Summary Experimental grassland communities (turves) were exposed to supplemental levels of UV‐B radiation (280–315 nm) at an outdoor facility, under treatment arrays of cellulose diacetate‐filtered fluorescent lamps which also produce UV‐A radiation (315–400 nm). Control treatments consisted of arrays of polyester‐filtered lamps, which allowed for exposure to UV‐A radiation alone, and arrays of unenergized lamps allowing for exposure to ambient levels of solar radiation.
Summary 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.
Abstract It has been suggested that field experiments which increase UV‐B irradiation by a fixed amount irrespective of ambient light conditions (‘square‐wave’), may overestimate the response of photosynthesis to UV‐B irradiation. In this study, pea ( Pisum sativum L.) plants were grown in the field and subjected to a modulated 30% increase in ambient UK summer UV‐B radiation (weighted with an erythemal action spectrum) and a mild drought treatment. UV‐A and ambient UV control treatments were also studied. There were no significant effects of the UV‐B treatment on the in situ CO 2 assimilation rate throughout the day or on the light‐saturated steady‐state photosynthesis. This was confirmed by an absence of UV‐B effects on the major components contributing to CO 2 assimilation; photosystem II electron transport, ribulose 1,5‐bisphosphate regeneration, ribulose 1,5‐bisphosphate carboxylase/oxygenase carboxylation, and stomatal conductance. In addition to the absence of an effect on photosynthetic activities, UV‐B had no significant impact on plant biomass, leaf area or partitioning. UV‐B exposure increased leaf flavonoid content. The UV‐A treatment had no observable effect on photosynthesis or productivity. Mild drought resulted in reduced biomass, a change in partitioning away from shoots to roots whilst maintaining leaf area, but had no observable effect on photosynthetic competence. No UV‐B and drought treatment interactions were observed on photosynthesis or plant biomass. In conclusion, a 30% increase in UV‐B had no effects on photosynthetic performance or productivity in well‐watered or droughted pea plants in the field.
Decomposing Quercus robur L. leaf litter was exposed for 64 weeks at an outdoor facility to supplemental levels of UV-B radiation (280-315 nm) under treatment arrays of cellulose diacetate-filtered fluorescent lamps which also produce UV-A radiation (315-400 nm). Litter was also exposed to UV-A radiation alone under control arrays of polyester-filtered lamps and to ambient levels of solar radiation under arrays of unenergised lamps. The treatment corresponded to a 30% elevation above the ambient erythemally-weighted level of UV-B radiation. Litter was sampled after 11, 39 and 64 weeks and was examined for differences in mass loss, decomposition constants (k), chemical composition and the abundances of saprotrophic fungi. No effects of UV radiation on k values were recorded, but after 11 weeks, percentage mass loss of litter exposed to UV-B radiation under treatment arrays was 3% lower than under control arrays and 2% lower than under ambient arrays. After 39 weeks, litters exposed to UV-A radiation under control arrays had 10% lower total nitrogen contents and 13% higher C:N ratios than those litters exposed beneath ambient arrays. At the last sampling, litters exposed to supplemental UV-B radiation had 5% higher carbon contents than those under ambient arrays. A 2.4-fold increase in the frequency of lamina particles of litter that were uncolonised by fungi was recorded in litter exposed to UV-B radiation under treatment arrays, compared to ambient arrays. The abundances of the saprotrophic fungi Cladosporium spp. and Acremonium persicinum (Nicot) W. Gams were decreased by 50% and 91%, respectively, under UV-B treatment arrays compared to ambient arrays, and the abundance of coelomycete conidiomata recorded on leaves was increased by 12% under treatment arrays, compared to ambient. Dactylella spp. were not recorded on litter exposed to UV-A radiation under control arrays and UV-A radiation applied under control and treatment arrays apparently increased the abundance of Polyscytalum fecundissimum Riess. on litter. UV radiation had fewer effects on the abundances of decomposer fungi that develop in leaf tissues than it did on those that develop on leaf litter surfaces. We conclude that increased fluxes of UV-B radiation as a result of stratospheric ozone depletion will have subtle but wide-ranging impacts on the decomposition of litters in oak woodlands.
