| Salt,alkali and drought are the main environmental factors limiting plant growth in northeastern China,and the high p H brought by alkaline salts,mainly sodium bicarbonate,can have a stronger inhibitory effect on plants.As an ornamental grass that can be wildly distributed in northeastern China,Hordeum.jubatum has strong tolerance to salinity and drought stress,and is an excellent herb that combines ornamental and ecological values.Previous studies have mostly focused on plant responses to saline and saline drought stresses and compared the differences in different stress factors,but there are few reports on the mechanisms of the effects of salt,alkali and drought stresses on plants using water potential as an entry point.In this study,the physiological mechanisms of response to salt,alkali and drought stresses were analyzed from the perspectives of ultrastructure,photosynthesis,osmoregulation,carbon and nitrogen metabolism and differential gene expression by simulating soil salt,alkali and drought environments at different isohydric concentrations.The results of the study can help to elucidate the stress tolerance mechanism of H.jubatum and provide scientific basis for improving the ecological environment quality and the application of ornamental grasses in urban gardens in Northeast China.The specific results of the study are as follows.(1)The growth of the leaves of H.jubatum deteriorated under salt,alkali and drought stresses of equal water potential.Scanning electron microscopy observed that the inner and outer stomatal length and width and stomatal opening of the leaves decreased under all three stresses,with the alkali stress group showing a 19.0%and 29.0%decrease in inner stomatal length and width,8.4%and 46.7%decrease in outer stomatal length and width,and 48.5%decrease in stomatal opening compared to the control group,which were significantly higher than those of drought and salt stresses(P<0.05).In addition,the wax density on the leaf surface as well as the density of glandular hairs on the abaxial surface of the leaf increased significantly and the length of glandular hairs was also the longest under alkali stress compared with the control,with greater changes under drought stress than salt stress.Transmission electron microscopy showed that the cells and internal organelles could maintain their basic integrity under salt stress;drought stress caused the separation of the cell wall and partial dissolution of the organelle membrane and endosol;alkali stress caused the most serious damage to the leaf cells and organelles,the cell wall had dissolved,and most of the internal organelles were disintegrated or scattered outside the cells.(2)Salt,alkali and drought stresses significantly reduced the biomass and water content of leaves of H.jubatum seedlings(P<0.05).With decreasing water potential,net photosynthetic rate(Pn),stomatal conductance(Gs)intercellular carbon dioxide concentration(Ci)and transpiration rate(Tr)showed a gradual decrease under all three stresses,while water use efficiency(WUE)gradually increased,with the most significant changes under alkali stress.Chlorophyll content tended to decrease with increasing concentration under all three stresses,which was most obvious under alkali stress.With decreasing water potential,salt,alkali and drought stresses significantly reduced the maximum photosystem II photochemical efficiency(Fv/Fm),the actual photosystem II chemical efficiency(ФPSII)and photochemical quenching coefficient(q P)of H.jubatum,which decreased by 15.4%,45.7%and 43.5%under alkali stress at the lowest water potential compared with the control,significantly higher than drought and salt stresses,respectively(P<0.05).The non-photochemical quenching coefficient(NPQ)gradually increased with decreasing water potential under drought stress and salt stress,while NPQ first increased and then decreased under alkali stress,reaching a maximum value of 1.19 at-0.47 MPa water potential.(3)Proline,soluble sugar,and soluble protein contents in leaves of H.jubatum under salt,alkali,and drought stresses gradually increased with decreasing water potential,with the alkali stress group increasing 88.0%,68.3%,and 77.7%,respectively,compared with the control at the lowest water potential,which was significantly higher than that of drought and salt stresses(P<0.05).With decreasing water potential,all three stresses increased the Na~+content in the leaves of H.jubatum,and the Na~+content increased most significantly under alkali stress(P<0.05).Both salt and alkali stresses decreased the K~+content in the leaves of H.jubatum with decreasing water potential,while it showed a trend of decreasing and then increasing with decreasing water potential under drought stress.The malondialdehyde content,ascorbate lyase(APX)and superoxide dismutase(SOD)activities gradually increased with decreasing water potential,while peroxidase(POD)and catalase(CAT)activities gradually decreased under salt,alkali and drought stresses in H.jubatum leaves,with the most significant changes in the alkali stress group(P<0.05).