Phytoactivity of secondary compounds in aromatic plants by volatile and water-soluble ways of release

Abstract

Phytoactivity should be expected as a generalized trait of secondary plant compounds if their primary role is defence against co-occurring plants, and volatilization should be their predominant way of release in dry climates while in wet climates water leaching should prevail. Bioassays were designed to compare the ability of volatiles and water-solubles of four aromatic species thriving in dry environments (Cistus salvifolius L., Foeniculum vulgare Miller, Myrtus communis L., and Rosmarinus officinalis L.) to affect germination and early growth evaluated by root and hypocotyl length and number of lateral roots, using cucumber as target species. No effects on germination were found, but volatiles and water-solubles showed the ability to reduce root growth, hypocotyl growth, and number of lateral roots of cucumber, with the former the more intensely affect and hypocotyls the less. Continuous release of volatiles did not differ from the non-renewable availability of water-solubles in what concerns the progressive loss of phytoactivity as the growth of cucumber seedlings proceeded. Inhibitory effects predominantly through volatiles, water-solubles, and almost equally by both ways of release were found in the four aromatic species investigated without a clear and systematic predominance of the volatile way of release, far from it. Phytoactivity was found predominantly in volatiles of M. communis, in water-solubles of C. salvifolius and R. officinalis, and almost equally in both types of release in F. vulgare. In spite of the generalized phytoactivity observed, the results of this experiment do not support, by the contrary, the hypothesis that volatilization is the preferential mode of release of phytoactive plant secondary metabolites in dry climates.