Physiological And Biochemical Response Of ‘Hanfu’ Apple Seedlings To Heavy Metal Copper Stress

In recent years,due to the unscientific application of copper(Cu)pesticides containing Cu in agricultural production,the content of Cu in some livestock and poultry feces exceeds the standard and sewage irrigation,excessive Cu is enriched in in orchards.Although Cu is a kind of micronutrient which is necessary for the growth and development of fruit trees,excessive Cu can can cause toxic effects on fruit trees.In this study,seedlings of ‘Hanfu’/Malus baccata Borkh.were cultivated in sand and exposed to 60,180,300,600 and 900 mg kg-1 Cu,and the basic Cu concentration in Hoagland nutrient solution was used as the control(CK).The physiological and biochemical responses of seedlings to different Cu concentration treatments were analyzed by physiological and biochemical techniques.The results of the present study revealed that the Cu uptake,translocation and accumulation,and the degree of phytotoxicity in seedlings,which will provide a theoretical basis for determining the threshold of Cu toxicity in ‘Hanfu’/Malus baccata Borkh.,so as to guide agricultural production and practice.The main results are as follows:After the seedlings were treated with different concentrations of Cu for 60 days,the plant growth was not significantly restrained when the concentration of Cu was less than 300 mg kg-1.Although there was no plant death during cultivation under 900 mg kg-1 Cu treatment,300 mg kg-1 Cu treatment inhibited the growth of ‘Hanfu’/Malus baccata Borkh.seedlings such as decreasing fine root,deepening of root browned and leaf yellowing,dwarfing the plant.When the concentration of Cu was less than 300 mg kg-1,the biomass,root configuration and photosynthetic parameters of ‘Hanfu’/Malus baccata Borkh.seedlings were not significantly affected,but under the treatment of high concentration(more than 300 mg kg-1),the parameters above showed a significant downtrend compared with the control.For example,the total root length,total root surface area and total root volume decreased by 35.95%,18.59% and 32.45%,respectively,under the highest concentration treatment compared with the control.Net photosynthetic rate and stomatal conductivity decreased by 57.81% and 61.55%,respectively.The maximum chemical efficiency and photosynthetic performance index decreased by 13.01% and 78.32% compared with the control,which was consistent with its growth condition.With the increase of Cu concentration,the content of Cu and the amount of total Cu in each tissue were significantly higher than those in the control.The bio-concentration factor(BCF)of root significantly increased when Cu concentration was higher than 180 mg kg-1,while the translocation factor of aerial part significantly decreased when Cu concentration was higher than 180 mg kg-1.The accumulation of soluble sugar and starch in the tissues of ‘Hanfu’/Malus baccata Borkh.seedlings decreased when the concentration of Cu was higher than 300 mg kg-1,and the accumulation of soluble sugar and starch increased when Cu concentration was lower than 300 mg kg-1.With the concentration of Cu increased,the content of active oxygen and malondialdehyde in the tissues gradually increased.In order to clear the excessive accumulated active oxygen,the content of free proline in each tissue was significantly accumulated,though the content of the total thiol and the soluble phenol in the roots showed a downtrend,and the total mercaptan and the soluble phenol in the stem are at the critical point of 300 mg kg-1,The change was increased and then decreased.The total mercaptan in the leaves decreased with the increase of the concentration of Cu,but the content of the soluble phenol showed an increasing trend,which decreased lightly at the highest concentration.Taking the Cu concentration of 300 mg kg-1 as the critical point,the content of GSH of each tissue and GSSG in root increased at first and then decreased,while the content of GSSG in stems and leaves decreased with the increase of Cu concentration.With the increase of Cu concentration,the contents of ASC and DHA in stems and leaves increased significantly,while the content of ASC in roots did not change significantly at low concentration,but increased significantly at high concentration,while the content of DHA increased significantly at low concentration and kept it almost the same at high concentration.Taking the Cu concentration of 300 mg kg-1 as the critical point,the activity of SOD in roots and stems,the activity of CAT in leaves,the activity of APX in roots and leaves,the activity of POD in roots and stems,and the activity of GR in roots increased at first and then decreased.Other enzyme activities showed a decreasing trend with the increase of Cu treatment concentration,indicating that when the concentration of Cu treatment was higher than 300 mg kg-1,the antioxidant enzyme system of ‘Hanfu’/Malus baccata Borkh.seedlings was restrained significantly.The results above showed that 300 mg kg-1 is the threshold of Cu stress on the ‘Hanfu’/Malus baccata Borkh.seedlings.There would be an obvious toxic effect when the concentration of Cu was higher than this concentration.