| Melia azedarach and Firmiana platanifolia are common deciduous trees in Chongqing.Both of these two trees have the characteristics of quick growth rate,developed root systems and strong anti-pollution ability,which are the excellent green tree species of city and mining area in Chongqing.Hg and Cd are two heavy metals with high toxicity,strong mobility and wide pollution range.This research takes 1-year-old Melia azedarach and Firmiana platanifolia as the test materials.The physiological tolerance and absorption and accumulation characteristics of the two tree species were studied through greenhouse hydroponics and pot experiment in the field under single factor stress of the solution Hg(1,5,10,20mg/kg),solution Cd(1,5,10,20mg/kg),soil Hg(light 1mg/kg,medium10mg/kg,heavy 50mg/kg)and soil Cd(light 1mg/kg,medium 10mg/kg,heavy50mg/kg)according to the standard for soil pollution risk control of agricultural land of soil environmental quality.According to the preliminary results,Melia azedarach,which performed well under Cd stress,and Firmiana platanifolia,which performed well under Hg stress,were potted with Ophiopogon japonicus respectively to study the tolerance and enrichment and transfer ability of Hg or Cd of the two tree species under the combined mode.In order to screening restoration tree species of Hg and Cd pollution and provide a efficient method for direct application.And the main research results are shown as follows:1 Tolerance and Characteristics of Absorption and Accumulation of Melia azedarach and Firmiana platanifolia to Hg and Cd(hydroponics)After 30 days of solution Hg stress,the root growth of Melia azedarach and Firmiana platanifolia in each treatment group was inhibited and the old roots rotted;The symtoms of Firmiana platanifolia were more severe than those of Melia azedarach The growth of ground diameter,root length and biomass of Melia azedarach and Firmiana platanifolia was inhibited on the whole.The SOD activity and soluble protein content of Melia azedarach and Firmiana platanifolia increased first and then decreased with the increase of the Hg concentration.he Hg contents of Melia azedarach and Firmiana platanifolia are both root > aerial parts.The Hg content and accumulation per plant of Melia azedarach reached the maximum at20mg/kg treatment group.The Hg content and accumulation per plant of Firmiana platanifolia reached the maximum at the 1mg/kg treatment group.The Hg transfer factor of Melia azedarach was greater than that of Firmiana platanifolia under the same treatment.After 30 days,the old roots of Firmiana platanifolia in each treatment group were browning and the number of new roots decreased.The leaves of Melia azedarach at 20mg/kg treatment group became chlorisis with plant height and root length Significantly inhabited.All concentrations of Cd inhibited the growth of Melia azedarach,while causing “Hormesis effect” of Firmiana platanifolia.With the increase of the Hg concentration,the SOD activity and soluble protein content of Melia azedarach and Firmiana platanifolia rose first and then decreased.The absorption and accumulation characteristics of Melia azedarach and Firmiana platanifolia to solution Cd: the Cd contents of Melia azedarach and Firmiana platanifolia are shown as root > aerial parts.At all Cd concentrations,the Cd contents and accumulations per plant of all parts of Firmiana platanifolia were significantly higher than those of Melia azedarach The accumulation per plant of Firmiana platanifolia reached 429.660 μg in the 1 mg/kg treatment group,which was 63.2 times that of the Firmiana platanifolia.The enrichment capacity of the solution Cd was Firmiana platanifolia > Melia azedarach.Comprehensive evaluation was carried out by principal component analysis and membership function method.The comprehensive evaluation value <D> of the resistance of Melia azedarach and Firmiana platanifolia to solution Hg and Cd was:Cd resistance of Firmiana platanifolia(0.466)> Cd resistance of Melia azedarach(0.400);Hg resistance of Melia azedarach(0.414)> Hg resistance of Firmiana platanifolia(0.377).The tolerance and accumulation capacity of Firmiana platanifolia to solution Cd was stronger than that of Melia azedarach;the difference in the comprehensive tolerance to solution Hg of Melia azedarach and Firmiana platanifolia was small and that of Melia azedarach was slightly stronger than that of Firmiana platanifolia.2 The tolerance and characteristics of absorption and accumulation of Melia azedarach and Firmiana platanifolia to Hg and Cd pollution(soil culture)The light and medium Hg promoted the growth of Firmiana platanifolia plant height and ground diameter and the accumulation of biomass,which had little effect on the growth of Melia azedarach.Under heavy