| Increasing salinization of agricultural soils is one of the most challenging issues faced by modern agriculture. In excess of 30% of cultivated soils are affected by salinity. Much of this salinization is attributable to the infiltration and accumulation of Na+ and plant nutrients. So, it should pay more much attention to the relationship between plant nutrients and tolerance ability of crop under salinity in the future. This thesis included three experiments totally, study on the physiological mechanisms of salt tolerance of different rice genotypes regulated by the cultivation conditions, plant nutrients and hormone under the field and phytotron (controlled water supply, temperature, light and nutrients supply) conditions during 2008-2010 year. It has been supplied some further basic theory evidence for the salt tolerant improvement and the crop production increasing. The main results showed as below:1 There is significant positive correlation between plant biomass, leaf area and salt tolerant ability of seedling. The larger amount of sugar has been transformed into starch, accumulated in plant tissues and has higher contents of starch and carbohydrate under normal condition; and inhibition the sugar transformed into starch, the larger amount of starch has been transformed into sugar under saline condition are the effective methods to improve the tolerant ability of rice, and it is the important reasons for tolerant genotype to increase tolerant ability. Plant maintained higher contents of chlorophyll a and chlorophyll a/b ratio is the physiological characters of salt tolerant genotypes, and it is the important reasons of salt tolerant ability increased. The differences of salt tolerant ability between tolerant genotypes and sensitive genotypes as affected by the contents of Na+, K+ and Na+/K+ ratio in plant tissues, which will directly affect the level of salt stress. The differences of Ca2+ and Mg2+ contents in plant tissues between normal and saline conditions are significant, and the differences between genotypes are significant also. Growth stages can be delayed by early salt stress, and the affect of salt stress can not be eliminated with the salt stress finished, it is the main reasons for the production decreasing.2 The effect of plant nutrients and cultivations on salt tolerant ability of different rice genotypes and its mechanisms are different. The salt tolerant ability of seedling can be improved by delaying the transplanting time and N application is because of the higher plant biomass and leaf area. Because of the special metabolic mechanisms of carbohydrate and the physiological characters of root, the tolerant genotype has higher salt tolerant ability under saline condition. There are higher chlorophyll a content and chlorophyll a/b ratio in plant tissues of tolerant genotype and N treatments, these are important physiological characters for salt tolerant ability increased. Compared with the root washed transplanting, seedling transplanting with soil can protect the root, so the passive absorption amount of sodium is decreased significantly; it is help plant to maintain higher nutrients absorption ability and higher plant biomass during the later growth period. The effect of salt stress on growth stage of salt tolerant genotype is smaller, and the effect of salt stress on growth stage of transplanting with soil is smaller also. These are the reasons of tolerant genotype and transplanting with soil treatment have higher production under salt stress condition.3 The salt tolerant genotypes (FL478 and IR651), when compared with the sensitive genotypes (IR29 and Azucena), had lower content of Na+ and lower ratios of Na+/K+, Na+/Ca2+ and Na+/Mg2+, but had higher content of K+ in plant under salt stress (100 mmol·L-1 NaCl), that's why tolerant genotypes have higher tolerant ability than sensitive genotypes. The content of Na+ and ratios of Na+/K+, Na+/Ca2+ and Na+/Mg2+ in plant can be declined significantly, the effect of salt stress on plant growth is alleviated significantly, the salinity tolerance of rice is improved significantly and the influence of Ca2+ treatment is higher than Mg2+ treatment, when the concentrations of Ca2+ and Mg2+ in culture solution were increased (60 mmol·L-1). However, the effect of K+ treatment on the above parameters is lower than Ca2+ and Mg2+ treatments respectively, that's why the effect of K+ treatment on salinity tolerance of rice is not significant. In addition, the salinity tolerance genotypes, when compared with sensitive genotypes, had higher amount of K+ distributed into the shoot, and had higher amount of Na+ accumulated at root, the salinity tolerance of root is higher; the contents of K+ in shoot is higher, and had lower contents of Na+ in shoot, the ratios of Na+/K+, Na+/Ca2+ and Na+/Mg2+ in shoot are decreased significantly also; however, the content of K+ in root is lower, while the ratios of Na+/K+, Na+/Ca2+ and Na+/Mg2+ in root are increased significantly.4 The content of Na+ and the ratios of Na+/K+, Na+/Ca2+ and Na+/Mg2+ in new plant tissues are lower than old tissues significantly under saline condition, but the content of K+ in new tissues is higher than old tissues. The content of Mg2+ in leaves is higher than other plant tissues significantly. The larger amount of Na+ is distributed into old plant tissues, while larger amount of K+ is distributed into new plant tissues, and maintained lower ratios of Na+/K+,Na+/Ca2+ and Na+/Mg2+ in new plant tissues is a effective method to increase the salt tolerant ability.5 Compared with sensitive genotype, the photosynthesis effective, transpiration effective, stomatal conductance and leaf water potential of tolerant genotype are higher under saline condition; so the biomass of tolerant genotype is higher than sensitive genotype, and the sodium content of tolerant genotype is lower than sensitive genotype. There is no significant difference of total sodium absorption between sensitive and tolerant genotypes, root biomass of tolerant genotype is higher than sensitive genotype, and the amount of sodium in root of tolerant genotype is higher than sensitive genotype, so the amount of sodium in shoot of tolerant genotype is lower than sensitive genotype. There is not enough time to product more salt tolerant mechanisms during the early salt stress stage, and the injury of plant which caused by large amount of sodium absorption at early stage can not be alleviated in the future. However, because of the large amount of ABA in root of tolerant genotype has been produced during the early salt stress stage, the stomatal conductance in leaves of tolerant genotype is decreased significantly, and the transpiration effective is decreased significantly also. The sodium absorption has been inhibited by these physiological responses, and the salt stress of rice plant is alleviated at a large scale.It can be concluded that, the salt tolerant ability can be affected by plant nutrients contents and nutrients metabolic significantly under saline condition; because of some special physiological indexes:carbornhydrate metabolic, chlorophyll content, plant nutrients content and distribution, salt tolerant genotype has higher tolerant ability under saline condition. Transplanting with soil, delaying the transplanting time and N application are effective methods to increase the ratio of seedling transplanting survival; the salt tolerant ability of seedling increasing is very important for the production increasing. The concentrations of Ca2+ and Mg2+ in culture solution are increased properly can improve the salt tolerant ability of plant significantly under saline condition, and the effective of Ca2+ is higher than Mg2+. Compared with sensitive genotype, the tolerant genotype has special root physiology characters, higher biomass and the regulation of plant hormone (ABA), that's why the tolerant ability of tolerant genotype is higher than sensitive genotype significantly under saline condition.
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