Phytochemical analysis of Asclepias syriaca L. leaf extract and its potential phytotoxic effect on some invasive weeds

Leaf extract of Asclepyas syriaca was utilized to investigate the phytotoxic effects on the germination and seedling growth of Amaranthus retroflexus L., Chenopodium album L., and Iva xanthifolia Nutt. and to evaluate the relative chlorophyll content and oxidative stress of these weeds growing in pots. The total content of phenols in the extract and its antioxidant activity were determined before bioassays. The TPC was 43.3 +/- 1.5 mg GAE g-1 dry extract, while IC50 value of DPPH radical scavenging activity was 0.651 mg ml-1 and FRAP value was 38.7 +/- 1.5 mu mol Fe2+ g-1 d.e., UHPLC-MS analysis was carried out to identify and quantify the major compounds in it. A total of 15 phenolic compounds in the extract were quantified, including 4 flavonoids, 7 hydroxybenzoic acids, and 4 hydroxycinnamic acids. Delphinidin was the most abundant (32.5 mg g-1 d.e.), followed by 4-hydroxybenzoic acid (1.810 mg g-1 d.e.), rutin (1.533 mg g-1 d.e.), p-coumaric acid (1.131 mg g-1 d.e.), quercetin (1.071 mg g-1 d.e.), gallic acid (0.946 mg g-1 d.e.) and protocatechuic acid (0.598 mg g-1 d.e.). The seed bioassay confirmed the inhibitory effect of leaf extract on the seed germination and early seedling growth of the tested weeds at all concentrations. In all tested plants the inhibition of seed germination ranged from 68 to 100%, while the inhibitory effect on early seedling growth (radical and shoot length) was >= 72% and up to 97-100% for root and 74 to 100% for shoot growth. The results obtained from the pot bioassays revealed significant oxidative stress and a decrease in the relative chlorophyll content. These findings evinced that A. syriaca leaf extract had a significant phytotoxic effect on tested weeds. However, further studies under field conditions are needed to confirm the presented results and to check its herbicidal potential.