The impact of increased solar ultraviolet-B (UV-B) exposure due to stratospheric ozone depletion can negatively affect plant growth and physiology, decreasing crop productivity. While some effects of prolonged elevated UV-B exposure on plants is clear, relatively little is known about the short-term effects of UV-B exposure, although, there are evidence of short-term UV-B increases that likely occur during summer. Two greenhouse experiments were conducted to examine the short-term effects of UV-B exposure on stomatal conductance (gs), UV-B absorbing compounds and photosynthetic pigment concentrations of soybean cultivars Glycine max [L.] Merr. cvs. Essex and Williams 82. Results showed that changes in leaf reflectance at 552 and 714 nm with UV exposure appear to be linked to UV-B induced alterations in pigment concentrations and the changes in reflectance seemed to be more dependent on the period of exposure rather than the UV-B dosage received. The UV-B exposed Williams 82 exhibited lower gs compared to UV-B exposed Essex throughout the experiment. The concentrations of carotenoids, chlorophyll a and total chlorophyll in leaf extracts were unchanged in response to an 18-h UV-B treatment in Essex but they increased significantly in Williams 82. Anthocyanin did not change significantly in either cultivar after the 18-h exposure. The 18-h UV exposure did result in substantially higher of UV-B absorbing compounds in Essex compared to Williams 82. Results of a 6-h UV-B exposure caused an induction of Chlorophyll a/b binding protein (CAB) and Phenyl ammonia lyase (PAL) in the irradiated leaves of Williams 82 and Essex and an up regulation in Chalcone synthase (CHS) in Williams but not in Essex. Further work should assess whether these short-term responses are related to the long-term UV-B mechanisms of damage and protection in soybeans and examine how the induction of genes are related to sensitivity of soybeans to UV-B stress.
The estimation of ultraviolet-A radiation across the earth's surface is needed to model plant productivity and future impacts of ultraviolet-B radiation to plant productivity. We have evaluated the quality of broadband ultraviolet-A (UV-A) irradiance measurements within a UV climate monitoring network in the USA and developed a model to estimate the UV-A irradiance from measurements of the global spectral irradiance at 368-nm. The model was developed from ½ hour interval measurements made during 2000 at three locations across the United States and evaluated from ½ hour measurements made during 2000 through 2002 at seven locations. The stability of the UV-A irradiance sensors across the two year period was evaluated by comparison of changes in UV-A sensor response to changes in 368 nm AOD across years on the same (+/-3) day referenced to the change in UV-A response to changes in 368 nm AOD on sequential days during 2000. Most of the seven UV-A sensors installed during 1999 and 2000 appear to have remained stable (within detectable bounds) through 2004. UV-A irradiance was modeled using measured global 368-nm irradiance and empirical functions defining UV-A and 368-nm irradiance relationships derived from a radiative transfer model. The theoretical pseudo two stream discrete ordinates radiative transfer model provided baseline irradiance relationships between UV-A irradiance and 368-nm spectral irradiance. The model estimated the UV-A irradiance at seven locations across the USA with a mean bias error of 0.5 W m-2 and a root mean squared error of 1.5 W m-2. The model error was comparable to the combined effect of previously-estimated UV-A and 368-nm irradiance measurement errors but greater than that of the UV-A sensor alone.
Ultraviolet radiation-B (UV-B) would increase due to the Ozone depletion. Global climatic factors, such as temperature, precipitation, evapotranspiration, soil moisture and CO2 content, are changing because of the increase of greenhouse emission and the destruction of ecosystems. Climate stress factors including enhanced UV-B irradiance have effects on crop production. Many studies have assessed the effects of enhanced UV-B on crops and impacts of global climatic change on crops separately. However, when UV-B effects were discussed, other environmental stress factors were generally neglected. It is well-known that crops in nature are seldom
affected by only a single stress factor, such as UV-B radiation. The impacts of enhanced UV-B radiation can be greatly increased or decreased by other environmental stress factors. In this paper, through field and plant growth chambers experiments, combined effects of enhanced UV-B radiation with other environmental stress factors
including solar visible light, temperature and soil moisture content on winter wheat were investigated. The experimental results showed that enhanced UV-B irradiance can restrain growth and development of winter wheat, which leads to reduction of plant height, leaf area, and slowing physiological activity and decreasing biomass and yield of winter wheat. The response of winter wheat to enhanced UV-B varied under different UV-B intensity and its combination with other environmental stress factors.
