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.     

A thirty percent increase in UV-B has no impact on photosynthesis in well-watered and droughted pea plants in the field

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 CO₂ 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 CO₂ 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.     

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.     

Herbivory on the pedunculate oak along an urbanization gradient in Europe: Effects of impervious surface,local tree cover,and insect feeding guild

Urbanization is an important driver of the diversity and abundance of tree‐associated insect herbivores, but its consequences for insect herbivory are poorly understood. A likely source of variability among studies is the insufficient consideration of intra‐urban variability in forest cover. With the help of citizen scientists, we investigated the independent and interactive effects of local canopy cover and percentage of impervious surface on insect herbivory in the pedunculate oak (Quercus robur L.) throughout most of its geographic range in Europe. We found that the damage caused by chewing insect herbivores as well as the incidence of leaf‐mining and gall‐inducing herbivores consistently decreased with increasing impervious surface around focal oaks. Herbivory by chewing herbivores increased with increasing forest cover, regardless of impervious surface. In contrast, an increase in local canopy cover buffered the negative effect of impervious surface on leaf miners and strengthened its effect on gall inducers. These results show that—just like in non‐urban areas—plant–herbivore interactions in cities are structured by a complex set of interacting factors. This highlights that local habitat characteristics within cities have the potential to attenuate or modify the effect of impervious surfaces on biotic interactions.     

UV‐B effect on Quercus robur leaf litter decomposition persists over four years

The effects of elevated UV‐B (280–315 nm) radiation on the long‐term decomposition of Quercus robur leaf litter were assessed at an outdoor facility in the UK by exposing saplings to elevated UV‐B radiation (corresponding to a 30% increase above the ambient level of erythemally weighted UV‐B, equivalent to that resulting from a c. 18% reduction in ozone column) under arrays of cellulose diacetate‐filtered fluorescent UV‐B lamps that also produced UV‐A radiation (315–400 nm). Saplings were also exposed to elevated UV‐A radiation alone under arrays of polyester‐filtered fluorescent lamps and to ambient solar radiation under arrays of nonenergized lamps. After 8 months of irradiation, abscised leaves were placed into litter bags and allowed to decompose in the litter layer of a mixed deciduous woodland for 4.08 years. The dry weight loss of leaf litter from saplings irradiated with elevated UV‐B and UV‐A radiation during growth was 17% greater than that of leaf litter irradiated with elevated UV‐A radiation alone. Annual fractional weight loss of litter (k), and the estimated time taken for 95% of material to decay (3/k) were respectively increased and decreased by 27% for leaf litter exposed during growth to elevated UV‐B and UV‐A radiation, relative to that exposed to UV‐A alone. The present data corroborate those from a previous study indicating that UV‐B radiation applied during growth accelerates the subsequent decomposition of Q. robur leaf litter in soil, but indicate that this effect persists for over four years after abscission.     

The Description of Clones of Quercus robur L. and Q. petraea (Matt.) Liebl. with Microsatellites and AFLP in an Ancient Woodland

Abstract: In densely populated areas autochthonous Quercus robur L. (pedunculate oak) and Q. petraea (Matt.) Liebl. (sessile oak) (Fagaceae) populations have been maintained as ancient devastated woodlands. The continuous cutting, grazing and resprouting of such woodlands has enabled the development of clonal structures. For conservation purposes, an analysis of the actual number, size and spatial distribution of clones is necessary, especially when there is an interest in genetic variation of the population. This study describes for the first time - based on microsatellite and AFLP™ analysis - clones in an autochthonous mixed Q. robur and Q. petraea population that has been coppiced and grazed for several centuries. Based on six microsatellite loci and 69 polymorphic AFLP markers, only 14 unique genotypes were detected in a plot that consisted of 80 trees. Clones were observed for both Q. robur and Q. petraea. The largest clone diameters were observed for Q. robur, with distances up to 5.8 m. The observed clone sizes may indicate the age of the trees.     

