Biological Responses Of Different Maize Varieties At Seedlings To Salt Stress And Mechanism Of Tolerance

China's soil salinization is one of the most significant constraining factors that affecting the stable development of maize production, salt stress restrains the seedlings growth, decline bud growth force, radicle few and short, enervate seedlings, low survival rate, serious affecting yield, growth and development. Therefore, improve the salt stress tolerance of maize varieties is one of the most efficient ways to development and utilize of this part of the resources. In this paper, 120 maize varieties select from Inner Mongolia and Liaoning province for the trial materials, through various indexes of the germination period identify and classify the salt stress tolerance, Screen out the typical varieties, and then cultivated these varieties in solution culture and sand culture, and then in the seedling period analyzed their salt tolerance difference under these two kinds of cultivate conditions, This research project has been studied on plant morphology, dry matter accumulation, osmotic adjustment matter, membranaceous peroxidation and protective enzyme activities, photosynthesis and fluorescence characteristics, etc., With a view to exploring the salt tolerance mechanism difference in different salt tolerance varieties as well as in different conditions of salt stress. The main conclusions are as follows:1. Under NaCl stress, the concentration of 180mmol / L can be used as identify and classify different salt tolerance varieties. In this concentration the cluster and analyze can be divided 120 maize varieties into five levels: high-tolerance, salt-tolerance, medium-sensitivity, salt-sensitivity and high-sensitivity. Under Na₂CO₃ + NaHCO₃ stress, the concentration of 90mmol / L +90 mmol / L can be used as identify and classify different salt tolerant varieties. In this concentration cluster and analyze 120 maize varieties can be divided these varieties into four levels: high-tolerance, medium-tolerance, medium-sensitivity, alkaline salt-sensitivity. From the results of the classifications, the neutral salt and the alkaline salt have significant difference between varieties at all levels.2. Under stress of NaCl, Na₂CO₃ + NaHCO₃, two maize varieties' height and leaf area changes trend are identical, and increased with the growth stage progressing and decreased with the concentration of salt stress reducing, and under salt stress the injury rate of salt-sensitive varieties greater than the salt-tolerant varieties. Under the stress of NaCl, the main radicle length, total root system length, root surface area, root volume are all increased with the growth stage progressing, except the concentration of 40mmol / L which can be boosts the growth when compare with the checks while the rest concentrations restrains the growth of maize. Under the stress of Na₂CO₃ + NaHCO₃, the main radicle length, total roots length, root surface area and root volume are increased with the advancing of growth periods, and increased if the salt concentration reduced, and the various indexes change range of the salt-tolerant varieties less than salt-sensitive varieties. With the advance of growth periods the average diameter of the root system is increasing, and they were decreasing with the salt stress concentration increased. Average diameter becomes greater when the severe of salt stress increased. These rules are more obvious in Na₂CO₃+NaHCO₃ stress than in the stress of NaCl.3. The effects of moderate NaCl stress which boosts the growth of roots system less than stems and leaves while under severe salt stress the roots system grow rapidly thus increase the root cap ratio. Under the stress of Na₂CO₃ + NaHCO₃, in the early growth periods either above ground parts or under ground parts' dry weight change difference isn't obvious, yet with the progressing of growth stages and the higher salt stress concentration the dry weight of plant tissue reduced more severely, and the root-cap ratio was increased with the concentration of salt stress increasing. The varieties of salt-sensitive varieties injury rate greater than salt-tolerant varieties.4. Two varieties of soluble protein, soluble sugar content, dissociate proline were increased with increasing concentration of NaCl, salt-sensitive varieties increase range less than salt-tolerant varieties. Along with the salt stress concentration increasing the dissociate amino acid content in the leaves will be increased, if the salt stress concentrations belowl20mmol /L the dissociate amino acid content significantly higher than other treatments. Both these two varieties, the soluble protein content in roots system and leaves increased with Na₂CO₃ + NaHCO₃ stress concentration increasing, the content of soluble protein in leaves were significantly higher than in the root system. Along with the salt stress concentration raising the soluble sugar content increased in leaves while there is slightly difference between leaves and roots system. Dissociate amino acids content of these two varieties' leaves increased with the concentration of salt stress increasing, in the root system, these two varieties the dissociate amino acids content significantly lower than the treatment which is below 0mmol / L concentration. Dissociate proline content in root system and leaves of these two varieties were significantly increased with the concentration of salt stress increasing, from absolutely content point of view, the dissociate proline content in the leaves was higher than in the root system.5. In these two varieties' leaves and root system, the relative conductivity increased with salt stress concentration increasing, yet relative conductivity in roots system has greater value. This performance rule is identical in both these two types of salt stress. Under the stress of NaCl, Na₂CO₃+NaHCO₃, the superoxide anion and malondialdehyde content in roots and leaves of the two varieties were increased with salt concentration increasing. From an absolutely content point of view, the content of the basic salt stress more than neutral salt content, this can be illustrate that the extent of membranaceous peroxide under basic salt stress is more serious than under neutral salt stress. The activity of three types of protection enzyme SOD, POD, CAT will be increasing along with the NaCl concentration stress increased, while the changing in leaves and roots trend to be identical. Under Na₂CO₃ + NaHCO₃ stress, the changes trend of POD, CAT is same as in the natural salt stress, but in the leaves, in the low concentration of alkaline salt stress the activity of SOD will be increase, yet it will be decrease in high concentration of alkaline salt stress.6. In two maize varieties' leaves, the content of chlorophyll a + b, chlorophyll a, chlorophyll b will be increased with the salt concentration decreasing, meanwhile, the ratio of a to b will be increase, this rule of performance is identical under the two types of salt-stress conditions. Maize leaf net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration, transpiration rate decreased with the lower salt concentration, the changes trend of photosynthetic rate, stomatal conductance and intercellular CO2 concentration were identical, It can be considered that stomatal limitation is one of the main reasons that lead to photosynthetic rate decline. Photosynthetic characteristics change caused by two kinds of salt stress also are consistent. The initial fluorescence F₀ of the two varieties are increased with the salinity concentration increasing, variable fluorescence Fv, Fv / Fm decreased with higher salinity concentrations. In terms of fluorescence characteristics change, the two types of salt stress change rules also are identical.