A Comparison of Antirrhinoside Distribution in the Organs of Two Related Plantaginaceae Species with Different Reproductive Strategies

A study of two related plants (Antirrhinum majus L. and Linaria vulgaris Mill.) containing the same defensive compound (the iridoid glucoside, antirrhinoside) but with reproductive strategies that differ during ontogeny was undertaken. Young leaves are important to plants due to their higher photosynthetic rates and, therefore, should be better protected with higher concentrations of defensive compounds such as antirrhinoside. Declining concentrations of antirrhinoside as leaves aged was found for A. majus but this was generally not the case for L. vulgaris. Concentrations of antirrhinoside in root tissue were low and constant throughout ontogeny for A. majus whereas for L. vulgaris root levels of antirrhinoside were high during the period when vegetative growth is its sole means of reproduction. Antirrhinoside in L. vulgaris roots declined relative to A. majus roots during budding and flowering. During flowering, significantly less antirrhinoside and relative biomass are devoted to L. vulgaris flowers than in A. majus. While these findings are consistent with Optimal Defense Theory (ODT) further work on the distribution of antirrhinoside and the effect of insect herbivory on plant fitness in other related species is needed.