Physilogical Mechanisms Of Alkali Tolerance In Three Economic Plants

At present, soil salinization has restricted the development of the global issues of agriculture, and affected the stability of global ecosystems and biological diversity. However, to date, more attention has been given to the problem of soil salinization, and few alkalization problems have been reported. In fact, salt and alkali stresses are two different stresses, which damaging actions to plants are different, and there are special responses to two stresses for plants. In present study, we chose three common economic plants (cotton, alfalfa, Sea buckthorn) which have certain salt-alkali-tolerant ability as the objects of study, under salt and alkali stresses in manual control, and compared effects of salt and alkali stress on both of them and physiological mechanisms by which they resist alkali stress. The main results and conclusions are as follows:1. The mechanism of alkali stress on three plantsThe findings of the three plants indicated that alkali stress on the inhibition of plant growth and photosynthesis was much greater than salt stress at the same salinity.. Alkali stress significantly inhibited root growth, leaf gas exchange disturbances and reduced the photosynthetic rate and transpiration rate, thereby inhibiting the growth and metabolism. Compared to alkali stress and salt stress, both stresses generally involves osmotic and ion stresses, but the difference is that alkali stress adds the high pH effect. This high pH damage caused by alkali stress might be major factor why alkali stresses are more destructive to plants growth than salt stress. This high pH can not only directly harm plant roots, root growth and damage to cell differentiation, alter the cell structure and stability of membrane, and interfere with the formation of transmembrane potential , resulting in root cell function and metabolism disorders, but the high pH can also cause phosphorus and calcium, magnesium and other important mineral elements to precipitate, and lead to lack of plant nutrients. To survive under alkali stress, plant species must carry out osmotic adjustment and maintain ion homeostasis, but also must resist this high pH. Therefore, the injury of alkali stress on the plant is more serious and complex than salt stress.2. A large of Na~+ accumulation under alkali stress might be main reason why the three plants growth and photosynthesis was lower than that of under salt stressThe results about contents of inorganic ion analysis showed that Na~+ content in shoot existed no significant difference between salt and alkali stresses when pH was low, but under the high pH stress the content under alkali stress was much higher than that of under salt stress. Because all three plants had certain pH regulation ability, under low-intensity stress, the harmful effect of high pH was resisted outside the roots and consequently the intracellular environment was not affected, here the response of plants to alkali stress was similar to salt stress. However, with increasing stress intensity, when stress exceeded the capacity of root adjustment, a large number of Na~+ influx into shoot of plants. Although the vacuole can store a certain amount of Na~+, when Na~+ content was more than the threshold value, protoplasm had to accumulated Na +, which resulted in the destruction of chloroplast lamellar structure, the decrease of chlorophyll content, the inactivation of enzymes, the decrease of stomatal conductance. It can be argued that Na~+ accumulation under alkali stress might be main reason why the plant growth and photosynthesis was lower than that of under salt stress. According to the results of this experiment, combined with existing research reports, we hypothesized that Na~+ accumulation under alkaline stress might also be related to possible decreased Na~+ exclusion, which deserves further study.3. Organic acids accumulation in three plants cells under alkali stress was a key physiological mechanism on ionic balance and pH regulationThe experimental results showed that none of the three plants under salt stress accumulated organic acids, but all of them accumulated a large of organic acids under alkali stress. For three plants under alkaline stress, Na~+ content increased, the total amount of inorganic anions decreased, which led to deficiency of intracellular negative charge, the damage of water molecular dissociation, eventually led to instability in the intracellular pH and interference of various metabolic activities. The large number of organic acids accumulation under alkaline stress might be a passive response to deficit of negative charge in cells, which played an important role in maintaining ionic balance and pH stability. Organic acids accumulation was the key physiological mechanism which decided the alkali tolerance of plants. In addition, organic acid metabolism regulation pathways in three different plants under alkali stress were different and different organic acids in the different plants might play different roles. Roots and leaves of cotton mainly accumulated malic acid and citric acid, but acetic acid was not detected; shoots of sea buckthorn mainly accumulated malic acid, citric acid and oxalic acid, but the contents of malic acid and citric acid were relatively small, acetic acid accumulation was enhanced significantly; roots and leaves alfalfa, mainly accumulated malic acid and citric acid, the content of tartaric acid was not many, but increased with increasing stress time, and no tartaric acid accumulation in cotton and sea buckthorn.However, the regulatory mechanism of organic acids may be extremely complex, possibly involving the citric acid cycle, photosynthetic carbon fixation, anaerobic respiration, and many other basic metabolic pathways. The regulatory process may occur in the transcription, translation and the proteins regulating the activity of various levels, which deserves further study.