PLANT CANOPY SHAPE AND THE INFLUENCES ON UV EXPOSURES TO THE CANOPY

The solar spectra at selected sites over hemispherical, conical and pi nnacle plant canopy models has been evaluated with a dosimetric techni que, The irradiance at the sites varies by up to a factor of 0.31 comp ared to the irradiance on a horizontal plane, The biologically effecti ve (UVBE) exposures evaluated with the dosimetric technique at sites o ver the plant canopy are up to 19% of that on a horizontal plane. Comp ared to a spectroradiometer, the technique provides a more practicable method of measuring the UVBE exposures at multiple sites over a plant canopy. Usage of a dosimeter at one site to provide the exposures at that site for different sun angles introduces an error of more than 50 %, Knowledge of the spectra allowed the UV and UVBE exposures to be ca lculated at each site along with the exposures to the entire canopies. These were dependent on the sun angle and the canopy shape, For plant damage, the UVBE was a maximum of about 1.4 mJ cm(-2) min(-1). Compar ed to the hemispherical canopy, the UVBE exposure for generalized plan t damage was 45% less for the pinnacle canopy acid 23% less for the co nical canopy. The canopy exposures could not be determined from measur ements of the ambient exposure.