| Soil cadmium pollution has become a serious environmental problem. Phytoextration is regarded as an environmentally friendly method for cleaning up soils contaminated with cadmium, mainly using hyperaccumulator or crops with chelator amendment, rarely using accumulating plant. Increase in soil heavy metal bioavailability and plant tolerance to metals was the key aspect for enhancing plant uptake and extraction efficiency of metals from soil. The present study investigated cadmium tolerance and accumulation characteristics of Elsholtzia argyi, a plant origining from a mining site, and explore the effects of allantoin, lactic acid and poly-lactic acid on plant growth and Cd removal by hydroponic and pot experiments with instrument and physiological-biochemical analyses. The main results were as follows:1The results of hydroponic experiment indicated that the dry biomass of stems, leaves, shoots and roots were similar to that of control underz?<40?M Cd, except for25?M Cd, shoot biomass was significantly higher (P<0.05) than that of control under this treatment. Leaf biomass of50?M Cd treatment and leaf, shoot, root and total plant biomass of100?M Cd treatment was significantly lower (P<0.05) than that of control. Shoot Cd concentration increased with the increase of Cd level in nutrient solution, shoot Cd concentrations of20,25,30and40?M Cd treatments were104.3,125.2,164.5, and237.9mg kg-1, respectively, which were higher than100mg kg-1, normally used as the threshold concentration for identifying the Cd hyperaccumulating plant. Translocation factors (ratio of shoot Cd concentration to root Cd concentration) of all Cd treatments were lower than1. It could be concluded that E. argyi origining from a mining site was identified as a Cd tolerant and accumulating plant species.2In the hydroponic test, Fv/Fo, Fv/Fm, qP of5-15?mol L-1Cd treatments and ?PS?, ETR, Fv'/Fm' of5-10?mol L-1Cd treatments were significantly higher (P<.05) than that of control, and these parameters were similar to control until50?mol L1Cd, but all above parameters were significantly (P<0.05) decreased at100 ?mol L-1Cd. Compared with control, Pn was significantly increased (P<0.05) under5-30?mol L-1Cd. However,50and100?mol L-1Cd significantly reduced (P<0.05) it. Gs was substantially decreased at50-100?mol L-1Cd. Cd-induced decrease of Pn at high Cd stress is not only connected to stomatal limitation but also to the inhibition some of primary photochemical processes. Maintain chlorophyll fluorescence and photosynthesis parameters were one of tolerance mechanisms of E. argyi under its Cd tolerance threshold (??40?M).3In the hydroponic test, compared with the control (40?M Cd without allantoin added), shoot biomass, root surface area, root volume and root diameter of E. argyi increased significantly (P<0.05) under5-30?M allantoin treatments. Net photosynthesis rate and SPAD value increased significantly (P<0.05) under5and10?M allantoin. Shoot Cd concentrations of5and10?M allantoin were significantly higher (P<0.05), so were shoot Cd accumulations of5-30?M allantoin. Compared with the control (50?M Cd without allantoin added), shoots biomass and Cd accumulation of all allantoin treatments were significant higher (P<0.05), so were SPAD value of5and10?M allantoin. The differences of shoot Cd concentration among all treatments were unsignificant. Based on50?M Cd stress, spraying allantoin (316,632and948?M) could significantly increase (P<0.05) shoot biomass and Cd accumulation. The differences of shoot Cd concentration among all treatments were unsignificant, except sparying316?M allantoin, which was significantly higher than the control. Allantoin with appropriate level can relieve Cd stress to E. argyi, promote plant growth and increase Cd accumulation in shoot.4In the hydroponic experiment, based on40?M Cd stress,?2000?M Cd stress significantly inhibited (P<0.05) the growth and shoot Cd accumulation of the plant. Further study indicated, shoot biomass of40?M lactic acid was significantly higher (P<0.05) than that of control (40?M Cd alone), but shoots biomass of200and400?M lactic acid treatments were similar to control. Shoots Cd concentration increased significantly (P<0.05) under40and200?M lactic acid, so did shoots Cd accumulation. Compared with control (20?M Cd alone), shoot biomass, root surface area, root volume, and number of lateral tips increased significantly (P<0.05) under the ratios of lactic acid/Cd were0.5,1.0, and2.0, so did net photosynthesis rate and SPAD value under the ratios of lactic acid/Cd were0.5,1.0. Shoot Cd concentration increased significantly (P<0.05) under the ratio of lactic acid/Cd was1.0, so did shoot Cd accumulation under all lactic acid treatments. Based on above results, appropriate level of lactic acid can promote photosynthesis of E. argyi, and increase Cd concentration and accumulation in shoot.5The results of four pot experiments with four artificially contaminated soil (e.g. vegetable garden soil added with5mg kg-1Cd, vegetable garden soil added with10mg kg-1Cd, rice soil added with5mg kg-1Cd, rice soil added with10mg kg-1Cd) indicated that shoot biomass, shoot Cd accumulation and Cd phytoextraction efficiency of E. argyi grown on vegetable garden soil amended with0.063or0.126g poly-lactic acid pot-1were significantly higher (P<0.05) than no poly-lactic acid amended, and shoot biomass, shoot Cd accumulation and Cd phytoextraction efficiency of0.126g poly-lactic acid pot-1treatment was significantly higher (P<0.05) than that of0.063g poly-lactic acid pot-1treatment. Shoot biomass, shoot Cd accumulation and Cd phytoextraction efficiency of E. argyi grown on above four types of soil amended with0.063or0.126g pot-1PLA1was significantly higher (P<0.05) than that of PLA2(molecular weight of PLA2is higher than PLA1). The differences of shoot Cd concentration among all treatments were unsignificant, except for0.126g pot-1poly-lactic acid amended to5mg kg-1Cd rice soil, which was significantly higher (P<0.05) than no poly-lactic acid amended. Poly-lactic acid enhanced Cd accumulation in shoot of E. argyi mainly by increasing its biomass. Amendment of0.126g pot-1PLA1was optimal for shoot Cd accumulation. |
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