| According to the soluble components of salt-alkaline grassland of the Northeast of China, eight-week-old seedlings of Puccinellia tenuiflora were stressed by exposure to 1:1 molar-ratio mixtures either of the two neutral salts NaCl and Na2SO4 or of the two alkali salts NaHCO3 and Na2CO3, which were simulated conditions of 0-24mmol/L salt stress or alkali stress. To identify the physiological mechanisms involved in this plant's resistance to alkali stress, the relative growth rates, dry matter accumulation, nitrogen metabolism, organic acid metabolism, anti-oxidative system, osmotic adjust and ionic balance to salt or alkaline stress were detected and measured.The experimental results and conclusions were as follows:1. Effects on Growth and Nitrogen Metabolism of P. tenuiflora under Salt and Alkali Stresses.The total biomass, relative growth rates, total nitrogen of P. tenuiflora under salt or alkali stress initially increased and then decreased. The results showed that lower salinity of neutral salts (≤120mmol/L) or alkali salts promoted the growth, dry matter accumulation, nitrogen absorption of P. tenuiflora, while higher salinity of neutral salts or alkali salts inhibited its growth and nitrogen absorption evidently, and their inhibition became more and more obvious with the salinity increased although alkali stress was more inhibitory than salt stress. Therefore, the effects of salt stress and alkali stress on nitrogen metabolism in P. tenuiflora correlated with effects on its growthe under salt stress and alkali stress.2. Relation between Anti-oxidative System and Salt- or Alkali- Resistance. in P. tenuifloraBoth salt stress and alkali stress led to the accumulation of malonaldehyde and the increasing of osmosis of electrolytes outside the membrane, however, alkali stress was predominantly obvious than salt stress. This demonstrated that the alkali stress was more destructive to membrane.The activities of superoxid dismutase (SOD), perodidase (POD) and catalase (CAT) increased under salt stress, while the activity of catalase decreased gradually although SOD activity increased by 1.4～1.6times of it and POD activity increased by 3.5 times of it. This demonstrated that the unique anti-oxidative system was one of main factors related to its salt or alkali resistance. 3. Accumulating Large Quantities of Organic Acids with Citric Acid As a Dominant Component were the Key Physiological Responses of P. tenuiflora to Alkali StressThe results indicate that organic acid concentrations in P. tenuiflora have no obviously changes and even decreased slightly. However, organic acids accumulate in detectable quantities only under alkali stress above 60 mM. Citric acid is always the dominant component of the accumulating organic acids in both the shoots and the roots. Therefore, the accumulation of organic acids, mostly citric acid, appears to be a specific physiological response of P. tenuiflora to alkali stress. Therefore, accumulating large quantities of organic acids with citric acid as a dominant component in P. tenuiflora under alkali Stress is closely related to the pH regulation and iron balance. Only alkali stress caused organic acids to accumulate, plants clearly shows that they have different physiological mechanisms under alkali stress and salt stress.4. The Characteristics of Organic Acid Accumulation in P. tenuiflora Roots Under Alkali StressThe organic acid concentration in the roots of P. tenuiflora under alkali stress was very low and has no obviously changes. The one of reasons is decided by the metabolism character in P. tenuiflora roots, and another reason is relative with is related to the secretary activity of the roots. Under alkali stress, the roots secreted citric acid which was the only organic acid detected. Calculation showed that the average amount of organic acid secreted each day was 5.92μmol per seedling. But it very obviously that P. tenuiflora roots adapt to alkali environment and pH regulation. Above all, the regulation roles of P. tenuiflora roots to high pH under alkali stress, perhaps were secretes organic acids and produce CO2 by respiration. The site of action is apoplast in cortex of root tips, instead of outside root epidermis5. Osmotic Adjust and Ionic Balance Roles under Salt and Alkali StressesThe result show that proline concentration under salt and alkali stresses increased obviously, but proline concentration was very low compared to total concentration. So proline contributed in a very limited way to total osmotic adjustment. Under salt stress, important osmolytes include Na+, K+, and Cl- are all, but under alkali stress, include Na+, K+, and organic acid.From the result of correlation analysis between organic acid concentration and other solute concentration, organic acid accumulation is closely correlated with Na+ concentration. Na+ concentration increased sharply under alkali stress, it induced iron balance was destroyed. Perhaps, these were primary reasons caused by organic acid accumulation. Percentage of the contribution of various free anions to total anions is deferent obviously. Under alkali stress, organic acid contributed 67.8% of total negative charge, while the total contribution of the inorganic anions was only 39%.So, the character of iron balance in P. tenuiflora is obviously different.Above all, organic acid accumulation, osmotic adjustment and maintaining ionic balance in P. tenuiflora under alkali stress, and adjust high pH by secreted organic acids from Rhyzosphere. Therefore, organic acid is the most key to regulate vivo and vitro.
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