Enhanced UV-B radiation due to stratospheric ozone depletion may have impacts on the productivity of agricultural crops. Which crop will be more sensitive to increased UV-B has received little attention. This paper presents a comparative study of the effects of supplemental UV-B on plant height, leaf area, biomass and yield among soybean, cotton, corn and wheat which were cultivated in fields in Nanjing, China. The experimental results showed that the four crops response to enhanced UV-B irradiation was shortened plant height, decreased leaf area and reduced biomass and yield of crops. Using the same criteria, the response of soybean and cotton to elevated UV-B is bigger than that of wheat and corn. RI (response index) is an integrated index which is the accumulation of relative change in plant
height, leaf area, biomass and yield, reflecting general impact of increased UV-B on crops. The results suggested that the RI for the four crops was minus, demonstrating a negative impact of enhanced UV-B on the crops. According to the RI, the soybean and cotton belong to
the sensitive plants category, wheat is a moderately sensitive plant and corn is a tolerant plant.
The effect of cloud cover on the amount of solar UV radiation that reaches pedestrians under tree cover was evaluated using a three-dimensional canopy radiation transport model. The UVB irradiance across a horizontal domain at the base of a regular array of spherical tree crowns of varying radius was modeled under the full range of sky conditions: clear, few clouds, scattered clouds, broken clouds, and overcast. Differences in crown radius created differences in crown cover (m) with resulting differences in portions of the domain in direct beam shade. The spatial mean relative irradiance and erythemal irradiance of the domain and the spatial mean relative irradiance (Ir) and erythemal irradiance in the shaded regions of the domain were determined for solar zenith angles of 15°, 30°, 45°, and 60°. The mean Ir and erythemal UV irradiance under skies with 4 octas or less was not remarkably different from that under clear skies. Broken cloud cover reduces the spatial mean irradiance by approximately 20% to 30% across the 15o to 60o solar zenith range. In the shade, the irradiance was greater under partly cloudy than under clear skies. Partial cloud cover did not greatly influence the irradiance in the shade of the canopies. Significant changes in erythemal irradiance in the shade did not occur except with cloud cover of 8 octas (overcast) with solar zenith angles less than 45°. Consequently the mean ultraviolet protection factor for vegetation canopies under partly cloudy skies (50% or less cloud cover) is nearly equivalent to that for clear sky days. Regression equations were developed to estimate the areally averaged relative irradiances across the entire domain and only the shaded regions of the domain for each cloud cover fraction as functions of the solar zenith angle and canopy cover. These equations were then used to predict the variation in erythemal irradiance received across a region of suburban Baltimore, Maryland.
