Studys On Adaptability Of White Clower (Trifolium Repens L) To Drought Stress

Currently, drought is one of the most serious environmental problems. And in arid andsemi-arid regions, drought is a major constraint imposed on plant survival and growth. In China, White clover (Trifolium repens L) has been planting as the pasture all along and plays more and more important role in city landscape and soil and water conservation in recently years. The shallow root system of white clover is sensitive to drought. Under dry condition,The root system of white clover is small, it is sensitive to aridity. Under the circumstance of aridity, the changes of growth and physiology and biochemistry character were more evident. The different varieties of White clover were used as the mode plants in the item, to study the reaction to the aridity of white clover and the discrepancy among different varieties by the methods of ecology, physiology and biochemistry. To provide the theory basis and scientific guidance for building the artificial pasture in the arid and semiarid zone, garden virescence in cities, and resume and reconstruction the degenerating ecology system. To provide the theory references for culturing and selecting the anti-aridity plants.1. Comparing among different varieties of white clover12 varieties of white clover were planted in the pots. To observe the growth status and drought endurance. It showed that the agricultural characters played clearly discrepancy among different varieties of white clover. Their capacities of drought endurance were also difference. Among which,'Zapican','Grasslands Huia','Nanouk','Korla','Haifa','Ladino','Beber'showed stronger capacity of drought endurance than'Milkanova','Gandalf','Sacremento','Titan','Rivendel'played a poor capacity of drought endurance. From the agricultural characters,'Haifa','Beber','Korla','Ladino','Nanouk','Rivendel'were capable of garden planting.2. Effect of daily dynamics of photosynthesis and chlorophyll fluorescence parametersin Trifolium repens leaves on the different drought stresses Daily dynamics of photosynthesis and chlorophyll fluorescence parameters in Trifolium repens cv. Haifa leaves under drought stress were studied. The results showed that under stress for 7 days, the curves of diurnal variation in Pn, Tr and Gs of Trifolium repens leaves were observed, and the chlorophyll fluorescence parameters such as conversion efficiency of primary light energy (Fv/Fm) of PSⅡ, the photochemical quenching coefficient (qP) and the quantum yield of PSⅡelectron transport (φPSⅡ) were decreased respectively when the relative soil moisture content was 25%. Nonstomatal limitation was the main reason responsible for Pn declined.3. Effects of soil water gradient on photosynthesis and root spatial distribution in two Trifolium repens cultivarsThe plant drought tolerance is one of the important parameters for selecting sustainable landscape plants. Trifolium repens Cultivars,'Haifa'and'Rivendel', were studied for their photosynthesis and root spatial distribution under different drought stresses. The results showed that there were no differences in plant photosynthesis or root growth between the two cultivars under adequate water supply. Net photosynthetic rates and water use efficiency increased, and the root growth was enhanced for'Haifa'with moderate drought stress in the 0 cm– 20 cm soil layer, but there was no significant change for'Rivendel'. Under extreme drought condition in 0 cm– 20 cm soil, and drought condition in 20 cm– 40 cm soil, the cultivar'Haifa'was affected only slightly at the first stage of the experiment and later recovered to a normal state, while'Rivendel'was much more adversely effected. Degree and time of the moisture stress increased the root/shoot ratios gradually for plants of'Rivendel'. Compared to'Rivendel', the root/shoot ratio for'Haifa'did not increase significantly given the same experiment conditions as'Haifa', but there were more deeper roots for'Haifa'than for'Rivendel'. Therefore, it is believed that'Haifa'was more drought resistant than'Rivendel'.4. Effects of continuous drought stress on osmotic adjustment substance and water metabolism of white cloverThe effects of continuous drought stress on osmotic adjustment substance and water metabolism of white clover (Trifolium repens'Haifa') were studied. The result proved, leaf dry matter was lower in water - stressed plants than in irrigated plants, but stolon and root dry matter were higher. During drought time, the water - stress plants had higher soluble sugar and proline content than irrigated plants. Relative water content in leaves of white clovers declined gradually during drought stress compared with controls(with water supply during treatment), the leaves wilted and died at last. Root distribution changed with soil water decreased, but the response varied with different varieties, the droght resistant variety had higher root lenghth densities in deeper soil, so it had higher ability to absorb soil water.5.Effects of drought stress on lipid peroxidation and clearing mechanism in white clovers leavesThe effects of drought stress on RPMP, MDA contents and the activities of MDHAR, DHAR, ASPX, GST, SOD, POD in white clovers leaves were studied. After submerging white clovers roots with PEG-6000 solution, drought stress increased MDA content and permeability of membrane in white clovers leaves. In slight drought stress, the white clovers could keep the balance of production and scavenging of active oxygen by heightening activities of the enzymes, and reduced the oxidic stress resulting from drought stress, At the same time reduced the content of MDA and permeability of cell membrane. In severe stress, activities of the enzymes in variety which was not resistant to drought increased at first, but they could not keep the balance of production and scavenging of active oxygen, white clovers were damaged. but the variety which was resistant to drought could keep the balance by increasing activities of the enzymes, then white clovers were not damaged.6. The change endogenesis hormones in white clover under drought stressThe content of ABA and IAA in the leaves of two white clover increased at different extent and the content CTK and GA were all down trend under drought strss. The general trend of coordinatiion among four endogenesis hormones was closeing the stoma and promoting the growth of root. Whereas the coordinatin mechanism of different white clover germplasm resources was different. Under serious drought strss, drought-resistance white clover had higher coordination ability in reducing grow rate to ensure surviving to avoid the damage of lack of water effectively.7. Effects of PEG stress on nutrient contents in root exudates and plasma membrane ATPase activity of white cloversThe plasma membrane (PM) permeability increased in white clover roots under PEG-6000 osmotic stress, and the amplitude of changes in drought-resistant cultivar was more sensitive than that in the drought-sensetive one. the contents of K+, Ca2+ and H+ in root exudates changed with the plasma membrane permeability. PM H+-ATPase activity increased at early stage under stress, but dropped at late stage under serious stress in drought-resistant cultivar. PM Ca2+-ATPase activity changed like PM H+-ATPase activity. These results show considerable correlation between the activities of PM H+-ATPase or PM Ca2+-ATPase and the drought tolerance of white clover.