Gravity counteracts light-induced inhibition of root growth

Abstract

THE view that roots are generally indifferent to light has recently been revised in the face of evidence that light can be inhibitory to the growth of some roots^1–10. The discovery^6,11 that light affects the production of an inhibitor prompted the suggestion⁸ that sensitivity to light in roots is related to the system controlling geotropic response, which is also dependent on the presence of an inhibitor^12,13. Since the onset of geotropic responsiveness in some roots is dependent on exposure to light^14–16, the suggestion seems well founded and attempts are being made to clarify the extent to which the mechanisms, triggered when light or gravity affects the root, are interrelated. The investigation described here was prompted by the report⁹ that roots of cress seedlings grown in Petri dishes in the light were stunted in comparison with similar seedlings in the dark. If light promotes geosensitivity, distorted growth could develop in a situation where the root is unable to realign itself, as for example when it is restricted to the horizontal plane. A study was therefore made of the influence of the plane of growth on the light-induced inhibition of root elongation in cress seedlings, from which it seems that although light does not enhance geosensitivity, light-induced inhibition is manifest only in horizontally grown roots and can be prevented by subjecting the roots to a force field.