Combined effects of enhanced UV-B radiation and additional nutrients on growth of two Mediterranean plant species

Seedlings of two Mediterranean plants, the slow-growing, evergreen sclerophyll Ceratonia siliqua L. and the fast growing drought semi-deciduous Phlomis fruticosa L., were grown for one year in the field at ambient or ambient plus supplemental UV-B radiation (equivalent to a 15% ozone depletion) and two levels of applied fertilizers (NPK). The effects on growth, morphological, anatomical and physiological parameters were measured at final plant harvest. Additional nutrients increased leaf nitrogen, improved growth and reduced the root/shoot ratio in both plants, yet these effects were more pronounced in the fast growing P. fruticosa. A nutrient-induced increase in chlorophyll content was also observed in this plant. The growth responses to UV-B radiation were different for the two species. Growth in C. siliqua was not affected by UV-B radiation at both nutrient levels and the same was true for P. fruticosa at low nutrients. However, at the high nutrient level, supplemental UV-B radiation improved growth in P. fruticosa, indicating a strong interaction between the treatments. Photosystem II (PSII) photochemical efficiency, methanol-extractable UV-B absorbing capacity, total phenolics and tannins were not affected by either treatment in both plants. It is concluded that nutrient levels can strongly modify the UV-B radiation effects on growth of P. fruticosa. We presume that this may be correlated to the fast growing habit of this species.     

Beneficial effects of enhanced UV-B radiation under field conditions: improvement of needle water relations and survival capacity of Pinus pinea L. seedlings during the dry Mediterranean summer

The possible mechanism(s) by which supplemental UV-B radiation alleviates the adverse effects of summer drought in Mediterranean pines (Petropoulou et al. 1995) were investigated with seedlings of Pinus pinea. Plants received ambient or ambient plus supplemental UV-B radiation (biologically equivalent to a 15% ozone depletion over Patras, 38.3° N, 29.1° E) and natural precipitation or additional irrigation. Treatments started on 1 February, 1994 and lasted up to the end of the dry period (29 September). In well-watered plants, UV-B radiation had no influence on photosystem II photochemical efficiency and biomass accumulation. Water stressed plants suffered from needle loss and reduced photosystem II photochemical efficiency during the summer. These symptoms, however, were less pronounced in plants receiving supplemental UV-B radiation, resulting in higher total biomass at plant harvest. Laboratory tests showed that enhanced UV-B radiation did not improve the tolerance of photosystem II against drought, high light, high temperature and oxidative stress. Enhanced UV-B radiation, however, improved the water economy of water stressed plants, as judged by measurements of needle relative water content. In addition, it caused an almost two-fold increase of cuticle thickness. No such UV-B radiation effects were observed in well-watered pines. The results indicate that the combination of water stress and UV-B radiation may trigger specific responses, enabling the plants to avoid excessive water loss and, thereby, maintain a more efficient photosynthetic apparatus during the summer. The extent of this apparently positive UV-B radiation effect would depend on the amount of summer precipitation. Abbreviations: DW – dry weight, F_v/F_m – ratio of variable to maximum fluorescence, A ₃₀₀ – absorbance at 300 nm, PAR – photosynthetically active radiation, PS II – photosystem II, RWC – relative water content, TCA – trichloroacetic acid, UV-B_BE – biologically effective ultraviolet-B radiation     

Reduction of ambient UV-B radiation does not affect growth but may change the flowering pattern of Rosmarinus officinalis L.

The effects of sub-ambient levels of UV-B radiation on the shrub Rosmarinus officinalis L. were investigated in a field filtration experiment in which the ambient UV-B was manipulated by a combination of UV-B transmitting and UV-B absorbing filters. As a result, the plants were receiving near-ambient or drastically reduced UV-B radiation doses. Drastic reduction of UV-B radiation had no effect on mean, total and maximum stem length, number of stems per plant, dry mass of leaves, stems and roots and leaf nitrogen and phenolic contents. However, flowering was more pronounced under reduced UV-B radiation during the winter period which coincides with ascending ambient UV-B radiation. In contrast, during autumn and early winter, a period which coincides with descending ambient UV-B radiation, flowering was unaffected by reduced UV-B radiation. We can conclude that natural UV-B radiation does not affect growth of Rosmarinus officinalis, but its reduction could influence the flowering pattern of the species.     