He-Ne激光和增强UV-B辐射对小麦幼苗类囊体捕光色素的影响

采用5mW.mm-2He-Ne激光辐照、10.08kJ.m-2d-1UV-B辐射及二者组合对冬小麦幼苗进行处理。通过测定叶绿体捕光色素含量和色素蛋白组成的变化,进一步探讨He-Ne激光对增强UV-B辐射后小麦幼苗类囊体捕光色素损伤的修复效应。循环处理小麦幼苗4d,利用90%乙醇和80%丙酮分别提取各处理组小麦幼苗叶片中的叶绿素,通过纸层析和分光光度法检测捕光色素含量的变化,并探讨不同处理对叶绿素与蛋白质结合牢固性的影响。利用柱层析法测定色素蛋白的主要成分。研究表明:与对照组相比,增强UV-B辐照后小麦幼苗捕光色素总含量降低了17.76%,叶绿素和蛋白质结合牢固度显著降低,色素蛋白的组成也发生变化,D1和D2蛋白质条带消失;而一定剂量He-Ne激光辐照可使增强UV-B辐射后的叶绿体色素含量增加约10.64%,但仍低于ck组约8.12%,叶绿素和蛋白质结合牢固度也显著高于B组,色素蛋白的组成与对照组相似。因此,低剂量的He-Ne激光辐照对增强UV-B辐射后小麦幼苗类囊体捕光色素的损伤具有促进修复效应。     

QTL analysis of leaf morphological characters in a Quercus robur full-sib family (Q. robur × Q. robur ssp. slavonica)

The distinction between white oak species (section Quercus sensu stricto ) is largely based on leaf morphological characters. There is, however, considerable within-species variation and no single species-diagnostic character, possibly due to phenotypic plasticity and/or underlying genetic variation. The aim of the present study was to identify quantitative trait loci (QTL) underlying the high within-species variation for leaf morphological characters in an F₁ full-sib family derived from a cross between Q. robur and Q. robur ssp. slavonica . In accordance with an earlier QTL mapping study in an intraspecific Q. robur full-sib family, polygenic inheritance was detected for leaf morphological characters that are used to discriminate between the species Quercus robur and Q. petraea . QTLs were distributed over ten linkage groups, showed a moderate effect in terms of phenotypic variance explained (PVE) in the mapping pedigree (3.6–9.6%), but accounted for a considerable amount of the parental differences. Co-localisation of QTLs on the same linkage group in different genetic backgrounds was found for the number and percentage of intercalary veins (NV, PV) on linkage group 3 and for NV on linkage group 5, revealing a high congruence in the relative QTL positions. The generally low correspondence of the other QTLs in the different mapping pedigrees may be an effect of the genetic background and of the environment. In conclusion, leaf morphological characters were found to be under polygenic control, and a comparison to earlier published results led to the identification of two QTLs that were stable across different genetic backgrounds.     

Influence of ultraviolet-B (UV-B) radiation on photosynthetic and growth characteristics in field-grown cassava (Manihot esculentum Crantz)

The effects of ultraviolet-B (UV-B between 290 and 320 nm) on photosynthesis and growth characteristics were investigated in field grown cassava (Manihot esculentum Crantz). Plants were grown at ambient and ambient plus a 5.5kJ m^?2 d^?1 supplementation of UV-B radiation for 95 d. The supplemental UV-B fluence used in this experiment simulated a 15% depletion in stratospheric ozone at the equator (0°N). Carbon dioxide exchange, oxygen evolution, and the ratio of variable to maximum fluorescence (F_v/F_m) were determined for fully expanded leaves after 64–76 d of UV-B exposure. AH plants were harvested after 95 d of UV-B exposure, assayed for chlorophyll and UV-B absorbing compounds, and separated into leaves, petioles, stems and roots. Exposure to UV-B radiation had no effect on in situ rates of photosynthesis or dark respiration. No difference in the concentration of UV-B absorbing compounds was observed between treatments. A 2-d daytime diurnal comparison of F_v to F_m ratios indicated a significant decline in F_v/F_m ratios and a subsequent increase in photoinhibition under enhanced UV-B radiation if temperature or PPF exceeded 35°C or 1800μmol m^?2 s^?1, respectively. However, UV-B effects on fluorescence kinetics appeared to be temporal since maximal photosynthetic rates as determined by oxygen evolution at saturated CO₂ and PPF remained unchanged. Although total biomass was unaltered with UV-B exposure, alterations in the growth characteristics of cassava grown with supplemental UV-B radiation are consistent with auxin destruction and reduced apical dominance. Changes in growth included an alteration of biomass partitioning with a significant increase in shoot/root ratio noted for plants receiving supplemental UV-B radiation. The increase in shoot/root ratio was due primarily to a significant decrease in root weight (–32%) with UV-B exposure. Because root production determines the harvest-able portion of cassava, UV-B radiation may still influence the yield of an important tropical agronomic species, even though photosynthesis and total dry biomass may not be directly affected.     