KEYWORDS: Ultraviolet radiation, 3D modeling, Buildings, Solar radiation models, Data modeling, Climatology, Vegetation, Meteorology, Geographic information systems, Cancer
Evaluating the impact of ultraviolet-B radiation (UVB) on urban populations would be enhanced by improved predictions of the UVB radiation at the level of human activity. This paper reports the status of plans for incorporating a UVB prediction module into an existing Urban Forest Effects (UFORE) model. UFORE currently has modules to quantify urban forest structure, urban tree volatile organic compound emissions, carbon storage and sequestration in urban vegetation, dry deposition of air pollutants on trees, tree influences on energy use for heating and cooling buildings, tree pollen allergenicity index, and replacement cost of trees. These modeled effects are based upon field sampling to characterize land use, vegetation cover, and building features. The field sampling includes recording of tree species, total height, height to base of live crown, and crown width on randomly selected 0.04-ha (0.1 acre) plots. Distance and direction from sampled trees to buildings are also measured. The input for UFORE modeling of effects includes hourly meteorological data and pollution-concentration data. UFORE has already been used in assessing the urban forest functionof 13 cities in the United States and 5 cities in other countries. The objective of the present work is to enable UFORE to predict the effect of different urban tree densities on potential average human exposure to UVB. The current version of UFORE is written using the Statistical Analysis System (SAS); a new version will be a user-friendly Windows application and will be available for wide distribution. Progress to date on the UVB module consists primarily of examining available modeling and data collection tools. Two methods are proposed for the UVB module. In Method 1, we will derive predicted UVB irradiance <Ib> at person height, that is, below the urban tree and building canopy, using gap fractions (sky view portions) measured from digitized fisheye photos taken from each of the UFORE plot centers during a UFORE field survey. A promising method for analyzing the photos is the use of Gap Light Analyzer (GLA). A human thermal comfort model will be used to determine the times when people would be comfortable outdoors in light attire, and UVB <Ib> will be determined for those times. Method 2 will be applied in cases where hemispherical photos cannot be made available, and for making predictions for cities where surveys have already been done. Method 2 will use a 3D canopy UV radiation transfer model to derive <Ib> based on tree canopy cover maps from GIS analysis of aerial color IR photographs or Landsat TM images. The UV module addition to UFORE will make it useful in epidemiology of UV-related human disease and assessment of UV benefits, such as in vitamin D production, and it will also facilitate consideration of UV exposure in urban forest management.
Studies into the biological effects of acute exposures requires understanding of the probability of an event of a given intensity and duration. The occurrence of high hourly biologically-effective UVB (UVBBE) exposures were evaluated for the 1997-2001 summer growing seasons (May-August) at five locations between 38° and 41° longitude across the continental United States. In general, the frequency of extreme hourly exposures decreased from west to east. The daily UVBBE exposures resulting from the extreme hourly UVBBE exposures were inversely correlated with TOC for that day. Hourly exposures in the upper 10% of all exposures occurred most frequently during June and July. There was a 30% probability of having a day with 2h of exposure in the upper 5% of UVBBE values across the entire USA. The probability of having two sequential days with 2h of high exposure was approximately 10% most locations. The probability of having six sequential days of 2h extreme exposure was approximately equivalent to two to three days of 4h extreme exposure. Based on this analysis, a reasonable exposure regime for acute UVB effects on plants is the insertion of a 1 kJm-2 h-1 UVBBE exposure for two to four hours into the ambient conditions for three sequential days.
Sorghum bicolor is grown in equatorial regions that have naturally high ultraviolet-B (UVB) exposures. To determine whether the increased wax production on the sorghum leaves and sheaths protects the plant by increased scattered radiation from the plant surface, the effects of wax amount on UVB reflectances were examined in greenhouse and field experiments involving three isolines of sorghum -- wild-type and two wax mutants. Reflectance of the wild-type sheath was found to be a result of the wax present while that on the mutant sheaths was not dependent on wax amount. Overhead UVB exposure corresponded with reduced sheath and increased leaf UVB reflectance for wild-type but negligible changes in both sheath and leaf reflectance for the two mutants. Although the sheath reflectances of wild-type were twice that of the two mutants, the negligible difference in leaf reflectance between isolines resulted in negligible differences in the canopy bi-directional reflectance, even at high view angles. The UVA canopy reflectance factors of the three sorghum isolines were measured at 0.03 at viewing angles near nadir on clear sky days. Predicted reflectance factors were calculated using the SAIL model then compared with the measured reflectance factors to evaluate the effect of sky diffuse fraction on the measured differences.
