Biological Spectral Weighting Functions Research Articles

How Realistically Does Outdoor UV-B Supplementation with Lamps Reflect Ozone Depletion: An Assessment of Enhancement Errors

Limitations in the realism of currently available lamps mean that enhancement errors in outdoor experiments simulating UV-B radiation effects of stratospheric ozone depletion can be large. Here, we assess the magnitude of such errors at two Finnish locations, during May and June, under three cloud conditions. First we simulated solar radiation spectra for normal, compared with 10% and 20% ozone depletion, and convoluted the daily integrated solar spectra with eight biological spectral weighting functions (BSWFs) of relevance to effects of UV on plants. We also convoluted a measured spectrum from cellulose-acetate filtered UV-B lamps with the same eight BSWFs. From these intermediate results we calculated the enhancement errors. Differences between locations and months were small, cloudiness had only a minor effect. This assessment was based on the assumption that no extra enhancement compensating for shading of UV radiation by lamp frames is performed. Under this assumption errors between spectra are due to differences in the UV-B effectiveness rather than differences in the UV-A effectiveness. Hence, conclusions about plant growth from past UV-supplementation experiments should be valid. However, interpretation of the response of individual physiological processes is less secure, so results from some field experiments with lamps might need reassessment.

Assessment of UV Biological Spectral Weighting Functions for Phenolic Metabolites and Growth Responses in Silver Birch Seedlings

In research concerning stratospheric ozone depletion, action spectra are used as biological spectral weighting functions (BSWFs) for describing the effects of UV radiation on plant responses. Our aim was to evaluate the appropriateness of six frequently used BSWFs that differ in effectiveness with increasing wavelength. The evaluation of action spectra was based on calculating the effective UV radiation doses according to 1-2) two formulations of the generalized plant action spectrum, 3) a spectrum for ultraviolet induced erythema in human skin, 4) a spectrum for the accumulation of a flavonol in Mesembryanthemum crystallinum, 5) a spectrum for DNA damage in alfalfa seedlings and 6) the plant growth action spectrum. We monitored effects of UV radiation on the concentration of individual UV absorbing metabolites and chlorophyll concentrations in leaves and growth responses of silver birch (Betula pendula) seedlings. Experiments were conducted outdoors using plastic films attenuating different parts of the UV spectrum. Chlorophyll concentrations and growth were not affected by the UV treatments. The response to UV radiation varied between and within groups of phenolics. In general, the observed responses of phenolic groups and individual flavonoids were best predicted by action spectra extending into the UV-A region with moderate effectiveness.

Metabolite specific effects of solar UV‐A and UV‐B on alder and birch leaf phenolics

AbstractWe measured the concentrations of ultraviolet (UV)‐absorbing phenolics varying in response to exclusion of either solar UV‐B or both solar UV‐A and UV‐B radiations in leaves of grey alder (Alnus incana) and white birch (Betula pubescens) trees under field conditions. In alder leaves 20 and in birch leaves 13 different phenolic metabolites were identified. The response to UV exclusion varied between and within groups of phenolics in both tree species. The changes in concentration for some metabolites suggest effects of only UV‐A or UV‐B, which band being effective depending on the metabolite. For some other metabolites, the results indicate that UV‐A and UV‐B affect concentrations in the same direction, while for a few compounds there was evidence suggesting opposite effects of UV‐A and UV‐B radiation. Finally, the concentration of some phenolics did not significantly respond to solar UV. We observed only minor effects on the summed concentration of all determined phenolic metabolites in alder and birch leaves, thus indicating that measuring only total phenolics concentration may not reveal the effects of radiation. Here, we show that the appropriate biological spectral weighting functions for plant‐protective responses against solar UV radiation extend in most cases – but not always – into the UV‐A region and more importantly that accumulation of different phenolic metabolites follows different action spectra. This demonstrates under field conditions that some of the implicit assumptions of earlier research simulating ozone depletion and studying the effects of UV radiation on plant secondary metabolites need to be reassessed.