Effect of Enhanced UV-B Radiation on Polyamine Content and Membrane Permeability in Cucumber Leaves

Cucumber plants (Cucumis sativus L., cv. Jingchun 3) were grown in a greenhouse under PAR illumination of 400–600 mol/(m² s) at 30/15°C (day/night) temperature. Two enhanced biologically effective UV-B radiation levels per day were applied: 8.82 kJ/m² (T₁) and 12.6 kJ/m² (T₂). Cucumber seedlings were irradiated 7 h per day for 25 days under T₁ and T₂. A comparative study of growth, membrane permeability, and polyamine content in cucumber leaves under T₁ and T₂ treatments was conducted. UV-B radiation resulted in the dose-dependent decrease in leaf area, dry weight of foliage, and plant height. The T₁ and T₂ treatments caused an increase in the contents of putrescine, spermine, and spermidine. However, the total polyamine content declined slightly when electrolyte leakage increased dramatically on the 18th day of treatment, especially after T₂ treatment. It can be concluded that polyamine accumulation in the cucumber leaves is an adaptive mechanism to the stress caused by UV-B radiation.     

Polyphenol deposition in leaf hairs of Olea europaea (Oleaceae) and Quercus ilex (Fagaceae)

The subcellular localization (cytoplasm, vacuoles, cell walls) of polyphenol compounds during the development of the multicellular nonglandular leaf hairs of Olea europaea (scales) and Quercus ilex (stellates), was investigated. Hairs of all developmental stages were treated with specific inducers of polyphenol fluorescence, and the bright yellow-green fluorescence of individual hairs was monitored with epifluorescence microscopy. During the early ontogenetic stages, bright fluorescence was emitted from the cytoplasm of the cells composing the multicellular shield of the scales of O. europaea. Transmission electron micrographs of the same stages showed that these cells possessed poor vacuolation and thin cell walls. The nucleus of these cells may be protected against ultraviolet-B radiation damage. The progressive vacuolation that occurred during maturation was followed by a shifting of the bright green-yellow fluorescence from the perinuclear region and the cytoplasm to the cell walls. The same trends were observed during the development of the nonglandular stellate hairs of Quercus ilex, in which maturation was also accompanied by a considerable secondary thickening of the cell walls. Despite the differences in morphology, high concentrations of polyphenol compounds are initially located mainly in the cytoplasm of the developing nonglandular hairs, and their deposition on the cell walls takes place during the secondary cell wall thickening. These structural changes during the development of the leaf hairs make them a very effective barrier against abiotic (uv-B radiation) and probably biotic (pathogenic) stresses.     

The effect of elevated UV-B radiation on herbivory of pea byAutographa gamma

Leaves exposed to above-ambient fluxes of ultraviolet-B (UV-B) radiation commonly contain increased concentrations of phenolic compounds which may influence herbivores. However, the hypothesis that elevated UV-B modifies herbivory, whether mediated by phenolics or other plant constituents, has rarely been studied experimentally. We investigated the responses of the mothAutographa gamma L. (Lepidoptera: Noctuidae) to pea (Pisum sativum L.) grown at a range of plant-effective UV-B fluxes. Although total phenolics did increase significantly with increasing UV-B, this change had little deleterious effect on the 5th instar larvae ofA. gamma. However, tissue nitrogen also increased with increasing UV-B. Increased nitrogen was correlated with an increase in the efficiency with which larvae utilized their food and in larval growth rate, but in a reduction in the amount of plant material consumed. The apparently major role of nitrogen in determining herbivore responses to changing UV-B demonstrates the risks in predicting such responses soley on the basis of changes in phenolics and other secondary metabolites.     

Solar UV-B radiation affects below-ground parameters in a fen ecosystem in Tierra del Fuego, Argentina: implications of stratospheric ozone depletion

Stratospheric ozone depletion caused by the release of chlorofluorocarbons is most pronounced at high latitudes, especially in the Southern Hemisphere (including the so‐called ‘ozone hole’). The consequent increase in solar ultraviolet‐B radiation (UV‐B, 280–315 nm) reaching the earth's surface may cause a variety of alterations in terrestrial ecosystems. Most effects might be expected to occur above‐ground since sunlight does not penetrate effectively below‐ground. Here, we demonstrate that solar UV‐B radiation in a fen in Tierra del Fuego (Argentina), where the ozone hole passes overhead several times during the Austral spring, is causing large changes of below‐ground processes of this ecosystem. During the third and fourth year of a manipulative field experiment, we investigated root systems in these plots and found that when the ambient solar UV‐B radiation was substantially reduced, there was a 30% increase in summer root length production and as much as a threefold decrease in already low symbiotic mycorrhizal colonization frequency of the roots compared with plots receiving near‐ambient solar UV‐B. There was also an apparent shift toward older age classes of roots under reduced solar UV‐B. Such large changes in root system behaviour may have decided effects on competition and other ecological interactions in this ecosystem.     

