulation, leaf ontogeny, flowering or productivity.     

Response of cotton and sorghum to several levels of subambient solar UV-B radiation: a test of the saturation hypothesis

Some have proposed that plant responses to above-ambient or supplemented levels of solar ultraviolet-B radiation (UV-B; 280–315 nm) are typically subtle because targets or receptors in plants become saturated. If true, in solar UV-B filter exclusion experiments we would expect that plant responses would level off or 'saturate' as doses approached ambient levels. To test this so-called 'saturation hypothesis' we examined the response of Gossypium hirsutum (cotton) and Sorghum bicolor (sorghum) to filter exclusions that provided five levels of biologically effective UV-B, ranging from 36 to 91% of ambient solar levels in Arizona, USA. UV-B dose had no effect on biomass production of either species. As UV-B dose increased or approached ambient, individual leaves of S. bicolor were smaller, but plants produced more tillers and leaves. In G. hirsutum , individual leaves as well as total plant leaf area were smaller, but plants produced more branches. Bulk concentrations of soluble UV-B absorbing compounds increased with UV-B dose in both species. Leaf epidermal UV-B transmittance, assessed with the chlorophyll fluorescence technique, declined with increasing UV-B dose, and was well correlated with bulk concentrations of soluble UV-B screening compounds. Bulk concentrations of insoluble or wall-bound UV-B absorbing compounds were not affected by UV-B dose. The intensity of UV-induced blue fluorescence from leaf surfaces was strongly correlated with bulk concentrations of wall-bound UV-B absorbing compounds, and this signal has the potential to provide a rapid, non-invasive method to estimate concentrations of these compounds, which are time-consuming to extract. While both species were responsive to solar UV-B, responses did not appear to become saturated as doses approached ambient levels. Rather, responses required a threshold dose of >70% of solar ambient UV-B levels before they became apparent.     

Is UV-B radiation affecting charophycean algae in shallow freshwater systems?

The objective of this study was to determine the effects of UV-B radiation on charophycean algae under natural conditions, since charophytes enhance water transparency in freshwater systems and levels of UV-B radiation have increased by ozone depletion. Potential and actual UV-B effects were studied by combining a glasshouse experiment in which plants were exposed to various levels of UV-B radiation and field measurements in two freshwater systems dominated by charophytes in the Netherlands. The glasshouse experiment showed that charophytes were sensitive to UV-B radiation. UV-B radiation negatively affected growth, while it increased levels of DNA damage in Chara aspera. Moreover, the charophytes did not seem to develop UV-B screens to protect against UV-B radiation since no increase in UV-B absorbing compounds was found. At field conditions, both spectroradiometrical measurements and DNA dosimeters showed that UV-B radiation was attenuated quickly in both freshwater systems, indicating that UV-B does not reach the submerged charophyte vegetation. However, specific conditions, like fluctuating water tables, may result in UV-B exposure to charophytes for certain periods annually.