Physiological Mechanism Of Melatonin Priming Regulating Root Hydraulic Conductivity Of Winter Wheat Seedlings Under Water And Salt Stress

Drought and salinization are the key factors restricting agricultural production in arid/semi-arid areas.It is of great significance to improve the drought resistance and salt tolerance of plants for maintaining plant growth and promoting the sustainable development of agricultural production.The root system is an organ that plants direct contact with soil and can sense the changes in the environment and respond to them.Root hydraulic conductivity indicates the ability of the root system to transport and conduct water,and regulating root hydraulic conductivity is helpful to improve the ability of plants to resist stress.In this study,winter wheat was taken as the research object,and three stress treatments(water stress（2%PEG6000+Hoagland nutrient solution）,salt stress（0.1%Na Cl+Hoagland nutrient solution）,and water-salt compound stress（2%PEG6000+0.1%Na Cl+Hoagland nutrient solution）and control treatment（Hoagland nutrient solution,CK）were set up to investigate the water-salt stress induced by melatonin.The main findings are as follows:1.Although melatonin and water priming decreased the root parameters of wheat seedlings under CK,they both increased the root hydraulic conductivity per plant and theoretical root hydraulic conductivity of wheat seedlings.Under CK,the proline content in the shoot of wheat seedlings increased by 23%,Na⁺/K⁺decreased by 3%,and malondialdehyde（MDA）decreased by 36%.Water priming increased the activities of superoxide dismutase（SOD）,peroxidase（POD）,and hydrogen peroxide（CAT）in the roots of wheat seedlings by 56%and 40%respectively,and the MDA content increased by 23%,respectively.2.Water stress reduced the root hydraulic conductivity of winter wheat seedlings,while melatonin significantly increased the root hydraulic conductivity per plant and theoretical root hydraulic conductivity.Compared with CK,water stress significantly reduced the root length of wheat seedlings by 56%,and the root hydraulic conductivity per plant and theoretical root hydraulic conductivity decreased by 27%and 77%respectively.The decrease of root hydraulic conductivity resulted in the decrease of nitrogen and K⁺absorption in the root tissue of wheat seedlings,the increase of soluble sugar and proline content of osmotic adjustment substances,and the significant increase of SOD and CAT activities in root tissue.Under water stress,melatonin priming increased the root length of wheat by 47%,the root hydraulic conductivity per plant by 53%,and the theoretical root hydraulic conductivity by 37.8%.The recovery of root hydraulic conductivity promoted the absorption of nitrogen and K⁺by wheat seedlings,increased the content of soluble protein and POD activity,and decreased the content of malondialdehyde（MDA）.3.Melatonin priming significantly increased the root hydraulic conductivity of winter wheat seedlings under salt stress.Salt stress significantly reduced the root length of wheat seedlings by 30%,and the root hydraulic conductivity per plant and theoretical root hydraulic conductivity decreased by 35%and 67%,respectively.The absorption of K⁺by wheat seedlings was reduced,and the Na⁺content in shoot tissue and root tissue of wheat was significantly increased by 3.4 times and 3.8 times,respectively,and the MDA content was also increased.Under salt stress,melatonin priming increased the root length of wheat by 25%,and the root hydraulic conductivity per plant and theoretical root hydraulic conductivity increased by 104%and 53%respectively.Melatonin priming reduced the starch content of wheat seedlings and promoted the recovery of root hydraulic conductivity.The recovery of root hydraulic conductivity of wheat seedlings increased the absorption of K⁺and decreased the absorption of Na⁺,which significantly increased the POD activity of roots.4.The combined stress of water and salt seriously inhibited the growth of wheat seedlings,and melatonin priming significantly increased the root hydraulic conductivity of wheat seedlings and promoted the recovery of wheat seedlings.The combined stress of water and salt significantly reduced the root length and theoretical root hydraulic conductivity of wheat seedlings by 96%and 99%,respectively,resulting in a significant decrease in the absorption of N and K⁺by wheat seedlings,an increase in the Na⁺content in root tissues and shoot tissues by3.6 times and 7.6 times,and a significant increase in the activities of osmotic regulators,POD,and CAT.Under the combined stress of water and salt,the root length of wheat seedlings was significantly increased by 76%,and the root hydraulic conductivity per plant and theoretical root hydraulic conductivity were restored to 0.444 ml·hr^-1·Mpa^-1and 0.441 mg·mm^-1·Kpa·s^-1.Melatonin priming increased the content of soluble protein and proline in root tissue and decreased the POD activity by 22%.