Sesquiterpene-rich Essential Oil From Polygonum pubescens: Chemical Composition and Phytotoxicity Study

Polygonum pubescens, known as drooping or lax knotweed is a tropical and subtropical annual herb belonging to Polygonaceae. The weedy and herbaceous nature, wide distribution and availability make this herb an attractive target for useful phytochemicals. This study aimed to investigate the volatile metabolites of this herb which remained unexplored. First, a detailed morphological study established its identity and distinguishing features with closely related Polygonum species. The leaves of this herb produced highest yield of essential oil (0.20%-0.35%) among three aerial parts (leaf, stem, and inflorescence) hydrodistilled. The oil composition was grossly similar across the tissues and exclusively dominated by three eudesmane and cadalane-type sesquiterpene hydrocarbons, namely, β-selinene (36.3%-43.7%, 221.8-254.5 mg/g), α-selinene (11.0%-14.4%, 74.2-86.5 mg/g), and δ-cadinene (29.0%-41.6%, 140.6-197.6 mg/g). This oil was practically composed of sesquiterpenoids which collectively contributed 97.9%-100.0% relative abundance. When phytotoxicity was evaluated, the oil dose-dependently (1.0-200.0 µg/mL) inhibited the growth of wheatgrass coleoptile. However, the inhibitory potency (59%-73%, 200.0 µg/mL) was significantly low for its consideration as an herbicidal candidate. The current study identified P. pubescens as a rich natural source of β-selinene and δ-cadinene, bearing significance in plant defense, chemical synthon and bioactive phytochemicals.