Phytotoxicity of volatiles from fresh and dry leaves of two Asteraceae shrubs: Evaluation of seasonal effects

In South Brazilian grasslands, the shrubs Heterothalamus psiadioides Less and Baccharis patens Baker show a dominance pattern that may be a consequence of allelopathy. In this study, the phytotoxic effects of H. psiadioides and B. patens and the seasonal variation in these effects were assessed. The effects of the volatiles released directly from fresh and dry leaves of the both species were evaluated on germination and growth of the target plants lettuce and onion in four seasons. The volatiles from H. psiadioides and B. patens inhibited the germination rate and speed of germination and reduced the shoot and root length of the target plants, with fresh leaves showing greater phytotoxicity. These parameters were more affected by H. psiadioides than by B. patens in all seasons and both plants showed a decrease in phytotoxicity in summer. It can be concluded that both shrubs possess allelopathic potential, with H. psiadioides showing stronger effects. Interseasonal variation of phytotoxicity should be taken into account in studies of potentially allelopathic plants.