Response Of Phytochelatins And Phenolics To Heavy Metals Of Aegiceras Corniculatum Seedlings

In a greenhouse sandy culture system we investigated the response of phtochelatins and phenolics metabolisms to heavy metals (Cd, Cu, and Zn) of mangrove (Aegiceras corniculatum) seedlings. Cd, Cu or Zn were added as chlorides to the nutrient solution at levels of 0, 5, 10, 20, 30 mg.Kg^-1, respectively. Systematic studies on Aegiceras corniculatum seedlings were carried on non protein thiols (NPT), glutathione (GSH), phytochelatins (PCs), total phenolics (TP), total condensed tannin (CT), extractable condensed tannin (ECT), protein-bound condensed tannin (PCT) and fibre-bound condesed tannin (FCT). The main purpose was to further reveal the functions of phtochelatins and phenolics in heavy metals tolerance in Aegiceras corniculatum seedlings. The results are as follows:1. Heavy metals (Cd, Cu and Zn) exposure increased non protein thiols (NPT) concentration in the leaves and roots of Aegiceras corniculatum seedlings, indicating disturbances in -SH metabolism. It was found that Cd was the most effective activator of NPT, whereas other heavy metals (Cu and Zn) activate it to a much lesser extent than Cd. We also found that the contents of NPT in roots were much higher than that in leaves. As to Cu and Zn, in the same exposure concentration, the content of NPT was not affected by the extending of time; As to Cd, distinct drop in the content of NPT was found at 30 g Kg^-1 treatment after 150 days of exposure. The variety of NPT in Aegiceras corniculatum seedlings might act as antioxidant and play an important role in tolerance of heavy metals.2. In response to heavy metals exposure, Aegiceras corniculatum seedlings showed significant induction of roots glutathione (GSH). However, in the present study, the level of GSH did not statistically change with the variety of heavy metals treatment levels; excepting at the 30 g Kg^-1 Cd treatment, the content of GSH was not affected by the extending of time. The exposure of Cd and Cu significantly increased the content of GSH in leaves of Aegiceras corniculatum seedlings, while no statistically difference was found at Zn treatments. Since PCs synthesis is regulated by the intracellular level of GSH, the increase of GSH might contribute towards increased in PCs in Aegiceras corniculatum seedlings.3. Phytochelatins (PCs) were also induced significantly under heavy metals exposure in comparison to control. Statistically differences in the content of PCs were found under different level of heavy metals treatments (0-30 g Kg^-1) in roots of Aegiceras corniculatum seedlings, while not in leaves. Although Cd increased both the PCs contents in roots and leaves of Aegiceras corniculatum seedlings, we did find that the levels of PCs declined drastically following the exposure time at high level treatments, which indicated that long-time and high level exposure of Cd might disturb the biosynthetic regulation of phytochelatins markedly. The increase of PCs might act as sorbent of dissociative heavy metals ion, and further affected tolerant metabolism in Aegiceras corniculatum seedlings.4. It was showed that heavy metals exposure changed the distributions of phytochelatins (PCs) in non protein thiol (NPT). Under Cd, Cu and Zn exposure, the PCs/NPT ratio were found to increase following the exposure time, which suggested that heavy metals induced the biosynthesis of PCs from NPT, and exposure time had some effects on this biosynthetic regulation.5. Foliar total phenolics (TP) of Aegiceras corniculatum seedlings was distinctly induced by heavy metals in compare with control, which implied disturbance of phenolic metabolism. The accumulations of TP might act as antioxidant and play a part in integrated mechanisms of heavy metals detoxification. At the range of 0-30 g Kg^-1, there was no statistically difference in the effects of different treatment of heavy metals to the contents of TP. At the same time, under the same treatment level, no statistically difference was found between Cd, Cu and Zn.6. At the range of 0-30 g Kg^-1, the foliar extractable condensed tannins of Aegiceras corniculatum seedlings increased significantly up to low levels of heavy metals exposure while at about 20 g Kg^-1, their level declined drastically.7. Cd, Cu and Zn had the same effects on the foliar protein-bound condensed tannins (PCT): higher treatment levels (>10 g Kg^-1) of heavy metals enhanced the contents of foliar PCT of Aegiceras corniculatum seedlings.8. In compare with extractable condensed tannins (ECT) and protein-bound tannins (PCT), fibre-bound condensed tannins (FCT) responded differently to heavy metals. Their contents declined drastically following the increase of heavy metals treatments.9. As the sum of extractable condensed tannins (ECT), protein-bound tannins (PCT) and fibre-bound condensed tannins (FCT), the foliar total condensed tannins (TCT) acted in the same way as total phenolics (TP) in response to heavy metals exposure. Heavy metals induced the concentrations of foliar TCT in Aegiceras corniculatum seedlings.10. It was showed that heavy metals exposure significantly changed the distributions of ECT, PCT and FCT in TCT. Under Cd, Cu and Zn exposure, the ECT/TCT ratio were found to increase following the exposure time, while FCT/TCT ratio declined significantly, which suggested that heavy metals disturbed the foliar phenolic metabolism of Aegiceras corniculatum seedlings. As a part of the antioxidant system, the variety of phenolics and condensed tannins might play an important part in heavy metal tolerance in Aegiceras corniculatum seedlings.