Effects of salt stress on physiological characteristics of two strains of Malus micromalus Mak. with different salt tolerance

Abstract: 【Objective】The Bohai Bay Rim and Loess Plateau of Northwest areas have sufficient sunlight and large temperature differences between day and night. They are the two dominant apple produc-tion areas in china. However, increasingly serious soil salinization has become an important factor re-stricting the development of the apple industry in these two major production areas. The salt tolerance of apples mainly depends on their rootstocks. Therefore, selecting apple rootstocks with strong salt toler-ance is one of the effective ways to solve the above problems. Malus micromalus Mak. has the advan-tages of cold resistance, salt-alkali tolerance, drought resistance, and good affinity with apple varieties. It is a common rootstock in Northern apple production areas. A salt-tolerant strain (No. NY) was ob-tained from previous study on the selection of salt-tolerance seedlings of Malus micromalus Mak. NY also performed well in the adaptability to the coastal area. Therefore it has high potential value for application. While its salt-tolerant mechanism is still unclear. In this experiment, the NY strain was used as the test material, and the common strain of Malus micromalus Mak. (No. PT) was used as the control to explore the changes in the physiological indexes of the two strains under different salt stress.【Methods】The plantlets of the NY and the PT nutrient pot were proliferated through tissue culture. The rooted  plantlets were grown in polyethylene plastic pots containing vermiculite and peat (V/V, 1∶1), watered with Hoagland nutrient solution, 4 plants per pots, 3 pots per treatment (ie 3 repetitions). 10 days later, the plantlets with proper growth were chosen for salt stress test. 6 NaCl concentrations (0, 2, 4, 6, 8, 10 g · L^-1) with Hoagland nutrient solution as the basic solutio were designed, . In order to reduce the salt stress reaction of the plants, the NaCl concentrations of each treatment was slowly increased (20% per day) for 5 days, and they reached the set salt concentration on the same day which was recorded as the first day of salt stress. After that, the Hoagland nutrient solution containing the corresponding NaCl con-centration was excessively irrigated every day, and the experiment was ended on the 40th day Six plants were collected for the each treatment of two strains, and dried, then the dry matter content of each plant above and below ground was measured. The leaves of the two strains of the each treatment were collect-ed for the determination of relative conductivity, malondialdehyde, soluble sugar, soluble protein and proline content, SOD, CAT and POD activities.【Results】With the increase of NaCl concentration, the total dry weight, above-ground dry weight, and underground dry weight of the two strains gradually de-creased, and the root-shoot ratio gradually increased. Salt stress inhibited the biomass accumulation of the two strains; with the increase of NaCl concentration, the relative conductivity value and malondial- dehyde content of the two strains gradually increased. Under high concentration (8 g · L^-1) NaCl stress, the increase of the above indicators of the NY strain was significantly lower than those of the PT strain ,It would be possible that the damage to the plasma membrane of NY strain was less than that of PT strain under salt stress; With the increase of NaCl concentration, the soluble protein and proline of the two strains showed an increasing trend. Under high NaCl stress, the above indicators of the NY strain reached a significant level of difference from the control, while the PT strain did not reach a significant level of difference; With the increase of NaCl concentration, the soluble sugar content of the NY strain increased first and then decreased, and the soluble sugar content of each treatment was higher than that of the control. While the soluble sugar content of the PT strain showed a slow increase trend. Under 8 g·L^-1 NaCl stress, the soluble sugar content of the PT strain was significantly higher than the control. With the increase of NaCl concentration, the SOD and CAT activities of the two strains both increased first and then decreased, and the POD activity showed a gradual decline. Under salt stress the SOD and CAT activities of the NY strain increased to a higher degree than those of the PT strain.【Conclusion】Under salt stress, the NY strain would relieves osmotic stress by increasing the content of osmotic adjustment substances. The NY strain may maintain the balance of active oxygen in plants by improving the activi-ty of SOD and CAT, and reduce the oxidative damage caused by salt stress. In turn, the damage to its cell membrane caused by salt stress would be reduced. As a result, the damage to its cell membrane caused by salt stress would be alleviated, and its salt tolerance would be enhanced.