| The soil-cultivated Aegiceras corniculatum seedlings were treated with various contents of Cd, and the effects on the physiological properties and the soil enzyme activities were measured. The physiological indexes included dissolved protein, dissolved sugar, starch, proline, anti-oxidation enzymes and MDA. The objectives were to probe into the transferring rule of Cd and the anti-heavy-metal mechanism in mangrove plant, and the variation of soil enzyme activities under Cd stress as well. The results were as follow:1. Under low Cd content (0.5 mg·kg-1), the A. corniculatum seedlings exhibited no obvious signs of being damaged when were treated 4 month; on the contrary, the heights and fresh biomass were promoted by low Cd content, with significant higher than those in CK. However, as the Cd contents increased from 0.5 mg·kg-1 to 50 mg·kg-1, the growth of seedlings was inhabited, with lower biomass and shorter root length. After treated 10 month, under 0.5～5 mg·kg-1 Cd content, the A. corniculatum seedlings were promoted than CK. The growth of seedlings was inhabited under high Cd content (30～50 mg·kg-1), show that tolerance of A. corniculatum seedlings should be increased when it growthed step by step.2. Low Cd content (0.5 mg·kg-1) accelerated the synthesis of chlorophylls. Whereas, the chlorophyll contents and the ratios of Chl (a/b) decreased with the increasing Cd contents. Meanwhile, the damage to Chla was heavier than to Chl b.3. The A. corniculatum seedlings adapted the stressing habitat at the cost of slower growth and more energy wasted. There existed a trend the starch content in leaf went down with the increasing Cd content. The seedlings under 50 mg·kg-1 treatment obtained starch contents of 20.46 mg·kg-1 in leaf and 3.75 mg·kg-1 in root, with ratios of 55.80% in leaf and 61.44% in root to CK respectively. Low Cd treatment (0.5 mg·kg-1) stimulated the starch synthesis in both leaf and root, with higher content than CK. Moreover, there were significant correlations between the starch and Cd (rroot= -0.904,p<0.01;rleaf= -0.871,p<0.05).4. To the mass proportions of dissolved sugar, dissolved protein and proline, there presented similar trends that at first the mass proportion increased and then decreased with the increasing Cd content. These indicated although the A. corniculatum seedlings were able to tolerate low mass proportion of Cd treatment, they would be harmed by higher treatments.5. In the Cd treatments of 5 to 50 mg·kg-1, the contents of dissolved sugar, dissolved protein and proline were all higher than CK, and significantly correlated with Cd contents, so that it seemed practicable to apply them to monitor Cd stress to A. corniculatum seedling. As a result, A. corniculatum seedlings improved tolerances to Cd stress through more accumulation of osmoprotectants and synthesis of dissolved protein.6. Under moderate and/or low Cd treatments, the activities of both SOD and POD all increased significantly. However, under high Cd treatments, the SOD activities in both root and leaf all decreased, and SOD activities in roots were significantly lower than CK while those in leaves were higher than CK. And under higher Cd treatments, the POD activities in both leaves and roots had little difference with CK. These results indicated Ag seedlings had certain degree of tolerance to Cd stress.7. In roots, MDA contents under any Cd treatments were all lower than CK. In leaves, MDA contents under high Cd treatment were a few higher than CK, but not significantly; moreover, MDA contents under moderate and/or low Cd treatments were significantly low than CK. These indicated the peroxidation of membrane fattiness was not the major factor damaging A. corniculatum seedling.8. Enrichment coefficients of Cd in parts of A. corniculatum seedlings showed the enrichment ability of root was higher than those of stem and leaf. And the Cd contents in root were 8.75～28.427 times and 7.15～29.05 times respectively to those in leaf.and stem The Cd content of ten-month-old seedlings was higher that of four-month-old seedlings. Furthermore, the enrichment abilities of A. corniculatum seedlings decreased with the increasing Cd content.9. Low Cd contents simulated the synthesis of urease, incertase and protease. However, the activities of urease, incertase, POD and protease all decreased with the increasing Cd content. The soil enzymatic activities were negatively correlated with Cd and the correlation were as follows: urease > protease> POD> incertasel> catalase. The maximum of inhibition rates could reach to 25% except catalase. It seemed reasonable to use urease as monitoring indexes to Cd pollution. No obvious variation was found in catalase activities.10. There were significant correlations among urease, incertase, POD and protease, while no correlation between catalase and anyone of the above four soil enzymes.
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