Over the past 15 years a number of studies have focused on characterising diurnal leaf movements that occur in a variety of plants in response to the sun's movement across the sky. It is now clear that these solar tracking leaf movements are triggered by a directional light stimulus and that these movements result in at least a partial regulation by the leaf of the intensity of the incident photon irradiance. The purpose of this chapter is to review what is known about the different kinds of leaf solar tracking movements, their impact on primary productivity, and the potential ecological roles of these phenomena.
Solar tracking is an expression applied to describe the heliotropic movements of both leaves and flowers; it denotes the ability of these structures to move in response to the diurnal change in the sun's position in the sky. Heliotropic movements are distinguishable from other directional types of growth by their rapidity, the reversibility and by the overnight resetting to face the morning sun (Yin, 1938). Two main kinds of diurnal movements are recognised: diaheliotropic movements in which the leaf lamina remain oriented perpendicular to the sun's direct rays and paraheliotropic movements in which the leaf lamina are oriented obliquely to the sun's direct rays (Ehleringer & Forseth, 1980). In the extreme cases of paraheliotropism, the leaf lamina may change from nearly perpendicular to the sun's rays to an orientation parallel to the sun's rays.