Effect of salinity on phenolic composition and antioxidant activity of halophytes

Abstract

Halophytes are well adapted to survive in harsh environmental conditions particularly saline lands as they possess distinctive regulatory mechanisms and secondary metabolites. These metabolites have been used in treating various diseases for hundred of years and also have various applications in pharmaceutical and other industries. Comprehensive knowledge about different factors (seasons, habitat, phenology and environment) affecting secondary metabolites and related biological activities of medicinal plants are lacking. To bridge this gap, current study was initiated with the analyses of polyphenols and antioxidant activity of different parts (leaf and stem) of 10 medicinal halophytes (i.e. Aerva javanica, Atriplex stocksii, Cressa cretica, Calotrpois procera, Heliotropium bacciferum, Ipomea pes-caprae, Salsola imbricata, Salvadora persica, Suaeda fruticosa and Thespesia populnea), collected from their coastal and inland habitats during different seasons (summer and winter) and phenological stages (vegetative and reproductive). This study revealed that plants accumulated significantly higher amounts of total polyphenols (TPC), flavonoids (TFC) and proanthocynadins (PC) in their leaves with considerable antioxidant activity, compared to stem. Plants accumulated greater proportion of polyphenols and higher antioxidant activity during drier season (winter) compared to wet season (summer). Similarly, higher polyphenols were found in reproductive stage compared to vegetative stage. The status of metabolites showed slight variation under different habitats compared to seasons. Moreover, all tested species exhibited strong correlations among all antioxidant activity assays, which indicated high radical scavenging and reducing power abilities of these plants. On the basis of polyphenol content plants were categorized into three groups i.e. high, moderate and low. Two plant species were selected for further detailed greenhouse experiments, one from high (T. populnea) and other from moderate (C.procera) category to analyze their salt induced ecophysiological responses as well as secondary metabolites (polyphenols) and antioxidant status. Both T. populnea and C. procera showed salt resistance up to 300 mM NaCl (~60% seawater salinity) and their biomass was not affected at moderate salinity (100 mM NaCl). Both species managed moderate salt stress by adjusting water balance, osmotic potential, selective absorption of K+ and oxidative stress management, while high (300 mM NaCl) salinity appeared damaging for both plants. High salinity resulted in significant decline in actual quantum yield (YII), photochemical quenching (qP) and electron transport rate (ETR). These photosynthetic limitations resulted in oxidative damage by increasing H2O2, MDA and electrolyte leakage ultimately 2

resulted in growth decline. However at moderate salinity, plants effectively managed heat dissipation (YNO), non-photochemical quenching (YNPQ), antioxidative enzyme activity (superoxide dismutase, catalase, glutathione reductase, guaicol peroxidase and ascorbate peroxidase) as well as secondary metabolite content, which in turn improved overall antioxidant status. Salt exposure markedly improved secondary metabolites in C. procera (e.g. natural antioxidants) with profound activities, while T. populnea maintained its antioxidant status at moderate salinity. HPLC analyses showed distinctive composition of phenolic antioxidants including pyrogallol, resorcinol, pyrocatechol, chlorogenic acid, coumaric acid, and rutin, which could be responsible for higher antioxidative performance of these species. This data highlights the importance of both tested species as rich sources of natural antioxidants, which may potentially replace synthetic chemicals. These naturally occurring chemicals are environment friendly which could be used as suitable alternatives in pharmaceutical and cosmetic industries.