Effects Of Exogenous Melatonin On Root System Of Lettuce Seedlings Under Salt Stress

The main agricultural facilities in our country are plastic greenhouses and sunlight greenhouses.Due to unreasonable water and fertilizer management,both substrate cultivation and soil cultivation are prone to salinization,which makes the plant roots under stress and seriously affects the growth and development of horticultural crops,thus causing great economic losses.Melatonin can promote the growth and development of plant lateral roots.Therefore,it is of great practical significance and application value to study the effect of exogenous melatonin on lettuce root growth and yield.In this study,' Hanyu' lettuce was used as the experimental material,and substrate cultivation was used to carry out two parts of experiments.The first part is the effect of melatonin of different concentrations on the root system of lettuce seedlings.Seven treatments are set up,namely CK and M0.01,M0.1,M1,M10,M50 and M100(M stands for melatonin treatment and 0.01 stands for melatonin concentration of 0.01?mol/L).The aim is to screen melatonin concentration suitable for lettuce root lateral root growth.In the second part,exogenous melatonin was used to alleviate the root development of lettuce under salt stress,and four treatments(CK: blank control;CY: 100 mmol/L sodium chloride solution;MT: 1?mol/L melatonin solution;MY: 1 ?mol/L melatonin and 100mmol/L sodium chloride solution).The effects of different treatments on root morphology,microstructure,expression of related genes LBD16 ? 18 ? 29 and endogenous melatonin hormone content of lettuce were studied.The test results are as follows:1.Root morphological indexes,root length,root diameter,root number and root surface area of lettuce seedlings changed significantly under different concentrations of melatonin.The concentration of melatonin had a concentration gradient effect on the root system of lettuce seedlings.Under the low concentration of 0.01 ?mol/L,the growth and development o the root system of lettuce seedlings had no difference from CK control.With the increase of concentration,the root length was significantly shortened but the number of lateral roots was significantly increased under the treatments of M0.1,M1 and M10.Under M1 treatment,the number of lateral roots was the largest.Under the high concentration M100 treatment,the root development of lettuce seedlings was seriously affected,which inhibited the growth and development of seedling roots.2.The effect of exogenous melatonin on the microstructure of lettuce under salt stress is mainly due to the significant increase of cambium thickness of lettuce root under MT treatment,which is 34% higher than CK.CY treatment is the lowest,only 70% of the thickness of CK treatment.In the development of vascular tissue of lettuce root,salt stress severely inhibited the development of vascular tissue cells of lettuce root,resulting in irregular cell development and disordered arrangement.After melatonin was applied,MY treatment significantly alleviated the inhibition of salt stress,and the vascular tissue cells had regular and orderly shapes,which proved that melatonin had a positive effect on alleviating the root development of lettuce under salt stress.3.Exogenous melatonin significantly affected the expression of genes related to root growth of lettuce seedlings under salt stress.LBD 16,LBD 18 and LBD 29 are specifically expressed in lettuce and play a very important role in regulating the growth and development of roots under the treatment of exogenous melatonin.After MT treatment,compared with CK,the expression level of LBD16 gene was significantly increased and the expression level of LBD18 gene was decreased.Compared with handling MY.The expression level of LBD 29 gene was significantly increased.At the same time,compared with CY and MY with salt stress,melatonin significantly lowered the expression levels of LBD18 and LBD29 genes.This indicates that melatonin plays an important role in up-regulating LBD16,thus enhancing the ability of cell division around xylem during lateral root formation and down-regulating LBD18 and LBD29 to change the morphogenesis of root vascular tissue.