| Summary: | Natural elicitors from macroalgae may affect plant secondary metabolites. Ulvan is a sulfated heteropolysaccharide extracted from green seaweed, acting as both a plant biotic protecting agent, and a plant elicitor, leading to the synthesis of signal molecules. In this work, the aqueous extract of <i>Ulva intestinalis</i> L., mainly composed of ulvan, was used as foliar-spraying treatment and its eliciting effect was investigated in basil (<i>Ocimum basilicum</i> L.) and parsley (<i>Petroselinum crispum</i> L.). Antioxidant metabolites (polyphenols and carotenoids), volatile compounds (both in headspace emissions and hydrodistilled essential oils), and hormones (jasmonic acid, salicylic acid, salicylic acid 2-O-β-D-glucoside, abscisic acid, and azelaic acid) were quantified. The foliar-spraying treatment with <i>U. intestinalis</i> extract increased salicylic acid and its β-glucoside in parsley; in basil, it induced the accumulation of jasmonic and abscisic acids, indicating the presence of a priming effect. In basil, the elicitation caused a change of the essential oil (EO) chemotype from methyl eugenol/eugenol to <i>epi</i>-α-cadinol and increased sesquiterpenes. In parsley EO it caused a significant accumulation of 1,3,8-<i>p</i>-menthatriene, responsible of the typical “parsley-like” smell. In both species, the phenylpropanoids decreased in headspace and EO compositions, while the salicylic acid concentration increased; this could indicate a primarily defensive response to <i>U. intestinalis</i> extract. Due to the evidenced significant biological activity, <i>U. intestinalis</i> extract used as an elicitor may represent a suitable tool to obtain higher amounts of metabolites for optimizing plant flavor metabolites.
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