| The world's arid and semi-arid regions account for about half of the arable land. In these areas, there is a shortage of water supply, sparse vegetation and sparse forest, ecological environment deterioratiosn, serious soil erosion, and frequent natural disasters. With global climate changes and local drought, more arid and semi-arid areas will be more seriously affected by drought stress. The influences of drought stress on plants are manifold, including biomass accumulation and allocation, photosynthetic pigments, protective substance and protective enzymes changes. The attenuation of the stratospheric ozone layer will directly lead in the enhancement of UV-B radiation (280-315 nm) onto the earth surface. This will inevitably affect the growth and development of the plants. Therefore, the enhanced UV-B radiation is bound to affect the response or sensitivity of plants toward drought stress.Fagopyrum dibotrys (D.Don) Hara is not only a kind of the national key protected wild plants of gradell, but also a plant with high pharmacological and clinical value. For its medicinal value, there is an increasing demand of the F. dibotrys. Fagopyrum esculentum Moench., with a high nutritional value, is widely cultivated in China. In this paper, the author has observed the influences of enhanced UV-B radiation in simulation and drought stress on the biomass accumulation and allocation, leaf photosynthetic pigments, leaf protective substance and protective enzymes of F. dibotrys and F. esculentum, with a comparison about the plasticity of the two buckwheat species under these conditions, in order to study the impact of the enhanced UV-B radiation on plants under drought stress, and so as to provide a theoretical basis for the protection and use of F. dibotrys and the cultivation of F. esculentum.1) Drought stress made the organs'biomass and total biomass accumulation decrease in F. dibotrys and F. esculentum. The decrease became more obvious with the deepening of drought stress. Biomass allocation to root increased significantly with the increasing of drought stress in F. esculentum while showed no obvious regularity in F. dibotrys. There was a trend that biomass allocation to stem and biomass allocation to leaves decreased with the deepening of drought stress in F. dibotrys and F. esculentum. In conditions of adequate water, enhanced UV-B radiation significantly reduced biomass accumulation to root, biomass accumulation to leaves and total biomass accumulation in F. dibotrys and F. esculentum. Enhanced UV-B radiation also reduced biomass accumulation to stem in F. dibotryswhile that of F. esculentum was not sensitive to it. Enhanced UV-B radiation also made biomass accumulation to inflorescence increase in F. esculentum. Enhanced UV-B radiation reduced biomass allocation to root of F. dibotrys while that of F. esculentum was not sensitive to it. Enhanced UV-B radiation could increase biomass allocation to stem in F. dibotrys and F. esculentum. Biomass allocation to leaves of F. dibotrys was not sensitive to enhanced UV-B radiation which reduced that of F. esculentum. Enhanced UV-B radiation could increase biomass allocation to inflorescence of F. esculentum. In conditions of moderate drought stress, enhanced UV-B radiation increased biomass accumulation to root in F. dibotrys and F. esculentum. Enhanced UV-B radiation could improve biomass accumulation to stem in F. dibotrys and F. esculentum, but this improvement reached a significant level only on specific processing time points. Enhanced UV-B radiation made biomass accumulation to leaves of F. dibotrys first decrease and then increase, but increased that of F. esculentum. Enhanced UV-B radiation increased biomass accumulation to inflorescences in F. esculentum. It increased biomass allocation to root in F. dibotrys but decreased that of F. esculentum. Biomass allocation to stem of F. dibotrys and F. esculentum was not sensitive to the enhanced UV-B radiation. Biomass allocation to leaves of F. dibotrys was not sensitive to the enhanced UV-B radiation which could improve biomass allocation to leaves of F. esculentum in a specific period. Biomass allocation to inflorescences of F. esculentum was not sensitive to the enhanced UV-B radiation. In the conditions of severe drought stress, enhanced UV-B radiation also increased biomass accumulation of root in F. dibotrys and F. esculentum. Enhanced UV-B radiation could lead in a certain increase to biomass accumulation to stem in F. dibotrys and F. esculentum, but this effect reached a significant level only on specific points in processing time. Biomass of leaves in F. dibotrys was not sensitive.to enhanced UV-B radiation which significantly increased that of F. esculentum. Enhanced UV-B radiation significantly increased biomass accumulation to inflorescences in F. esculentum. Biomass allocation to root of F. dibotrys was not sensitive to enhanced UV-B radiation which increased that of F. esculentum. Enhanced UV-B radiation had no significant effect on biomass allocation to stem in F. dibotrys and F. esculentum. Biomass allocation to leaves of F. dibotrys was not sensitive to enhanced UV-B radiation which significantly increased that of F. esculentum. Enhanced UV-B radiation reduced biomass allocation to inflorescences in F. esculentum. From the perspective of the phenotypic plasticity index of biomass traits, in UV-B radiation or drought conditions, plasticity of biomass traits of F. esculentum was higher