Changes in phenolic profile, soluble sugar, proline, and antioxidant enzyme activities of Polygonumequisetiforme in response to salinity

This study intended to focus on the effect of different levels of NaCl (0–400 mM, at regular intervals every 100 mM) on the growth, secondary metabolites, and antioxidant enzyme activities of Polygonum equisetiforme, a wild plant with medicinal and industrial uses, which often grows in harsh environmental conditions. Our results showed that biomass production significantly decreased with salinity, while the leaf relative water content declined only at high levels of NaCl concentration (300–400 mM). In contrast, these higher doses of salt resulted in a significant increase in malondialdehyde (MDA) content. Both proline and soluble sugar (e.g., fructose, glucose, and sucrose) contents were enhanced under saline conditions. The methanolic extracts of the shoots included 10 flavonoids and 9 phenolic acids. The total phenolic acids (TPA), total flavonoid compounds (TFC), and total phenolic compounds (TPC) increased with salinity, particularly at 300 mM NaCl. An increase in TPA resulted especially from an increase in quinic, gallic, and protocatechuic acids (phenolic compounds), followed by quercetin-3-O-galactoside, catechin, and epicatechin (flavonoid compounds). Superoxide dismutase (SOD) activity increased only at high salinity levels (>200 mM), while glutathione reductase (GR), guaiacol peroxidase (GPX), catalase (CAT), and ascorbate peroxidase (APX) activity increased with salinity level. A positive significant correlation between antioxidant DPPH and TPA, TFC, TPC, CAT, and APX suggests a vital protective role in controlling oxidative stress through the scavenging process. Consequently, our results indicated that P. equisetiforme shoots are rich in secondary metabolites, especially phenolic compounds with high potential antioxidant activities. It can be considered a salt-tolerant species able to survive at salinity levels up to 300 mM NaCl.