Plant Growth Inhibitory Activities of Iridoidglucosides

Abstract

The mode of action and structure-activity relationship of iridoidglucosides were examined.
1) Among the tested iridoidglucosides, asperuloside, geniposidic acid, geniposide and aucubin were isolated from higher plants, and genipin and deacetyl asperulosidic acid methyl ester were derived from the isolated glucosides (Fig. 1).
2) The plant growth inhibitory activities of the iridoidglucosides were stronger in the case of radicle elongation compared with the growth of the shoots. The inhibitory effects of the iridoidglucosides were stronger in rice seedlings than in lettuce seedlings (Fig. 2 and 3).
3) The inhibitory activities of paederosidic acid, geniposidic acid, aucubin and catalpol, which have a -OH group at the 6-position and/or -COOH group at the 4-position were reduced compared with those of asperuloside which has a lactone ring. On the other hand, deacetyl asperulosidic acid methyl ester and gardenoside, which have a -COOCH₃ group at the 4-position were found to be inactive at several dose levels or which other glucosides inhibited effectively the growth of seedlings of lettuce and rice (Fig. 2 and 3).
4) The glucosides also showed a strong growth inhibitory activity against catchweed seedlings, which the inhibitory activity of asperuloside accumulated at high concentrations in the catchweed tissues was markedly reduced (Table 1).
5) An aglycone, genipin derived from geniposide showed a strong inhibitory activity on growth and germination compared with the glucosides (Fig. 2, 3 and 4).
6) Inactive, deacetyl asperulosidic acid methyl ester was not affected by enzymatic hydrolysis with β-glucosidase. Based on these data, it appears that the inhibitory action of iridoidglucosides occurs after conversion into aglycone by hydrolysis in vivo.