The Study On Physiological Responses Of White Clover Under Drought Stress

White clover (Trifoliumre Pens L. ) belongs to the Leguminosae and the genus Trifoiium. White clover contains high content of proteins and other nutrients making it as a very excellent grazing grass; and its beautiful shape and long-lasting green leaves also makes it as a major lawn grass. It also has the ability to fix nitrogen. The cultivation of white clover can increase soil fertility and the ground green coverage rate, and control the growth of weeds, maintain soil temperature and reduce the erosion of soil.Nowadays, with the rapid advancing of urbanization and the New Countryside Construction in China, the prospects of cultivating white clover, both as grazing and as lawn grass, seems promising. However, the basic research is needed because white clover has a very narrow ecological amplitude, is not very resistant to stresses (e.g. drought and water-logging), and prefer warm and humid climates. Therefore, In order to promote the development of white clover industry the physiological and biochemical studies of Trifolium repens L in stresses are quite necessary.In this paper, Trifolium repens.cv.Rivendel was used as experimental material which was exposed to different concentrations of mannitol to simulate natural drought stress. Some growth indices, photosynthetic parameters, antioxidative enzyme activity and changes of membrane permeability were measured and analyzed under different concentrations of simulated drought stress. We could primarily confirm that to a certain extent white clover was hurt physiologically by drought stress and how white clover adapted to drought stress physiologically by comparing differences of physiological responses Trifolium repens.cv.Rivendel exposed to different degrees of drought stress, which could provide a theoretical basis on large-scale cultivation of this turfgrass in the arid and semi-arid region. The main results were as follows:1. Effects of drought stress on the growth indices and relative water content of Trifolium repens In this experiment, mannitol solution was used in order to simulate drought stress. The results showed that the dry weight and fresh weight of white clover decreased under different concentrations of mannitol, but decreasing degrees were not the same. When treated with 400 mM mannitol, the decreasing rate of dry weight and fresh weight was faster than that of controls and 200 mM mannitol. At the same time, the relative content of water of the leaves also decreased, the leaf growth was reduced, and with increasing of the treatment time, the leaves appeared wilting. This indicated that under the conditions of drought stress, white clover were more sensitive to water deficiency, and drought stress affected dry matter accumulation, which resulted in plant growth inhibition.2. Effects of drought stress on photosynthesis of white clover seedlingsWe measured the change of photosynthetic parameters (net photosynthesis rate, stomatal conductance, intercellular CO2 concentrations) under mannitol simulating drought stress conditions in this experiment. And the results showed that the changes of photosynthesis rate were different under different conditions. Net photosynthetic rate increased at the beginning of the treatment and then declined with 200mM mannitol treatment, which was consistent with changes of stomatal conductance under the same condition. But under 400mM mannitol stress, net photosynthetic rate always decreased, while changes of stomatal conductance behaved differently yet. At the end of treatment, changes of stomatal conductance continued to increase, which suggested that non-stomatal limitation for the decrease of net photosynthetic rate. The decrease of chlorophyll content and increase of chlorophyll a/b ratio indicated that the decrease of chlorophyll was one of factors resulting in decrease of photosynthetic rate. During short time of drought stress, intercellular CO2 concentrations declined. However, with increasing of the treatment time, the intercellular CO2 concentrations increased.3. The effects of drought stress on antioxidant enzyme activity and cell membrane permeability of white clover seedlingsUnder different degrees of mannitol stress, SOD as well as POD activity increased with increasing of the treatment time under condition of 400mM mannitol treatment. The up-regulation of SOD and POD activity under drought stress can effectively reduce ROS, thus reduce their damage. Though under 400mM mannitol stress, SOD and POD activity was enhanced during a short period, then they decreased sharply, demonstrating that 400 mM mannitol had severely destroyed the normal physiological reactions of cells. White clover growth was markedly reduced and some tissue and cells died under this condition. The significant increase of plasm-membrane permeability indicated that the membrane system was severely damaged, which was possibly related to the sharp decrease of SOD and POD activity.4. The effects of drought stress on the content of free proline of white clover seedlings Proline is an important compatible solute in plants under environmental stresses.Therefore, the change of free proline content in leaves of white clover under mannitol simulating drought stress conditions was determined in this experiment. And the results showed that free proline content inceased with the increase of mannitol osmotic stress. The praline content significantly increased under mild water stress for 6 days, indicating that white clover was able to accumulate a great amount of proline to cope with drought stress. Although the proline content of white clover plants treated with 400 mM mannitol was 2.3 times of that of the control, white clover did not show more resistance because of physiological metabolism disorder resulting from severe water shortage, which suggested that increase of the praline content under severe drought stress the result of physiological metabolism disorder rather than adaptation. These results also demonstrated that white clover is not a high drought-tolerant plants.