Illuminance, stomatal opening, and photosynthesis in sorghum and cotton

Abstract

The relationships between illuminance, leaf resistance, and net photosynthesis were studied in sorghum. Some comparisons were made between varieties chosen on the basis of possible differing environmental adaptation, and with cotton whose photosynthetic response is known to differ from that of sorghum. The after-effects of varying periods and degrees of shading were examined for sorghum. Net photosynthesis of sorghum was not light saturated at 11,000f.c. and increases above intermediate illuminance arises almost entirely from increasing stomatal opening. At lower intensities other factors become important, but leaf resistance still exerts the major control. In contrast, leaf resistance in cotton was much lower at all illuminances, and was relatively little affected by changing illuminance. Light saturation of cotton at 5000 f.c. was associated with constant leaf resistance, while at lower intensities, declining net photosynthesis must depend mainly on factors other than stomatal closure. Variation between four sorghum varieties in response to illumination was attributed solely to differences in leaf resistance. Stomata opened slowly after shading at below 3000 f.c., and at a rate independent of the duration of shading. Shading in the afternoon had greater and more persistent effects. Net photosynthesis was affected according to stomatal aperture.