| Drought stress is a major environmental factor affecting plant growth,development and yield on a global scale.spermidine and melatonin are two important plant hormones that have important regulatory effects on plant growth and development,yield formation and stress tolerance,but there has been little clarity on their effects on plant responses to drought stress.Hordeum jubatum L.is an ornamental grass with high stress tolerance and ecological importance in Northeast China.In this study,H.jubatum was used as the test material,and different concentrations of exogenous spermidine(0 mmol/L,0.5 mmol/L,1.0 mmol/L,1.5mmol/L)and melatonin(0 mmol/L,0.1 mmol/L,0.2 mmol/L,0.3 mmol/L)were applied in equal amounts by root application in a pot test design and in a greenhouse through natural drought treatment(0,5,10,15 d of drought)by natural water control,the root system conformation,anatomical structure and physiological parameters were determined and metabolomic analysis was performed to investigate the effects of exogenous phytohormones(spermidine and melatonin)on the root system of H.jubatum under drought stress using a combination of physiological,anatomical and histological techniques,this study provides a scientific basis for exploring the mechanism of drought resistance and the function of exogenous hormones in H.jubatum,and also provides an important basis for the cultivation and management of H.jubatum in arid areas.The main findings are as follows:(1)Exogenous spermidine and melatonin promote the growth of the root system in drought-stressed H.jubatum.The response to drought stress was gradually increased as the duration of drought increased.The root length,root surface area,average root diameter,root fork genus and total root volume all showed a gradual increase with the addition of different concentrations of exogenous spermidine,while the specific root length first decreased and then increased and the root weight density showed a gradual decrease.The average diameter,the number of root bifurcations and the total volume of the root system of H.jubatum with different melatonin concentrations showed a gradual increase,the surface area of the root system showed an increasing trend followed by a decreasing trend,and the specific root length and root weight density gradually decreased.The application of 1.0 mmol/L exogenous spermidine and 0.2 mmol/L exogenous melatonin could effectively promote root growth during0-10 d of drought,and 1.5 mmol/L exogenous spermidine and 0.1 mmol/L exogenous melatonin could effectively promote root growth during 11-15 d of drought.(2)Exogenous spermidine and melatonin enhanced the antioxidant capacity and maintained the osmotic equilibrium of the root system under drought stress.Under drought stress,Superoxide dismutase(SOD)activity in the roots of H.jubatum showed an increasing trend followed by a decreasing trend,the activity of catalase(CAT),peroxidase(POD)and ascorbate peroxidase(APX)increased gradually,and the osmoregulatory substances(malondialdehyde,proline,soluble sugars and soluble proteins)increased gradually with the duration of drought.The application of exogenous spermidine and melatonin significantly increased the activity of SOD,CAT,POD,APX and the content of proline,soluble sugars and soluble proteins,while the content of malondialdehyde decreased significantly.The application of 1.0-1.5 mmol/L exogenous spermidine was more effective in enhancing the antioxidant capacity of the root system of H.jubatum,and the application of 0.1 mmol/L exogenous melatonin was more effective in maintaining the osmotic balance of the root system of H.jubatum.(3)Exogenous spermidine and melatonin increased the transport capacity,increased the cortical thickness and reduced the lignification of the root system of H.jubatum under drought stress.From 0 to 10 d of drought,the root diameter,duct diameter and cortical thickness of H.jubatum showed an increasing trend,and from 11 to 15 d of drought,they all showed different degrees of decrease.The application of 1.0 mmol/L exogenous spermidine and 0.1 mmol/L exogenous melatonin to drought stress(0-10 d)was able to improve the transport capacity,cortical thickness and lignification of the roots of H.jubatum.The application of 1 mmol/L exogenous spermidine and 0.2 mmol/L exogenous melatonin to drought stress(11-d)improved the transport capacity,cortical thickness and lignification of the root system.(4)On the basis of the physiological and root anatomical analyses in the previous chapter,a 10-d natural water loss treatment with more obvious stress effects,and 1.5 mmol/L spermidine and 0.2 mmol/L melatonin as exogenous hormone application treatments with better alleviating effects were selected and further metabolomic analyses were conducted.Further KEGG enrichment analysis revealed that drought stress significantly affected the metabolic processes in the root system of H.jubatum,especially on linolenic acid metabolism,phenylalanine metabolism,aminoacyl t-RNA biosynthesis and ABC transporter protein synthesis.In contrast,269 and 238 metabolites were significantly altered in the roots of H.jubatum after spermidine and demelatonin application,respectively.In the arginine treatment,the differential metabolites were mainly enriched in three metabolic pathways,including phenylalanine metabolism,phenylpropanoid biosynthesis and linolenic acid metabolism,whereas the melatonin treatment metabolites were mainly enriched in the tricarboxylic acid cycle and linoleic acid metabolism.Although both spermidine and melatonin treatments were able to improve the drought tolerance of H.jubatum roots to a certain extent,the mechanisms of action of the two exogenous hormones were distinctly different.Under drought stress,the application of exogenous spermidine significantly increased the metabolism of phenylalanine,phenylpropanoid biosynthesis process linolenic acid,which on the one hand promoted the biosynthesis of phenolic compounds with antioxidant properties,and on the other hand also improved the cell membrane flow of the root system under drought stress,significantly alleviating the oxidative damage caused by drought stress,while promoting cell membrane fluidity,and to some extent maintaining the physiological metabolic processes of H.jubatum under drought stress.The physiological and metabolic processes of the root system of wheatgrass under drought stress were maintained to a certain extent.In the melatonin application treatment,the exogenous hormone application significantly enhanced the tricarboxylic acid cycle and linoleic acid metabolic processes,promoting the production of adenosine triphosphate(ATP)on the one hand,and influencing the synthesis of various enolenic aldehydes and enols on the other,enhancing the energy metabolic processes in the root system of H.jubatum,producing more energy to help it adapt to the energy deficit caused by drought stress,and increasing the production of various enolenic aldehydes and enols.It also increased the biosynthesis of various alkenals and alkenols,which regulated the growth and development of the root system under drought stress. |
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