UV Exposure Regimes Greenhouse studies

The greenhouse UV-B treatment was based on the exposures found in the conterminous United States. Analysis of the probability of having four hours in a day with an upper 5% daily exposure of 8 kJ m UV-Bbe (based on the action spectrum of Caldwell, 1971) occurs approximately 5% of the days in June and July (Grant and Slusser, 2002). During the early part of June, the soybean crops are in early vegetative stages. A 3-day sequence of such exposure levels occurs approximately 1% of the time (Grant and Slusser, 2002). An 8 kJ m UV-Bbe exposure, when distributed over 6 hours of the day as a square wave exposure, corresponds with an exposure of 1.33 kJ m

"2 h-1 UV-Bbe. The use of short-term exposures minimizes the problems surrounding variability in exposure relative to outside solar radiation levels as reported by Musil et al. (2002), although a few of the days during these experiments were heavily overcast and therefore had relatively little solar radiation incoming through the greenhouse glass. These days will be mentioned below.

Since the response of plants to UV-B irradiance is influenced by the amount of

UV-A irradiance, greenhouse experiment exposures were made using a UV lamp array composed of eight UV-A lamps (UV-340, Q Panel Lab Products, Cleveland, OH) and eight UV-B lamps (UV-B-313, Q Panel Lab Products) alternately arranged on a frame positioned 0.6 m above the seedlings. The UV-C radiation was screened out by placing two layers of 0.08 mm CA films below the frames (Fig. 16.1). The CA films were solarized for six hours prior to use and replaced every three to four days to maintain the appropriate spectral irradiance. Ultraviolet-B exposures were determined from prior measurements of the irradiance with distance from the lamp.

Wavelength (nm) Wavelength (nm)

Figure 16.1 Approximate spectral exposure distributions for experimental schemes. Panel (a): Spectral distribution of above-greenhouse solar irradiance (red solid line), irradiance in the control area (light black solid line), and irradiance at plant height in experimental setup (dark black solid line). Values derived from lamp manufacturer-provided spectra for the UV-B-313 and UV-340 lamps, laboratory-measured cellulose acetate filter, and solar spectrum from a TUV model run for solar zenith angle of 30° and total ozone column depth of 320 DU. Panel (b): Spectral irradiance above exclosure plastic filter (red solid line), under UV exposure filter (dark black solid line), and under the control filter (light black solid line)

Wavelength (nm) Wavelength (nm)

Figure 16.1 Approximate spectral exposure distributions for experimental schemes. Panel (a): Spectral distribution of above-greenhouse solar irradiance (red solid line), irradiance in the control area (light black solid line), and irradiance at plant height in experimental setup (dark black solid line). Values derived from lamp manufacturer-provided spectra for the UV-B-313 and UV-340 lamps, laboratory-measured cellulose acetate filter, and solar spectrum from a TUV model run for solar zenith angle of 30° and total ozone column depth of 320 DU. Panel (b): Spectral irradiance above exclosure plastic filter (red solid line), under UV exposure filter (dark black solid line), and under the control filter (light black solid line)

Ultraviolet-A exposures were a result of both lamp enhancement and ambient solar radiation penetration to the plants. The lamp enhancement of UV-A was determined from the ratio of UV-A to UV-B emitted by the lamps times the UV-B exposure. The ambient UV-A reaching the plants was calculated from the ambient measurement of spectral irradiance at 368 nm made at the W. Lafayette, IN UV-B Climate Monitoring Station located within 8.5 km of the greenhouse. The UV-A irradiance was calculated from the 368 nm irradiance according to Grant and Slusser (2005). Ultraviolet-A radiation penetrates the greenhouse glass. The measured light transmittance through the greenhouse glass panels was above 80% for wavelengths above 350 nm; below 350 nm the transmittance rapidly reduced to 10% at 320 nm and less than 1% at 310 nm. When the bank of UV lamps is overhead, the UV-A penetration is only 25% of that above the greenhouse (Fig. 16.1). When there is no bank of lights overhead, the penetration of UV-A is 48% of that overhead. The relatively low apparent penetration of UV-A is due to the wall influences or adjacent greenhouse rooms and protective plastic shields for people walking through the greenhouse of the UV lamp irradiance.

Since the response of plants to UV-B irradiance is influenced by the simultaneous receipt of PPFD, greenhouse experiments also measured and controlled the PPFD. Photosynthetically active radiation exposure during the experiments was a result of both lamp enhancement and ambient solar radiation penetration to the plants. Photosynthetic photon flux density levels for plants in both the control and UV areas were supplemented by three 400-W high pressure sodium lamps that provided 180 ^moles m s at plant height. The ambient PPFD reaching the plants was calculated from ambient measurement made at the W. Lafayette, IN UV-B Climate Monitoring Station. When the bank of UV lamps is overhead, the PPFD penetration is only 25% of that above the greenhouse. When there is no bank of lights overhead, the penetration of PPFD is 48% of that overhead.

Exposures varied with the experiment. In the 2004 experiment, the lamp banks were positioned to provide 1.3 kJ m

" h_1 UV-Bbe and 0.65 MJ m h_1 UV-A for 6 hours as a square wave centered on solar noon, while in the 2007 and 2008 experiments, the lamps were positioned to provide 1.5 kJ m h UV-BBE and

0.75 MJ m h UV-A for 8 hours as a square wave cantered on solar noon

The response of plants to UV irradiance depends on the amount of UV received which can vary over the day as the soybean plants adjust their leaf orientation in response to the position of the sun (Rosa and Forseth, 1995; Bawhey et al., 2003). Leaf orientations were modelled according to Bawhey et al. (2003), and the leaf exposures to both UV radiation and PAR were calculated assuming the lamp bank represented a diffuse radiation source. The orientation of leaves influences the exposure of plants to UV radiation. In these studies, we calculated the daily fractional UV-B exposure using the assumption of isotropic sky radiance distributions (ISO) as described in Bawhey et al. (2003). It was estimated that Essex received 7% greater UV-B exposure than Williams 82 over the UV treatment exposures.

Field study

The 2004 field study utilized plastic filters mounted on 1.2 m by 3.7 m frames to prevent UV radiation from reaching plants grown under the plastic. Plants were grown around the experimental plot in identical planting density to limit the solar radiation penetration to the experimental plots from the side. The control exposure was that under a Llumar filter plastic while the ambient "treatment" exposure was that under a polyethylene filter plastic. Since ambient exposure conditions prevailed, the UV-A exposures were determined from the measurement record of the USDA UV-B Monitoring and Research Station's Ultraviolet Multifilter Rotating Shadowband Radiometer (UV-MFRSR) measuring the 368 nm solar radiation according to Grant and Slusser (2005). The UV-BBE exposures were based on column ozone, solar zenith angle (SZA), and measured erythema irradiance determined from the measurement record of the USDA UV-B Monitoring and Research Station's Yankee Scientific Incorporated UV-B-1 radiometer. The mean daily exposure of UV-Bbe under the Llumar (control) averaged 27 Jm-2d-1, while

that under the polyethylene averaged 4.6 kJm d . The mean daily exposure of

UV-A under the Llumar (control) was 2.49 MJm d , while that under the

polyethylene was 13.5 MJm d . The PAR exposure under the two plastic filters

was nearly identical; under Llumar it was 61.7 mol m d and under polyethylene it was 61.7 mol m-2d-1.

Since the exposure to the plant depends on leaf orientations, we calculated the fractional UV-B exposure over the course of the experiment using the assumption of anisotropic sky radiance distributions as described in Bawhey et al. (2003). It was estimated that Essex received only 2% greater UV-B exposure than Williams 82.

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