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Oat (Avena sativa L) seeds and seedlings were subjected to saline and alkaline stress (from 9∶1 molar ratios of NaCl∶Na2SO4 and NaHCO3∶Na2CO3, respectively) to determine their effect on growth indices, and physiological parameters, to elucidate the mechanism of saline and alkaline stress (high-pH) damage to oat seedlings and the physiological adaptive mechanism of oat seedlings to both stresses, especially alkaline stress.The results showed that alkaline stress had a more severe effect on Oat seedlings than saline stress, which was demonstrated by greater reduction in Relative Growth Rate, Water Content and root system activity.Under saline stress, the changes of net photosynthetic rate (Pn), stomatal conductance (gs), and transpiration rate (E) were no significantly effect, and they were lower than those of control only at salinity over 80 mM.Under alkaline stress,Pn, gs, and E decreased sharply with rising salinity.The high-pH under alkaline stress may be by damaging the photosynthetic apparatus and stomatal conductance to restrict photosynthesis processes in oat seedlings, and causing photoinhibition and impede the absorption of light energy and cause the activity of the maximal photochemical efficiency of PSⅡ reactor in decreased.High pH is the key feature of the alkaline stress that is different from saline stress.Oat seedlings might enhance carbohydrates synthesis to remedy the shortage of inorganic anions, indicating that carbohydrates metabolic regulation might play a central role in the intracellular pH adjustment of oat seedlings.