Antioxidant responses to drought stress in pennyroyal (Mentha pulegium L.)

Mentha pulegium L. (Lamiacea) is a valuable medicinal and aromatic plant found in humid and arid bioclimatic regions of Turkey. Drought stress is a growing concern for the future of agriculture, as well as the most common abiotic stress factor affecting the biochemical processes of plants and seriously damaging crop productivity. The aim of the study was to evaluate the effects of drought stress on the activity of enzymatic antioxidants (polyphenol oxidase - PPO, peroxidase - POD, ascorbate peroxidase - ASPX, catalase - CAT) and some ecophysiological (total chlorophyll content, chlorophyll a and b, carotenoid) responses in M. pulegium grown in pots under greenhouse conditions. In addition, oxidative stress markers were analysed to determine whether drought stress causes oxidative damage in pennyroyal. The plants were exposed to water stress during the 3rd, 6th and 10th days. All enzymatic antioxidant activities of plants under stress were increased compared to control plants. While there was a significant increase in PPO and POD activities in the first days of drought treatment, the prolongation of the treatment period resulted in a significant decrease in these activities. In addition, drought significantly increased lipid peroxidation (294%), hydrogen peroxide (158%) and proline (3172%) content compared to controls. These results show that drought treatment and duration significantly affect antioxidant enzyme activities, lipid peroxidation, hydrogen peroxide and proline content. DPPH (2-diphenyl-1-picrylhydrazyl), ABTS (2,20’-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid)) radical scavenging activity, Fe+2 (FRAP) reducing activity and total phenolic content analysis were performed to analyse the effects of drought stress on antioxidant properties of pennyroyal plant. In addition, the decrease in photosynthetic pigment content in parallel with the prolongation of the drought period due to oxidative damage shows that this valuable medicinal and aromatic plant has low tolerance to drought.