The Role Of Ascorbic Acid In Alleviating Aluminum Toxicitv In Wheat (Triticum Aestivum L.) Genotypes Differing In Aluminum Tolerance

It was reported that soils with pH lower than 5.5 accounts for 30% of the world land, and 50% of the world arable land. In China, acidic soil distributed across 15 provinces, accounting for 203 square KM, and representing 21% of its arable land. Soil acidization arose from the application of acidic fertilizers and the industrialization process. Previous study indicated that when Al concentration is lower than 10μmol/L in soil, plant root growth is inhibited due to the problems in nutrients and water uptake. Normally, Al concentration varys between 10 to 100μmol/L in acidic soils. Therefore, Al toxicity is thought to be the most important inhibitor limiting crop productivity. Although there are many reports on the Al stress and its toxicity mechanism, little is known about the physiolical and molecular mechanism on the plant response againt Al toxicity.1. Mechanisms for the effects of Al on AsA content and As A metabolic enzymes activities between wheat genotypes differing in Al toleranceHydroponic experiments were carried out to study the effects of Al on AsA metabolism for two wheat genotypes varying in Al tolerance. The results indicated that the dose-dependent experiment of 24 h exposure to 0,10,30,50μmol/L AICl3 indicated that the root elongation of Jian-864 and Yangmai-5 was inhibited with the increasing Al concentrations. The greatest difference in relative root elongation (RRE) between the two genotypes was observed at the concentration of 30μmol/L AlCl3, with the RRE 52% of Jian-864 and only 28% of Yangmai-5. This shows Yangmai-5 was more sensitive to Al than Jiang-864. In addition, The Al content and monodehydroascorbate (MDA) content of Jian-864 and Yangmai-5 was promoted with the increasing Al concentrations. The greatest difference in Al content and MDA content between the two genotypes was observed at the concentration of 30μmol/L. So,30μmol/L AICl3 concentration is chosen for the following study.The dose-dependent experiment indicated that the AsA content, L-galactono-1,4-lactone dehydrogenase(L-GalLDH), monodehydroascorbate reductase(MDHAR), dehydroascorbate reductase(DHAR), ascorbate peroxidase(APX), AsA/DHA radio and ascorbate oxidase (AAO) activities in 0-10 mm root apex of two genotypes increased responding to the increasing Al concentration. However, Jian-864 is still superior to Yangmai-5. The results indicated that Al can increase the AsA content, induce the activity of enzymes related to metabolism and stimulate the antioxidant system response to Al stress. Meanwhile, AsA synthesis capability and metabolic enzyme activity closely associated with the differences in Al tolerance in different wheat genotypes.2. Mechanisms of the effects of Al on AsA content and AsA metabolic enzymes activities in different wheat genotypes with AsA applicationHydroponic experiments were carried out to study the effects of AsA catabatic mechanism and AsA metabolism for two wheat genotypes exposed, to 0.5 mmol/L AsA pretreatment for 6 h. The results indicated that the two wheat genotypes of relative root elongation (RRE) were increased significantly with AsA pretreatment, but Al content in roots is reduced. After AsA pretreatment, AsA content, AsA/DHA ratio, L-GalLDH, MDHAR DHAR, APX and AAO activity in wheat rose markedly. And the rate of increase of AsA content, MDHAR, DHAR, APX and AAO activity in Yangmai-5 was greater than the ones in the Jian-864. The results indicated that exogenous AsA can accelerate endogenous AsA, DHA and their metabolic enzymes involvingin synthesis, promote wheat root elongation and reduction of the aluminum absorption and relieve the Al inhibition in root apex.3. Mechanisms of the effects of Al on AsA content and AsA metabolic enzymes activities in different wheat genotypes with L-Gal applicationHydroponic experiments were carried out to study the effects of AsA catabatic mechanism and AsA metabolism for two wheat genotypes exposed to 0.5 mmol/L L-Gal pretreatment for 6 h. The results indicated that After L-Gal pretreatment, the RRE of two wheat genotypes were increased significantly, but Al content in roots is reduced. And AsA content, AsA/DHA ratio, L-GalLDH, MDHAR DHAR, APX and AAO activity in wheat were also promoted. The results indicated that exogenous L-Gal can promote the biosynthesis of endogenous AsA, DHA and their metabolic enzymes involved in synthesis, reduce the absorption of aluminum content and relieve the Al inhibition in root apex.