Ultraviolet radiation from the sun, especially the UVB (280 to 320 nm), has important roles in urban ecosystems, including effects on human health. Broadband UVB radiation is being continuously monitored in the city of Baltimore, MD as part of a long-term ecological research program, the Baltimore Ecosystem Study. This paper compares above-canopy broadband UVB irradiance at the Baltimore station to broadband UVB irradiance at a more-rural station 64 km SE (at Wye Research Center in Queenstown, MD) and a station characterized as suburban within the Baltimore-Washington metropolitan area, 42 km SW (at Beltsville Agricultural Experiment Station). The Baltimore data are from the initial 14 months of measurements there. The solar radiation monitoring station in Baltimore is located on a 33-m-tall building on a high point with no significant obstructions to sky view. The broadband instruments, all of which were provided by the USDA UVB Monitoring and Research Program, were calibrated in the same facility, the NOAA Central UV Calibration Facility in Colorado. In general, UVB irradiances at the three sites were similar. Over all conditions, Baltimore and the suburban site measured 3.4% less irradiance than the rural site. This difference is within the anticipated ±3% calibration uncertainty of the broadband pyranometers. On the 59 days with cloud-free conditions at all three sites, the average differences between measured UVB at the three sites was even smaller; Baltimore measured 1.2% less irradiance than the rural site. On the clear days, differences between total daily irradiance and the trend of daily irradiance through the year were clearly related to total column ozone as indicated by the EPTOMS satellite. High aerosol optical thickness strongly reduced daily UVB dose; whereas [SO2] had no influence. Surface O3 increased with increasing UVB dose when [NO2] exceeded 10 ppb.
A decrease in stratospheric ozone would lead to increases in Ultraviolet-B irradiances reaching the earth’s surface. The effect of UV-B enhancements on plants includes reduction in grain yield, alteration in species competition, decrease in photosynthetic activity, susceptibility to disease, and changes in plant structure and pigmentation. A substantial number of studies have been conducted that have evaluated the potential consequences of an increase in UV-B radiation on many plants, but there are few studies that consider the changes of plant growth curve under increased UV-B radiation. This study evaluated availability of the four existing plant growth models under ambient level of UV-B radiation and supplemental UV-B radiation for soybean and winter wheat plants. Both of the Monomolecular and Divided simulation models proved not suitable for use in simulating plant growth under supplemental UV-B radiation. The Logistic model can be used to simulate plant growth in early growing stages and in the condition of Ck and T1 while the Gompertz model simulates plant growth better under T2 and T3 for soybean crop. For winter wheat crop, both of the Logistic and Gompertz models can be used to simulate plant growth under supplemental UV-B treatments.
Impacts of UV radiation on humans, plants and animals involve both acute exposures to high levels of UV radiation and chronic exposures to moderate to high levels of UV radiation. However exposure estimates are often limited by the relatively low density of ground-based measurements of solar ultraviolet irradiance. This study relates the spacing of measurements to the accuracy of estimating daily exposure (chronic exposure) and maximum daily exposure over a 7-day interval (acute exposure) to solar UV. 300 nm, 368 nm, and UVB broadband measurements using multi-filter rotating shadow band radiometers and a broadband radiometer were paired by day for locations within a climate region. The variability in exposures in the various wavebands was evaluated using spatial statistics and interpolations made using kriging methods. The spatial correlation of the 300 nm and UVB acute exposures were greater than the correlation for the chronic daily exposures for distances up to 500 km. Conversely, the spatial correlation of the 368 nm acute exposure was greater than that for the chronic daily exposures for distances up to 500 km. A decrease in the variability in the acute over chronic exposure resulted in an improved ability to estimate the acute exposure across the full 1100 km domain. Limitations to the interpolation of 368 nm acute exposure were assumed to be due to local cloud effects on the radiation. Improvements in the ability to interpolate chronic 300 nm, 368 nm, and UVB exposures from the present USDA UVB Climate Network will require longer exposure periods.
