Effects of altitudinal gradient on daily rhythm of antioxidant capacity and dynamic photoinhibition in Marrubium vulgare

Several studies have been conducted focusing on antioxidant capacity of medicinal plants; however information is lacking on the daily rhythm of antioxidant capacity as well as photochemical reactions in medicinal plants under natural conditions. Thus, in this study, changes in phenolic acids, PSII functioning and antioxidant enzymes activities in response to low (1,100 m) and high (2,200 m) elevation sites in Marrubium vulgare L. were investigated. Plants were harvested and analyzed in a temporal manner; and for the latter analysis, leaf samples were frozen immediately in liquid N2 and stored in it until assay. High-altitude plants exhibited higher phenolics and carotenoids than that in low-altitude plants, which was associated with higher phenylalanine ammonia-lyase (PAL) activity. Results indicated that the absolute amounts of antioxidant enzymes activities were considerably greater in high-altitude plants than low-altitude plants. High-altitude plants showed the rapidly reversible inhibition of PSII reaction centers (dynamic photoinhibition) associated with an increase in non-photochemical rate constant (kN), which was important photoprotection mechanism under high-mountain conditions. Surprisingly, the activities of antioxidative enzymes, the accumulation of hydrogen peroxide (H2O2), & the free radical scavenging activities using DPPH radical-scavenging in the leaves of high-altitude plants showed diurnal rhythm during the course of day. Finally, we found that the ability of dynamic photoinhibition was an important photoprotection process during periods of high solar radiation in the high-altitude Marrubium vulgare plants.