Carbon dioxide fixation in orchid aerial roots

Abstract

Acidity fluctuation, CO 2 gas exchange, δ 13 C value, PEP carboxylase and RuBP carboxylase activities in aerial roots of selected thick‐leaved orchid hybrids ( Arachnis and Aranthera ) were studied. Both aerial roots and leaves showed acidity fluctuation over a 24 h period. Dark acidification in aerial roots was enhanced at low temperature (15°C). Aerial roots had δ 13 C values close to those of leaves which have been previously demonstrated to possess crassulacean acid metabolism. Variation in δ 13 C values along the length of the roots was observed; the root tip having a less negative δ 13 C value (—13.34%‰) than the older portions of the roots (—14.55%‰). There was no net CO 2 fixation by aerial root, although 1432 CO 2 fixation was observed in light and in darkness. The pattern of fluctuation in activities of PEP carboxylase and RuBP carboxylase in aerial roots was similar to that obtained for the leaves. In both aerial roots and leaves, PEP carboxylase activity was several times higher than that of RuBP carboxylase.