Salt-induced alternations in the content of secondary metabolites and antioxidant responses of guava seedlings

Guava (Psidium guajava L.), is adapted to an extensive range of soils, however, its development is strongly affected by salinity. Environmental stresses are of the major and influential factors in the amount of secondary metabolites, especially phenolic compounds present in plants. The purpose of this investigation was to study the leaf antioxidant and flavonoid contents (catechin, quercetin and rutin) of guava seedlings grown under normal growth conditions or exposed to 100 mM salinity through irrigation water. Leaf samples collected 12 weeks after salinity stress treatment and total phenol, antioxidant capacity, malondialdehyde, catalase, peroxidase and protease activities and high performance liquid chromatography were examined.  According to the results, salinity enhanced the amount of total phenol from 15.31 mgg-1 GA (control) to 32.12 mgg-1 GA. The MDA value improved in guava leaves from 0.42 mg g-1FW under non-stress conditions to 0.76 mgg1 FW under salinity. The most catalase (182.11 μmol min-1g-1FW), protease (9.70 μmol min-1g-1 FW) and peroxidase (19.99 μmol min-1g-1 FW) activities were observed in salt exposed plants. The antioxidant capacity of leaf extract was 62.90% in control plants, which enhanced to 77.46% after salt treatment. The most abundant flavonoid composition of leaf extract was rutin (770 μg mg-1DW) in the control treatment and catechin (1110 μg mg-1DW) in salt exposed plants. We concluded salinity stress, in addition to a noticeable increase in the activities of catalase, peroxidase and protease enzymes; enhanced phenol, catechin and quercetin contents.