Penetration of photosynthetically active and ultraviolet radiation into alfalfa and tall fescue canopies

Ultraviolet-B radiation (UV-B, 280-320 nm) reaching the earth's surface has deleterious effects on plants. The degree of susceptibility to UV-B is dependent on the amount of energy present in longer wavelengths of ultraviolet-A radiation (UV-A, 320-400 nm) and photosynthetically active radiation (PAR, 400-700 nm). This study was conducted to quantify the UV and PAR light environment and describe the UV-B/UV-A and UV-B/PAR ratios above and below developing vegetative canopies. Transmitted irradiant flux densities of UV-B, UV-A, and PAR in a developing alfalfa (Medicago sativa L.) canopy and a tall fescue (Festuca arundinacea Schreb.) canopy were measured at varying solar zenith angles under clear and overcast sky conditions. Extinction coefficients for average transmittance differed for alfalfa and tall fescue; a single equation for each waveband and canopy/sky condition sufficed to describe the average transmittance. Canopy structure, LAI, and, to a lesser degree, the extent of direct and diffuse radiant energy were found to influence penetration more than sun angle. An envelope of transmittances defined by equations representing the maximum and minimum light transmittance illustrates the variability in transmittances and was broadest under clear skies and narrowed with decreasing wavelength. Leaf area altered the average ratios of above-canopy UV-B/UV-A and UV-B/PAR ratios. The average ratios of above-canopy UV-B/UV-A and UV-B/PAR ratios varied slightly with year and sky condition. Differences between the two canopies indicate the need to consider canopy architecture in determining the amount of light penetrating a canopy in the UV-B, UV-A, and PAR.