Effects Of Exogenous Salicylic Acid On Absorption,Accumulation And Tolerance Of Cadmium In Iris Hexagona W.

Iris hexagona is an ornamental hydrophyte introduced from foreign country at the end of 1980s, which has a good promotion prospects. It is also an ideal material for phytoremediation because of dual functions of ornamental and accumulation of heavy metals. Based on the research of absorption, accumulation and tolerance of Cd, the improving resistance to Cd by exogenous SA was furtherd studied through SA pretreatment and SA simultaneous treatment, which not only provides the theoretical reference for the mechanism of SA alliviavtion Cd toxicity but also provides a pracitical method to improve resistance to Cd in Iris hexagona.In order to clarify uptake, accumulation and tolerance of Cd in Iris hexagona, we studied the effects of 0, 10, 100, 500 μM Cd on growth morphology, chlorophyll content,photosynthetic parameters,Cd content, the activities of antioxidant enzymes and the cell structures by hydorponics. The results showed that: 10 μM Cd treatment had no significant effect on the growth of Iris hexagona. With the increase of Cd stress, dry weight of leaf or root and root length were significantly reduced along with the decreased chlorophyll content and photosynthetic rate. POD, SOD activities in leaves increased significantly at 100 μM Cd treatment(P < 0.05) but decreased at 500 μM Cd, while CAT activity decreased and MAD content increased both in100 μM Cd and 500 μM Cd. Cd contents in leaves,basal stems and roots were all increased with the increase of Cd concentration and Cd contents in basal stems and roots were significantly higher than that in leaves. The concent of Cd reached 147.67μg g-1 dry weight after 10 μM Cd treatment for 14 days in basal stems and roots. Under Cd stress, N content in leaves decreased. K relative content in leaf, stem and root of decreased significantly while P, S relative content increased. These results indicated that the abilities of resistance, absorption and accumulation of Cd in Iris hexagona were strong in low concentration of Cd and the underground part of basal stems and roots were the main position of accumulation. Cd stress not only affected the N, S, P metabolism but also had an important influence on the absorption of K. Though the growth damage imerged, the antioxidant system worked well in Iris hexagona at 100 μM Cd stress,and 500 μM Cd stress was beyond the range of tolerance for Iris hexagona with wilting leaves, damaged cell structure and the death of roots.On the basis of the above research, this paper further studied the effects of exogenous SA on the growth of Iris hexagona through SA pretreatment and SA simultaneous treatment.In comparison with 100 μM Cd treatment alone, the growth, dry weight and oxidizing capacity in roots were improved after SA pretreatment, while the dry weight, chlorophyll content, photosynthetic rate, transpiration rate, stomatal conductance of leaves were improved obviously by SA pretreatment under 500 μM Cd stress. It suggested that SA pretreatment could alleviate Cd toxicity and the roots were more sensitive to toxicity alleviation than leaves. In addition, after SA pretreatment, Cd content had no significant change comparing with the same concentration of Cd treatment alone, but the contents of N,S, P was promoted. It suggested that the improvement of growth by SA pretreatment wasn’t attributed to reducing the absorption of Cd but to keeping the ion absorption equilibrium and enhancing the tolerance to Cd. Compared with the same concentration of Cd treatment alone, CAT, SOD, POD activities and Pro content increased(P < 0.05) while MDA content decreased both in leaves and roots. At the same time, the ultrastructures of mesophyll cells were improved significantly. Especially, root tip cells possessed many small vesicles after SA pretreatment and many of them packaged high electron density granules. These results indicated that S A pretreatment could increase the activities of antioxidant enzymes, depress lipid peroxidation and maintain the stability of biomembrane, thus alleviate the damage of cell structure. The improvement of chloroplast structure was a main reason for photosynthesis raise after SA pretreatment and the increase of vesicles and depositing precipitates in it might be a improtant mechanism of SA-induced detoxification in plants.After SA simultaneous treatment, the dry weight, chlorophyll content, photosynthetic rate, transpiration rate, stomatal conductance, intercellular CO2 in leaves were lower than 100 μM Cd or 500 μM Cd treatment alone, but the growth, dry weight, oxidizing capacity in roots were significantly improved. So the reaction of leaves and roots to S A simultaneous treatment was different and the presence of SA enhanced Cd inhibition of photosynthesis. N content decreased in leaves, the relative content of Cd, P, S increased and the relative content of K was significantly decreased after SA simultaneous treatment. It indicated that SA simultaneous treatment increased Cd uptakes and further inhibited the absorption of N and K, which was probably the reason for photosynthesis decrease and growth inhibition in shoots by SA simultaneous treatment. After SA simultaneous treatment, CAT, SOD, POD were also more active and the content of MDA was lower than the same concentration of Cd treatment alone, which indicated that SA simultaneous treatment could activate antioxidant system too but that was not as effective as SA pretreatment, consistent to the degree of improvement in root and leaf cell structure.On the basis of SA pretreatment alleviating the toxicity of Cd, we further studied the effects of leaf proteome expression under 100μM Cd stress and exogenous SA pretreatment. Compared with the control group, protein pattern was significantly different under 100 μM Cd stress in Iris hexagona leaves and there are 77 protein spots differentially expressed more than 1.5 times including up 39, down 28, new 10. Compared to the control,there are only 18 differential protein expression spots with SA pretreatment followed by 100 μM Cd treatment. The results showed that SA pretreatment can significantly improve the matching rate of the 2-DE protein spots with the control, which further confirmed that SA pretreatment could significantly reduce the differential protein expression caused by Cd stress and alleviate Cd toxicity. 54 associated protein were identified by MALDI-TOF-MS among 102 differential protein expression spots in three exogenous SA and Cd treatment.These proteins were involved in photosynthesis, the energy and substance metabolism,detoxification and antioxidant effect, amino acid and protein metabolism. Among them,ascorbate peroxidase and heat shock protein 70 cognate play an important role in alleviating Cd toxicity by SA.In conclusion, Iris hexagona is a kind of aquatic flower with dual functions of ornamental and accumulation of Cd, which has strong tolerence to Cd stress below 100 μM and could resist and absorb Cd in generally polluted water. SA root pretreatment could alleviate Cd toxicity in Iris hexagona and the reason might be the result of many hands.