Quantitative measures of leaf orientation and heliotropic response in sunflower, bean, pepper and cucumber

Abstract For single unstressed plants, the distribution of leaf area as a function of azimuth angle shows a trend towards a preferred azimuth which can be supported statistically. Young sunflower leaves, and to a limited extent bean leaves, follow the sun and the preferred azimuth therefore changes from east to west during the day. With pepper and cucumber a significant trend towards a northeasterly azimuth was found irrespective of the sun's position. Distributions are given in terms of the cosine of the angle between the sun's beam and normals to the surfaces of the leaves, and the phase angle between the leaf-normal projection on the solar plane and the sun's beam. Calculations show that the heliotropism of sunflower leaves and to some extent of bean leaves, significantly increases the radiation intercepted by these species throughout the day compared with that expected for a random leaf distribution.