Physiological Responses Of Chloris Virgata To Various Salt-alkaline Mixed Stress

One of ecological problems people are facing is land salt-alkalinization. In saline and sodic soils, Na⁺, Ca²⁺, Mg²⁺, and K+ are the main cations of dissoluble mineral salt, and Cl-, SO₄^2-, HCO₃^-, CO₃^2-, and NO₃^- are the corresponding main anions. These ions all come from neutral salts or alkaline salts. We can further classify the natural salt stress, in terms of the salt characteristics, into neutral salt stress, alkaline salt stress, and mixed salt stress. For alkalinized grassland in the northeast of China, salinization and alkalization frequently co-occur, and its ecological destroy is bigger than that of simple salinization. Consequently, salt-alkaline mixed stress is the actual main problem in this area.Chloris virgata was a native alkali resistant halophyte, and was used as the material of this experiment. Twenty five kinds of natural salt-alkaline ecological conditions with different salinity and alkalinity (the range of total salt concentration 60-300mmol/L, pH 6.53-10.61) were simulated by mixing NaCl, Na₂SO₄ , NaHCO₃ and Na₂CO₃ in various proportions. Seedlings of C. virgata were treated under these salt-alkaline conditions. Twelve strain indexes, including relative growth rate (RGR),water content, chlorophyll content, electrolyte leakage rate, and contents of inorganic and organic solutes, were determined. On the basis of determination, the relationships between the stress acting factors (salinity and pH) and each strain index were mathematically analyzed to explore the characteristics of salt-alkaline mixed stresses acting on C. virgata and the responses of C. virgata to them.The results showed as follows. (1) The seedlings of C. virgata still survived and remained growth at least when the salinity was up to 300 mmol/L, and pH 10.57. This showed that C. virgata was a high alkali resistant halophyte, and provided a credible evidence for its ecological distribution. (2) All the strain indexes were changed regularly. With increasing the intensity of salt-alkaline mixed stress, relative growth rate, water content, and chlorophyll content decreased; the leakage rate of electrolyte and the contents of Na⁺, Cl^-, SO₄^2-, organic acid, and citric acid increased; K+ content decreased firstly and then increased. The content of proline increased only with increasing salinity. Besides, these indexes showed a common feature, namely there were not too great differences in the strain among A, B, C three groups (pH under 8.92), and were great differences in strains between groups D and E or between A, B, C and D, E (pH above 8.92). This implied that the critical point of C. virgata adapted to alkali stress might be about pH 9. In other words, the effect of alkali stress on the seedlings of C. virgata was relatively weak when pH below this point; whereas turned to great when pH over this point. The result of two-way variance analysis (ANONA) indicated that the effects of salinity and alkalinity on twelve indexes were all significant (P < 0.01), and interactions between salinity and pH were all evident (RGR P < 0.05, others P < 0.01). This outcome meant that neutral salt stress and alkaline salt stress not only coexisted but also interacted between them. (3) The osmotic adjustment of C. virgata was achieved by two main ways: one was the accumulations of inorganic ions and soluble organic solutes in plant; another was reduction of water potential of cell by decreasing the water content of plant. Na⁺, Cl^-, SO₄^2- were the main inorganic osmotica, and Na+ was dominant among them. Proline and organic acid were the most important organic osmotica. And accumulation trait of organic acid in C. virgata was evidently different from Aneurolepidium chinense and Puccinellia tenuiflora. It was not only promoted by alkali stress but also by salt stress, especially under high salinity and alkalinity. Accumulated organic acid functioned not only as an osmotica in osmotic adjustment, but also as anion in ion balance and pH adjustment in C. virgata. The result showed that organic acid is the main resource of anion to maintain ions balance and to steady pH in the seedlings of C. virgata.