Terrestrial ecosystems, increased solar ultraviolet radiation, and interactions with other climate change factors

There have been significant advances in our understanding of the effects of UV-B radiation on terrestrial ecosystems, especially in the description of mechanisms of plant response. A further area of highly interesting research emphasizes the importance of indirect UV radiation effects on plants, pathogens, herbivores, soil microbes and ecosystem processes below the surface. Although photosynthesis of higher plants and mosses is seldom affected by enhanced or reduced UV-B radiation in most field studies, effects on growth and morphology (form) of higher plants and mosses are often manifested. This can lead to small reductions in shoot production and changes in the competitive balance of different species. Fungi and bacteria are generally more sensitive to damage by UV-B radiation than are higher plants. However, the species differ in their UV-B radiation sensitivity to damage, some being affected while others may be very tolerant. This can lead to changes in species composition of microbial communities with subsequent influences on processes such as litter decomposition. Changes in plant chemical composition are commonly reported due to UV-B manipulations (either enhancement or attenuation of UV-B in sunlight) and may lead to substantial reductions in consumption of plant tissues by insects. Although sunlight does not penetrate significantly into soils, the biomass and morphology of plant root systems of plants can be modified to a much greater degree than plant shoots. Root mass can exhibit sizeable declines with more UV-B. Also, UV-B-induced changes in soil microbial communities and biomass, as well as altered populations of small invertebrates have been reported and these changes have important implications for mineral nutrient cycling in the soil. Many new developments in understanding the underlying mechanisms mediating plant response to UV-B radiation have emerged. This new information is helpful in understanding common responses of plants to UV-B radiation, such as diminished growth, acclimation responses of plants to UV-B radiation and interactions of plants with consumer organisms such as insects and plant pathogens. The response to UV-B radiation involves both the initial stimulus by solar radiation and transmission of signals within the plants. Resulting changes in gene expression induced by these signals may have elements in common with those elicited by other environmental factors, and generate overlapping functional (including acclimation) responses. Concurrent responses of terrestrial systems to the combination of enhanced UV-B radiation and other global change factors (increased temperature, CO2, available nitrogen and altered precipitation) are less well understood. Studies of individual plant responses to combinations of factors indicate that plant growth can be augmented by higher CO2 levels, yet many of the effects of UV-B radiation are usually not ameliorated by the elevated CO2. UV-B radiation often increases both plant frost tolerance and survival under extreme high temperature conditions. Conversely, extreme temperatures sometimes influence the UV-B radiation sensitivity of plants directly. Plants that endure water deficit stress effectively are also likely to be tolerant of high UV-B flux. Biologically available nitrogen is exceeding historical levels in many regions due to human activities. Studies show that plants well supplied with nitrogen are generally more sensitive to UV-B radiation. Technical issues concerning the use of biological spectral weighting functions (BSWFs) have been further elucidated. The BSWFs, which are multiplication factors assigned to different wavelengths giving an indication of their relative biological effectiveness, are critical to the proper conduct and interpretation of experiments in which organisms are exposed to UV radiation, both in the field and in controlled environment facilities. The characteristics of BSWFs vary considerably among different plant processes, such as growth, DNA damage, oxidative damage and induction of changes in secondary chemicals. Thus, use of a single BSWF for plant or ecosystem response is not appropriate. This brief review emphasizes progress since the previous report toward the understanding of solar ultraviolet radiation effects on terrestrial systems as it relates to ozone column reduction and the interaction of climate change factors.

The Use of Wavelength‐selective Plastic Cladding Materials in Horticulture: Understanding of Crop and Fungal Responses Through the Assessment of Biological Spectral Weighting Functions

Plant responses to light spectral quality can be exploited to deliver a range of agronomically desirable end points in protected crops. This can be achieved using plastics with specific spectral properties as crop covers. We have studied the responses of a range of crops to plastics that have either (a) increased transmission of UV compared with standard horticultural covers, (b) decreased transmission of UV or (c) increased the ratio of red (R) : far-red (FR) radiation. Both the UV-transparent and R : FR increasing films reduced leaf area and biomass, offering potential alternatives to chemical growth regulators. The UV-opaque film increased growth, but while this may be useful in some crops, there were trade-offs with elements of quality, such as pigmentation and taste. UV manipulation may also influence disease control. Increasing UV inhibited not only the pathogenic fungus Botrytis cinerea but also the disease biocontrol agent Trichoderma harzianum. Unlike B. cinerea, T. harzianum was highly sensitive to UV-A radiation. These fungal responses and those for plant growth in the growth room and the field under different plastics are analyzed in terms of alternative biological spectral weighting functions (BSWF). The role of BSWF in assessing general patterns of response to UV modification in horticulture is also discussed.

