Ion Balance Regulation Mechanism Of Static Magnetic Field Enhancing Salt Stress Tolerance Of Poplar

Saline soil is widely distributed all over the world,and Na Cl is considered to be one of the most important components that restrict the growth of poplar in saline soil.The initial effects of salt stress are ion toxicity and osmotic stress,but secondary reactive oxygen species injury significantly inhibits plant growth and development.In recent years,researchers have found that the application of magnetic field higher than the intensity of geomagnetic field can stimulate the growth and yield of several agronomic plants.Under stress conditions,magnetic treatment is also a simple choice to improve crop growth traits and yield.Although a large number of studies on magnetic effects have been carried out so far,they are mainly focused on animal medicine and plant seed physiology,and the research on magnetic biological effects and plant stress resistance is still lack of theoretical support.Using poplar as plant material,the effects of external steady magnetic field（SMF）on ion balance and active oxygen metabolism in poplar roots under salt stress were studied.The main results are summarized as follows:（1）SMF treatment can alleviate salt stress injury of poplar seedlings under Na Cl stress.Na Cl treatment of 100m M significantly inhibited the growth of poplar seedlings,while SMF treatment promoted the growth of poplar roots,which significantly increased both root fresh weight and root length.（2）SMF treatment can reduce the stress damage caused by Na Cl by increasing the level of osmotic substances and maintaining the homeostasis of antioxidant system.The results showed that the contents of MDA and proline in poplar roots did not change with the loading of SMF,but the content of MDA and proline in poplar roots significantly decreased under salt stress under SMF treatment.Under salt stress,ROS levels and Ca²⁺concentration detected in poplar root cells in the control group were significantly higher than those in the SMF group,and SMF treatment could maintain homeostasis of reactive oxygen species and Ca²⁺in root cells.At the same time,the results showed that stable magnetic field could activate reactive oxygen species and calcium signals in poplar root cells without salt stress.In addition,SMF treatment significantly increased SOD,CAT and POD contents in poplar roots,and increased SOD and CAT activities in poplar roots under salt stress.The CAT activity of the plants in the control group under salt stress was significantly lower than that in the non-salt stress control group,while the plants in the SMF treatment group maintained the CAT level under salt stress.（3）SMF can maintain ion balance in poplar roots under salt stress.SEM-EDS results showed that the content of Na⁺increased significantly,while the content of K⁺and Ca²⁺decreased significantly in the control group under 100m MNa Cl stress.The increase of Na⁺content in roots of SMF treatment group was lower than that of control group under salt stress,and the contents of K⁺and Ca²⁺,the ratio of K⁺/Na⁺and Ca²⁺/Na⁺had no significant change.However,in the absence of Na Cl treatment,SMF treatment resulted in the loss of cations in poplar root cells.（4）Under salt stress,SMF could change the ion flux at root tip of poplar seedlings and maintain the balance of Na⁺,K⁺and Ca²⁺ions in roots.The results of NMT measurement showed that SMF treatment could effectively block the entry of Na⁺into cells and prevent the massive loss of intracellular H⁺in poplar seedling roots under salt stress,and this ability was related to the enhancement of the activities of Na⁺/H⁺exchanger and H⁺-ATPase in plasma membrane.Furthermore,in the absence of salt treatment,SMF treatment significantly reduced the net influent flux of H⁺,but did not change the flux of Na⁺.Both ST and LT salt treatments resulted in significant net K⁺outflow from poplar roots in the control group.SMF treatment reduced the degree of Na Cl induced plasma membrane depolarization and prevented K⁺outflow from poplar root cells.In the absence of Na Cl stress,the root system of SMF treated plants showed significant K⁺outflow.In the absence of salt stress,there was no significant difference in net Ca²⁺flux between the control group and the SMF-treated group.ST salt stress caused significant Ca²⁺inflow to outflow,and LT salt stress reduced Ca²⁺inflow.However,salt stress of ST and LT did not result in significant Ca²⁺efflux from the root system of poplar treated with SMF.On the contrary,LT salinity significantly increased the net flux of Ca²⁺,reaching-66.0597pmol cm^-2s^-1.（5）SMF treatment regulates reactive oxygen species homeostasis and ion flux by improving transcription activity of specific genes in root leaves.The transcription levels of poplar rboh F genes were measured by q PCR.The results showed that SMF up-regulated the expression levels of rboh F,rboh D,rboh C,HA5 and NHX1 in roots.Meanwhile,SMF up-regulated the expression levels of rboh F,rboh D,rboh C,HA5,NHX1,NHX6,CSD2 and MSD1in leaves.In conclusion,salt stress can cause the imbalance of ion proportion in poplar root cells,and then cause the disorder of reactive oxygen species system.However,SMF can enhance the activities of H⁺-ATPase and Na⁺/H⁺exchanger in the plasma membrane,so that poplar can maintain the ion balance in root cells under salt stress.Meanwhile,SMF treatment can maintain the homeostasis of reactive oxygen species and Ca²⁺in root cells,so that poplar can obtain less ion toxicity and osmotic stress under salt stress.It can improve the ability of poplar to withstand salt stress.