[The influence of low and high oxygen concentrations on oxygen and carbon dioxide gas exchange of Amaranthus and Phaseolus leaves during illumination]

[O2- and CO2-exchange of illuminated leaves and the CO2/O 2 ratios at normal and low CO 2 partial pressure]

The O2- and CO2-exchange of detached, illuminated leaves from 18 species of plants grown in the Botanical Garden at the University of Frankfurt was measured at 22° C in an open gas stream (400 ppm CO2 supplied to the assimilation chamber) at both 2% O2 and 21% O2.At a low partial pressure of oxygen (2%) the ratios of CO2 uptake to O2 evolution were in all cases near unity, with a mean of 0.93±0.01. In normal atmosphere (21% O2) the CO2/O2 ratios were higher than those in 2% O2 and averaged 1.26±0.01.The results are interpreted as showing that oxygen is involved in the regulation of metabolism and may cause a shift in the direction of carbon flow in photosynthesizing tissue.