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.