Dosimetric measurement of the visible and UV exposures on field grown soybean plants

Abstract

This paper describes a dosimeter system for measuring both the PAP (photosynthetically active photons) (400-700 nm) and ultraviolet-B (UVB) (280-320 nm) exposures in the supplemental UVB irradiation of field grown soybean (Glycine max [L.] Merr.) plants. At the V2 growth stage, the dosimeters positioned at the same position and orientation as the trifoliate leaves of the plants that were measured received 12 to 38% more PAP and 5 to 82% more UVB than the unifoliate leaves. For the crop maturity stage, the plants exposed to high levels of UV irradiance (high UV treatment) received approximately 40% more UVB on a horizontal plane at the top of the plant canopy compared to the control group of plants (control treatments). For the other measurement sites over the plants that were orientated at 45 degrees to the vertical in the north, south, east and west directions, the additional amount of UVB for the supplemental treatment compared to the corresponding sites for the plants in the control treatments varied between -39% and 37%, due predominantly to the shading provided by the other plants. Furthermore, the supplemental UVB changed the natural partitioning of UVB and PAP and the ratio of PAP to UVB over the plants. All these variations to the UVB and PAP over the plant canopy cannot be predicted by exposure measurement in the wavebands on a horizontal plane. Consequently, for the case of the complex topography of plants, the dosimeters described in this paper have the advantage of allowing the exposures to be measured simultaneously at multiple sites that are at any orientation.