Effects Of Exogenous Plant Hormones On Physiological And Biochemical Characteristics Of Cucumber Seedlings Under Cadmium Stress

With the rapid development of urbanization,industrial"three wastes"emissions and large-scale application of pesticides and fertilizers,soil heavy metal pollution has become increasingly serious.Soil cadmium（Cd）pollution has attracted more attention.Cd in soil enters the plant,it will cause different degrees of damage to the growth and development of plants.Cd accumulated in plants also harms animal and human health through food chain transmission.Plant hormones such as 2,4-epibrassinolide（2,4-EBL）and salicylic acid（SA）play important roles in alleviating the stress of heavy metals on plant growth.Cucumber（Cucumis sativus L.）is the main variety of protected vegetable cultivation,but it is vulnerable to heavy metal pollution due to its weak resistance.At present,the possible mitigation effect and molecular mechanism of single or combined action of 2,4-EBL and SA on the growth of cucumber seedlings under Cd stress are not very clear.This study intended to explore the effects of exogenous 2,4-EBL,SA single and their combined treatment on physiological characteristics and molecular mechanism of cucumber seedlings under Cd stress on the basis of growth characteristics,photosynthesis,antioxidant system and gene expression through foliar spraying,so as to explore suitable exogenous regulatory substances for alleviating the toxicity of heavy metals in plants.The main results were as follows:（1）The effects of different concentrations of 2,4-EBL and SA on the growth of cucumber under 100μmol·L^-1Cd stress were studied.The results showed that appropriate concentrations of2,4-EBL and SA could increase plant height,root length,biomass and photosynthesis of plants under Cd stress.The Cd content in shoot and root of plants was reduced,and the inhibitory effect of Cd on cucumber growth were reduced.Among them,2,4-EBL at 0.5μmol·L^-1concentration and SA at 100μmol·L^-1concentration had the most significant alleviation effects.Hormones at too high or too low concentration caused inhibition of cucumber growth.The results showed that the effects of 2,4-EBL and SA on photosynthesis of cucumber under Cd stress were mainly non-stomatal limiting factors.When the concentration of 2,4-EBL was 0.5μmol·L^-1,the Cd content in shoot and root of cucumber decreased by 56.62%and 9.86%,respectively.When the concentration of SA was 100μmol·L^-1,the Cd content of cucumber root decreased by 10.02%.（2）The effects of 0.5μmol·L^-12,4-EBL and 100μmol·L^-1SA on the physiological and biochemical characteristics of cucumber seedlings induced by Cd were further analyzed.The results showed that exogenous 2,4-EBL and SA could effectively relieve the Cd toxicity of cucumber by single or combined application,but the effects of the three treatments on the plant were different.2,4-EBL significantly promoted the root length and biomass of cucumber treated with Cd.The effect of SA on plant height was more significant.Combined application could significantly promote cucumber root biomass.In addition,the time effect of spraying different hormones on cucumber was different.Single or combined application of exogenous 2,4-EBL and SA can reduce the content of Cd in shoot and root of plants 4-6 days after Cd stress,and only reduce the content of Cd in shoot and root on 2 days after Cd stress.Compared with Cd treatment,Both single treatment of 2,4-EBLand the combined application of two hormones reduced the Cd transfer coefficient on some treatment days,and the transfer coefficient was the lowest on both the2nd and the 4th day of treatment.By analyzing HMA3 and HMA4 gene expression under different treatments,it was found that Cd treatment significantly up-regulated HMA3 and HMA4 gene expression levels.The expression levels of HMA3 and HMA4 genes were down-regulated by single and combined application of 2,4-EBL and SA,and the combined treatment had the largest decrease,which was 55.42%and 44.75%,respectively.The results showed that 2,4-EBL and SA alone or in combination could inhibit the transport of Cd²⁺from root to shoot in plants.（3）Single or combined application of exogenous 2,4-EBL and SA could increase chlorophyll content,net photosynthetic rate and decrease intercellular carbon dioxide concentration of cucumber on some days under Cd stress.The net photosynthetic rate of cucumber under 2,4-EBL treatment was significantly increased by 69.89%and 46.13%compared with Cd treatment on the 2nd and 4th day of stress,respectively.SA treatment significantly increased net photosynthetic rate of cucumber on the 6th day of stress,and increased by 49.88%compared with Cd treatment.These results indicated that the difference of the effects of 2,4-EBL and SA on cucumber photosynthesis was related to the treatment time.The OJIP curves of cucumber leaves were also changed by different hormone treatments,and the values of chlorophyll fluorescence parameters Fv/Fm,Fv/Fo,PI_ABS,φEo andΨo increased,suggesting that exogenous 2,4-EBL and SA could reduce the damage of PSII reaction center and receptor side caused by Cd stress,increase the photochemical efficiency of PSII,and improve the photosynthetic performance of the plant.（4）Single or combined application of exogenous 2,4-EBL and SA up-regulated the gene expression levels of Cu/Zn-SOD,POD and CAT in cucumber seedlings under Cd stress.The gene expression levels of Cu/Zn-SOD and CAT in cucumber seedlings under Cd stress were the highest after combined treatment of hormones,reaching 1.64 and 1.07,respectively.POD gene expression was the highest after SA treatment,reaching 1.55.Similar to gene expression,SOD,POD and CAT activities of cucumber leaves and roots were increased by different hormone treatments,but the degree of increase of antioxidant enzymes was different.Single or combined application of2,4-EBL and SA could also reduce MDA content and proline content in cucumber leaves and roots,and increase soluble protein content in cucumber leaves under Cd stress.These results indicated that 2,4-EBL and SA could improve the defense of cucumber antioxidant system,reduce membrane peroxidation,maintain cell membrane integrity,and effectively alleviate Cd toxicity of cucumber seedlings.（5）Combined application of 2,4-EBL and SA had a synergistic effect on reducing Cd accumulation and Cd transport gene expression,and improving chlorophyll content,SOD activity and part of antioxidant enzyme gene expression in cucumber under Cd stress,and this effect was affected by the time of Cd treatment.In other parameters,it was between 2,4-EBL and SA treatments.In addition,the spraying concentration of 2,4-EBL or SA in agricultural production should be controlled.