Study On Rhizosphere Characteristics And Root Tolerance In Cadmium Enrichment Plant Amaranthus Hybridus L.

Cadmium(Cd)can be absorbed and accumulated by plant roots and moved in plant tissues.There are significant differences between the rhizosphere and non-rhizosphere under Cd stress.We used a pot experiment to investigate the growth and rhizosphere charateristics under different concentrations of Cd supplyin Amaranthus hybridus L.Meanwhile,in order to reveal the mechanisms involved in Cd tolerance and detoxification in root of A.hybridus,we through the short-term nutrient solution experimental to study physiological response of root under Cd stress.The main results showed as follows:(1)The plants height and biomass showed a trend of increased first and decreased later with Cd concentration increasing.At the Cd concentration of 5 mg kg-1,the plants height of A.hybridus was as high as 109 cm,and the shoots biomass was significantly increased 17.11%(P<0.05).Compared with the control,the plant height and roots biomass decreased significantly at the Cd concentration>50 mg kg-1(P<0.05),and the shoots biomass decreased significantly at the Cd concentration ? 25 mg kg-1(P<0.05).(2)The Cd concentration and Cd accumulationin of the shoots and roots increased significantly with Cd concentrationincreasing(P<0.05),and all reached the maximum at concentration of 150 mg kg-1,Cd concentration for 204.94 and 183.27 mg kg-1,Cd accumulation for 1153.06 and 192.05 ?g plant-1.The translocation factor and bioconcentration factorof A.hybridus were close to 1 or greater than 1 at each concentration,which has a strong ability of accumulation and translocation.(3)The pH in rhizosphere soil were lower than that in non-rhizosphere at each concentration,and showed a trend of downward with Cd concentration increasing.The soluble and water-soluble organic carbon concentration in rhizosphere soil all increased with increasing concentrations of Cd,while the readily oxidizable carbon showed a trend of decreased first and increased later.The DTPA extractable Cd and continuous extractable Cd concentration in rhizosphere soil was lower than that in non-rhizosphere.During Cd concentration increasing,exchangeable fraction Cd in rhizosphere soil increased from 49.33%to 64.66%,and other fraction Cd decreased.It showed that the rhizosphere environment of A.hybridus conducive to Cd activation and absorption.(4)The soil enzyme activities in rhizosphere soil were higher than those in non-rhizosphere.During Cd concentration increasing,the phosphatase and urease activities in rhizosphere soil showed a trend of first upward and downward later,but the catalase activity continued to decline.The catalase,urease and phosphatase activties of were significantly lower than that of control in the non-rhizosphereat concentration of 100 mg kg-1,25 mg kg-1 and 50 mg kg-1 respectively.While the catalase and phosphatase activities in rhizosphere soil were no significant compare with the control,and urease activity was downward significantly at the Cd concentration of 100 mg kg-1.The enzyme activities in rhizosphere soil were maintained at a high level,which had some alleviation effect on the toxicity of Cd.(5)The microbial biomass carbon concentration,microbial biomass nitrogen concentration,basal respiration and microbial(bacteria,fungi and actinomycetes)numbers in rhizosphere soil were higher than those in non-rhizosphere,and the metabolic quotientin rhizosphere soil was lower than that in non-rhizosphere soil.The results indicated that the characteristics of rhizosphere microorganisms in rhizosphere soil of A.hybridus has a certain effect on Cd tolerance and improving the bioavailability of Cd.(6)The root length,root surface area and root volume of A.hybridus showed a trend of first increased and decreased later with Cd concentration increasing,and all reached the maximumat the concentration of 10 mol L-1.The average diameter of root was no significant differences compared with the control at each concentration.The activity of root had concentration and time effect under Cd stress,but there were no significant difference compared with the controlat the low concentration(<25 ?mol L-1)and short time(3d)Cd treatment.(7)There was no significant oxidative stress on the root of A.hybridusat at the low Cd concentration(<25 mol L-1)and a short time(3 d).The malondialdehyde,superoxide anion and hydrogen peroxide concentration in the root of A.hybridus were increased with increasing of Cd concentration and treatment time,which caused membrane lipid peroxidation increased seriously.(8)With increasing Cd concentration and treatment time,the osmotic adjustment substance concentrationin the root of A.hybridus was generally increased first and decreased later.High concentration(>100 mol L-1)and long time(5 d or 7 d)Cd stress caused APX and GR activity decreased significantly(P<0.05).The glutathione and ascorbic acid(ASA)concentration increased with the increase of Cd concentration.The ASA concentration was decreased just at treament time of 7 d and the concentration of 150 mol L-1.The results indicated that osmotic adjustment and antioxidant defense were the important physiological mechanisms of root tolerance under Cd stress.