Field Testing of Biological Spectral Weighting Functions for Induction of UV‐absorbing Compounds in Higher Plants†¶

ABSTRACTAction spectra are typically used as biological spectral weighting functions (BSWF) in biological research on the stratospheric ozone depletion issue. Despite their critical role in determining the amount of UV supplied in experiments, there has been only limited testing of different functions under realistic field conditions. Here, we calculate effective radiation according to five published BSWF and evaluate the appropriateness of these BSWF in representing the induction of UV‐absorbing compounds. Experiments were carried out in the field using both ultraviolet‐B radiation (280–320 nm) supplementation and selective filtering of solar UV radiation. For the four species tested, BSWF that extend into the ultraviolet‐A radiation (320–400 nm) (UV‐A) with moderate effectiveness best represented the observed results. When compared with the commonly used generalized plant response, these BSWF suggest that simulations of ozone depletion will require more radiation than in the past experiments. However, they imply lower radiation supplements than a new plant growth BSWF that has a greater emphasis on UV‐A wavelengths.

Ecosystem UV-B experiments in terrestrial communities: a review of recent findings and methodologies

Ecosystem-level UV-B experiments have revealed a variety of responses in terrestrial communities. These include decreases in the growth of some plant species (usually small changes but occasionally >25%), both decreases and increases in herbivory, occasional altered decomposition patterns, changes in populations of fungi and invertebrates, and morphological changes in the growth patterns of mosses. These field experiments have been pursued both with solar UV-B exclusion using selective filters and with supplementation of UV-B using lamps. For UV-B-exclusion studies it is critical that the UV-B-excluding and UV-B-transmitting filters pass the same amount of photosynthetically active radiation (PAR, 400–700 nm) and also infrared. Canopy photosynthesis models indicate that even small differences in PAR transmittance can, under many circumstances, have significant effects on plant carbon gain. It is also important that filters in UV-B-exclusion studies allow precipitation to pass uniformly to the plots unless there is some form of irrigation underneath the filters. For UV-B-supplementation studies with lamps, many factors are involved in the realism of the ozone depletion simulation. We believe the major lamp-system error is excessive supplementation of UV-B by timer-controlled lamps when weather conditions (primarily clouds) decrease ambient solar UV-B irradiance. The choice of biological spectral weighting functions (BSWF) used in adjusting lamp flux is also critical in determining the level of ozone depletion simulated. In addition, shading by the lamp arrays influences plant growth and becomes particularly important when BSWF with substantial weighting in the UV-A are employed.

Field testing of UV biological spectral weighting functions for higher plants

In biological research on the ozone depletion issue, action spectra are typically used as biological spectral weighting functions (BSWF). There has been little testing, however, of the appropriateness of different functions under realistic field conditions. Here we quantitatively evaluate a new BSWF for plant growth response to UV radiation, and other action spectra potentially usable as BSWF, in two seasons of field experiments. We utilized supplemental UV‐B radiation with various combinations of filtered solar UV‐A radiation. Our new BSWF, which we call the UV plant growth weighting function, indicates responses in the UV‐A waveband. In field tests it proved to be the most appropriate weighting function for the responses we measured in oat (Avena sativa L. cv. Otana) over the two field seasons in these studies. A 10‐day field experiment with canola, Brassica rapa L. cv. Goldrush, also suggested substantial effects of both UV‐B and UV‐A radiation on growth. If this new UV plant growth BSWF proves to be appropriate for many plant species, it represents a lower radiation amplification factor (RAF) than several previously used weighting functions for plants. This means less change in biologically effective UV for each increment of ozone column change. However, the lower RAF also means that experiments utilizing supplemental UV treatments have supplied milder doses of additional UV than intended, since they were based on BSWF that overemphasized UV‐B and ignored the UV‐A region. While we await continued field testing of BSWF on diverse taxa, we recommend: (1) reporting weighted irradiance with our new BSWF, and other appropriate functions; (2) continued reporting of the generalized plant weighted UV for continuity and comparative purposes, since it has been so widely used.

