Heartwood Formation And Physiological Metabolic Changes Of Xylem Parenchyma Calls In Eucalyptus Cloeziana

Eucalyptus cloeziana grows rapidly,and has a large proportion of is heartwood.Because heartwood has beautiful color,corrosion resistance and good stability,it can be used to make solid wood furniture,flooring and so on.The formation of Eucalyptus cloeziana heartwood and the changes of physiological metabolism of xylem parenchyma cells were studied.In this study,the stem analysis method was used to study the variation of the proportion between sapwood and heartwood.The structure and chemical composition of sapwood and heartwood parenchyma cells beside Eucalyptus cloeziana were detected by electron microscope and ultra-high performance liquid chromatography-mass spectrometry.The differential metabolites and metabolic pathways of sapwood and heartwood were analyzed by metabolomics.The results are as follows:(1)Tree age and tree height had significant effects on sapwood and heartwood of Eucalyptus cloeziana.In radial direction,tree age significantly affected Eucalyptus cloeziana heartwood radius,heartwood area,and heartwood rate and sapwood width.The amount of heartwood increases with the increase of tree age.And the sapwood width decreased with increasing age between 5 and29 years,and tended to be stable when it grew to a certain age.On the contrary,sapwood area did not differ significantly among trees of different ages.In the axial direction,the Eucalyptus cloeziana heartwood amount and sapwood area decreased gradually with increasing of tree height.Among them,the growth of heartwood radius is consistent with the height of trunk,while the width of sapwood varies little with tree height,fluctuates greatly at the base,and tends to be relatively stable after 5.3m.As a tree species with faster heartwood formation,Eucalyptus cloeziana begins to form heartwood when the cross-sectional peeling diameter is about 4.66 cm,and then the diameter increases by about 0.52 cm per1 cm heartwood radius.(2)The amount of starch grains,the morphology of nucleus and the physiological structure of other organelles changed significantly during the formation heartwood.The maximum ratio of nuclear length to width was 2.49 in sapwood and 1.73 in transition zone.The nucleus morphology changed from oval to round and disappeared after entering heartwood.The cells in sapwood are rich in starch grains,lipid droplets,large central vacuoles and other organelles.Starch grains and many organelles have disappeared in the transition zone.By the time of heartwood,many empty cavities appeared in parenchyma cells and protoplasts disappeared wall in this sapwood.The striatum can be clearly observed on parenchyma cells,while part of the amorphous substance is attached in the transition zone,while a large amount of amorphous material has been deposited on the pit membranein the heartwood.(3)There were differences in sapwood,transition wood,heartwood extract and lignin content of Eucalyptus cloeziana.The holocellulose and α-cellulose contents of the sapwood were higher than those of the heartwood and transition wood.The lignin content was analyzed by confocal Raman microscopy,and it was found that the change pattern of lignin content was: heartwood > transition wood > sapwood,heartwood extractives content is higher than sapwood,and the content of polyphenol in sapwood,transition wood and heartwood was 8.31mg/g,21.96 mg/g and 30.69 mg/g,respectively.21 phenolic compounds were identified by UPLC-MS/MS,including ellagic acid,Ellagic acid-pentoside,ellagic acid-rhamnoside and Gallic acid.The main phenolic component was ellagic acid,with contents of 0.57 mg/g,2.17 mg/g and 3.26 mg/g in sapwood,transition wood and heartwood,respectively.The content of ellagic acid in heartwood was 485.79% and 49.82% higher than that of sapwood and transition wood,respectively.(4)The metabolites in the xylem of Eucalyptus cloeziana were analyzed and compared by UPLC-TOF-MS analysis combined with PCA and OPLS-DA statistical analysis.A total of 899 chromatographic peaks were obtained in positive ion mode and 291 were obtained in negative ion mode.A total of 88 differential metabolites were identified,of which 19 metabolites of sapwood and69 metabolites of heartwood were higher than those of heartwood and sapwood respectively.Sapwood is mainly composed of primary metabolites,including carbohydrates,amino acids and other metabolites.The main metabolites of heartwood are flavonoids and fatty acids,in which the metabolite with the greatest difference in the increase of heartwood is Medicarpin,which may be the key component in wood color development.In this study,we analyzed the structural and metabolic changes of xylem parenchyma cells during the heartwood formation of Eucalyptus cloeziana,and provided scientific guidance for the regulation of heartwood formation of Eucalyptus cloeziana through multiple channels.