Effects Of Soil Stress Mediated By Water Potential On Root Morphology And Physiological Metabolism Of Hordeum Jubatum Seedlings

Hordeum jubatum L.is a plant of Gramineae barley family,which has strong reproductive ability and extensive ecological adaptability.Its inflorescence is green or purplish,light and soft in shape,and has excellent ornamental effect.H.jubatum has strong adaptability and extensive management,so it has high application and promotion value in landscaping.It is suitable to be a good resource plant for soil and water conservation and ecological restoration,and plays an important role in the process of ecological restoration and urban greening in the north,As halophyte,H.jubatum also has high salt tolerance,and its roots can absorb and accumulate a large amount of salt and reduce the salt content of the soil.Based on this,this paper systematically studied the effects of salt,alkali and drought stress on the root configuration and physiological metabolism of H.jubatum barley grass under the same water potential,and discussed the physiological response of the root system of H.jubatum to different stress types under the same water potential,which is of great significance to further explore the stress resistance mechanism of H.jubatum barley grass and promote the development of excellent ornamental grass varieties in Northeast China.The results showed that:（1）With the decrease of water potential,the total root length,total surface area,total volume,root tip number and fork number decreased gradually under salt,alkali and drought stress,and the root system architecture was significantly inhibited under-0.8 MPa,and the inhibition degree was alkali stress>salt stress>drought stress.Under alkali stress,the total root length was 145.83 cm,which decreased by 55.12%compared with the control.Compared with saline-alkali stress,the root system of H.jubatum barley grass is more resistant to drought stress,which can be resisted by increasing the number of forks,and the number of forks is the largest at-0.2 MPa,which increases by 9.52%.With the decrease of water potential,the fractal dimension of saline-alkali stress gradually increased but lower than that of the control group,and the fractal dimension of drought stress decreased significantly,while the root fractal abundance was only affected by water potential and had no obvious response to the type of stress and the interaction between them.In this study,through the classification of different root diameter grades,it was found that the inhibitory effect of stress on the roots of different diameter grades was significantly different,and the inhibition effect of 0-0.1 mm diameter root system was the most significant.Compared with the control group,the fine root length,root surface area,root volume and root tip number decreased significantly with the decrease of water potential,and the root length decreased by 64.44%,63.27%and 32.73%respectively under-0.8 MPa.Under high water potential stress,the inhibition degree of salt stress,alkali stress and drought stress on fine roots was from large to small,and with the decrease of water potential,the inhibitory effect of alkali stress on roots gradually increased.（2）Salt,alkali and drought stress all changed the anatomical structure of the root system to deal with the root damage caused by soil stress under various water potentials.After stress,the root diameter increased to a certain extent,and the number of vessels began to decrease.The epidermal cells of roots gradually separated and damaged from the original tight arrangement,the intercellular space gradually increased,while the size and thickness of cortical cells gradually increased.Root anatomical parameters,such as root diameter,cortical thickness,stele diameter,vessel average diameter and the ratio of cortical thickness to stele radius,were significantly affected by water potential,stress type and their interaction.At the same time,the interaction among the anatomical parameters was significant.The effects of saline-alkali stress and drought stress on the anatomical structure of roots were obviously different:with the decrease of water potential,the root diameter,the thickness of cortex and the diameter of stele decreased gradually under saline-alkali stress.The average diameter of vessel decreased gradually,and the ratio of cortex thickness to stele radius increased gradually.On the contrary,.the ratio of cortex thickness to stele radius decreased gradually but significantly higher than that of the control group under drought stress,and increased by 72.71%,48.14%and 14.96%under-0.2,0.4 and-0.8 MPa,respectively.The inhibition effect of root anatomical structure under alkali stress was the most significant（P<0.05）,and the inhibition effect was aggravated with the decrease of water potential.Under the condition of-0.8 MPa,the root system under alkali stress was the most toxic,the thickness of cortex increased by 6.21%,and the diameter of stele decreased by 28.95%.（3）With the decrease of water potential,the contents of MDA and proline in roots of H.jubatum showed an upward trend under salt,alkali and drought stress,and the contents of MDA were alkali stress,salt stress and drought stress from large to small,and the contents of MDA and proline increased the most with the decrease of water potential.With the decrease of water potential,the content of soluble sugar in roots showed a downward trend under saline-alkali stress.The content of soluble sugar decreased the most under alkali stress and the lowest under-0.8 MPa,which decreased by 40.15%.However,it showed a gradual upward trend under drought stress.With the decrease of water potential,the Na⁺content of root system of H.jubatum increased significantly under saline-alkali stress and decreased gradually under drought stress,and the Na⁺content under alkali stress was significantly higher than that under salt stress,while the root K⁺content decreased under salt,alkali and drought stress,and the K⁺content under saline-alkali stress was significantly lower than that under drought stress.With the decrease of water potential,the activities of SOD,POD and APX increased under salt,alkali and drought stress,while the activity of CAT decreased with the decrease of water potential.Alkali stress forced the root POD and APX to increase significantly while maintaining higher SOD activity.Under alkali stress,the SOD activity was the highest under-0.8 MPa,which was 6.64 U mg^-1.（4）LC-MS was used to detect the changes of root metabolites of H.jubatum after salt,alkali and drought stress treatments under-0.8 MPa.The results of modeling analysis showed that different stress treatments caused the changes of root metabolic map of H.jubatum.Under positive and negative ion mode,the results of principal component analysis（PCA）showed that the trend of each treatment group was obvious,and results of orthogonal partial least square discriminant analysis（OPLS-DA）showed that salt,alkali and drought stress were significantly separated from the control.There is a total of 36 differential metabolites were detected,which were enriched in KEGG database.The results showed that there were 8,9 and 12 metabolic pathways in salt,alkali and drought stress,respectively,of which 5 metabolites were rich in metabolites in the three stresses.The metabolites included Citrate cycle（TCA cycle）,arginine and proline metabolism,alanine,aspartate and glutamate metabolism,mainly related to amino acids and organic acid metabolites.Citrate cycle（TCA cycle）,aspartate and glutamate metabolism and arginine biosynthesis were the most significant metabolic changes caused by saline-alkali stress.The roots of H.jubatum resist saline-alkali stress by enhancing the citrate cycle and synthesizing reduced glutathione,nucleic acid and amino acids,while the roots not only enhance the synthesis of related amino acids under drought stress,but also respond to drought stress by synthesizing linolenic acid.