Research On The Salt-Alkali Resistance Mechanism Of Alfalfa And Oat

Salinization and alkalization of soil is one of the widespread and increasing environmental crisises. It also seriously restricts the development of agriculture and stockbreeding. It is widespread recognized that alkalization of the soil may be a more severe problem than salinization. Researching on the plant response to salt and alkali stresses is very important for improving crop and pasture production under such conditions. According to the present condition of salt-alkali soil, we used oat and alfalfa as the experimental materials and explored the effects of six type of separate salts on seed germination of alfalfa, effects of mix salt-alkaline stresses on biochemistry in seed germination of alfalfa and, effects of mixed salt-alkali stresses on seedlings of alfalfa, effects of salt stress or alkali stress on biochemistry in seedlings of oat, and response mechanism of oat under three types of salt, e.g. NaCl、NaHCO₃ and Na₂CO₃. The results listed as follows:（1） Effects of six types of salt stresses on germination of alfalfa: with the increasing salinity stress, when CaCl₂ concentration was between 25-75 mmolL^-1, germination rate, germination vigor, germination index and vitality index, were all higher than that of the control, indicating that CaCl₂ can promote seed germination; but when CaCl₂ concentration was higher than 100 mmolL^-1, it had a inhibiting role in germination, which was similar to all kinds of the concentration of the other five types of salt. It was obvious that Na₂CO₃ significantly inhibited germination vigor. Low concentration of Na₂CO₃ strongly restrained germination, especially at 50mmolL^-1 salinity in which seed can not germinate. When salinity concentration in the soil was very high, and reached the impatience threshold of the seeds, they can not germinate again. The salt-tolerance of alfalfa seeds was different, i.e. MgCl₂>CaCl₂>NaCl >Na₂SO₄>NaHCO₃>Na₂CO₃. With the increasing concentration, alfalfa plumule,radicle growth were all restricted. At the same salinity stress, total biomass in alfalfa was influenced according to CaCl₂> NaCl> Na₂SO₄> Na₂CO₃. With the increasing salinity concentration, the proline contents in alfalfa seedling significantly increased in six types of salts but insignificantly under different salt type.（2） Effects of mixed salt-alkaline stresses on seed germination of alfalfa: 6 treatments were designed by two neutral salts （NaCl and Na₂SO₄） and two alkaline salts （NaHCO₃ and Na₂CO₃） in different molar ratio, and also based on the increasing alkalinity. Within each group, six concentrations were used. Total 36 mixed stress treatments （various salinity, alkalinity and pH） were set. Germination index were measured under the pH values of treatment solutions ranged from 7.03 to 10.68. The results showed that with the increasing salinity, germination rate, germination vigor and seedling length all decreased significantly. Membrane permeability and proline content increased, solute sugar content decreased; Na+ content increased, K+ content decreased, Cl-、SO₄2- content firstly increased and then decreased. The interactions of salinity and alkalinity was obvious, had a seriously synergism role. In addition, Alfalfa was more sensitive to alkalinity.（3） Effects of mixed salt-alkaline stresses on seedlings of Alfalfa: According to the complicated salt composition of local soil in Northeast China, 6 treatments were designed by two neutral salts （NaCl and Na₂SO₄） and two alkaline salts （NaHCO₃ and Na₂CO₃） in different molar ratio, and also based on the increasing alkalinity. Within each group, 5 concentrations were used. Total 30 mixed stress treatments （various salinity, alkalinity and pH） were set. The pH values of treatment solutions ranged from 7.03 to 10.32 and salinity of treatment solutions ranged from 24 to120 mmolL^-1. We used these treatment solutions to stress seedlings of alfalfa, and then measured the physiological and ecological indexes. The results showed that physiological index of alfalfa seedlings had regular changed in response to the mixed salt-alkaline stresses, all of the seedlings could survive when pH < 8.30, however, when pH>9.69, the seedlings were all died. Survival rate, belowground biomass and aboveground biomass of the seedlings were affected by salinity, pH and their interactions.（4） Responses to salt stress and alkali stresses of oat: two neutral salts （NaCl and Na₂SO₄） and two alkaline salts （NaHCO₃ and Na₂CO₃） were both mixed in a 2:1 molar ratio and used as A and B groups, respectively. Six total salt concentration gradients (48^-168 mmolL^-1) were set in each group, which simulated 12 mixed salt conditions with different salinities and alkalinities （pH value）. Stress treatments were carried out on oat seedlings and the effects of saline and alkaline stresses on the growth and physiology of oat seedlings were discussed. The results showed that biomass, moisture content and chlorophyll content decreased and cell membrane permeability significantly increased under alkaline stress. Saline stress did not have obvious effect on pH value in the tissue fluids from stem, leaves and root, but alkaline stress increased pH value in the tissue fluid of root, and high concentration alkaline stress caused mass mortality of seedlings. The contents of Na+, Na+/K+, SO₄2- increased more and the contents of K+, NO3- and H2PO4- decreased more under alkaline stress than under salt stress; the increment of Ca2+ was greater under saline stress than that under alkaline stress; the Cl- content was increased obviously under saline stress but had little change under alkaline stress. The increment of proline was largely increased under alkaline stress, and the content of organic acid was remarkably increased under alkaline stress but kept the same under saline stress. The results indicate that high pH caused by alkaline stress created more harmful effects on growth and physiological changes of oat seedlings especially broke the pH stability in root tissue fluid. Physiological response mechanisms and adaptive strategy of oat seedlings under saline stress and alkaline stress were different, which mainly took the way of accumulating organic acid, proline and SO₄2- under alkali stress but accumulating Cl-, proline and SO₄2- under saline stress.（5） Responses of oat to three types of salt stress: NaCl、NaHCO₃ and Na₂CO₃ were applied as three groups （A, B and C）, respectively. Every group included 5 salinity gradients (24^-144 mmol L^-1). Oat seedling was treated and growth and physiological indexes were measured. We explored the effects of NaCl、NaHCO₃ and Na₂CO₃ stress on growth and physiological indexes in belowground and aboveground parts of the seedlings. The results suggested that under three types of saline stress, biomass , number of tiller, water content, chlorophyl content, K+ and Ca2+ all decreased, and the extent of the decrease was that Na₂CO₃>NaHCO₃>NaCl. Na+, Na+/K, SO₄2-, proline content and membrane permeability all increased, their extent was Na₂CO₃>NaHCO₃>NaCl. Under NaCl stress, no changes were found in tissue fluid pH and organic acid in both shoot and root, Cl- content increased. Under NaHCO₃ and Na₂CO₃ stresses, organic acid increased significantly, and the extent of decrease in belowground biomass, water content, K+, Ca2+, Cl-, SO₄2- were higher than aboveground parts. Na+, Na+/K+, proline and organic acid all increased, but the increasing extent was lower than aboveground parts. Meanwhile, high concentration of NaHCO₃ and Na₂CO₃ caused root pH and organic acid content increased significantly, resulting in lots of seedlings death. The survival rate of oat indicated that tolerance limit of oat was 96mmolL^-1 under Na₂CO₃, 144mmolL^-1 under NaHCO₃. We can summarize that NaHCO₃ and Na₂CO₃ stresses had a greater effect on growth and physiology of oat seedlings, especially interrupted the pH stability of root. In order to resist the harm effects of alkaline stress, oat accumulated lots of organic acids, proline, and changed the distribution proportion between aboveground and belowground parts to adapt to salt-alkali environment. Oat accumulated proline, Cl- and SO₄2- under NaCl Response mechanism and adaptive strategy was different among the three salts for oat.