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.     

Decreased Ultraviolet-B Radiation Alters the Vertical Biomass Distribution in Cocksfoot

A greenhouse experiment was conducted to investigate whether small differences in UV-B irradiance would lead to changes in competition between cocksfoot (Dactylis glomerata L. cv. Athos) and white clover (Trifolium repens L. cv. Mervi). Plants were grown in greenhouses covered with different thicknesses of UV-transmittant plexi (3 and 5 mm) resulting in 82 % and 88 % of ambient UV-B radiation. Aboveground biomass was harvested at 4-week intervals and the vertical distribution of biomass, leaf thickness and specific leaf area were determined. Tillering, stubble and root biomass and crop height were also measured. There was only one significant effect: at 88 % of ambient UV-B radiation a larger fraction of the biomass was present in the lower layers and a smaller fraction was present in the upper layers.     

Enhanced UV-B radiation,flower attributes and pollinator behaviour in Cistus creticus: a Mediterranean field study

The aim of this investigation was to examine the reasons for the higher pollination success in Cistus creticus under enhanced UV-B radiation (Stephanou & Manetas 1998). Thus, a selected array of floral attributes as well as the frequency and duration of insect visits were studied in the field under ambient or ambient plus supplemental UV-B radiation, simulating a 15% ozone depletion over Patras (38.3° N, 29.1° E). Video-recording revealed two categories of visitors, i.e. true pollinators (bees) and nectar thieves. The frequency of visits to both control and UV-B treated plants was the same and independent of whether the UV-B tubes were on or off during video recording. UV-B radiation had no effect on gross floral morphology (petal surface area, number of pollen grains, stamens and ovules, optical properties of petals and stamens), yet nectary size was almost doubled. In addition, the duration of insect visits was significantly longer on UV-B treated plants, provided that the UV-B tubes were off during monitoring. The differences were abolished during the part of the day that the tubes were on, indicating that the insects were annoyed by supplemental UV-B radiation. These results are consistent with the nectaries producing larger quantities of nectar, which caused the insects to stay longer on flowers of UV-B treated plants and improved pollination success.     

SOYBEAN GROWTH RESPONSES TO ENHANCED LEVELS OF ULTRAVIOLET-B RADIATION UNDER GREENHOUSE CONDITIONS

Soybean (Glycine max [L.] Merr. cv. Essex) was grown in an unshaded greenhouse under three levels of biologically effective ultraviolet-B (UV-B_BE) radiation (effective daily dose: 0, 11.5 and 13.6 kJ m^–2) for 91 days. Plants were harvested at regular intervals beginning 10 days after germination until reproductive maturity. Mathematical growth analysis revealed that the effects of UV-B radiation varied with plant growth stage. The transition period between vegetative and reproductive growth was the most sensitive to UV-B radiation. Intermediate levels of UV-B had deleterious effects on plant height, leaf area, and total plant dry weight at late vegetative and reproductive stages of development. Specific leaf weight increased during vegetative growth but was unaffected by UV-B during reproductive growth stages. Relative growth, net assimilation, and stem elongation rates were decreased by UV-B radiation during vegetative and early reproductive growth stages. Variation in plant responses may be due in part to changes in microclimate within the plant canopy or to differences in repair or protection mechanisms at differing developmental stages.     

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.     

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.     

The effects of seasons, exposure, enhanced UV-B radiation, and water stress on leaf epicuticular and internal UV-B absorbing capacity of Cistus creticus: a Mediterranean field study

