Modification of paraheliotropic leaf movement in Phaseolus vulgaris by photon flux density

Abstract. The effects of modification of photon flux density (PFD, 400-700 nm) on paraheliotropic leaf movement were examined in Phaseolus vulgaris L. under controlled environmental conditions. The cosine of the angle of incidence to directional PFD (cos(i)), a measure of leaf movement, was linearly and negatively related to PFD. That is, leaflets progressively oriented away from a direct light beam in response to increasing PFD. The minimum PFD causing paraheliotropic movement was approximately 25 μmol m⁻² s⁻¹. When PFD was varied, tissue temperature changed due to an altered energy balance. Since a change in pulvinus temperature can affect leaf movement, experiments were conducted to distinguish the effects of PFD signal and pulvinis temperature. Leaflets oriented to reduce incident PFD levels in response to increasing PFD (either white light or blue light) when pulvinis temperature was kept constant. From these results, we conclude that changes in PFD signals alone can control paraheliotropic leaf movements. Phaseolus vulgaris grown outdoors oriented their leaflets to face towards the sun in the morning and again in late afternoon, but avoided the sun's direct rays at midday. This diurnal pattern of paraheliotropic leaf movements can be explained on the basis of known paraheliotropic movements in response to PFD and air temperature.