Hg stress,the root biomass of Melia azedarach and Firmiana platanifolia decreased.With the increase of Hg concentration,the soluble protein and MDA content of Firmiana platanifolia rose first and then decreased,while the MDA content of Melia azedarach persistently increased with the soluble protein persistently decreased.The Hg content and accumulation of Melia azedarach and Firmiana platanifolia are both root > aerial parts.The Hg content and accumulation of Melia azedarach reached the maximum in the heavy treatment groupe while those of Firmiana platanifolia reached the maximum in the medium treatment group.The Hg accumulation of Melia azedarach and Firmiana platanifolia was 270.130 μg and 66.022 μg.The ability of enrichment of soil Hg was Firmiana platanifolia > Melia azedarachAfter 10 days of heavy Cd stress,the leaves of Melia azedarach was chlorosis with veins twisted;the leaves of Firmiana platanifolia showed black spots with the veins swelled and turned red.These symptoms disappeared after 60 days.The growth of Melia azedarach and Firmiana platanifolia showed the “Hormeswas effect”.Under light and medium Cd stress.The difference between the total biomass of Firmiana platanifolia and the control was not significant under heavy Cd stress.With the increase of Cd concentration,the chlorophyll content of Melia azedarach and Firmiana platanifolia increased first and then decreased,the MDA content persistently increased.The Cd content and accumulation in each part of Melia azedarach and Firmiana platanifolia increased with the increase of Cd concentration,and all reached the maximum in the heavy treatment group.The Cd accumulation of Melia azedarach and Firmiana platanifolia per plant were 162.590μg and 269.483 μg.The Cd transfer factor of Firmiana platanifolia was larger than 1 under light and medium Cd stress.The ability of enrichment and transport of soil Cd was Firmiana platanifolia > Melia azedarachThe comprehensive evaluation value <D> of the tolerance of Melia azedarach and Firmiana platanifolia to soil Hg and Cd was: Hg resistance of Firmiana platanifolia(0.553)> Hg resistance of Melia azedarach(0.376);Cd resistance of Firmiana platanifolia(0.436)> Cd resistance of Melia azedarach(0.307).In conclusion,neither Melia azedarach nor Firmiana platanifolia were hyperaccumulator of Hg or Cd.The comprehensive tolerance of Melia azedarach to soil Hg was weaker than that of Firmiana platanifolia while the absorption and accumulation capacity of Melia azedarach to soil Hg was significantly stronger than that of Firmiana platanifolia;the tolerance,enrichment and translocation capacity of Firmiana platanifolia to soil Cd were stronger than those of Melia azedarach and the difference was significant in the heavy treatment group.3 Effect of Ophiopogon japonicus on the growth and enrichment characteristics of Melia azedarach and Firmiana platanifolia under Hg and Cd stressAt all Hg concentrations,the combination with Ophiopogon japonicus significantly promoted the extension and thicken of the main root,the incerase of biomass in each part and plant height of Melia azedarach The root and total biomass increased the most in the medium group,which were 1.65 and 1.29 times that of the control,respectively and the difference was significant.When Hg was50mg/kg,the combination with Ophiopogon japonicus significantly reduced the Hg content in the root and Hg accumulation per plant of Melia azedarach,which were0.50 and 0.62 times of the control with extremely significant difference.At all Cd concentrations,combination with Ophiopogon japonicus promoted the extension and thicken of the main root and incerased the roots and total biomass of Firmiana platanifolia.The roots and total biomass increased the most in the medium treatment group,which were 1.93 times and 1.62 times of the control.The difference was extremely significant.Combination with O.japonicus significantly increased the Cd transfer factor of Firmiana platanifolia at all concentrations and the accumulation of Cd in each part of Firmiana platanifolia in the heavy treatment group.The accumulation of Cd in the aerial parts and the whole plants increased the most in the heavy treatment group,incerasing by 57.04% and 18.63% compared with the control with extremely significant difference.As a common garden plant in southwest China,Firmiana platanifolia can be grown in combination with Ophiopogon japonicus,which can effectively promote the growth of Firmiana platanifolia and increase its ability to accumulate and transport Cd in the soil,to be used for phytoextraction or phytostabilization in heavy(50mg/kg)Cd contaminated soil and may play a certain role in absence of super-enriched tree species. |
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