| In this study, a hybrid sorghum (Sorghum bicolor L.) variety Longza5was used asexperimental material, individual or combined treatments of different dosages of UV-Bradiation (UV-B) and different concentrations of24-Epibrassinolide (EBR) were applied onsorghum seedlings at two-leaf stage, and phenotypic, physiological and biochemical changes ofsorghum seedlings were examined. To investigate:1. Effects of enhanced Ultraviolet B Irradiation on the growth of Sorghum seedlings2. Effects of EBR on the growth of sorghum seedlings3. Effects of EBR on the remission of UV-B stress of sorghum seedlingsThe main results are as follows:The enhanced UV-B radiation on the surface of our planet due to thinning of ozone layerexerts differential effects on all kinds of terrestrial plants. In this study, a low dose (0.4J·m-2)UV-B radiation could improve sorghum powdering seeds phenomenon, and the germination rateincreased0.23%compared with the control group at4℃; the breeding sorghum (Sorghumbicolor L.)variety Longza5was used and the40μw·cm-2UV-B radiation treatment wasconducted on sorghum seedlings at two-leaf and one-heart stage, at a time course of0h,3h,6h,9h,12h and followed by a2d recovery. Photosynthetic parameters were measured by aphotosynthetic apparatus, and the activities of antioxidant enzymes were detected at the sametime as well. Our results revealed that, as the dosages of UV-B increased, significantly higheranthocyanin content, leaf browning injury, internode shortening and dwarfing, and significantlyreduced fresh weight and dry weight were observed; chlorophyll and carotenoid contentsignificantly reduced and net photosynthetic rate and chlorophyll fluorescence parametersdecreased; stomatal conductance, intercellular CO2concentration and transpiration rate showedtrends "down-up-down"; activities of SOD and GR presented a "down-up" change, activitiesof POD and CAT demonstrated "down-up-down" trends, and APX, GPX had a "up-down-up" pattern. It worth to note that a sharp decline in net photosynthesis in sorghum seedlings wasobserved at6h UV-B treatment (equals to2.4J·m2), and the turning point was also found forother photosynthetic parameters and activities of antioxidant enzymes at the same time point. Insummary, the photosynthesis system including photosynthetic pigment content, net photosynthetic rate and chlorophyll fluorescence parameters of sorghum were in linearcorrelation with the radiation intensity; the antioxidant system showed different responses toUV-B radiation below or above6h of treatment: ASA-GSH cycle was more sensitive tolow-dose UV-B radiation, while high-dose UV-B radiation resulted in serious damage on plantenzymatic and non-enzymatic antioxidant systems and photosynthesis, reducing substancesaccumulation and anthocyanin deposition, leaf browning and necrosis, plant dwarfing and evendeath.In addition, EBR could prevent the powdering seeds phenomenon at4℃and7℃, thepreventive effects were more obvious when the concentration of EBR was within a certainrange(5×10-3~1.0×10-2mg·L-1), the resulting germination rate increased7.45%and38.60%compared with the control group. In this study, four different concentration of EBR treatmentswere conducted on Sorghum seedlings for24h, and photosynthetic parameters, photosyntheticpigments content, and soluble protein content were measured at the two-leaf-one-heart stage. Theresults showed that when the concentration of EBR increased, the plant height, root length, freshweight, dry weight, and soluble protein content presented a trend of increased first and thendecreased. Net photosynthetic rate, stomatal conductance, intercellular CO2concentration andtranspiration rateo showed an “up-down†trend, some of the changes were significant,chlorophyll and carotenoid content also showed an “up-down†trend, It is worth to note thatwhen the concentration of EBR was5.0×10-3mg·L-1all above mentioned parameters reached thepeak. EBR could not only promote the growth of plants, but also alleviate the damage caused bya variety of stresses. The remission effect of exogenous EBRon UV-B stress was studied, andfound that with the increasing of EBR concentration, plant height, root length, shoot dry weight,underground fresh and dry weight and soluble protein content followed the “up-down†trends;anthocyanin content and MDA showed “up-down-up†changes; the above-ground fresh weight,photosynthetic parameters, photosynthetic pigment, chlorophyll fluorescence, antioxidantenzyme activity presented “down-up-down†trends. The results demonstrated that differentconcentrations of EBR enhanced the photosynthetic and antioxidant capacity of sorghumseedlings under UV-B stress, and alleviated the harmful effects of UV-B stress on sorghumseedlings.
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