A biological spectral weighting function for ozone depletion research with higher plants

Biological spectral weighting functions (BSWF) play a key role in assessing implications of stratospheric ozone reduction. They are used to calculate the increase in biologically effective solar UV radiation due to ozone reduction (radiation amplification factor, RAF), assess current latitudinal gradients of solar UV radiation, and compare solar UV radiation with that from lamps and filters used in experiments. As a basis for a BSWF, we developed an action spectrum for growth responses of light‐grown oat (Avena sativa L. cv. Otana) seedlings exposed to narrowband UV radiation from a large double water‐prism monochromator. Five UV wavelength peaks were used (275, 297, 302, 313 and 366 nm) in the absence of any visible radiation. Growth responses were measured from 1 to 10 days after the treatments. At all these wavelengths, the UV radiation inhibited height growth, including the height at which the first leaf separated from the stem. Radiation at all wavelengths used, except the one UV‐A wavelength, promoted the length of the second leaf. The resulting action spectrum closely resembles the commonly used generalized plant response function except that it indicates continued sensitivity into the UV‐A region. When used as a BSWF for the ozone depletion problem, this new function for plant growth would suggest substantially less impact of ozone depletion because it results in only a modest increment of biologically effective UV for a given level of ozone depletion (a lower RAF). Yet this new BSWF also suggests that experimental treatments based on previous BSWF with less emphasis on the UV‐A may have resulted in simulations of less pronounced ozone depletion than intended. The validity of this new BSWF with UV‐A sensitivity, designated the UV plant growth weighting function, was verified in field experiments as described in the companion paper.

Both solar UVA and UVB radiation impair conidial culturability and delay germination in the entomopathogenic fungus Metarhizium anisopliae.

The entomopathogenic hyphomycete Metarhizium anisopliae has been used in programs of agricultural pest and disease vector control in several countries. Exposure to simulated solar radiation for a few hours can completely inactivate the conidia of the fungus. In the present study we determined the effect of exposures to full-spectrum sunlight and to solar ultraviolet A radiation at 320-400 nm (UVA) on the conidial culturability and germination of three M. anisopliae strains. The exposures were performed in July and August 2000 in Logan, UT. The strains showed wide variation in tolerance when exposed to full-spectrum sunlight as well as to UVA sunlight. Four-hour exposures to full-spectrum sunlight reduced the relative culturability by approximately 30% for strain ARSEF 324 and by 100% for strains ARSEF 23 and 2575. The relative UV sensitivity of the two more sensitive strains was different under solar UV from that under ultraviolet B radiation at 280-320 nm (UVB) in the laboratory. Four-hour exposures to solar UVA reduced the relative culturability by 10% for strain ARSEF 324, 40% for strain ARSEF 23 and 60% for strain ARSEF 2575. Exposures to both full-spectrum sunlight and UVA sunlight delayed the germination of the surviving conidia of all three strains. These results, in addition to confirming the deleterious effects of UVB, clearly demonstrate the negative effects of UVA sunlight on the survival and germination of M. anisopliae conidia under natural conditions. The negative effects of UVA in sunlight also emphasize that the biological spectral weighting functions for this fungus must not neglect the UVA wavelengths.

Scaling Plant Ultraviolet Spectral Responses from Laboratory Action Spectra to Field Spectral Weighting Factors

Biological spectral weighting functions (BSWF) play a key role in calculating the increase of biologically effective solar ultraviolet-B radiation (UVV- BE ) due to ozone reduction, assessing current latitudinal gradients of UV-B BE , and comparing solar UV-B BE with that from lamps and filters in plant experiments. Plant UV action spectra (usually determined with monochromatic radiation in the laboratory with exposure periods on the order of hours) are often used as BSWF. The realism of such spectra for plants growing day after day in polychromatic solar radiation in the field is questionable. We tested the widely used generalized plant action spectrum since preliminary data from an action spectrum being developed with monochromatic radiation for a cultivated oat variety indicate reasonable agreement with the generalized spectrum. These tests involved exposing plants to polychromatic radiation either from a high-pressure xenon arc lamp in growth chambers or in the field under solar radiation with supplemental UV-B lamps. Different broad-spectrum combinations were achieved by truncating the spectrum at successively longer UV wavelengths with various filters. In the growth chamber experiments, the generalized plant spectrum appeared to predict plant growth responses at short (<310 nm) wavelengths but not at longer wavelengths. The field experiment reinforced these conclusions, showing (in addition to the expected direct UV-B effects) both direct UV-A effects and UV-A mitigation of UV-B effects.