Effects of UV-B Radiation on Physiological, Phytochemical and Biochemical Features of Sugar Beet (Beta vulgaris)

رحیم¬زاده کاروانسرا, پریسا and رضوی, دکتر سید مهدی (2019) Effects of UV-B Radiation on Physiological, Phytochemical and Biochemical Features of Sugar Beet (Beta vulgaris). Doctoral thesis, University of Mohaghegh Ardabili.

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Abstract

Research Aim: The aim of this study was to evaluate physiological, biochemical and phytochemical responses of BR1 cultivar of sugar beet to three different UV-B radiations (3.042, 6.084 and 9.126 KJm-2d-1) in order to assess its resistance against this environmental stress. Research method: In this study, the seeds of BR1 cultivar of sugar beet (Beta vulgaris L.) were sterilized with 10% sodium hypochlorite for 10 min and then soaked in distilled water. After 4 days, approximately 90% of seeds were germinated. All sugar beet plants were grown in a growth chamber at 25/20 ºC (day/night) with a 16 hours light /8 hours dark photoperiod for 30 days. Then, plants were divided into 4 groups including control and three experimental groups exposed to 3.042, 6.084 and 9.126 KJm-2d-1 UV-B radiation respectively for one week. All plants were watered twice a week with Hogland’s solution. Findings: The UV-B induced a significant decrease in growth, displayed as reduced height (14%) and fresh and dry weights (17% and 19%, respevtively). This reduction was associated with diminishing photosynthetic rate (86%), relative chlorophyll content (55%), photosynthetic pigments content, soluble sugar contents (11%) and maximum quantum yield of PSII (32%). On the other hand, flavonoids and total phenolic compounds content, amounts of betalains (betanin and betaxanthin) antioxidant enzymes activities, total protein content, compatible solutes, proline, glycine betaine, total free amino acids and total betalain content were increased under UV-B treatments, representing mechanisms by which the plants coped with the stress. Besides, the antioxidant activity of sugar beet leaves methanolic extraction increased under UV-B radiation. Evaluation of betanin and betaxanthin antioxidant activities also revealed that these pigments cause α, α-diphenyl-β-picrylhydrazyl free radical inhibition and betaxanthin has a higher antioxidant activity than betanin. The oxidative stress upon UV-B treatment was evident by increased malondialdehyde content; however, root length, fresh and dry weighs, leaf relative water content and content of hydrogen peroxide were not affected in UV-B exposed plants. We also found out that UV-B radiation didn’t cause DNA fragmentation in sugar beet leaves cells, but it modified the electrophoretic patterns of the plant proteins with creating some new electrophoretic bands in sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis. Conclusion: Growth and photosynthetic performance of BR1 were negatively affected by UV-B. However, UV-B radiation caused antioxidant activity increase in sugar beet plants through accumulation of secondary metabolites such as betalains and flavonoids as well as antioxidant enzymes activity elevation. Enhancement of antioxidant activity causes plant resistance against oxidative stress. The studied sugar beet variety, BR1, seems to be suitable particularly for areas with high doses of UV-B irradiation.

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