Allelopathic Potential of Mangroves from the Red River Estuary against the Rice Weed Echinochloa crus-galli and Variation in Their Leaf Metabolome

Mangroves are the only forests located at the sea–land interface in tropical and subtropical regions. They are key elements of tropical coastal ecosystems, providing numerous ecosystem services. Among them is the production of specialized metabolites by mangroves and their potential use in agriculture to limit weed growth in cultures. We explored the in vitro allelopathic potential of eight mangrove species’ aqueous leaf extracts (Avicennia marina, Kandelia obovata, Bruguiera gymnorhiza, Sonneratia apetala, Sonneratia caseolaris, Aegiceras corniculatum, Lumnitzera racemosa and Rhizophora stylosa) on the germination and growth of Echinochloa crus-galli, a weed species associated with rice, Oryza sativa. Leaf methanolic extracts of mangrove species were also studied via UHPLC-ESI/qToF to compare their metabolite fingerprints. Our results highlight that A. corniculatum and S. apetala negatively affected E. crus-galli development with a stimulating effect or no effect on O. sativa. Phytochemical investigations of A. corniculatum allowed us to putatively annotate three flavonoids and two saponins. For S. apetala, three flavonoids, a tannin and two unusual sulfated ellagic acid derivatives were found. Some of these compounds are described for the first time in these species. Overall, A. corniculatum and S. apetala leaves are proposed as promising natural alternatives against E. crus-galli and should be further assessed under field conditions.