The seasonal variation in leaf epicuticular and internal (cellular)UV-B absorbing compounds was followed in field-grown shrubsof Cistus creticus L. Fully exposed and permanently shaded (undera canopy of evergreen sclerophylls) individuals were used. Nearmonthly measurements were performed for three years. In addition,a 2-year field experiment was established, in which fully exposedplants received ambient or ambient plus supplemental UV-B radiation,simulating a 15% ozone depletion over Patras (38.3 N, 29.1E). A second, six-month field experiment investigated the effectsof additional summer irrigation on UV-B absorbing compounds.It was found that: (a) exposed plants had considerably higherconcentrations of UV-B absorbing compounds (both epicuticularand internal), irrespective of season, (b) exposed and shadedplants exhibited dramatic and yearly consistent seasonal fluctuationsin epicuticular material with a c. 18-fold difference betweenthe summer maxima and winter minima. Internal UV-B absorbingcompounds showed the same seasonal trends, but with far lesspronounced differences, (c) In the 2-year field experiment withenhanced UV-B radiation, an increase in both epicuticular andinternal UV-B absorbing capacity was evident only during thesummer dry period of the first year. Later on, however, andup to the termination of the experiment, the differences wereabolished, (d) Well-watered and water-stressed plants showedthe same levels of both epicuticular and internal UV-B absorbingcompounds. Although the results indicate that concentrations of epicuticularand, to a lesser extent, internal UVB absorbing compounds fluctuateaccording to the seasonally and site-dependent environmentalstresses, no indications for an effect of water availabilitywere obtained. Therefore it may be concluded that the main determinantof the levels of UVB absorbing compounds is the intensity ofsolar radiation. The relative influence of the various spectralregions of solar radiation cannot be assessed from the presentresults. It was predicted, however, that the effects of increasedsolar UV-B radiation, resulting from the expected ozone depletionover the next decades, will be negligible and probably maskedby the much larger seasonal and site-dependent changes. Key words: Epicuticular, exposure, seasons, UVB absorbing compounds, UVB radiation     

Growth enhancement of soybean (Glycine max) upon exclusion of UV-B and UV-B/A components of solar radiation: characterization of photosynthetic parameters in leaves

Exclusion of UV (280–380 nm) radiation from the solar spectrum can be an important tool to assess the impact of ambient UV radiation on plant growth and performance of crop plants. The effect of exclusion of UV-B and UV-A from solar radiation on the growth and photosynthetic components in soybean (Glycine max) leaves were investigated. Exclusion of solar UV-B and UV-B/A radiation, enhanced the fresh weight, dry weight, leaf area as well as induced a dramatic increase in plant height, which reflected a net increase in biomass. Dry weight increase per unit leaf area was quite significant upon both UV-B and UV-B/A exclusion from the solar spectrum. However, no changes in chlorophyll a and b contents were observed by exclusion of solar UV radiation but the content of carotenoids was significantly (34–46%) lowered. Analysis of chlorophyll (Chl) fluorescence transient parameters of leaf segments suggested no change in the F _v/F _m value due to UV-B or UV-B/A exclusion. Only a small reduction in photo-oxidized signal I (P700⁺)/unit Chl was noted. Interestingly the total soluble protein content per unit leaf area increased by 18% in UV-B/A and 40% in UV-B excluded samples, suggesting a unique upregulation of biosynthesis and accumulation of biomass. Solar UV radiation thus seems to primarily affect the photomorphogenic regulatory system that leads to an enhanced growth of leaves and an enhanced rate of net photosynthesis in soybean, a crop plant of economic importance. The presence of ultra-violet components in sunlight seems to arrest carbon sequestration in plants. An erratum to this article can be found at     

Effects of spring and summer levels of UV-B radiation on the growth and reproduction of a temperate perennial forb

Narrow-leaved plantain (Plantago lanceolata L.), a perennial forb, flowers for virtually the full length of the growing season in temperate latitudes and as a result it is exposed to widely variable intensities of shortwave (UV-B) radiation. In order to determine effects of spring and summer levels of UV-B exposure on growth and development, representatives of 42 maternal families were grown for 85 days at 3.2 and 6.4 kJ m^–2 day^–1 BE₃₀₀, levels corresponding to early spring and mid-summer in central Illinois. Impacts on early vegetative stages were most pronounced; early vegetative growth was decreased by higher levels of UV-B and both leaf angle (a measure of erectness) and leaf hair density were increased. At harvest, vegetative growth was significantly affected by higher levels of UV-B as well; the mass of senescent leaves and crown tissue were both decreased. Although exposure to higher levels of UV-B decreased inflorescence number by nearly 15%, it did not significantly alter reproductive biomass. Significant variation attributable to maternal families was present in nearly all measurements and the range of variation among families was wider than among UV-B treatments. A marginally significant (p=0.07) maternal family by UV-B interaction was found for the number of inflorescences, suggesting that, within populations of this plant, some small amount of genetic variation exists to allow for differential reproductive performance under a regime simulating spring and summer differences in UV-B exposure. For the most part, however, in this cosmopolitan species the level of adaptation to natural levels of variation in UV-B radiation does not differ dramatically among maternal families.     

