| Cadmium (Cd) is one of the common elements in heavy metal pollution; it can obviously inhibit the growth and development of plants. Salicylic acid (SA) is a kind of plant endogenous hormone, and plays a very important role in resistance to biotic or abiotic stresses. To assess the role of different SA treatment in alleviating Cd toxicity in Louisiana iris, the effects of1μM SA treatment (pretreated or soaked) on biomass, activities of antioxidant enzymes, root activity, proline and MDA content, photosynthesis parameters and chlorophyll content in Louisiana iris were studied by hydroponic experiment. Furthermore, ultrastructure of chloroplasts and root cells with SA treatment were also observed. The results obtained were as follows:1. In Louisiana iris, Cd stress inhibited the root activity, CAT activity and increased POD and SOD activity, and increased the content of proline and MDA also. When pretreated with1μM SA, the growth and physiological activities of Louisiana iris were alleviated under Cd stress, and the mitigative effect was the best under high Cd treatment (500μM). The root activity, leaf CAT activity, root CAT activity, leaf POD activity, root POD activity, leaf SOD activity, root SOD activity, leaf MDA content, root MDA content, leaf proline content and root proline content were1.76,1.09,1.25,1.36,1.15,1.20,1.05,0.47,0.81,1.31and1.37times higher than those of no SA treatment under Cd stress separately. However, when soaked with1μM SA, the antioxidant enzyme activities, root activity and proline content in Louisiana iris also increased, and the MDA content decreased, but the effects were not as good as pretreated with1μM SA.2. With the increase of Cd concentration, the chlorophyll concent, net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) of Louisiana iris decreased with no SA treatment, accompanied with the increase of intercellular CO2concentration (Ci). Under1μM SA pretreatment, the chlorophyll content, Tr, Gs and Pn of Louisiana iris significantly increased while the Ci decreased, which indicated that SA pretreatment could counteracted the Cd-induced inhibition on photosynthesis. Although SA soaking could also improve the chlorophyll content, but the Pn,Tr Gs and Ci all decreased, which indicated that SA soaking could not alleviate the Cd inhibition on photosynthesis.3. Under500μM Cd treatment, the chloroplast became deformed severely and the membrane and the stroma lamellar structure were disorganized. When pretreated with1μM SA, the chloroplast membrane and the stroma lamella still kept integrated, though there were some distortion, ambiguity and loose in stroma lamella. So pretreatment could obviously alleviate the injury in ultrastructure of chloroplast, while there was no obvious effect when soaking seeds with1μM SA. Root cells exposed to concentrations of100μM of Cd exhibited pronounced damage, such as reduce of mitochondrial cristae, serious plasmolysis, greatly damage of membrane system and highly concentrated cytoplasm. In addition, a few electron-dense granules precipitated in large vesicles. When pretreated with1μM SA, the plasmolysis could be significantly alleviated, the number of mitochondrial cristae and vacuoles increased. In addition, SA (1μM) soaking could also increase the number of vesicles and relieve the plasmolysis in root cells, but the nuclei were severely deformed and chromatin highly concentrated, which showed that the alleviating effects of SA soaking on Cd toxicity in Louisiana iris were not good as that of SA pretreatment. |
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