Solute Accumulation And Ion Balance Traits In Shoots Of An Alkali-tolerant Halophyte Kochia Sieversiana Under Salt-alkaline Mixed Stress

The salinization of soil is a widespread environmental problem and an important factor in limiting agricultural productivity. For inland salt-alkalinized soil, especially salt-alkalinized steppes in the northeast of China, salinization and alkalization frequently co-occur, consequently, a salt-alkaline mixed stress is the main problem of salt-alkalinized soil. A naturally alkali-resistant halophyte, Kochia sieversiana(Pall.) C. A. M., was chosen as the materials for this experiment to probe theoretically the traits of the salt-alkaline mixed stress on plants and the adaptative mechanisms of plants to it not only, but also to provided the foundation for the ecological recovery of salt-alkalinized land and for the utilization of natural alkali-tolerant genes.The seedlings of an alkali-tolerant halophyte K. sieversiana were treated under 25 salt-alkaline mixed conditions with different alkalinities and salinities, which were established by mixing two neutral salts (NaCl and Na2SO4) and two alkali salts (NaHCO3 and Na2CO3) at various proportions. Nine physiological indices including the solute contents of K+, Na+, Cl- ,SO42-,NO3-, betaine, proline, and organic acid in shoots and relative growth rate (RGR) were determined, the components of organic acid were analyzed, in order to explore the traits of growth, osmotic adjustment, and ionic balance in the seedlings of K. sieversiana under salt-alkaline mixed stress.The results showed that the growth of K. sieversiana seedlings was stimulated at moderate neutral salt stress (80mmol·L-1), and under other stress conditions its RGR decreased gradually with increasing salinity or alkalinity. In the experimental condition, the highest stress intensity of that K. sieversiana could resist reached to 400 mmol·L-1 of salinity and pH 10.7, which proved that K. sieversiana was a halophyte species with high alkali tolerance. Under 25 salt-alkaline mixed conditions with different alkalinities and salinities, the values of K+/Na+ in shoots were basically steady (approximate 0.49). It indicated that there was not a competition between K+ and Na+ to be absorbed. It could be considered that the relative stabilization of K+/ Na+ in K. sieversiana shoots under various salt-alkaline mixed stresses was a key physiological trait for maintaining K nutrition and ionic balance in the cell. The significant accumulation of proline occurred only under the intensity of salt-alkaline mixed stress was over 240mmol·L-1 or pH 9.8, and its quantity was merely about 1/74 of betaine. It implied that the proline accumulation in the shoots of K. sieversiana might be not a physiological response to osmotic stress, but resulted from metabolism being interrupted by high stress intensity or from an adaptive response with special physiological function.The osmotic adjustment mechanisms in the shoots of K. sieversiana were not different obviously under various salt-alkaline mixed stresses. The shared essential features were that inorganic ions and organic acid accumulated largely in the vacuoles, and betaine accumulated in cytoplasm. Na+ and K+ were the dominant contributors to positive charge in cell, the ratio of their contribution percents was 2:1 nearly, and retained unchanging basically. The intracellular negative charge was mainly from organic acid, Cl-, NO3-, and SO42-. Among them, the organic acid was a dominant contributor, its contribution percent to negative charge was as high as 60.63-71.09%. Its contribution percent was not significant changed with increasing salinity, but increased with increasing alkalinity.The experimental results also showed that the trait of organic acid accumulation in K. sieversiana was different from that in Aneurolepidium chinense and Puccinellia tenuiflora in them the organic acid accumulation occurred only under alkali stress. The accumulation of organic acid in K. sieversiana was promoted by salt stress not only, but also promoted by alkali stress. The organic acid accumulated, with oxalic acid as a dominant species, plays an important role in pH adjustment and maintain pH stabilization in vivo not only, but also, as a dominant contributor to negative charge, is more important for maintaining ion balance in cell.