Synergism of polygodial and trans-cinnamic acid on inhibition of root elongation in lettuce seedling growth bioassays.

A bicyclic sesquiterpene dialdehyde, polygodial did not inhibit root elongation up to a concentration of 12.5 microg/ml in a lettuce seedling assay: trans-Cinnamic acid inhibited the elongation by 50% at 1.2 microg/ml (8.1 microM). The inhibitory activity of trans-cinnamic acid was enhanced 17-fold when used in combination with 6.25 microg/ml (26.5 microM) of polygodial. A decrease in the pH of the medium was observed during normal seedling growth, indicating transport of protons from the cells by a plasma membrane H+-ATPase. The inhibitory effect of trans-cinnamic acid on the elongation was reduced to some extent in 2 mM phosphate buffer (pH 7.0) during seedling growth. Although polygodial did not inhibit the activity of H+-ATPase in the plasma membrane fraction of roots in normally growing seedlings, a decrease in activity was found in the fraction obtained from seedlings incubated with 20 microg/ml of polygodial. These results suggest that polygodial functions synergistically with trans-cinnamic acid in the inhibition of root elongation via restriction of proton transport from the cytoplasm of germinated cells.