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 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.     

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     

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.     

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.     

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.     

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.     

Effects of quercetin and enhanced UV-B radiation on the soybean (Glycine max) leaves

The possible ameliorative effects of quercetin on soybean [Glycine max (L.) Merr.] leaves exposed to UV-B radiation were conducted in greenhouse. The symmetrical leaves supplied with quercetin solution (0.2%, 1%) were exposed to UV-B radiation (0, 3.5, 6.5 kJ m⁻² d⁻¹). 0.2% quercetin ameliorated leaf photosynthesis, improved leaf water content (LWC), and decreased lipid oxidation. The unfavorable effect on photosynthetic parameter was displayed in 1% quercetin treatment. The effect of quercetin on phenylalanine ammonia lyase (PAL) activity varied with the quercetin concentration, UV-B radiation intensity and leaf development. In the later development polyphenol oxidase (PPO) activity was increased significantly by quercetin treatments. We suggested that quercetin with suitable concentration could serve as UV-B protective agent partly due to its antioxidant capacity.     

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     

Variability in tolerance to UV-B radiation among Beauveria spp. isolates

Solar radiation, particularly the UV-B component, negatively affects survival of entomopathogenic fungi in the field. In an effort to identify Beauveria spp. isolates with promise for use in biological control settings with high insolation, we examined 53 Beauveria bassiana isolates, 7 isolates of 4 other Beauveria spp. and Engyodontium albus (=Beauveria alba). The origins of these fungi varied widely as to host/substrate and country, but approximately 30% of these isolates were B. bassiana from ticks in Brazil. A preliminary trial with three B. bassiana isolates (Bb 19, CG 310 and CG 481) at several UV-B dosages indicated that 2h of weighted UV-B irradiance at 978mWm(-2) (providing a total dose of 7.04kJm(-2)) allowed separation of isolates into low, medium or high UV-B tolerance. This dose, therefore, was selected as a single dose to compare UV-B tolerances of all 60 Beauveria spp. isolates. There was high variability in tolerance to UV-B radiation among the B. bassiana isolates, ranging from virtually zero tolerance (e.g., Bb 03) to almost 80% tolerance (e.g., CG 228). In addition, surviving B. bassiana conidia demonstrated delayed germination; and this is likely to reduce virulence. Conidia of the other species were markedly more sensitive to UV-B, with E. albus (UFPE 3138) being the least UV-B tolerant. Among B. bassiana isolates originating from 0 degrees to 22 degrees latitudes, those from lower latitudes demonstrated statistically significant greater UV-B tolerances than those isolates from higher latitudes. Isolates from above 22 degrees , however, were unaffected by latitude of origin. A similar analysis based on host type did not indicate a correlation between original host and UV-B tolerance. The identification in this study of several B. bassiana isolates with relatively high UV-B tolerance will guide the selection of isolates for future arthropod microbial control experiments.     

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.     

Effects of supplementary UV-B radiation on development of damping-off in spinach caused by the soil-borne fungusFusarium oxysporum

The effects of UV-B radiation (290–320 nm) on development of damping-off of spinach (Spinacia oleracea) caused by the fungusFusarium oxysporum were examined in a growth cabinet. The incidence of disease greatly increased when experimental plants were grown in visible radiation with supplementary UV-B radiation. This increase was suppressed by increasing the irradiation of visible radiation.Fusarium oxysporum was isolated from the roots of all damping-off plants and the roots of some unwilted plants, indicating that spinach infected with the pathogen did not necessarily suffer from damping-off in 15d. Supplementary UV-B radiation suppressed the increase in growth components such as the number of leaves, the plant height and the fresh weight of aboveground plant parts, but did not affect the fresh weight of roots. The ratio of the number of plants infected with pathogen to the total number of plants was over 80% irrespective of light conditions. It was suggested that the defense response of spinach to this pathogen was greatly influenced by the physiological state of aboveground plant parts resulting from supplementary UV-B radiation.     

Time sequence of the damage to the acceptor and donor sides of photosystem II by UV-B radiation as evaluated by chlorophyll a fluorescence

The effects of ultraviolet-B (UV-B) radiation on photosystem II (PS II) were studied in leaves of Chenopodium album. After the treatment with UV-B the damage was estimated using chlorophyll a fluorescence techniques. Measurements of modulated fluorescence using a pulse amplitude modulated fluorometer revealed that the efficiency of photosystem II decreased both with increasing time of UV-B radiation and with increasing intensity of the UV-B. Fluorescence induction rise curves were analyzed using a mechanistic model of energy trapping. It appears that the damage by UV-B radiation occurs first at the acceptor side of photosystem II, and only later at the donor side.     

