Leaf orientation and sunlit leaf area distribution in cotton

Abstract

The diurnal leaf orientation behaviour of row-planted cotton plants ( Gossypium hirsutum L. cv. ‘DES 119’) and its relationship to sunlit leaf area distribution at three stages of development were studied in the field. Electromagnetic digitizing was used for plant geometrical structure measurement for three periods of 2 h during the day. Cotton leaves showed a diaheliotropic response throughout the day. This heliotropic behaviour varied according to growth stage. In addition to changes in orientation, leaves also moved in space. The distance moved by a leaf between two observation times increased with stage of development, in agreement with petiole and blade lengths. Sunlit leaf area distribution varied according to stage of development. Analysis of interception showed that probability of light interception was greater in the morning and in the afternoon than at noon. Without a diurnal change in canopy structure, cotton plants would intercept less direct radiation in the morning and in the afternoon. Leaf dispersion was regular during the first and last stage of development, but it was clumped during the intermediate stage. Leaf dispersion was more regular in the morning and the afternoon than at noon.. Leaf dispersion, however, changed primarily with sun direction and not with canopy structure. This would indicate that small changes in leaf location do not significantly affect light interception. Finally, the ecological significance of diaheliotropism and the implications for modelling light interception in cotton plants are discussed.