DEPENDENCE OF CANOPY LIGHT-DISTRIBUTION ON LEAF AND CANOPY STRUCTURE FOR 2 COTTON (GOSSYPIUM) SPECIES

The light environment within a crop canopy is an important determinant of the potential photosynthetic activity of the crop. The photon flux environment within the crop is influenced by the total incoming radiation and the structure of the plant canopy. In this study, the light environment within two economically important species of cotton (Gossypium, sps.) that show diverse growth habits was examined at different growth stages. G. hirsutum had very regular leaf shapes throughout the growing season, and was diaheliotropic. G. barbadense had large, fairly flat leaves early in the season that progressively became more cupped at increasing mainstem positions, and had no heliotropic response. These differences in leaf shapes and solar tracking altered the leaf and canopy light environment. Estimated total diurnal light intercepted by upper canopy leaves was found to be lower for G. barbadense than for G. hirsutum, due in part to the cupping of the leaf surface. Alternatively, the cupping of the G. barbadense leaves resulted in a more erectophile canopy, increasing PFD to lower canopy leaves in the G. barbadense canopy particularly later in the season relative to that PFD observed in the G. hirsutum canopy. This redistribution of light to lower canopy leaves has been suggested to be beneficial by increasing net canopy photosynthesis. The cupping of the G. barbadense leaves, in addition to distributing light over a greater photosynthetically active area, may be beneficial by reducing photoinhibition. Additionally, the extreme cupping of the G. barbadense leaves results in illumination of the abaxial surface during portions of the day, which may further increase the photosynthetically active area and contribute to total canopy carbon uptake.