Effects Of Exogenous Proline On Physiological Characteristics Of Lonicera Japonica Under Cadmium Stress

The heavy metal cadmium（Cd）is a highly toxic pollutant.The absorption of cadmium by plants will reduce the activity of enzymes in plants,cause metabolic disorders,change the permeability of cell membrane,hinder the growth of roots,and inhibit the absorption of water and nutrients by plants.Cadmium can also be transferred to the human body along with the food chain,causing serious harm to human health.Proline is the most water-soluble amino acid in plants.It is an important organic substance with osmotic adjustment function and plays an important role in improving and enhancing plant stress resistance.Lonicera japonica Thunb.has the characteristics of long life,fast growth,well-developed root system,wide adaptability,strong resistance,and less strict requirements on soil,which may be an important material for new environmental protection soil remediation.In order to better realize phytoremediation technology and reduce or avoid cadmium damage in soil,this study used L.japonica as material and exogenous applied two different concentrations of proline(10 mmol·L^-1and 20 mmol·L^-1)at three concentrations of cadmium(50 mg·kg^-1,100 mg·kg^-1and 200 mg·kg^-1).The effects of exogenous proline on Cd stress in L.japonica were studied from three aspects of growth index,physiological characteristics and gene expression.The main results were as follows:（1）The plant height and dry weight of L.japonica were significantly increased under low concentration of cadmium stress,while the plant height and root length were significantly inhibited under high concentration of Cd stress,while the leaf area and fresh weight were significantly decreased.Exogenous proline treatment effectively alleviated the inhibitory effect of high concentration of cadmium on L.japonica,and the plant height,root length,dry weight and leaf area were significantly increased.After the absorption of cadmium,a large amount of cadmium will accumulate in the root of L.japonica,part of the cadmium can be transferred to the aboveground part of L.japonica.The application of proline could reduce the content of cadmium in roots,stems and leaves of L.japonica.（2）Under cadmium stress,the production rate of O₂^·-and the content of hydrogen peroxide in honeysuckle were significantly higher than those in the control,and a large amount of reactive oxygen species was accumulated.The activities of superoxide dismutase（SOD）and peroxidase（POD）increased.The content of malondialdehyde（MDA）,relative conductivity,endogenous proline content,soluble sugar and soluble protein content increased.Under cadmium stress,the contents of ascorbate peroxidase（APX）,monodehydroascorbate reductase（MDAR）,glutathione（GSH）and non-protein thiols（NPT）of L.japonica increased.Exogenous proline increased the activities of antioxidant enzymes and the contents of ascorbic acid（ASA）and GSH,significantly reduced the level of reactive oxygen species,and alleviated the oxidative damage.Exogenous Proline can induce more accumulation of endogenous proline,soluble sugar and protein in L.japonical,significantly increase the content of NPT in L.japonica under cadmium stress,promote the chelation of Cd²⁺,effectively alleviate the toxic effect of cadmium stress on L.japonica,and further improve the tolerance of L.japonica to cadmium.（3）Under the influence of cadmium stress,L.japonica changed at the gene level.Under cadmium stress,AP2-24 and Ljb ZIP1 genes were down-regulated,while Ljdof7 genes were up-regulated.Exogenous proline can promote the expression of ljbzip1 gene,but has limited effect on Lj Dof7 gene.Low concentration of exogenous proline may be more conducive to the increase of AP2-24 gene expression.In conclusion,L.japonica has a certain tolerance to cadmium,and the growth of L.japonica will be inhibited under high concentration of cadmium stress.Under cadmium stress,the excessive ROS accumulated in L.japonica,which threatened the oxidation system and the oxidative damage of cells.After exogenous proline application,the growth status of L.japonica was restored to some extent,the activities of antioxidant enzymes were increased,and the contents of antioxidant and chelating agent were increased,which reduced the toxicity of cadmium and alleviated the oxidative stress.