Effects of flower size and number on pollinator visitation to wild radish,Raphanus raphanistrum

Plant traits that increase pollinator visitation should be under strong selection. However, few studies have demonstrated a causal link between natural variation in attractive traits and natural variation in visitation to whole plants. Here we examine the effects of flower number and size on visitation to wild radish by two taxa of pollinators over 3 years, using a combination of multiple regression and experimental reductions in both traits. We found strong, consistent evidence that increases in both flower number and size cause increased visitation by syrphid flies. The results for small bees were harder to interpret, because the multiple regression and experimental manipulation results did not agree. It is likely that increased flower size causes a weak increase in small-bee visitation, but strong relationships between flower number and small-bee visitation seen in 2 years of observational studies were not corroborated by experimental manipulation of this trait. Small bees may actually have responded to an unmeasured trait correlated with flower number, or lower small-bee abundances when the flower number manipulation was conducted may have reduced our ability to detect a causal relationship. We conclude that studies using only 1 year, one method, or measuring only one trait may not provide an adequate understanding of the effects of plant traits on pollinator attraction.     

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     

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.     

Effects of UV-B radiation on terrestrial plants and ecosystems: interaction with CO2 enrichment

UV-B radiation is just one of the environmental factors, that affect plant growth. It is now widely accepted that realistic assessment of plant responses to enhanced UV-B should be performed at sufficiently high Photosynthetically Active Radiation (PAR), preferably under field conditions. This will often imply, that responses of plants to enhanced UV-B in the field will be assessed under simultaneous water shortage, nutrient deficiency and variation of temperature. Since atmospheric CO₂ enrichment, global warming and increasing UV-B radiation represent components of global climatic change, interactions of UV-B with CO₂ enrichment and temperature are particularly relevant. Only few relevant UV-B× CO₂ interaction studies have been published. Most of these studies refer to greenhouse experiments. We report a significant CO₂ × UV-B interaction for the total plant dry weight and root dry weight of the C₃-grass Elymus athericus. At elevated CO₂ (720 mol mol^-1, plant growth was much less reduced by enhanced UV-B than at ambient atmospheric CO₂ although there were significant (positive) CO₂ effects and (negative) UV-B effects on plant growth. Most other CO₂ × UV-B studies do not report significant interactions on total plant biomass. This lack of CO₂ × UV-B interactions may result from the fact that primary metabolic targets for CO₂ and UVB are different. UV-B and CO₂ may differentially affect plant morphogenetic parameters: biomass allocation, branching, flowering, leaf thickness, emergence and senescence. Such more subtle interactions between CO₂ and UV-B need careful and long term experimentation to be detected. In the case of no significant CO₂× UV-B interactions, combined CO₂ and UV-B effects will be additive. Plants differ in their response to CO₂ and UV-B, they respond in general positively to elevated CO₂ and negatively to enhanced UV-B. Moreover, plant species differ in their responsiveness to CO₂ and UV-B. Therefore, even in case of additive CO₂ and UV-B effects, plant competitive relationships may change markedly under current climatic change with simultaneous enhanced atmospheric CO₂ and solar UV-B radiation.     

Increased solar UV-B radiation may reduce infection by arbuscular mycorrhizal fungi (AMF) in dune grassland plants: evidence from five years of field exposure

An area of coastal dune grassland, dominated by the gramineous species Calamagrostis epigeios and Carex arenaria, was exposed to enhanced levels of UV-B radiation during a five year period. These species showed reduced AM-fungal infection percentages in their roots. In C. epigeios AM infection was reduced by 18%, C. arenaria showed a reduction by 20%. The major effect of enhanced UV-B on AM associations was a reduction of the number of arbuscules. This indicates a reduction in the exchange of nutrients between the symbionts. Since the effect of UV-B on AM associations may result from altered flavonoid levels in the root exudates of the host plants, flavonoid levels in the roots were investigated. No detectable flavonoid concentrations were found in the roots of C. epigeios and C. arenaria. Less effective AM associations can have pronounced negative effects on biodiversity and nutrient dynamics of the dune grassland ecosystem. The possible mechanisms causing these indirect effects of elevated UV-B on below ground AM associations are discussed. We conclude that UV-B induced changes in plant hormone levels are more likely to be the mechanism reducing AMF infection than UV-B induced alterations in flavonoid concentrations in the root exudates of the host plant.