Perception of a unilateral light stimulus

Abstract

An organism can detect light direction given a gradient in light intensity within the organism. This gradient, which may be measured temporally or spatially, can be produced by screening or by refraction. The ramifications of the method of producing the gradient are potentially great, with possible effects on the shape of dose-response curves and action spectra. Two biological systems, amoebal phototaxis in Dictyostelium and phototropism by monocot seedlings, illustrate some potential problems. In the former system, no obvious mechanism exists for producing a substantial internal gradient in light intensity. This indicates our lack of knowledge concerning the amount of gradient necessary for an organism to measure light direction. In the latter system, it is evident that a gradient in light intensity is established by screening for second positive phototropism . However, screening may not be the method used for first positive phototropism. The implications of refraction as the mechanism involved in first positive phototropism are sufficiently great to warrant a thorough examination of the role of screening and refraction in first positive phototropism.