The Cytological Mechanism Of Internode Elongation During The Rapid Elongation Growth In Phyllostachys Edulis ’Pachyloen’ Culms

Elongation growth is the key process for the growth and development of bamboo,which is closely related to biomass.The bamboo can reach their final height quickly due to the elongation of individual internodes.Phyllostachys edulis ‘Pachyloen’ is the only one excellent culm and shoot producting variety with thicker culm wall and higher biomass of Phyllostachys edulis,and its some internodes are shorted.The mechanism of internode elongation during rapid high growth was studied by histological section,cytochemical localization and enzyme-linked immunosorbent assays（ELISA）.The main results are as follows:According to the anatomical characteristics of bamboo culms,the development of internode elongation can be divided into four stages: meristematic stage,initial elongation stage,rapid elongation stage and maturity stage.At the meristematic stage,cells contained central and big nucleus,dense cytoplasm and rich organelles.Cells were in vigorous division at this stage.At the initial elongation stage,the cells in the upper part of internode began to elongate earlier than that in other parts of the same internode.The cells of internode still undergo vigorous cell division.And the internode elongation is mainly due to the cell division at the initial elongation stage.During the rapid elongation,most of the cells were elongated and the primary cell wall became thicker,while cells elongation in the basic part of internode was not significant,but its nucleus began to be marginalized.So internode elongation is mainly due to the cells elongation at rapid elongation stage.At the maturity stage,the cell elongation finished and secondary cell walls thickened obviously.During the rapid high growth,the node plays an important role in the transverse transport of internode,which provides rich material and energy for the growth of internode.In the process of internode elongation,typical intercalary meristem cannot be founded,and internode elongation was mainly caused by the increase of cell numbers and cell elongation.The results of cytochemical studies showed that Ca²⁺ distributed in vacuole in the process of internode elongation.And Ca²⁺ increased in the cytoplasm at the rapid elongation stage,which transferred to the edge of plasma membrane and cell walls.Ca²⁺ content in vacuole of cells in the basic part of internode decreased with internode elongation.With internode elongation finished,Ca²⁺ distributed on degradation and around starch grains,which was different in cells of upper part of internode.There is a close relationship between cell elongation and cell wall expansion.Ca²⁺ plays an important role in signal transduction.During the elongation of internode,the plasma membrane,mitochondrial membrane and transport vesicles always have high ATPase activity,which indicates that ATPase is involved in the secretion and transport of cell wall materials,so ATPase can promote cells elongation.The phloem ganglion cells in the node remained strong physiological state.Ca²⁺ is mainly distributed in the vacuole of phloem ganglion cells,and ATPase activities always can be found in the plasma membrane,nuclear membrane,plasmodesmata and transport vesicle membrane.In the node without diaphragm,the number of phloem ganglions increased,while the activity of ATPase was stronger showing that phloem ganglions in the node are related to transverse transport during the internode elongation process.The analysis of plant endogenous hormones showed that the content of gibberellin（GA）and ribosylzeatin（ZR）were the highest during the meristematic stage,and decreased with the elongation of internode.The content of auxin（IAA）and brassinolide（BR）were higher at the initial elongation stage and rapid elongation stage.There was no significant change of abscisic acid（ABA）during the whole internode elongation.The content of ZR decreased gradually with the elongation of internode,indicating that ZR was related with cell division.At the rapid elongation stage,the content of IAA was higher in the upper and middle part of internode,while the content of GA was lower.The change of IAA content was related with cell elongation.The content of GA was the highest at the meristematic stage,which indicated that GA was closely related to cell division.The content of BR was higher in the upper and middle part of internode during the initial elongation stage,and increased in the basic part of internode with the elongation of internode,which meaned that BR was related to cell elongation.The content of IAA and BR were significantly changed in the node,which played an important role in the internode elongation.According to the study of the above four aspects,we draw the following conclusions: Cells in the basic part of internode do not have typical characteristics of intercalary meristem cells.During the rapid growth,internode elongation is mainly caused by the increase of cell numbers and cell elongation.At the initial elongation stage,internode elongation is mainly caused by the increase of internode cell numbers.During the rapid elongation,internode elongation is mainly caused by cell elongation.The node plays an important role in the transverse transport of internode during internode elongation.