A major limitation in predicting the ultraviolet-B (UVB) irradiance on humans, plant leaves and flowers and aquatic organisms is the difficulty in determining the UVB under partly cloudy sky conditions. This study analyzes the UV diffuse fractions under partly cloudy and clear conditions for nine locations in the USA over a period of 1997 through 1999. Radiation measurements, made as part of the United States Department of Agriculture UVB Monitoring Program using multi-filter rotating shadow band radiometers, were paired with cloud cover and other atmospheric measurements made with National Weather Service Automated Surface Observation Systems within 30 km of the radiation measurement location to evaluate the accuracy of using a relatively simple model to describe the diffuse fraction of UV radiation under partly-cloudy skies. The diffuse fraction was modeled as the summation of clear and overcast sky diffuse fractions, weighted by the probability of the sun's direct beam being obstructed or not for a given cloud cover fraction. For the nine locations evaluated, the model had a mean bias error (MBE) of 0.0037 and a root mean squared error (RMSE) of 0.0361. Simplifying the model by assuming a diffuse fraction of 1 for the overcast sky resulted in a slightly higher error (MBE error of -0.0045 and an RMSE of 0.0387). Model errors were greatest for low solar zenith angles and high cloud fractions. The greatest error, associated with overcast sky conditions, appeared to be a result of scattering off the clouds during the period of time where the sun's beam was unobstructed. Error analysis also showed that the diffuse fraction of partly cloudy skies when the sun's beam was not obstructed is well approximated by the clear sky condition to within approximately 0.1, supporting the use of aerosol optical thickness estimates by Langley plot under partly cloudy skies.
A decrease in stratospheric ozone may result in a serious threat to plants, since biologically active short-wavelength ultraviolet-B (UVB 280-320 nm) radiation will increase even with a relatively small decrease in ozone. Experimental work has shown that various cultivars and species respond to UVB in different ways. To determine the physiological effects on plants of any increases in UVB radiation, the irradiances at the potential sensitive plant surface need to be known. Numerical models are needed to calculate UVB irradiance. This paper compares spatially and temporally averaged measurements of UVB canopy transmittance (Tcanopy, irradiance below canopy/irradiance above canopy) to that predicted by three models. Maize was selected as the canopy for the study because direct measurements of leaf area and leaf angle distribution are manageable. The models can be applied to other plants including urban trees, though other methods of characterizing leaf area and angle distributions generally would be used. Using measurements of canopy parameters as inputs to the numerical scheme, the models attempt to simulate the UVB Tcanopy that the UVB sensors measure. The purpose of this paper is: (1) to describe the models developed for calculating UVB irradiances (as measured by UVB Tcanopy) at given positions in maize canopies; and (2) to report the results of experimental tests of the models.
The stratospheric ozone decrease has heightened concern over the ecological implications of increasing solar UVB radiation on agricultural production and natural plant ecosystems. UVB is absorbed and can damage many important plant species through a variety of interacting mechanisms. The effects of enhanced UVB exposure on yield and yield formation of winter wheat associated with photosynthetic activity and total biomass development were investigated in this study. The overall experimental design was three UVB treatments (two supplemental UVB treatments and an ambient level) and three replicates of each treatment. The results suggested that the supplemental UVB can cause the decrease of yield of winter wheat up to 24% with 11.4% increased UVB. The key growing stage which cause the yield decrease is during the jointing-spike formation. This study also investigated the effects of supplemental UVB on production of dry matter, allocation, LAI, total biomass, and photosynthetic activity of winter wheat. The combined UVB effect with other environmental stress factors on wheat was discussed in this study.
Many of the effects of ultraviolet radiation (UVR) on people and their environment--damage to various materials, survival of insects and microbial pathogens, growth of vegetation, and adverse or beneficial effects on human health--are modified by the presence of trees. Human epidemiological investigations generally consider exposure as given by indices of UVR irradiance on horizontal surfaces in the open. Though many people are exposed to UVR while reclining at a beach or swimming pool, thus experiencing irradiance on essentially horizontal surfaces in the open, exposure to UVR during daily routines in urban areas may also be important in affecting human health. Tree influences on UVR irradiance, particularly in the UVB, can differ substantially from influences on the visible portion of the solar spectrum. Trees greatly reduce UVB irradiance in their shade when they obscure both the sun and sky. Where trees obscure the sun but leave much of the sky in view, UVB irradiance will be greater than suggested by the visible shadow. In small sunny areas near trees that block much of the sky from view, UVB irradiance is reduced substantially, whereas visible irradiance may be nearly as great or slightly greater than in the open.
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