Studies of grain production in Sorghum bicolor (L. Moench). VI.* Profiles of photosynthesis, illuminance and foliage arrangement. 4. Profiles of Photosynthesis, Illuminance and Foliage Arrangement

Grain sorghum was grown at two population densities in the field, and photosynthetic rates compared at noon. Profiles of photosynthesis were established by combining measurements of 12CO2 exchange and 14CO2 uptake. Canopy structure and light penetration were measured. Factors responsible for the superiority of the higher density population were evaluated. Photosynthesis–radiation responses of leaves were similar between the populations and there was little difference in total light interception. The high density population had leaves which were more vertically displayed, more uniformly dispersed, smaller in both length and width, and distributed over a greater height of canopy. Light was therefore more uniformly distributed down the profile, and coefficients of light extinction were lower. Associated with this was a higher leaf area index. The overall consequence was the distribution of intercepted radiation over a larger leaf surface, at a lower illuminance and therefore a higher efficiency of photosynthetic conversion, resulting in greater total photosynthesis. ___________________ ** Part V, Aust. J. Agric. Res., 26: 31 (1975).