The USDA UV-B Monitoring and Research Program (UVMRP), Colorado State University, Fort Collins, CO, USA, has measured the actual UV exposure at the earth's surface for over a decade. This is an important tool for assessing the quantity and quality of radiation that plants are exposed to over time. It also enables a risk assessment of seedlings and older plants for injury development as well as assessing which plants are capable of developing sufficient defense mechanisms at different growth stages. The UV source (for wavelengths 300, 305, 311, 317, 325, 332 and 368 nm) was custom designed by William Durham of the UVMRP for specific UV-A and UV-B wavelengths (designated the Durham lamp) and assembled by the UVMRP, as originally described in Warpeha et al. (2008). Light is redirected by a mirror that reflects incident UV radiation, and allows the near infrared and infrared to pass out via a light-tight exhaust port so seedlings are not affected. Condensing lenses collect the reflected UV light and define the beam, which subsequently passes through one of seven narrow band filters of type Ultraviolet Multifilter Rotating Shadowband Radiometer (UVMFRSR). The narrow band-pass filters (Barr Associates, Westford, MA, USA) have a 2 nm FWHM at the following centered wavelengths: 300, 305, 311, 317, 325, 332 and 368 nm. The lengths of irradiation were selected to avoid reciprocity failure.
All treatments consisted of a total fluence (dose) of 10 ^mol m (Warpeha and Kaufman, 1990; Warpeha et al., 1991) as is normally used for photobiology studies. Our doses of UV do not exceed brief periods of direct sunlight at specific wavelengths estimated from measurements made at noon in June at the UVMRP Illinois site as discussed in Warpeha et al. (2008).
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