Total reflectance of ultraviolet and photosynthetically effective wavelengths was measured for a range of different leaf types. Two approaches were employed. Firstly, reflectance of monochromatic wavebands at 330 and 680 nm was measured for a total of 45 different species covering a wide range of genera. In the second, specific leaf types that displayed different degrees of reflectance were treated to remove hairs and waxes that contributed to their reflectance. Selected waxy and non-waxy leaves were also studied in more detail over the spectral range 270-500 nm. It was found that both pubescence (presence of hairs) and glaucousness (presence of a thick epicuticular wax layer) had marked effects on total reflectance. Pubescent leaves tended to be more effective in reflecting longer wavelengths than ultraviolet radiation. The extent of this effect depended on hair type. Glaucous leaves demonstrated that surface waxes were very effective reflectors of both UV and longer wavelength radiation.

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A polychromatic irradiation system for the outdoor exposure of plants or other samples to additional UV and longer-wavelength radiation has been constructed and tested. The system provides a range of wavebands in the ultraviolet-B radiation (280-315 nm) and ultraviolet-A radiation (315-400 nm) spectral regions, and the irradiance is fully adjustable. The performance of the lamp and filter system after over 4000 h of use is described; a brief description of filter degradation after over 30,000 h of use is also presented. Cellulose acetate filters were found to be adequate for some purposes in the UV waveband, but rigid plastic and glass-based filters were superior in terms of stability and the range of spectral combinations available. They also exhibited relatively small changes in transmittance with time. The system is also convenient for the measurement of differential polychromatic action spectra.

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