Regulatory Effecte Of Exogenous Salicylic Acid On Helianthus Tuberosus L.under Copper Stress And The Mechanism Of Copper Tolerance

Helianthus tuberosus L.as an eco-environment-friendly cash crop which can adapt to all kinds of unfit conditions with a high biological yield,is industrially planted in many regions of China.However,the increasing pollution of soil copper has led to a fall in the yield and quality of H.tuberosus,and the safety of edibles is questioned,hindering the natural growth and promotion.However,as a plant hormone and signal molecule,exogenous salicylic acid（SA）can reduce membrane lipid peroxidation,and trigger systemic acquired resistance by regulating the photosynthesis and mineral absorption of plants,and participating in the the antioxidant defense system.In view of this,this study uses copper-tolerant H.tuberosus of Xuzhou and copper-sensitive H.tuberosus of Weifang as experimental materials and chooses 1 m M SA mitigation concentration sprayed on the plants grown at 300 mg/kg soil copper stress concentration,to explore the mechanism of how exogenous SA alleviates the copper stress on H.tuberosus.To this end,this study analyzed the effects of exogenous SA on the photosynthesis of H.tuberosus,the antioxidant system of the aboveground and underground parts,and the internal factors of the various organs of H.tuberosus under copper stress.The findings are as follows:1.Under copper stress,the net photosynthetic rate（A）,transpiration rate（E）and stomatal conductance（gsw）of H.tuberosus leaves decreased as the intercellular carbon dioxide concentration（Ci）decreased,indicating that copper affects plant performance through stomata restriction photosynthetic rate.With the accumulation of the treatment time,the values of Ci,A,E and gsw showed a decreasing trend in each treatment group.At the same time,the contents of chlorophyll a,chlorophyll b and carotenoids in H.tuberosus reduced after copper treatment（chlorophyll b reduced most shapely）,in the three cycles,the H.tuberosus of Xuzhou in the Cu treatment group was 41.62%,83.77%,and 56.35%of the CK group,and H.tuberosus of Weifang was 66.55%,64.58%,and 68.42%,respectively;accompanied by a significant increase in the initial fluorescence（F₀）and non-photochemical quenching coefficient（q _N）and a decrease in the maximum photochemical quantum yield（Fv/Fm）,electron transfer efficiency（ETR）and photochemical quenching coefficient（q _P）content.However,with the application of exogenous SA,the values of all photosynthetic indicators such as Ci,A,E,and gsw recovered to the level of the controlled group,indicating that copper stress reduces the photosynthetic capacity of H.tuberosus by regulating the degree of stomata opening,inhibiting chlorophyll synthesis and reducing photosynthesis efficiency.However,the application of SA can effectively alleviate such adverse effects of copper on the photosynthesis of H.tuberosus.2.After the copper treatment,the ascorbic acid（AsA）content decreased with the glutathione（GSH）value increasing in the leaves of H.tuberosus,while a large amount of active oxygen accumulated in the root tip tissue.In the leaves,the activity of the superoxide dismutase（SOD）,catalase（CAT）and ascorbate peroxidase（APX）declined all,while there was a significant increase in the activity of peroxidase（POD）enzyme.A consistent change of CAT and POD was observed in the root system.This indicates that the copper toxicity hinders the process of active oxygen removal in the cells by influencing the activity of various antioxidant enzymes.As the exogenous SA was applied,the synthesis of AsA promoted,and GSH increased sharply on the21st day,which compared with the previous cycle,the growth rates of H.tuberosus in the two regions reached 248.59%and 209.18%respectively,and various antioxidant enzymes in the leaves and root tips also returned to the control group concentration.However,contrary to the changes of SOD and APX in leaves,the activity of these two enzymes in root tips increased in the copper treatment group,and decreased after the addition of exogenous SA.This may be related to the fact that exogenous SA did not directly affect the root system,proving indirectly that SOD and APX may be the key enzymes of H.tuberosus against copper toxicity.Therefore,exogenous SA can effectively regulate the antioxidant enzyme system of H.tuberosus in the leaves and roots to enhance the tolerance of H.tuberosus to copper by initiating the AsA-GSH cycle to avoid excessive accumulation of active oxygen in the body.3.The Cu content in the ground and roots of H.tuberosus increased with the copper treatment but decreased with the addition of exogenous SA,which the copper content in H.tuberosus Xuzhou of roots,stems and leaves decreased by 43.06%,30.69%and 38.13%,while H.tuberosus of Weifang decreased to 47.85%,53.51%and 37.36%,respectively,indicating the Cu²⁺accumulation effect in H.tuberosus was reversed.At the same time,the cation exchange capacity of H.tuberosus was changed by copper toxicity,which weakened the absorption capacity of K,Ca,Mg and Zn nutrients in the roots,stems and leaves of H.tuberosus.However,the addition of exogenous SA can increase the content of K,Ca and Mg in various organs under copper toxicity,but it has little effect on the absorption and transport of Zn.Therefore,when copper stress destroys the absorption and transport of nutrient elements in H.tuberosus and the dynamic balance among various ions,the application of exogenous SA can regulate the integrity and stability of the plasma membrane,restore the absorption of mineral elements,and reduce the nutrient deficiency in H.tuberosus to aviod the symptoms of ion poisoning.4.This study found that the root tip cell wall of H.tuberosus contains polysaccharides,fats,proteins,saturated hydrocarbons and aromatic substances,of which sugars are the most important constituents of the H.tuberosus cell wall by FTIR to analyze the functional groups in the root tip cell wall of H.tuberosus.The absorption peaks of hydroxyl and amino groups became sharper and narrower under copper treatment,but after the addition of exogenous SA,the shape of the absorption peaks recovered,indicating that exogenous SA can adjust the hydroxyl and amino bonds in polysaccharides to remission transform of the cell wall composition under copper.Under copper treatment,the characteristic absorption peaks of the ester carbonyl C=O bond,the C=C bond in the lignin aromatic ring and the carboxylate COO-asymmetric stretch bond on the cell wall of H.tuberosus root cell wall shifted on different levels,and the absorbance ratio of the three functional groups to the absorption peak of-CH₃bond increased.With the application of exogenous SA,the ratio further increased,and the displacement of the three functional group absorption peaks decreased,indicating that exogenous SA also increased the content of functional groups,and weaken the role of hydrogen bonds in the functional groups of the cell wall,to enhance the binding force with metals,thereby alleviating the toxic effect of heavy metal copper on H.tuberosus.Based on the above research results,this study confirms that exogenous SA can avoid the accumulation of excessive metal copper in the body of H.tuberosus by promoting the absorption and transport of mineral nutrients in the body and regulating the changes in root tip cell wall functional groups.At the same time,exogenous SA can alleviate the inhibitory effect of copper on plant growth and enhance the tolerance to copper toxicity by improving the antioxidant and photosynthesis capacity of H.tuberosus.