| The depletion of the ozone led to the increase of ultraviolet-B(UV-B) with biological effects in the earth’s surface. In the first experiment of our study, the effects of enhanced UV-B radiation on growth, development, antioxidant capacity, photosynthesis and chemical components contents were studied in the leaves of two flue-cured tobacco cultivars-"Yunyan87"(Y87) and "Honghuadajinyuan"(HD). Selected cultivars were exposed to UV-B radiation at ambient(control, CK), ambient plus5.30kJ·m-2·d-1(T1) and ambient plus8.50kJ·m-2·d-1(T2) levels that respectively simulated24.65%and39.53%ozone depletions under summer solstice field conditions in Kunming City (located at25°04’ N and102°73’E). The study was meant to provide theoretical basis for the formation mechanism of quality of flue-cured tobacco leaves in different ambient UV-B regions. The results were as follows:Low UV-B radiation significantly promote the growth of two flue-cured tobacco varieties, but also to promote reproductive developmental process of flue-cured tobacco, make squaring period, initial time of flowering and full-bloom stage shorten. The plant height and stem diameter of low UV-B radiation were the largest, whereas high UV-B radiation were the least. Influential tendencies of UV-B radiation on internode and plant height of HD are just the same. And the internode of Y87was decrease with enhanced UV-B radiation. The length, width and leaf area of HD were increase with enhanced UV-B radiation, whereas those of Y87were the largest under the low UV-B radiation condition. The low UV-B radiation promoted the increase of leaf area index, weight per unit leaf area and weight per leaf of both cultivars. Compared to control, the high UV-B radiation decreased weight per unit leaf area and weight per leaf of both cultivars, and increased leaf area index. The thickness of palisade tissues, leaf thickness, the ratio of palisade tissues thickness to spongy and to leaf thickness of low UV-B radiation were the largest, whereas high UV-B radiation were the least. Change in plant height. internode and leaf area in response to UV-B radiation indicated that Y87might be more sensitive to UV-B stress. The low UV-B radiation can improve the growth and anatomical structure of tobacco leaves. UV-B radiation remarkably enhances MDA content level of two flue-cured tobacco varieties leaves. With the enhancement of UV-B radiation, two flue-cured tobacco varieties leaves of SOD and POD. CAT activity increased significantly, and soluble protein content also increased significantly, and proline content decreased significantly. Low UV-B radiation promoted polyphenol oxidase activity and UV-B absorbing substances of two flue-cured tobacco varieties increased, but high UV-B radiation made polyphenol oxidase activity and UV-B absorption material by certain inhibition. Low UV-B radiation also promoted chlorophyll a, chlorophyll b and total chlorophyll content of two flue-cured tobacco varieties to increase, while high UV-B radiation made chlorophyll a, chlorophyll b and total chlorophyll content decreased due to the broken chlorophyll structure by high UV-B radiation. Effect of enhanced UV-B radiation on the carotenoid content of two varieties was inconsistent, the carotenoid content of HD under low UV-B radiation condition was the highest, with the further enhancement of the UV-B radiation and its content decreased, while the carotenoid of Y87is no difference between the enhanced UV-B radiation and control. Responses of two flue-cured tobacco varieties of photosynthetic rate, transpiration rate and stomatal conductance to enhance UV-B radiation were different. Photosynthetic rate, stomatal conductance and transpiration rate of HD increases with the enhancement of UV-B radiation, while photosynthetic rate, stomatal conductance and transpiration rate of Y87are highest under low UV-B radiation condition, those are lower under high UV-B radiation condition. Enhance UV-B radiation on intercellular CO2concentration of two flue-cured tobacco varieties has almost the same response, as low UV-B radiation to reduce, high UV-B radiation to increase. Water soluble total sugar and catechol levels declined markedly in leaves of both cultivars with two doses of UV-B radiation compared with CK. However, total nitrogen, potassium, free amino acids, chromoplast pigment and flavonoid levels increased in leaves of both cultivars. While nicotine level increased in the leaves of Y87, it decreased in the leaves of HD under two doses of UV-B radiation compared with CK. Changes in water soluble total sugar and total nitrogen levels in response to UV-B radiation suggested that Y87was more sensitive to UV-B stress than HD.Through short-term UV-B radiation experiment of22flue-cured tobacco varieties seedling showed that enhanced UV-B radiation on most of flue-cured tobacco varieties the plant height, stem circumference, maximum leaf length, maximum leaf width, shoot dry weight, underground dry weightand fresh weight has a significant inhibitory effect, and higher than the degree of inhibition of the field period of the same varieties. UV-B radiation made leaf membrane lipid peroxidation of most of flue-cured tobacco varieties increased significantly, mainly to improve the activity of superoxide dismutase and polyphenol oxidase, as well as increasing proline and UV absorbing compounds to protect the flue-cured tobacco leaves from reactive oxygen and membrane lipid peroxidation harm. UV-B radiation significantly inhibited the chlorophyll a and carotenoids of most of the flue-cured tobacco varieties. This is also why the growth of most of the flue-cured tobacco varieties were inhibited. Through the comprehensive evaluation to UV-B sensitivity of growth response index and total chlorophyll content change value of the flue-cured tobacco varieties seedling, the results showed that Y87and Y85in seedling stage to UV-B patience were relatively strong, and the KRK26and HD patience were relatively weak, this result is different from the field period.Through field experimengt of the effect of long-term enhanced UV-B radiation on growth and photosynthesis of the different flue-cured tobacco varieties showed that enhanced UV-B radiation on growth and photosynthesis of flue-cured tobacco varieties have a remarkable effect, UV-B radiation have a stimulating effect on some varieties, but for most varieties inhibited. Enhanced UV-B radiation has little effect to most of varieties of leaf length, maximum leaf width and stem circumference, has a stimulating effect on the number of varieties than inhibit the number of varieties. The response of plant height and pitch to UV-B radiation is consistent, the inhibition of the number of varieties is higher than the number of varieties of the stimulation. Enhanced UV-B radiation on most varieties of chlorophyll a, chlorophyll b, total chlorophyll, photosynthetic rate, transpiration rate, stomatal conductance and chlorophyll fluorescence has minimum inhibition. The ratio of chlorophyll a and chlorophyll b, carotenoids, carotene and total chlorophyll ratio, the intercellular CO2concentration, the maximum chlorophyll fluorescence and the maximum quantum efficiency had stimulative effect. The total chlorophyll and photosynthetic rate, photosynthetic rate and stomatal conductance, photosynthetic rate and transpiration rate, total chlorophyll and the lowest chlorophyll fluorescence, chlorophyll a and chlorophyll b ratio and the maximum quantum efficiency, the maximum chlorophyll fluorescence and the maximum quantum efficiency are closely related. Three kinds of results, among UV-B response index, total chlorophyll content changes with Fv/Fm change value to evaluate flue-cured tobacco varieties to UV-B sensitivity are less consistent. Taken together, Fv/Fm change value and the results of the evaluation of the actual production are close, but the results had little relation with the origin of flue-cured tobacco varieties. |
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