| 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≤<40μM Cd, except for25μM Cd, shoot biomass was significantly higher (P<0.05) than that of control under this treatment. Leaf biomass of50μM Cd treatment and leaf, shoot, root and total plant biomass of100μM 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 of20,25,30and40μM 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 of5-15μmol L-1Cd treatments and ΦPSΠ, ETR, Fv’/Fm’ of5-10μmol L-1Cd treatments were significantly higher (P<.05) than that of control, and these parameters were similar to control until50μmol L1Cd, but all above parameters were significantly (P<0.05) decreased at100 μmol L-1Cd. Compared with control, Pn was significantly increased (P<0.05) under5-30μmol L-1Cd. However,50and100μmol L-1Cd significantly reduced (P<0.05) it. Gs was substantially decreased at50-100μmol 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 (小≤40μM).3In the hydroponic test, compared with the control (40μM Cd without allantoin added), shoot biomass, root surface area, root volume and root diameter of E. argyi increased significantly (P<0.05) under5-30μM allantoin treatments. Net photosynthesis rate and SPAD value increased significantly (P<0.05) under5and10μM allantoin. Shoot Cd concentrations of5and10μM allantoin were significantly higher (P<0.05), so were shoot Cd accumulations of5-30μM allantoin. Compared with the control (50μM Cd without allantoin added), shoots biomass and Cd accumulation of all allantoin treatments were significant higher (P<0.05), so were SPAD value of5and10μM allantoin. The differences of shoot Cd concentration among all treatments were unsignificant. Based on50μM Cd stress, spraying allantoin (316,632and948μM) could significantly increase (P<0.05) shoot biomass and Cd accumulation. The differences of shoot Cd concentration among all treatments were unsignificant, except sparying316μM 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 on40μM Cd stress,≥2000μM Cd stress significantly inhibited (P<0.05) the growth and shoot Cd accumulation of the plant. Further study indicated, shoot biomass of40μM lactic acid was significantly higher (P<0.05) than that of control (40μM Cd alone), but shoots biomass of200and400μM lactic acid treatments were similar to control. Shoots Cd concentration increased significantly (P<0.05) under40and200μM lactic acid, so did shoots Cd accumulation. Compared with control (20μM 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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