(4)The sucrose content of H.jubatum leaves under salt,alkali and drought stresses varied with decreasing water potential,with the content showing a trend of decreasing followed by increasing under salt stress,while the opposite trend was observed for drought and alkali stresses,with the content under alkali stress being significantly higher than the salt and drought stresses(P<0.05).Fructose and glucose contents gradually increased with increasing stress concentration under all three stresses,except at-0.47 MPa water potential the alkali stress was significantly higher than the salt and drought stresses(P<0.05).The content of starch gradually decreased,with the most significant changes in the alkali stress(P<0.05).With the increase of stress concentration,the activities of enzymes related to carbon metabolism and gene expression levels showed an increasing trend.Nitrate,ammonium and total nitrogen contents gradually decreased with increasing stress concentrations under the three stresses,while free amino acid contents gradually increased,the most significant changes were observed in the alkali treatment groups(P<0.05).Glutamine synthetase(GS)and glutamate synthase(GOGAT)activities gradually increased with stress concentration under the three stresses,while nitrate reductase(NR)activity gradually decreased,and the expression levels of related enzyme genes(Hj GS,Hj GOGAT and Hj NR)changed in the same way,and the most significant changes were observed under alkali stress.(5)The number of differentially expressed genes in the salt,alkali drought and stress groups compared to the control was 1599 11935 and 3311 respectively,indicating that the leaves of H.jubatum were most sensitive to alkali stress.GO enrichment analysis showed that most of the differential genes for the three stresses compared to the control were distributed in entries localized to several major parts of metabolic processes,cellular components,signaling,membrane systems,and transport binding.The results of TOP30GO entries showed that both drought and salt stress significantly inhibited the abscisic acid signaling process,but jasmonic acid as a signaling molecule effectively mobilized various defense physiological processes in the H.jubatum leaves under drought conditions.The KEGG enrichment analysis revealed that processes such as signaling,lipid metabolism,carbohydrate metabolism and amino acid metabolism played important roles in the response of H.jubatum leaves to the three stresses.All three stresses promoted the starch sucrose metabolism and inhibited the phenylpropane metabolism.A significant enrichment of down-regulated differentially expressed genes in the ribosomal pathway was observed only under alkali stress in TOP20 KEGG entries,suggesting that alkaline stress inhibits a large number of R proteins in ribosomes to participate in the regulation of leaf phenotype and ultrastructure..The results of the above study showed that the leaves of H.jubatum adapted to salt,alkali and drought environments by changing their ultrastructure,and that the leaf cells completely ruptured and a large number of organelles disintegrated under alkali stress.Salt,alkali and drought stresses reduced the photosynthetic performance of H.jubatum leaves by weakening the light energy absorption capacity and impeding photosystem II(PSII)electron transfer.The causal factors for the decrease in photosynthetic rate under the three stresses were different.The high p H caused by alkali stress destroyed the thylakoid membrane on the chloroplast,resulting in increased degradation of D1 protein,which interfered with the photoprotection mechanism dependent on the lutein cycle,and had the strongest inhibitory effect on the photosynthetic rate.In addition,the accumulation of osmotic adjustment substances and small molecule sugars,the increase of APX and SOD activities and the increase of carbon metabolism enzyme activities were all strategies for maintaining osmotic potential and accelerating sugar metabolism to supply the energy required for the whole carbon metabolism process under three stresses,and the alkali stress groups had the most significant changes.A large number of differential genes in transcriptional sequencing under three stresses were enriched in membrane system,starch and sucrose,amino acid metabolism and other items,and the up-regulated expression of a large number of differential genes in light repair items under alkali stress also echoed the whole research results.Based on the results of physiological process and transcriptome analysis,the response of H.jubatum to salt,alkali and drought stress was different.The tolerance of salt stress was higher than that of drought and alkali stresses,and alkali stress had the most serious effects in H.jubatum. |
PDF Full Text Request