Abstract
Hypocotyl growth in Cucumis sativus L. cv Ridge Greenline is inhibited by increasing blue light (B) fluence rate in a near log linear fashion once a low fluence threshold is exceeded. Deviation from log linearity at the highest fluence rate used here is due to light perceived by the cotyledons and this effect is assigned to phytochrome. This response can be removed by Norflurazon treatment, without affecting the rest of the fluence response curve.There is also some activation of phytochrome by lower fluence rates of B, an effect which contributes to the overall inhibition of growth. Responses to photostationary state and cycling rate indicate, however, that B does not primarily act via phytochrome, but through a specific blue light photoreceptor.