Abstract Quercus robur L. saplings were exposed in an outdoor experiment to supplemental levels of UV‐8 (280–315 nm) radiation using treatment arrays of cellulose diacetate‐filtered fluorescent lamps that also produce UV‐A (315–400 nm) radiation. Saplings were also exposed to UV‐A radiation alone using control arrays of the same lamps filtered with polyester and to ambient levels of radiation, using arrays of unenergized lamps. The UV‐B treatment was modulated to maintain a 30% elevation above the ambient level of UV‐B radiation, measured by a broad‐band sensor weighted with an erythemal action spectrum. Saplings exposed to UV‐B radiation beneath treatment arrays developed thicker leaves than those beneath ambient and control arrays. Despite the fact that supplemental levels of UV‐A radiation were only a small percentage of ambient levels, apparent UV‐A effects were also recorded. Significant increases in sapling height, lammas shoot length and herbivory by chewing insects were observed under treatment and control arrays, relative to ambient, but there were no differences between the responses of saplings under treatment and control. These data imply that supplemental UV‐A radiation or other effects associated with energised lamps can significantly affect plant growth parameters and herbivory in outdoor studies. We conclude that the results from current outdoor UV‐B supplementation experiments that lack control exposures using polyester‐filtered lamps need to be interpreted with caution and that future supplementation experiments should include appropriate controls.
Abstract The release of certain man‐made chemicals has led to recurrent, seasonal destruction of ozone in the upper atmosphere, allowing more solar radiation in the UV‐B waveband to reach the Earth. Consequently, many amphibians may suffer increased exposure to UV‐B at various stages in their lives. Embryonic stages of species which spawn in the spring, in shallow, open water, are at high risk of increased exposure. We exposed newly fertilized eggs of one such species, Rana temporaria L., to sunlight with and without supplemental UV‐B. We used outdoor arrays of lamps to simulate the increase in UV‐B which might result from previously documented ozone depletion. From immediately after fertilization to when hatchlings began feeding, ambient solar UV‐B, weighted for DNA‐damaging potential, was supplemented by ≈ 81% in 1995 and 113% in 1996. These levels of supplementation approximated the increase in solar UV‐B expected to result from losses of 21% and 25%, respectively, of the total amount of ozone in the atmospheric column, relative to pre‐ozone‐depletion values. We found no evidence that these additions of UV‐B radiation increased the incidence of mortality or overt developmental abnormality among embryos. We stress the need for appropriate dosimetry in studies of effects of UV‐B on organisms.
Summary Quercus robur saplings were exposed at an outdoor facility in the UK to supplemental levels of UV‐B radiation (280–315 nm) under arrays of cellulose diacetate‐filtered fluorescent lamps which also produced UV‐A radiation (315–400 nm). Saplings were also exposed to supplemental UV‐A radiation under arrays of polyester‐filtered lamps and to ambient levels of solar radiation under arrays of unenergized lamps. The UV‐B treatment was modulated to maintain a 30% elevation above the ambient level of erythemally weighted UV‐B radiation. Naturally occurring infections by oak powdery mildew ( Microsphaera alphitoides ) were more abundant, and developed more rapidly, on lammas leaves of saplings which were exposed to treatment levels of UV‐B radiation than on leaves of saplings exposed to supplemental UV‐A or to ambient levels of solar radiation over 12 weeks in summer and autumn 1996. An analysis of leaf photosynthetic capacities revealed that M. alphitoides infection reduced the quantum efficiency of photosystem (PS) II by 14% at moderate irradiance. Although there was no direct effect of UV‐B radiation on PSII photochemistry, exposure of saplings to supplemental UV‐A radiation under polyester‐filtered lamps resulted in a 17.5% decrease in PSII quantum efficiency, compared with saplings exposed to ambient solar radiation. The results from our study suggest that photosynthesis of Q. robur may be constrained by exposure to UV‐B radiation in the natural environment through impacts on the abundance of M. alphitoides .
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