Soybean seeds (Glycine max L., cultivars Clark and Clark-magenta) were planted in plots (4.5 m x 2.5 m) at the USDA Agricultural Research Center (ARS), South Farm, Beltsville, MD, with a spacing between rows of 0.4 m, and a density of ~35 seeds m-1. One row (4.5 m) of Clark seed was planted next to one row of Clark-magenta seed, and bordered by additional rows of soybeans to simulate field-crowding conditions and to create even sunlight exposure and shading. After germination, plants were thinned to ~25 plants m-1 (see (Teramura et al., 1990) for additional information regarding the field sites). In order to reduce water or nutrient stress that can mask the response to UV-B radiation, the plants were watered daily when natural precipitation did not occur, and the plots were fertilized before planting according to recommendations from the Maryland Soil Testing Laboratory.
Ultraviolet-B radiation was provided in a squarewave system by Q-Panel (Cleveland, OH) with UVB-313 sunlamps (12 lamps per plot, 30 cm apart) suspended over and perpendicular to the rows of soybean plants (Teramura et al., 1990). Lamps suspended over the supplemental UV-B plants were wrapped with either cellulose diacetate (CA) which transmits UV-B radiation down to 290 nm, or with polyester filters that block almost all UV-B below 316 nm. Therefore, plants received similar levels of PAR and UV-A radiation, but differed in exposure to UV-B radiation. Spectral irradiance beneath the lamps was measured with an Optronic Laboratories Inc. (Orlando, FL) Model 754 spectroradiometer. The height of the lamps above the plants was maintained at approximately 75 cm.
All filters were presolarized for eight hours, and changed weekly for CA and every two weeks for polyester to compensate for filter degradation. Plants in the field received either ambient UV-B (polyester) or ambient UV-B plus a maximum of 5 kJ m-2 d-1 UV-Bbe (Biologically Effective UV-B when weighted with the action spectrum of Caldwell (1971)). This represented the maximum supplemental level of UV-B radiation that would be received at College Park, MD, during a cloudless day at the summer solstice, with a 25% reduction in ozone, according to the model derived by Green et al. (1980), with later derivations by Björn and
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