Relation of respiration to CO 2 fixation by Aranda orchid roots

Abstract

Net CO 2 fixation in the light was observed only in freshly harvested Aranda root sections 20 cm or more from the tip at 15°C. No net CO 2 fixation was observed at higher temperatures. The rate of CO 2 fixation at 350 ppm (15°C) increased with light intensity and became saturated at about 300 μE·m −1·sec −1. The roots evolved CO 2 in the light and in the dark when placed in CO 2-free air. CO 2 evolution in the light did not increase with increasing O 2 concentration, and glycolate oxidase activity was not detected in roots. The contribution of photorespiration to CO 2 effux in the light was negligible. The absence of net CO 2 fixation in the light and in the dark by Aranda orchid roots is attributed to a relatively high rate of dark respiration. The rate of CO 2 fixation by the orchid roots can be estimated from the CO 2 gas exchange in ambient air vs that in CO 2-free air.