Physiological Mechanism Of Salt-Alkaline Tolerance In Kochia Sieversiana

Soil salinization has become a global environmental problem. Soil alkalization and salinization frequently co-occur. The alkali stress and salt stress are two distinct kinds of stresses actually. Alkali-stress more complex and serious destructive effect on ecological environment than that salt-stress. In present study, we chosen an alkali-tolerant halophyte Kochia sieversiana as the test organism, probed the physiological mechanisms of K. sieversiana to salt and alkali stresses, especially, we revealed oxalic acid accumulation and distribution of K. sieversiana and its physiological contribution to the adaptability of K sieversiana to saline and alkaline conditions. On the basis of these improved the theoretical basis for the treatment of salt-alkalinized habitats.1. The mechanisms of ion balance and osmotic adjustment in Kochia sieversiana under salt- and alkali-stressIn the present study, seedlings of Kochia sieversiana were exposed to the following conditions: non-stress, salt stress （molar ratio of NaCl: Na₂SO₄= 1:1, salinity: 200mM） and alkali stress （molar ratio of NaHCO₃: Na₂CO₃= 1:1, salinity: 200mM）. The solutes accumulation and the differences in solutes distribution under salt- or alkali- stress were monitored in order to investigate the dynamic process and distribution differences of osmotic adjustment and ion balance in K. sieversiana during responding to salt and alkali stresses. Our work revealed the physiological mechanisms in K. sieversiana responding to salt and alkali stresses more deeply.The contributions of different solutes to osmotic adjustment and ion balance were different, and their contributions under salt- or alkali- stress were discrepant too. But anyhow Na⁺, K⁺, organic acid, and Cl^- were the main osmolytes under salt stress, while Na⁺, K⁺, organic acid were the main osmolytes under alkali stress. Whether under salt stress or under alkali stresses, Na⁺ and K⁺ were the main contributors to positive charge, and organic acid was the dominant contributor to negative charge.There were differences among different parts of K. sieversiana in the osmotic adjustment and ion balance, but the common point: K⁺, Na⁺ and organic acid were the main osmolytes in different parts of K. sieversiana. The positive charge mainly came from Na⁺ and K⁺, and the negative charge mainly came from organic acid in different parts. Remarkably, the contributions of organic acid to osmotic adjustment and ion balance was concentrated in leaves. Organic acid play physiological role preferentially in leaves, on which photosynthesis productivity was based, and ensured to maintain normal physiological metabolism in whole plant. The effects of organic acid on salt and alkali stresses were different, and its effect was weaker under salt stress. That was because that Cl- or SO42- were accumulated selectively in shoots, which accumulation process consumed less energy. The selective absorption of inorganic ions in K. sieversiana to adapt to salt stress is an important survival strategy, while the accumulation of organic acids was one of the key 2. The mechanisms of organic acids metabolism in K. sieversiana under salt and alkali stressesThe characteristic of mechanisms of organic acids metabolism under salt and alkali stresses were discussed by analyzing the dynamic accumulation and differences of distribution of organic acids in K. sieversiana.The results showed that different parts of K. sieversiana differed in organic acids metabolic regulation mechanisms of adaptability to alkali conditions. Oxalic acid was the main constituent in shoots, especially mature leaves; malic acid was the main organic acid in old stems; and succinic acid was the dominant organic acid in roots. However, for the whole plant, oxalic acid was the main organic acid observed in plants, and it was also the main organic acid observed in plants under either salt or alkali stress. The root secretion experiment indicated that oxalic acid was not detected in root exudates. Oxalic acid played its physiological action in functional leaves on which photosynthesis productivity is dependent on, and ensured plant had normal physiological metabolic. The accumulation of oxalic acid was one of the significant endogenous factors which determined the adaptive characteristics of both salt-resistance and alkali-resistance for K. sieversiana.3. Physiological contribution of oxalic acid and the characteristics of its metabolism in K. sieversiana under salt and alkali stressesThe contribution of oxalic acid to salt-alkalinized soil adaptability and the characteristic its metabolism were identified by studying the physiological action of oxalic acid and change of its related enzyme activities. The results showed that oxalic acid anion was not only an organic osmotic regulator, but also a negative charge donator, playing leading roles in maintaining ion balance and pH adjustment in vivo. Results also showed that the oxalic acid content in K. sieversiana and its decomposition （OXO way） were not entirely relevant, but mainly depended on its synthesis. K. sieversiana were exposed to salt stress and alkali stress after 24 hours, GO, ICL and PEPcase activities increased significantly. The analysis of correlation showed that GO was a key enzyme for oxalic acid biosynthesis. Therefore, this proved that GO was the key limiting factor for the accumulation of oxalic acid.K. sieversiana in evolution formed physiological mechanisms to adaption of alkali and salt stresses, mainly by increasing the activity of key enzyme GO to mobilize the metabolism of oxalic acid-based regulation, in order to achieve comprehensive purposes of osmotic adjustment, ion balance and pH regulation.