Localization and antioxidant capacity of flavonoids in Halophila johnsonii in response to experimental light and salinity variation

Abstract

The threatened seagrass Halophila johnsonii contains UV-absorbing flavonoids localized in the cuticle and cytosol of epidermal cells in both abaxial and adaxial leaf surfaces. Previous studies indicate that these compounds exhibit variations in response to salinity stress and may have physiological roles other than UV protection. In this study, controlled light and salinity mesocosm experiments were performed to investigate effects of salinity variation (35, 25 and 15) with or without light reduction in order to elucidate possible physiological roles of flavonoids in H. johnsonii. Response variables were measured over short (one day) and extended (three week) time periods and included flavonoid content and localization, Trolox equivalent antioxidant capacity (TEAC), and chlorophyll fluorescence parameters. Confocal laser scanning microscopy (CLSM) indicated that distribution of flavonoids in the cuticle and cytosol of abaxial and adaxial leaf epidermal cells did not change in response to the salinity or light treatments. Likewise, salinity and light did not have a significant effect on total flavonoid content overall, although within day 1, flavonoid content was significantly higher among shade treatments. Among salinities, TEAC values were significantly higher for salinity 15 at 14 and 21days; TEAC values were similar between shaded and unshaded treatments. Neither salinity nor light had significant treatment effects on individual photosynthetic parameters, but a reduction of salinity and light did have a significant interactive effect as rapid light curves (RLCs) for salinity 15 and 25 shade treatments were significantly lower than all other treatments. Flavonoids were significantly correlated with TEAC values, suggesting a possible antioxidant function for cytosolic flavonoids in H. johnsonii. However, a sunscreen role for cuticular flavonoids in this species was not supported as total flavonoid content tended to be higher in shaded plants and intracellular flavonoid distribution was unaffected by light conditions.