UV-B AND PAR IN A GRASS (LOLIUM-PERENNE L) CANOPY

The penetration of natural and artificial UV-B-BE (Biologically Effect ive UV-B, Caldwell 1971) and PAR (400-700 nm) in a grass canopy with i ncreasing LAI was followed during 2 months. Overall, the transmission of UV-B-BE sunlight is significantly higher than of PAR sunlight. This is mainly due to the higher proportion of diffuse light in the UV-B. Under cloudy conditions no difference between UV-BBE and PAR could be found. Sun angle and intensity of the radiation were less important in determining the penetration of light. Artificial light penetrates muc h more through the canopy, resulting in higher irradiation levels in t he lower part of the canopy, but a lower UV-B-BE/PAR ratio (since UV-B transmittance of the leaves is lower). The UV-B-BE/PAR ratio reaching the leaves was influenced by LAI, sun angle, percent diffuse light an d leaf angle. The large differences in UV-B-BE/PAR ratio per unit leaf area under natural and artificial light conditions are important in u nderstanding the influence of UV-B on plants.