Phytotoxic secondary metabolites produced by plants have been studied as possible sources of bioherbicides. However, several environmental factors can change the secondary metabolism of plants, and thus, the production of these compounds. Among these factors, seasonality can be a source of variation in the content of all classes of secondary metabolites. In this study, we evaluated the phytotoxic effect of aqueous extracts of leaves from Byrsonima intermedia A. Juss. (Malpighiaceae), Gochnatia polymorpha (Less.) Cabrera (Asteraceae), Luehea candicans Mart. (Tiliaceae), Miconia chamissois Naudin (Melastomataceae), and Qualea cordata Spreng (Vochysiaceae) (species of the Brazilian savanna), collected both during the dry and the rainy season, on germination and seedling growth of maize and cucumber. The analysed parameters were affected by all leaf extracts collected during both seasons. However, a greater phytotoxic effect was observed when leaves were collected during the dry or during the rainy season, depending on the savanna species studied, on the target species, and on the parameters evaluated. The phytochemical screening of all extracts was also evaluated and allowed the identification of anthraquinones, triterpenoids, saponins, and tannins. The composition of extracts differed between the seasons for all species. This study highlights the importance of considering savanna seasonality when studying the phytotoxicity of the species of this biome.

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Medicinal plants produce a high diversity of secondary metabolites with different biological activities, which are commonly evaluated when prospecting for bioherbicides. We analyzed the phytotoxic activity of organic extracts from the leaves of five medicinal species, Byrsonima intermedia, Moquiniastrum polymorphum, Luehea candicans, Miconia chamissois, and Qualea cordata. Phytotoxicity was evaluated on the initial growth of cucumber seedlings through tests with different concentrations of hexane, ethyl acetate, and methanol extracts. The results showed that all organic extracts and all concentrations affected cucumber development, with methanol extracts generally showing the greatest negative effect on the initial growth of the target species. The only exception was for M. chamissois extracts, in which the hexane extract had the greatest phytotoxicity. Furthermore, the organic extracts were subjected to preliminary phytochemical analysis, revealing the widespread presence of alkaloids along with other chemical classes. All the study species are thus potential candidates for use as natural herbicides.

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