The Key Physiological Response to Alkali Stress by the Alkali-Resistant Halophyte Puccinellia tenuiflora is the Accumulation of Large Quantities of Organic Acids and into the Rhyzosphere

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. To identify the physiological mechanisms involved in this plant's resistance to alkali stress, the relative growth rates, the quantities and compositions of organic acids accumulated and secreted through the roots into the rhyzosphere, the concentrations of inorganic ions, proline and other solutes accumulating in the shoots were measured. The results show that the organic acid constituents in the shoots and roots were much the same. These were predominantly malic acid, oxalic acid, citric acid and succinic acid. The total concentration of organic acids in the shoots increased strongly with increasing alkali stress. However, these either did not increase or they decreased slightly with increasing salt stress. Of the four organic acids, the concentration difference between salt- and alkali-stressed plants was most striking for citric acid. This became the dominant organic acid component under alkali stress. Results show that proline is the main organic osmolyte, whereas the contribution of betaine to osmotic adjustment is insignificant under either salt or alkali stress. The main organic acid accumulated was not only an important organic osmotic regulator, but also an important negative charge contributor, playing important roles in ionic balance and pH adjustment. The concentrations of Na+, K+, Cl- and of organic acid were 80.7% of all solutes under salt stress. The concentrations of Na+, K+, Cl- and of organic acid were 85.4% of all solutes under alkali stresses. The ionic balance was disrupted by the strong increase in Na+ content under alkali stress. This perhaps explains why large amounts of the organic acids were accumulated. The organic acid concentration in the roots was lower than in the shoots. The roots secreted citric acid into the rhyzosphere only under alkali stress, secretion of the other organic acids was not detected. Therefore, citric acid secreted from the roots probably plays an important role in pH adjustment in the rhyzosphere of P. tenuiflora.