Studies On The Mechanism Of Salt Resistance Of Two Halophytes And Amelioration Of Saline Soil By Salt-resistance Plants

Soil salinization represents an increasing resource and environmental problem. Although the world's land surface occupies about 13.2×109ha,25% of the area are saline soil. Meanwhile, secondary soil salinization becomes more and more serious. Great effort has been undertaken to improve this problem in recent twenty years. Numerous studies have investigated the mechanism of salt resistance of plant using only NaCl, but two vital aspects have been neglected:alkali and salt-alkali mixed stress. Neutral (NaCl and Na2SO4) and alkaline (NaHC03 and Na2CO3) salts are two distinct stresses for plants, and should be defined as salt and alkali stress, respectively. When saline soil contains CO32- and/or HCO3-, it causes injury to plants not only through salt stress but also through alkali stress. In fact, soil salinization and alkalinization frequently co-occur in nature, so the conditions in natural salt-alkali soil are very complex. Therefore, the problem of mixed salt-alkali stress should be recognized and investigated as thoroughly as that of salt stress. The studies on the the response of plant to mixed salt-alkali stresses in special halophytes could indicate the mechanism of salt-alkali resistance of plant and be also helpful to amelioration of saline soil by biological remediation.Studies on amelioration of saline soil have been conducted for more than 100 years. There are three main methods:engieering remediation, chemical remediation, and biological remediation. Although engineering and chemical remediation could be both helpful for saline soil, meanwhile accompanied with side-effects. For example, when using fresh water for salt-leaching, necessary mineral elements for plant were also rinsed out. Meanwhile, chemical remediation could lead to secondary pollution. The cost of these two methods is higher. Biological remediation has remarkable advantages such as low cost, good disposal effect, low environmental impact, and can not lead to secondary pollution and damage to soil environment.Seeds and seedlings of Spartina alterniflora and Suaeda salsa were treated under different conditions of salinity and alkalinity by mixing NaCl, NaHCO3, Na2SO4, and Na2CO3 at various proportions.The results of seed experiments are showed as follows, respectively.(1) Seed germination percentages and germinate rates of S. alterniflora and S. salsa were not significantly reduced at low salinity, and then decreased with increased salinity at each pH group.(2) Under high salinity, seed germination percentages and germinate rates of S. alterniflora and S. salsa were both lower; Conversely, uner low salinity, they were both higher. Ungerminated seeds germinated well after being transferred to distilled water from treatment solutions, indicating that seeds could remain viable in high salt-alkali habits. Therefore, seeds of S. alterniflora and S. salsa are well adapted to salt-alkali habitats via a high capacity for germination recovery. For these two species, a fraction of moistened ungerminated seeds builds up a seed bank in the presence of salinity, and germinated under suitable germination conditions opportunistically.(3) Stepwise regression analysis indicates that salinity is the dominant factor, while pH and buffer capacity are less important for salt-alkaline mixed stress.The results of seedling experiments are showed as follows, respectively.(1) The results showed that S. alterniflora was capable of surviving all treatments under low pH (pH<8.30) regardless of the levels of salinity, and the highest salinity was 600 mmol L-1 ([Na+]=900 mmol L-1), which indicate its high salt resistance; Under moderate alkalinity, it was also capable of surviving, which indicate its moderate alkali resistance. S. salsa also had high salt resistance, and the highest salinity was 360 mmol L-1([Na+]=480 mmol L-1). Compared to S. alterniflora, S. salsa had higher alkali resistance, and was capable of surviving under pH=10.40.(2) Relative growth rate (RGR), root activity, and electrolyte leakage rate (ELR) were all increased with increasing salinity and pH. Furthermore, for salt-alkali mixed stress, it is concluded that reciprocal enhancement between salt and alkali stress is a characteristic feature.(3) The content of Na+ increased, while K+ decreased with increasing salinity and pH in S. alterniflora and S. salsa seedlings, suggesting competitive inhibition between absorptions of Na+ and K+. The contents of proline in S. alterniflora and S. salsa were both increased with increasing salinity and pH, suggesting that alkali stress may also cause accumulation of this solute for pH balance in plant body. However, the content of carbohydrates increased in S. alterniflora, while decreased in S. salsa with increasing salinity and pH. Thus, the mechanism of carbohydrates accumulation in halophytes responding to salt-alkali stress may be varied and deserves further investigation.(4) The combined action of salinity, pH, and buffer capacity should be considered when evaluating the strengths of mixed salt-alkali stress.We cultivated more than twenty salt resistance grasses including the native species S. salsa, and species introduced from USA in natural coastal saline soil. After three years'cultivation, five species including S. salsa, Elytrigia elongata, Festuca elata, Bouteloua dactyoides, Psathyrostachys juncea grew well in natural coastal saline soil. The objective of this study was to determine the effect of the selected plants cultivation on the physical, chemical and microbiological properties of natural coastal saline soil. The results showed that plant growth altered microclimate, increased litter input, and hence, improved soil moisture, contents of total carbon and N, available P and K, numbers of microorganism, activities of urease, dehydrogenase,β-glucosidase activities, and decreased soil bulk density and soil electrical conductivity (salt content). Plant cultivation could be an important role in the restoration of the coastal saline soil, and severe salinization in the coastal saline land could be reversed by a management approach of establishing suitable vegetations on a large spatial scale.