The growth,flower properties and demography of Anthemis arvensis exposed to enhanced UV-B radiation

The winter annual species Anthemis arvensis L. (Asteraceae) was grown for 3.5 months in the field under ambient or ambient plus supplemental UV-B radiation, simulating a 15% ozone depletion over Patras (38.3° N, 29.1° E). Enhanced UV-B radiation had no effect on the methanol extractable UV-B absorbing capacity of leaves, phenological and morphometric parameters of anthesis (flowering time, anthesis duration, head life span, number of heads per plant, number of tubular and ligulate florets per head, area per ligulate floret). Concerning the optical properties of heads, enhanced UV-B radiation had no significant effect on the extractable absorbance of both floret types nor on the spectral reflectance of the tubular florets. However, under UV-B supplementation the white ligulate florets exhibited a slight, statistically significant decrease of reflectance in the visible region of the spectrum. This may be due to structural changes of the floret surface, since microscopic examination under SEM revealed the papillae of the adaxial epidermal cells to be swollen. The above ground dry mass measured at plant harvest was not affected but a significant increase in root biomass (and accordingly in root/shoot ratio) was observed. We conclude that Anthemis arvensis is resistant against UV-B radiation damage. The possible consequences of UV-B induced structural changes on floret epidermis are discussed.     

Stratospheric Ozone Depletion: High Arctic Tundra Plant Growth on Svalbard is not Affected by Enhanced UV-B after 7 years of UV-B Supplementation in the Field

The response of tundra plants to enhanced UV-B radiation simulating 15 and 30% ozone depletion was studied at two high arctic sites (Isdammen and Adventdalen, 78° N, Svalbard).The set-up of the UV-B supplementation systems is described, consisting of large and small UV lamp arrays, installed in 1996 and 2002. After 7 years of exposure to enhanced UV-B radiation, plant cover, density, morphological (leaf fresh and dry weight, leaf thickness, leaf area, reproductive and ecophysiological parameters leaf UV-B absorbance, leaf phenolic content, leaf water content) were not affected by enhanced UV-B radiation. DNA damage in the leaves was not increased with enhanced UV-B in Salix polaris and Cassiope tetragona. DNA damage in Salix polaris leaves was higher than in leaves of C. tetragona. The length of male gametophyte moss plants of Polytrichum hyperboreum was reduced with elevated UV-B as well as the number of Pedicularis hirsuta plants per plot, but the inflorescence length of Bistorta vivipara was not significantly affected. We discuss the possible causes of tolerance of tundra plants to UV-B (absence of response to enhanced UV-B) in terms of methodology (supplementation versus exclusion), ecophysiological adaptations to UV-B and the biogeographical history of polar plants     

Elevated UV-B radiation incident on Quercus robur leaf canopies enhances decomposition of resulting leaf litter in soil

We examined whether the exposure of Quercus robur L. to elevated UV-B radiation (280–315 nm) during growth would influence leaf decomposition rate through effects on litter quality. Saplings were exposed for eight months at an outdoor facility in the UK to a 30% elevation above the ambient level of erythemally weighted UV-B radiation under UV-B treatment arrays of fluorescent lamps filtered with cellulose diacetate, which transmitted both UV-B and UV-A (315–400 nm) radiation. Saplings were exposed to elevated UV-A alone under control arrays of lamps filtered with polyester and to ambient radiation under unenergised arrays of lamps. Abscised leaves from saplings were enclosed in 1 mm² mesh nylon bags, placed in a Quercus–Fraxinus woodland and were sampled at 0.11, 0.53, 1.10 and 1.33 years for dry weight loss, chemical composition and saprotrophic fungal colonization. At abscission, litters from UV-A control arrays had ≈ 7.5% higher lignin/nitrogen ratios than those from UV-B treatment and ambient arrays (P < 0.06). Dry weight loss of leaves treated with elevated UV-B radiation during growth was 2.5% and 5% greater than that of leaves from UV-A control arrays at 0.53 and 1.33 years, respectively. Litter samples from UV-B treatment arrays lost more nitrogen and phosphorus than samples from ambient arrays and more carbon than samples from UV-A control arrays. The annual fractional weight loss of litter from UV-B treatment arrays was 8% and 6% greater than that of litter from UV-A control and ambient arrays, respectively. Regression analyses indicated that the increased decomposition rate of UV-B treated litters was associated with enhanced colonization of leaves by basidiomycete fungi, the most active members of the soil fungal community, and that the frequency of these fungi was negatively associated with the initial lignin/nitrogen ratio of leaves.     

