The hydrodistillation of fresh leaves of P. anisum resulted in four EO with yields of 0.22% for spring (EO-Sp), 0.26% for summer (EO-S), 0.27% for autumn (EO-A), and 0.21% for winter (EO-W). Compounds identification ranged from 16 to 3 different ones and from 93.29 to 99.49% of identification (Table 1). BMDB (Figue 1) is reported as the major compound for all samples.
As published in the literature, EO are associated with plant communication, attraction of pollinators, seed dispersers, and defense against microorganisms, and herbivory (Machado et al., 2021). Various factors affect the qualitative and quantitative production of EO, and these changes are related to the genotype of the species, which has undergone several plastic adaptations throughout the evolutionary process due to interactions with biotic and abiotic factors (Pereira et al., 2021).
The spring essential oil showed greater chemical diversity (16 identified compounds) and lower BMDB percentage (51.34%) compared to other seasons. In spring, the specimens were at the beginning of the reproductive period, with flowers and fruits, and the end was recorded in summer. According to Machado et al. (2021), during the reproductive period, in order to ensure the completion of the reproductive cycle, specimens of the genus Piper L. synthesize compounds with anti-herbivory and pollinator-attracting action. The author highlights the presence of linalool and E-nerolidol in the reproductive phenophase of Piper mollicomum Kunth as powerful attractors, as well as 1,8-cineol as a significant herbivore repellent. This may justify the greater chemical diversity of the EO from spring. The same was observed in the spring EO of P. anisum, with a great relative percentage of 1,8-cineol (17.64%) and the lowest content of BMDB (51.34%) compared to other seasons. According to Giuliani et al. (2018), the ecological demands experienced by plants produce responses that are modulated in terms of energy economy, which has been categorized as the drain effect. The competition between the mevalonic acid and/or 2-C-methyl-D-erythritol 4-phosphate and shikimic acid pathways takes place to mediate microclimatic factors (Ramos et al., 2020). Furthermore, the relatively low percentage (78.41%) recorded for BMDB in the autumn sample, compared to the samples collected during summer (99.12%) and winter (96.86%), could potentially be attributed to the presence of 1-propyl-3,4-methylenedioxybenzene. This compound shares the same biosynthetic pathway as BMDB.
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It is noteworthy that the seasonal variation of the EO from this species has not been previously studied. As formely mentioned, BMDB compound has been identified in the chemical composition of the EOs of P. anisum from different locations (Moreira and Kaplan, 1997; Pereira et al., 2021). Also, BMDB has been described in the chemical composition of the EO of Piper corcovadensis (Miq.) C.DC., a species taxonomically very similar to P. anisum (de Lira Pimentel et al., 2021).
Regarding molluscicidal activity, the mortality rate of snails was lower for EO-Sp (30% mortality), whereas EO-S, EO-A, and EO-W led to 100% mortality, all samples at a concentration of 100 mg/mL. The isolated BMDB at a concentration of 25 µg/mL (BMDB25) caused 10% mortality, while at 38 µg/mL (BMDB38) resulted in 70% mortality. At concentrations of 50 and 75 µg/mL (BMDB50 and BMDB75), the mortality rate reached 100% (Table 2, Fig. 2).
Considering these results, molluscicidal assays performed with BMDB at concentrations of 25, 38, 50, and 75 µg/mL resulted in an LC50 value of 35.05 ppm, demonstrating significant molluscicidal action against snails of the B. glabrata. This is the first report of this biological activity for BMDB in the literature.
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The lowest activity observed for the spring EO can be explained in terms of a higher relative percentage of 1,8-cineole, as well as a lower of BMDB. According to studies carried out with the species Eugenia punicifolia (HBK) DC. and Hyptis dilatata Benth, both of which contain 1,8-cineol as the major compound, they did not exhibit any biological activity against the snail species B. glabrata. In contrast, Lippia acutidens Mart. and Lippia gracilis Schauer, species that do not contain this terpenoid, showed bioactivity against the mollusk (Ribeiro et al., 2021). Although Ribeiro et al. (2021) isolated 1,8-cineol for direct testing on B. glabrata snail species, no biological activity was observed.
Compounds with a C6-C3 chemical skeleton, known as arylpropanoids, have been demonstrated to be effective molluscicides in several studies (Kumar et al., 2008; Maia and Andrade, 2009; Do Carmo et al., 2012; Sousa et al., 2014). The compound BMDB, which has a C6-C4 skeleton, is synthesized through the same biosynthetic pathway as the C6-C3 compounds and are chemically related to arylpropanoids. In a biological assay against the snail species B. glabrata, eugenol obtained from the species Cinnamomum zeylanicum Blume, resulted in an LC50 of 18.62 µg/mL (Gomes et al., 2019). These data suggest that aromatic rings, methylenedioxy group, and lateral carbon chains (allyl and/or alkyl) may contribute to the biological action against the snail species B. glabrata. Compounds with methoxy or hydroxy groups in place of methylenedioxy are demonstrated to be more active, such as eugenol (Gomes et al., 2019).
Nevertheless, the biological mechanisms leading to the death of snails of the species B. glabrata are still unclear. Despite that, cellular-level actions, inhibition of enzymatic action, or electrostatic interactions with phospholipids in the cytoplasm are suggested as possible causes (Ribeiro et al., 2021).