ABSTRACT.This study aimed to analyze oxidative stress produced in cucumber seeds and seedlings when exposed to aqueous extract from dried leaves of leucaena, as well as its effect on the germination behavior, early growth and the antioxidant enzymes activity. It was evaluated the percentage, the speed index, the average time, the frequency and germination synchronization, the root length and shoot, as well as the catalase and peroxidase enzymes activity. There was no significant inhibitory effect of the leaf extracts on the germination percentage. However, there was delay in the seeds germination, as the extract proportion increased. A stimulatory effect of the extract compared to the shoot length was observed, however the root growth was significantly reduced. The catalase activities had a peak at 24 hours after soaking the seeds, however, the activities were reduced in seedlings. The peroxidase activity was low in the seeds and increased in the seedlings at 168 hours after immersion. The results suggest that there was oxidative stress due to allelochemicals present in the leaves extracts from leucaena, verified by germination and initial growth changes, causing alterations in the plants rootlets.Keywords: allelopathy, reactive oxygen species, antioxidant enzymes.Enzimas antioxidativas de sementes de Cucumis sativus são moduladas por extratos de Leucaena leucocephala RESUMO. Este trabalho objetivou analisar o estresse oxidativo produzido em sementes e plântulas de pepino quando submetidas a extrato aquoso de folhas secas de leucena bem como, seu efeito sobre o comportamento germinativo, crescimento inicial e atividade de enzimas antioxidantes. Foram avaliadas a porcentagem, índice de velocidade, tempo médio, frequência e sincronização da germinação, comprimento de raiz e parte aérea e atividade das enzimas catalase e peroxidase. Não houve efeito inibitório significativo dos extratos sobre a porcentagem de germinação. No entanto, houve atraso na germinação das sementes, à medida que se aumentou a proporção do extrato. Foi observado efeito estimulatório do extrato em relação ao comprimento da parte aérea, porém o crescimento da raiz foi reduzido significativamente. As atividades da catalase tiveram pico às 24 horas de embebição das sementes, tendo sido reduzido nas plântulas. No entanto, a atividade da peroxidase foi baixa nas sementes e teve aumento nas plântulas, às 168 horas após a embebição. Os resultados sugerem que houve estresse oxidativo devido aos aleloquímicos presentes nos extratos foliares de leucena, verificado pelas alterações na germinação e de crescimento inicial, o que causou alterações nas radículas das plântulas.Palavras-chave: alelopatia, espécies reativas de oxigênio, enzimas antioxidantes.
Allelopathy is recognized as an important ecological mechanism that influences plant dominance and succession through the action of chemicals released by plants into the environment. This study aimed to identify allelochemical groups and the existence of the allelopathic potential of Brachiaria brizantha and Megathyrsus maximus in Parapiptadenia rigida seeds and seedlings. Hexane, ethyl acetate and methanolic extracts were prepared for phytochemical screening and germination bioassays. Aqueous extracts at concentrations of 10 %, 5 % and 2.5 % were prepared and applied monthly to P. rigida plants. Although allelochemical groups were identified in the extracts, they did not interfere with P. rigida seed germination. The aqueous extracts also did not show allelopathic potential in the development of P. rigida.
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