The combined effects of CO2 concentration and solar UV-B radiation on faba bean grown in open-top chambers

The response of faba bean seedlings to the combined effects of increased atmospheric CO2 concentrations ([CO2]) and solar UV-B irradiance was studied using open-top chambers transparent to UV-B radiation. The purpose of the study was to determine whether effects of increased [CO2] on growth and physiology are modified by the present solar UV-B fluence rate in the Netherlands. Seedlings were exposed to 350 or 700 micromoles mol-1 CO2. At both [CO2], solar UV-B irradiance was either present or reduced using polyester foil opaque to UV-B radiation. To obtain information on the time dependence of increased [CO2] and UV-B radiation effects, three harvests were performed during the experiment. CO2 enrichment resulted in increased biomass production at all harvests. At the final harvest, UV-B radiation did not affect biomass production but a significant decrease was observed after 14 d of treatment. A reduction of the UV-B fluence increased shoot length at both [CO2] throughout the experiment. UV-B radiation slightly altered biomass allocation. Plants grown at reduced levels of UV-B radiation invested less biomass in flowers and more in stem material compared to plants grown at ambient UV-B levels. CO2 enrichment resulted in a stimulation of net photosynthesis after 26 and 38 d of treatment. UV-B reduction did not alter this response. After 26 d of treatment, photosynthetic acclimation to CO2 enrichment was observed, which was probably the result of accumulation of carbohydrates in the leaves. After 38 d, photosynthetic acclimation was no longer present. The UV absorbance of methanolic leaf extracts was increased by CO2 enrichment only. Both CO2 enrichment and solar UV-B reduced the transmittance of radiation through intact attached leaves. Interaction between [CO2] and UV-B radiation was limited to UV-A transmittance of leaves. Under prevalent experimental conditions, UV-B radiation did not affect the measured physiological parameters. Most open-top chambers used for climate change research are constructed of materials which do not transmit UV-B radiation. Our results indicate that part of the 'chamber effects' on plant height often described in the literature might be explained by the absence of solar UV-B radiation in these chambers.     

Effects of elevated ultraviolet radiation on Quercus robur and its insect and ectomycorrhizal associates

Saplings of pedunculate oak (Quercus robur L.) were exposed at an outdoor facility to modulated levels of elevated UV-B radiation (280–315 nm) under treatment arrays of cellulose diacetate-filtered fluorescent lamps which also produced UV-A radiation (315–400 nm). Saplings were also exposed to UV-A radiation alone under control arrays of polyester-filtered lamps and to ambient levels of solar radiation under arrays of unenergized lamps. The UV-B treatment corresponded to a 30% elevation above the ambient level of erythemally weighted UV-B radiation. Sapling growth and the occurrence of associated organisms were examined over two years. In both years, leaves of saplings exposed to UV-B treatment were thicker and smaller in area relative to leaves exposed to ambient and control levels of radiation. UV-B treatment also retarded bud burst at one sampling in the first year of the study. Some responses were recorded which were common to both treatment and control arrays, implying that UV-A radiation, or some other factor associated with energized lamps, was responsible for the observed effects. Saplings under treatment and control arrays were taller in the first year of the study, suffered greater herbivory from chewing insects, and had lower root dry weights and greater insertion heights of secondary branches than saplings exposed to ambient levels of radiation. Exposure of saplings to elevated UV-A radiation alone under control arrays increased estimated leaf volumes in the second year of the study and reduced the number of secondary branches and the total number of branches per sapling after two years, relative to both treatment and ambient arrays. There were no effects of elevated ultraviolet radiation on shoot or total plant weight, root/shoot ratios, stem diameter, the numbers or insertion heights of primary or tertiary branches, total leaf number, timing of leaf fall or frequency of ectomycorrhizas. Our study suggests that any increases in UV-B radiation as a result of stratospheric ozone depletion will influence the growth of Q. robur primarily through effects on leaf morphology.     

