Application of extracts from Caribbean seaweeds improves plant growth and yields and increases disease resistance in tomato and sweet pepper plants

The efficacy of alkaline extracts from Acanthophora spicifera, Gracilaria ornata, Codium taylorii, Caulerpa serrulata, and Sargassum vulgare was assessed both in vitro and in planta under greenhouse conditions. Seaweeds were collected from the coast of Trinidad, WI. Alkaline extracts were tested for their efficacy in promoting plant growth and performance in tomato (Solanum lycopersicum, Hybrid-61) and sweet pepper (Capsicum annuum, Amrit) and for their efficacy in controlling infections caused by Alternaria solani and Xanthomonas campestris pv. vesicatoria. Germination assays indicated that seed bio-priming with seaweed extracts lead to a significant increase in seed germination percentage (up to 21.7%), germination index (up to 21.86%) and seedling vigour index (up to 105.8%) compared to hydro-primed seeds. Mean germination time was also significantly reduced (up to 40.6%) in seaweed bio-primed seeds. Chlorophyll and carotenoid contents significantly increased (up to 18.3 and 42.5%, respectively) in seedlings that were bio-primed with seaweed extracts. Greenhouse trials showed that plants foliar sprayed with 0.5% v/v seaweed extract had significantly fewer incidences of bacterial spot (22.9-49.1% and 24.6-49.2% for sweet pepper and tomato, respectively) and early blight (32.8-48.3% and 31.9-47.5% for sweet pepper and tomato, respectively). Furthermore, seaweed extract treatments caused a significant increase in the fruit yield of tomato (118.2-181.8%) and sweet pepper (47.1-70.6%). In vitro antimicrobial assay revealed that all seaweed extracts had no direct antimicrobial effect against a panel of phytopathogens. However, upon foliar treatments with the extracts, the DNA levels of A. solani and X. campestris pv. vesicatoria were significantly lower compared to the controls. Seaweed extract-treated tomato and sweet pepper plants had significantly higher activities of chitinase, peroxidase, polyphenol oxidase, phenylalanine ammonia-lyase, glucanase and higher levels of total phenols. Concurrently, the upregulation of marker genes involved in the salicylic acid, jasmonic acid, and ethylene-mediated defence pathways was also observed in seaweed extract-treated plants. The above results point towards the elicitor-like activity of seaweed extracts on plants' metabolic processes. This study, therefore, highlights the beneficial effects of extracts from Caribbean seaweeds and thus supports their potential use in plant systems as a sustainable crop input.