| More and more attention is being paid to the methods of using fast-growing native woody plants for cleaning soil polluted by cadmium (Cd). Both the physiological adaptation to heavy metals and phyto-remediation capacity are two important aspects for choosing phytoremediation material. In our study, Cinnamomum camphora (belonging to Lauraceae), C. longepaniculatum, Toona ciliata (belonging to Meliaceae), T. sinensis as model species were used to detect the effects of three levels of Cd (CK,0.12 mg·kg-1 soil; T1,2.5 mg·kg-1 soil; T2, 5 mg·kg-1 soil) on morphological growth, metal accumulation and distribution, physiological and biochemical traits, nutrient status, and Cd accumulation and allocation. Our aim was to compare the differences in metal tolerance and remediation capacity among above species, and to screen the most excellent candidate for phyto-remediation for Cd. The second experiment was to detect the effects of chelators (EDTA, citric acid, or both of them) addition on physiological adaptation and phyto-remediation efficiency of the selected candidate. This research might provide theoretical and scientific basis for phyto-remediation of the Cd-polluted soil in the hilly area and for utilization of valuable native woody species. The main results are as follows:(1) when compared to the controls, T1 treatment decreased the pigment contents, photosynthetic rate (Pn) and the biomass (especially for fine roots) to a different degree in both C. longepaniculatum and T. ciliata, and the decrease in such parameters under T2 was significant, whereas there was no obvious negative effects of T1 or T2 exerted to both C. camphora and T. sinensis. There was no effective response in antioxidant enzymes, such as superoxide dismutase (SOD) and peroxidase (POD), in C. longepaniculatum and T. ciliata when exposed to T1 or T2, and some of them even depressed, which might induce excessive accumulation of the reactive oxygen species (ROS), and result in injuries of the cell membrane, reflected by the increase of malondialdehyde (MDA), while the activities of SOD or POD increased significantly in C. camphora and T. sinensis, convenient for quenching the excess ROS. In general, the N content in different organs of above woody species was hardly affected by T1 or T2 treatments, but P and K was affected obviously. For instance, in C. longepaniculatum, both T1 and T2 significantly decreased the P content in all of organs, and the K content in new stems, old stems, coarse roots and fine roots. In T. ciliata, both T1 and T2 significantly decreased the K content in all of organs, and the P content in the leaves. However, the nutrient status was not affected negatively in C. camphora and T. sinensis when exposed to Cd pollution. Under both T1 and T2, the Cd concentration in all organs of each species increased to a different degree, especially for the fine roots and the leaves. In respect to the amount of Cd accumulation, for all of the plants, the roots accumulated most of the Cd. The amount of Cd accumulation in C. camphora was 875.43 μg g-1, more than C. longepaniculatum’s 791.98 μgg-1, T. sinensis’s 786.97 μgg-1, T. ciliata’s 345.51 μgg-1.(2) when compared to the conditions exposed to Cd pollution alone (10 mg g-1 of dried soil), the combined treatments associated with additive EDTA, citric acid, or both of them did not significantly affect the morphological growth, pigment content, and Pn in C. camphora, except for an increase in biomass of fine root and photosynthesis rate under the treatment associated with citric acid alone. In contrast, the addition of various chelators all increased the activities of SOD and POD, and thus mitigated the impairment of the excessive ROS, reflected the stability of MDA. There were certain negative effects on N, P and K nutrient status when the chelators were added. For example, compared to Cd stress alone, there was a decrease in N content in fine roots, P content in stems, K content in both leaves and stems, when both of EDTA and citric acid was applied. Taken together, EDTA addition alone induced the minimum negative effects to C. camphora,, and the maximum negative effects were resulted from combined addition of both EDTA and citric acid. However, both EDTA addition alone and combined addition significantly increased the Cd concentration and accumulation amount in C. camphora, and thus improved the phyto-remediaiton capacity.In conclusion, in our study, we compared the differences in biomass accumulation, photosynthesis rate, metabolism in reactive oxygen species, nutrient status, and heavy metal accumulation capacity among four common native species. The biomass accumulation, photosynthesis rate, nutrient uptake and partitioning was not obviously affected in C. camphora when exposed to Cd-polluted soils. C. camphora also exhibited a more effective antioxidant enzyme system to scavenge excess ROS, and showed a prospective capacity in Cd accumulation. Therefore, among the conducted woody species, C. camphora is considered to be the most excellent candidate for phytoremediation. Thus, C. camphora was used as the materials to detect the effects of chelator addition on its physiological adaptation and Cd accumulation capacity. EDTA application alone did not obviously affected various physiological and biochemical traits, and nutrient status of C. camphora, but enhanced Cd accumulation amount greatly. Therefore, the application with EDTA alone could be considered to be an effective method for improving phyto-remediation efficiency of Cd. |
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