Effect of UV-B radiation on the shoot dry matter production and stable carbon isotope composition of two Arabidopsis thaliana genotypes

An application of stable carbon isotope analysis to the mechanistic interpretation of ultraviolet-B (UV-B) effects on growth inhibition is described that is particularly useful for small plants such as Arabidopsis thaliana that are not well suited for gas exchange studies. Many investigators use tissue δ¹³C, relative abundance of ¹³C and ¹²C, as a proxy for water use efficiency and as an indicator of environmental effects on stomatal behaviour and on photosynthesis during growth. Discrimination against ¹³C is enhanced by both high stomatal conductance and damage to photosynthetic machinery. Because the thinning of the stratospheric ozone layer is permitting more UV-B to enter the biosphere, the mechanisms of action of UV-B radiation on plants are of particular current interest. Arabidopsis thaliana wild-type Landsberg erecta (L er ) and the UV-B-sensitive mutant fah I , deficient in UV-absorbing sinapate esters, were grown in a controlled environment and exposed to UV-B^BE doses of 0 or 6–7 kJ m⁻² day⁻¹. UV-B exposure decreased dry matter production and δ¹³C in both genotypes, but growth inhibition was generally greater in fah I than in L er . The fah I mutant also had less leaf greenness than L er . Changes in leaf tissue δ¹³C were detected before growth inhibition and were evident in treatments of both genotypes that did not cause marked growth effects. This suggests that the effects of UV-B contributing to increased carbon isotope discrimination in L er may have been primarily associated with high stomatal conductance, and in fah I with both high stomatal conductance and damage to photosynthetic machinery.     

Isoprene synthase activity and its relation to isoprene emission in Quercus robur L. leaves

Pedunculate oak ( Quercus robur L.) is known as a strong isoprene (2-methyl-1,3-butadiene) emitter. Diurnal changes in isoprene emission were determined by branch enclosure measurements. In contrast to the diurnal cycle in emission rates, specific isoprene synthase activity in the leaves remained unchanged. Based on in vitro enzyme activity and its temperature dependency, an isoprene synthesis capacity at specific leaf temperatures was calculated. The comparison of these 'leaf temperature-dependent enzyme capacities' and the measured emission rates revealed that the enzyme activity of isoprene synthase is comparable to the observed isoprene emission rates. In addition, variation in the isoprene synthase activity of the leaves due to changes in light intensity during leaf development was investigated. A 50% reduction of light intensity by shading of single branches reduced isoprene synthase activity by ≈ 60% compared with full sunlight. The calculation of isoprene synthesis capacities based on enzymatic data obtained under optimum reaction conditions, corrected for actual leaf temperature and related to leaf surface area, provides a sound basis for predicting the isoprene emission potential of plants.     

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m^?2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO₂ concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bis-phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO₂-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO₂ assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO₂ assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO₂ assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO₂ assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO₂ assimilation.     

Impacts of urbanization on insect herbivory and plant defences in oak trees

Systematic comparisons of species interactions in urban versus rural environments can improve our understanding of shifts in ecological processes due to urbanization. However, such studies are relatively uncommon and the mechanisms driving urbanization effects on species interactions (e.g. between plants and insect herbivores) remain elusive. Here we investigated the effects of urbanization on leaf herbivory by insect chewers and miners associated with the English oak Quercus robur by sampling trees in rural and urban areas throughout most of the latitudinal distribution of this species. In performing these comparisons, we also controlled for the size of the urban areas (18 cities) and gathered data on CO₂ emissions. In addition, we assessed whether urbanization affected leaf chemical defences (phenolic compounds) and nutritional traits (phosphorus and nitrogen), and whether such changes correlated with herbivory levels. Urbanization significantly reduced leaf chewer damage but did not affect leaf miners. In addition, we found that leaves from urban locations had lower levels of chemical defences (condensed and hydrolysable tannins) and higher levels of nutrients (nitrogen and phosphorus) compared to leaves in rural locations. The magnitude of urbanization effects on herbivory and leaf defences was not contingent upon city size. Importantly, while the effects of urbanization on chemical defences were associated with CO₂ emissions, changes in leaf chewer damage were not associated with either leaf traits or CO₂ levels. These results suggest that effects of urbanization on herbivory occur through mechanisms other than changes in the plant traits measured here. Overall, our simultaneous assessment of insect herbivory, plant traits and abiotic correlates advances our understanding of the main drivers of urbanization effects on plant–herbivore interactions.