Physiological Characterization Of Rice Seedlings Responding To Aluminum Stress

Aluminum is one of the major constraints limiting rice productivity in acidic soil, which inhibits growth and minerals absorption of rice root. In the research, IR24, Jinyou 725, Liang-you-pei-jiu at germination and four-leaf stage were employed to carry out hydroponical culture coupled with aluminum stress, and then the physiological characteristics of rice seedlings responding to aluminum stress had been explored, using organelle preparation, SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electroph- oresis) and RP-HPLC (reverse phase high performance liquid chromatography) techniques. The main results obtained are as follows:1. The experiment was conducted with the germinated rice seedlings exposed to 0, 0.075mmol/L, 0.10mmol/L, 0.30mmol/L, 1.5mmol/L, 3.0mmol/L and 7.5mmol/L aluminum stress. It demonstrated that, under the lower concentration aluminum treatment such as 0.075 and 0.10mmol/L, there was little increase or decrease in the root length, fresh root weight of single plant and root activities ofjinyou 725 and liang-you-pei-jiu, and a little decrease in that of IR24 compared with their respective control, showing no significant difference between different treatments. Under such higher concentration treatment as 0.30mmol/L, 1.5mmol/L, 3.0mmol/L and 7.5mmol/L, the three indices above of the three cultivars were decreased significantly, especially that of IR24 did. The growths of the three varieties tested were all arrested under 7.5mmol/L aluminum stress. Therefore, it suggested that 0.30mmol/L was the critical aluminum concentration to evaluate the rice germplasm to aluminum tolerance. Finally, jinyou 725 and liang-you-pei -jiu were considered to be the more tolerant cultivars; whose relative growth indices were better than that of IR24.2. On the basis of tolerance difference confirmation of the three culitivars during germination after the stress treatment of 0.50mmot/L and 7.5mmol/L aluminum, the effect of aluminum on soluble protein in rice shoots was screened by SDS-PAGE, four polypeptides with molecular weight of about 80.8kD, 78.5kD, 52.9kD and 16.4kD had been found to decline under aluminum treatment, suggesting that they were probably associated with aluminum stress, but there was no novel polypeptide induced under aluminum stress.3. Aluminum stress experiment was done with the addition of exogenous organic acids such as citrate, malic acid and succinic acid to the culture solution. It showed that, the growth of rice roots was inhibited by 1.5 mmol/L aluminum, the activities of POD (peroxidase) and APX (ascorbate peroxidase) in mitochondria of rice roots were increased remarkably and that of CAT (catalase) changed little. After the addition of exogenous organic acids, aluminum toxicity was obviously alleviated. The activities of POD and APX tumed out to be declined and that of CAT changed little, suggesting that the metabolism homeostasis of ROS (reactive oxygen species) was changed by aluminum stress and the activities of antioxidant enzymes in mitochondria of the germinated rice root were regulated by exogenous organic acids and thus aluminum toxicity was then alleviated.4. Such morphological characters as root length, shoot length, new root length, flesh shoot weight of single plant and root activity were all decreased after aluminum treatment for ten days on rice seedlings of four-leaf stage. IR24 showed obvious decline in the above five indices than the other two Al-tolerant cultivars jinyou725 and liang-you-pei-jiu, that is to say, IR24 was injured more badly than the other two cultivars. Meanwhile the respiration rate of the two hybrid rice cultivars was determined and we found that the cyanide-resistant respiration rate of them had a rise-fall tendency as aluminum increased, while the total respiration rate varied from each other.5. After the rice seedlings of four-leaf stage had been treated with aluminum for ten days, H₂O₂ (hydrogen peroxide) content in mitochondria increased when the concentration of aluminum were increased and the activities of antioxidant enzymes in mitochondria varied differently. The activities of POD increased first and then decreased in Al-sensitive cultivar but increased all the way in Al-tolerant cultivar as the concentration of aluminnum increased, and the differences between them under different aluminum stress were found to be significant. The change of APX showed a similar tendency to that of POD. The activities of CAT changed slightly under 0.50mmol/L and increased sharply under 7.5mmol/L aluminum stress. Meanwhile, the activities of CAT increased more obviously in Al-sensitive cultivar than Al-tolerant one.6. The content of organic acids were measured by RP-HPLC in rice root of four-leaf stage after ten days' aluminum stress. The content of glutamic acid was increased under 0.50mmol/L and decreased under 7.Smmol/L aluminum stress, showing a upside-down "V" shape. The content of phosphoenolpyruvate (PEP) was increased as aluminum concentration increased and it was probably involved in the stimulation of the cyanide-resistant respiration to some extent. However, the content of citrate in rice roots showed a tendency of being decreased as the concertration of aluminum increased, which was probably due to its exudation for the detoxification of aluminum externally, reduction of its synthesis, decomposition or transformation into other substance. By research on the items above, we can draw a conclusion that there was a close relationship between respiration and aluminum tolerance of rice, the dynamic change of the activities of antioxidant enzymes in mitochondria of rice root played an important role in aluminum tolerance of plants and the metabolism of organic acids was associated with the adaption of rice to aluminum stress.