Document Type : Research Paper
Authors
1 Biology Department, Faculty of Basic Sciences, Payame Noor University, Tehran, Iran
2 Professor, Department of Biology, Payame Noor University, I. R. of Iran
Abstract
Background and objectives: In nature, plants are continually exposed to abiotic and biotic stresses. Under altitudinal gradients, the environmental change in temperature, radiation, and moisture negatively affects plant growth. Among all the environmental factors that change with the altitude, high radiation is the most important factors limiting the plant growth. In such a condition, plants have developed various morphophysiological and biochemical adaptations, which are defined as photoprotection mechanisms, to avoid the harmful effects of higher intensities of light radiation. In this study, for the first time the role of high light stress and UVB on the regulation of photoprotection mechanisms in two ecotypes of Nepeta crassifolia plants was investigated. Indeed, a certain amount of UVB radiation on medicinal plants can be a suitable way to obtain a significant content of secondary metabolites and thus increase their medicinal and economic values.
Methodology: Three to five seeds of Nepeta crassifolia were transferred and sown on the surface of the cylindrical plastic pots (30 cm in diameter and 40 cm in depth) containing sandy soil mixed with peat moss and perlite for 12 weeks, and were irrigated with distilled water every 7 days. Plants were grown at 25 °C day/14 °C night temperatures, with 16 h photoperiods and a daytime photon flux density of 400 μmol m–2 s–1, in an environmentally controlled growth chamber for a period of three months prior to the start of experiments. The mature plants were then exposed to irradiation with UVB (control, UVB, 15 and 30 kJ m-2 d-1) and high-intensity light (HL, 800 µmol m-2 s-1) stress for further 10 days. Measurements of chlorophyll fluorescence (which were described in the following section) and other analysis were performed on the fully expanded and mature leaves. For the latter analysis, leaf samples were freezed immediately in liquid N2 and stored until assay.
Results: After exposure to UVB and HL stress, high altitude (3500 m) plants exhibited more tolerance to photoinhibition as compared to low-altitude plants. Under stress combination, low-altitude (2400 m) plants exhibited lower photochemical activity, which was associated with the down-regulation of chlorophyll, and consequently the occurrence of photoinhibition. The photoinhibition of PSII under combined stress was assessed by the analysis of maximal quantum yield of photosystem II (Fv/Fm), which was consistent with the decrease in the activity of oxygen-evolving complex (Fv/Fo). Increased tolerance in high altitude plants was accomplished by higher levels of phenol content under UVB and HL stress coupled with higher levels of carotenoids, and consequently higher photochemical functioning.
Conclusion: To sum up, exposure of high altitude plants to HL and UVB radiation caused further increases in the activity of phenylalanine ammonia lyase (PAL) enzyme as well as the accumulation of phenolic, rosmarinic acid and chlorogenic acid (CGA), which play a key role in enhancing resistance to combined stress in comparison to low-altitude plants. The present findings provide important information about the selection of resistant ecotypes to stress conditions with more valuable compounds. The results of this study are useful for scientific, agricultural and industrial purposes.
Keywords
- Chlorogenic acid
- ecotypes
- phenylalanine ammonia lyase
- photochemical activity
- Nepeta crassifolia
- UVB radiation
Main Subjects
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