than that of F. dibotrys, and i.e. F. esculentum demonstrated a higher plasticity of biomass traits than F. dibotrys did.2) Chlorophyll a, chlorophyll b and total chlorophyll contents in F. dibotrys and F. esculentum reduced with the severity of drought. In conditions of adequate water, enhanced UV-B radiation reduced chlorophyll a, chlorophyll b and total chlorophyll contents in F. dibotrys and F. esculentum. While under drought stress, chlorophyll a contents of F. dibotrys was not sensitive to enhanced UV-B radiation which could increase chlorophyll a in F. esculentum, chlorophyll b and total chlorophyll contents in F. dibotrys and F. esculentum. The ratios chlorophyll a/ chlorophyll b of F. dibotrys decreased with the severity of drought stress and the decrease was most significantly in the medium-term of the dealing process. Under moderate drought stress, in the medium-term of the dealing process, the ratios of chlorophyll a/chlorophyll b F. esculentum was improved. In whether adequate water or drought stress conditions, the ratios of chlorophyll a/ chlorophyll b was not sensitive to enhanced UV-B radiation. Leaf carotenoid concentratiosns both in F. dibotrys and F. esculentum decreased with deepining drought stress. Only in adequate water conditions, enhanced UV-B radiation increased leaf carotenoid concentratiosns of F. dibotrys. In other water conditions, leaf carotenoid concentratiosns in F. dibotrys and F. esculentum were not very sensitive to enhanced UV-B radiation. In the middle and late dealing process, ratioss of carotenoid/chlorophyll in both F. dibotrys and F. esculentum showed a rising trend with the deepening drought and reached a significant level in the late stage of the dealing process. In adequate water conditions, enhanced UV-B radiation significantly increased ratios of carotenoid/ chlorophyll in F. dibotrys, while in other water conditions, ratioss of carotenoid/chlorophyll in F. dibotrys and F. esculentum were not very sensitive to enhanced UV-B radiation. From the perspective of the plasticity index of photosynthetic pigments, in UV-B radiation or drought conditions, plasticity of photosynthetic pigments in F. dibotrys was higher than that of F. esculentum, and i.e. F. dibotrys demonstrated a higher plasticity of photosynthetic pigments than F. esculentum did.3) SOD activity, total flavonoids contents, MDA contents and soluble sugar contents in F. dibotrys and F. esculentum increased significantly with the deepening drought stress. POD activity rose with the deepening drought stress, reaching a significant level in the medium-term of the dealing process, while in the early and last stages, this effect seemed to wane. In the early and medium stages of the treatment, free proline contents rose with the deepening drought stress but showed no obvious regularity in the later stage. In the early stage of the treatment, soluble protein contents of F. dibotrys rose with the deepening drought stress, but decreased in the medium and late stages. In the early stage of the treatment, soluble protein contents of F. esculentum increased with the severity of the drought, while it was just the opposite issue in the late stage of the treatment. In conditions of adequate water, enhanced UV-B radiation increased SOD activity, total flavonoids contents, free proline contents and MDA contents of F. dibotrys and F. esculentum. In the early and late stages of the treatment, POD activity of F. dibotrys was increased by enhanced UV-B radiation which decreased POD activity of F. esculentum in late stage of the treatment. In the early and late stages of the treatment, soluble protein contents ofF. dibotrys was increased by enhanced UV-B radiation which decreased that of F. esculentum. Enhanced UV-B radiation reduced soluble sugar contents of F. dibotrys and F. esculentum. In conditions of drought stress, enhanced UV-B radiation increased SOD activity of F. dibotrys in the medium-term of the treatment while decreased it in the early and late stages of the treatment. In the late stage of the treatment, enhanced UV-B radiation increased POD activity of F. dibotrys, while decreased that of F. esculentum. Enhanced UV-B radiation decreased total flavonoids contents of F. esculentum in the late stage of the treatment. Enhanced UV-B radiation increased free proline contents of F. dibotrys, while decreased that of F. esculentum in the medium-term of the treatment. Enhanced UV-B radiation increased MDA contents in F. dibotrys and F. esculentum. In conditions of moderate drought stress, enhanced UV-B radiation increased soluble protein contents of F. dibotrys. In the conditions of severe drought stress, enhanced UV-B radiation increased soluble protein contents of F. dibotrys in the early and medium-term of the treatment, but decreased it in the late stage of the treatment. Soluble protein contents of F. esculentum were consistently not sensitive to enhanced UV-B radiation. Enhanced UV-B radiation significantly reduced soluble sugar contents of F. dibotrys, which of F. esculentum was affected only in severe drought stress. In the UV-B radiation and drought treatment, F. dibotrys showed a higher plasticity in protective enzyme and protective substance than of F. esculentum did.In short, enhanced UV-B radiation would not intensify the injury of F. dibotrys and F. emarginatum by drought, but help to improve their resistance to drought to some extent.
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