Effects of enhanced ultraviolet-B radiation on plant nutrients and decomposition of spring wheat under field conditions

Spring wheat (Triticum aestivum) was grown in the field under ambient and supplemental levels of ultraviolet-B (UV-B, 280–315 nm) radiation to determine the potential for alteration in plant nutrients, decomposition, leaf quality and dry matter yield. Supplemental UV-B radiation simulating a 12, 20 and 25% stratospheric ozone depletion significantly decreased dry matter yield, but had no significant impact on harvest index. UV-B radiation resulted in an increase of the concentrations of N and K in all plant parts; changes of the concentrations of P, Mg, Fe and Zn varied in a tissue-dependent manner, as the decrease of P in leaves and stems, and its increase in spikes and grains. The mass of N, P, K, Mg, Fe and Zn in various plant parts and whole plant was generally decreased except leaf N mass was increased by enhanced UV-B radiation. Enhanced UV-B radiation decreased the concentrations of soluble carbohydrates in leaves and increased that of holocellulose and soluble proteins. After 60 and 100 days of decomposition of leaves and stems in the field, enhanced UV-B radiation stimulated the loss of organic C. As a consequence, the nutrient content of soils might be less diminished under enhanced UV-B radiation.     

Enhanced UV-B radiation increases the reproductive effort in the Mediterranean shrub Cistus creticus under field conditions

Seedlings of the Mediterranean shrub Cistus creticus L. were grown in the field under ambient or ambient plus supplemental UV-B radiation (simulating a 15% ozone depletion over Patras, 38.3°W, 29.1°E) for 20 months. During this period, measurements of photosynthetic capacity, photochemical efficiency of PS II, chlorophylls and carotenoids were performed once per season. Supplemental UV-B radiation had no significant effect on these parameters nor on the total, above ground biomass accumulation, plant height and leaf specific mass measured at plant harvest. It was observed, however, that UV-B supplementation increased the number of seeds per fruit as well as mean individual seed mass. As a result, seed number and total seed mass per plant were considerably increased. Germination rates of produced seeds were not affected. We may conclude that C. creticus is a UV-B resistant plant whose competitive ability may be improved by enhanced UV-B radiation through an increase in its reproductive effort and a higher contribution to the seed bank.     

The reduction of aboveground Calamagrostis epigeios mass and tiller number by enhanced UV-B in a dune-grassland ecosystem

Since early May 1997 dune-grassland vegetation in the Netherlands has been exposed to enhanced levels of ultraviolet-B (UV-B) radiation. Expected increases in the amount of biologically effective UV-B (UV-B_BE) upon a reduction of the stratospheric ozone layer with 15% were calculated and artificially supplemented.In June and September 1998, above- and belowground vegetation samples were taken. Of the dominant species Calamagrostis epigeios and Carex arenaria aboveground mass accumulation, leaf weight (LW), leaf area (LA), specific leaf area (SLA) and tiller number were assessed separate from the remaining vegetation.The results of our study indicate alterations in the vegetation structure due to UV-B supplementation. In June, a significant reduction due to UV-B supplementation in number of tillers and aboveground dry weight per soil area unit was found for C. epigeios. As C. epigeios is the most dominant species of the dune-grassland, these effects indicate a change in vegetation structure due to UV-B enhancement. Indications of UV-B effects on other parameters, such as the number of tillers of C. arenaria and aboveground plant dry weight of the group of species other than C. epigeios and C. arenaria, may also represent changes in vegetation structure. The LA and LW data show the same patterns as the mass accumulation trends. No significant UV-B effects on the SLA of the species or of the total vegetation could be found.Trends in patterns of species dry weight accumulation and partitioning of dry weight between species groups are different in June and September. This may indicate seasonal dependence of UV-B responses. Also, the consistently reducing trend in total and aboveground plant dry weight may indicate deleterious effects of UV-B on total plant matter accumulation. Possible causes of observed trends and effects are discussed.