| Haloxylon ammodendron,belongs to Amaranthaceae,Haloxylon Bunge,is the constructive species of vegetation in the Guerbantunggut desert.The plant is highly adaptable to extremely high/low temperature,drought and salinity in arid desert ecosystem.The structure of wood(xylem)can be adapted to different ecological environments during the long-term growth and development of trees.Moreover,Nonstructural carbohydrates(NSC)stored in xylem parenchyma tissues(ray parenchyma and axial parenchyma)may be the potential limiting factors for the growth,development as well as survival of trees.Consequently,studies on xylem anatomy,NSC storage and radial transport function of xylem ray will be helpful to understand the population dynamics of H.ammodendron and its evolutionary adaptation to the desert environment.This paper takes the H.ammodendron xylem as the research materials,sample plots are set up in five regions,including Jinghe,Wusu,Shihezi,Caijiahu and Qitai,on the southern Guerbantunggut desert,three edaphic types(gravel soil,sandy soil and loam)are selected,the ecological anatomy and ray function of the H.ammodendron xylem are studied by field investigation and pot experiment.The results are as follows.The transition between dark brown layer and light brown layer is distinct in the growth rings of H.ammodendron,which belongs to abrupt change.The xylem of H.ammodendron has ray tissues,uniseriate or multiseriate.The wall of ray cells is obvious thick with the thickness of 2.85μm～3.08μm.Both wide vessels and narrow vessels exist in the plant xylem,2～18 vessels are aggregated to form pore clusters,which can ensure the safety and efficiency of water transport in H.ammodendron.Intervessel pits are alternate or near opposite,there is spiral thickening in inner wall with wall thickness of 3.15μm～5.53μm,the pits between spiral thickening are uniseriate.The fibers are thick-walled,ranging from 2.64μm～2.97μm.The anatomy of H.ammodendron xylem shows superstrong xerophytic characteristics,and the sandy soil type has stronger drought resistance than the loam type.For most anatomical characteristics,more variation is explained by region than by tree(within-region variation).With the decrease of precipitation,the ray density,ray height,diameter of vessel and wall thickness of vessel in H.ammodendron raise significantly(P<0.05),but the wall thickness of ray cell reduce(P<0.05).Xylem responds to low precipitation by increasing ray density and ray height to improve the storage of water and starch to combat drought stress in Jinghe.In an arid region(Jinghe),H.ammodendron has wide vessels to improve conductivity.Moreover,they increase the vessel wall thickness to decrease its sensitivity to embolism caused by drought stress.Both average temperature in January and maximum wind velocity from April to June are significantly related to the wall thickness of the fiber(P<0.05),indicating that a thicker fiber cell wall could provide higher mechanical resistance to steady stems and improve vessel conduction in low temperature and strong wind conditions.These results indicate that there is high adaptation in the xylem anatomical structures of H.ammodendron in response to different desert climates and also explain the adaptation of the plant to a wide range of Asian and African deserts.The simulated pot experiment shows that rays appear when the H.ammodendron seedlings at the 1-month-old age,and the stem of H.ammodendron is semi-lignified.During the seedling stage,the ray height and width,wall thickness of ray cell and area of ray cell in H.ammodendron enhance rapidly from April,May to June,and stabilize in July and August,which may be caused by seasonal changes in environmental factors.From pith to outside in xylem of H.ammodendron adults,the ray width and area of ray cell enhance significantly with the increases of tree ring ages(P<0.05).The anatomical characteristics of rays in H.ammodendron show significantly seasonal and annual dynamic changes.The radial transport rates of NSC,soluble sugar and starch in H.ammodendron xylem increase significantly with the tree age(P<0.05).In different seasons,the radial transport rate of NSC is higher in May and October than in July(P<0.05).The radial transport rate of soluble sugar in H.ammodendron is the highest in May,and then in July,and there is a significant difference exists among seasons(P<0.05).The starch radial transportation rate decreases from May to October,and a significant difference is found among the seasons(P<0.05).There is a significant positive correlation between vessel length and the radial transport rate of soluble sugar(P<0.05),vessel diameter and the radial transport rate of soluble sugar(P<0.01),ray height and the radial transport rate of soluble sugar(P<0.01),vessel length as well as diameter and the radial transport rate of starch(P<0.01),vessel diameter and the radial transport rate of NSC(P<0.01)as well as ray height and the radial transport rate of NSC(P<0.01).The radial transport function of H.ammodendron xylem is related to the anatomical structure of xylem ray and vessel,and shows obvious seasonal dynamics.The highest values of ray density(4.16 n/mm～2),ray proportion(7.0%),NSC(5.87%),soluble sugar(1.87%)and starch(4.0%)are found in gravel soil,the values in loam are just the opposite.The ray proportion is positively correlated with NSC(P<0.01)and starch(P<0.05),and a negative correlation is found between the wall thickness of ray cell and NSC(P<0.01).These results indicate that H.ammodendron cope with the severe conditions,through decreasing the wall thickness of ray cell,increasing ray proportion and NSC.The wall thickness of ray cell in Jinghe(2.85μm)is significantly lower than that in Shihezi(3.08μm)(P<0.01),but the wall thickness of vessel in Jinghe(5.53μm)is significantly higher than that in Shihezi(5.15μm).In Shihezi region,totally 795differentially expressed proteins are upregulated and 421 differentially expressed proteins are downregulated.The phenylpropanoid biosynthesis,photosynthesis,glycolysis/gluconeogenesis,carbon metabolism,starch and sucrose metabolism,metabolic pathways,plant hormone signal transduction,cysteine and methionine metabolism,amino sugar as well as nucleotide sugar metabolism etc.promote cell wall biosynthesis of H.ammodendron xylem through providing substrates or energy.During the process of cell wall biosynthesis in the H.ammodendron xylem,the proteins(including PREDICTED:non-specific lipid-transfer protein-like protein At5g64080-like[Vitis vinifera],PREDICTED:probable non-specific lipid-transfer protein AKCS9-like[Citrus sinensis]and beta expansin EXPB2.1[Mirabilis jalapa])first make the cell wall loosening,extension as well as expansion,and the xyloglucan endotransglycosylase/hydrolase 1 cut off and link the xyloglucan chains.Then the photosystem I P700 apoprotein A1,reversibly glycosylated polypeptide 1 and GDP-mannose-3′,5′-epimerase etc.involve in the cellulose,hemicellulose and pectin biosynthesis of the cell wall through providing the components or energy.Finally,the proteins in the phenylpropanoid biosynthesis promote the lignification of cell walls and complete the biosynthetic process of cell walls.In conclusion,our results indicate that significant adaptation of both the anatomical features and ray storage as well as radial transport function in H.ammodendron xylem to variations of desert environment.Environment variations can also result in the changes of differentially expressed proteins associated with the cell walls of the H.ammodendron xylem.The regulatory genes related to thick-walled tissues(especially ray cell)in H.ammodendron can be further explored and their functions be identified,and the genes can be applied to the improvement of timber forest,which is of great practical value in improving the wood properties of timber forest. |
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