Effects of Light and Darkness on Gaseous Exchange of Bean Leaves

Abstract
Gaseous exchange fates of excised bean leaves (Phaseolus vulgaris) were determined in light and darkness using mass spectro-metric techniques. Labelled oxygen (O2 18) and carbon dioxide (C13O2) were employed in order to measure simultaneously the photosynthetic and respiratory rates in the light. Illumination at 300 ft-c depressed the rate of CO2 evolution and accelerated the rate of O2 consumption relative to rates measured in darkness. Increasing the light intensity to 1500 ft-c did not modify these rates further. As the leaves approached maturity, the rates of respiratory CO2 evolution and O2 consumption in the dark decreased. When measured in the light, however, the rate of CO2 evolution remained constant while the rate of O2 consumption declined with maturity.