The Influence of Light on Geotropism in Cress Roots

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Abstract
Light affects the growth and orientation of roots of cress seedlings (Lepidium sativum L. cv. Curled). The effects are manifest either as increased rates of geotropic curvature or, if the roots are horizontal, as distorted and crinkled forms of growth. Blue, red, and far-red irradiation can bring about these effects, but with differences of detail: at equal fluence rates during the period of geostimulus, blue is more effective than red at increasing the rate of geocurvature; however, with irradiation prior to a geostimulus, only the stimulatory effects of red irradiation persist for 2–4 h of darkness. Short periods (5 min) of radiation, if given at the time of geostimulus, enhance geocurvature, again with blue most, and far-red least, effective, but there are no clear indications of red/far-red reversibility. The possibility of there being more than one photosystem responsible for the effects of white light on the geotropic responsiveness of roots is discussed.