Abstract: [Objective] In this study, we continuously measured the root pressure of intact seedlings of 84 K popular (Populus alba×P. glandulosa) with the pressure transducer technique, and investigated relationships between diurnal rhythm of root pressure and transpiration and xylem sap osmotic osmolytes, in order to reveal the diurnal rhythm of root pressure and its occurrence pattern in intact plants under natural conditions.[Method] A hole was drilled through the bark into the xylem of the stemnear the base, and a compression fitting was inserted into the hole and connected to a pressure transducer. With this method, we measured in situ the diurnal rhythm of root pressure of intact popular seedlings in field. We also measured diurnal variations of the leaf water potential, as well as the osmotic potential, ions and soluble sugarscontent in xylem sap from roots and stems.[Result] The root pressure of intact seedling could be measured in situ by side hole opening method, and the result was reliable.The root pressure of intact plantshad similar diurnal rhythm to that obtained with the decapitated root systems, that is, root pressure was high in daytime and low at night. However, the maximum root pressure of intact plants was relatively lower than that by the decapitation method, which might be due to the influence of transpiration. The reduction of water potential caused by transpiration in the morning did not conceal the root pressure. Until noon,when water potential descended to a threshold,the root pressure declined rapidly. There existed a top down osmotic potential gradient in xylem sap across popular stem,and it was lower in the upper position and higher in the bottom of the stem.The gradient maintained at any point within a day, particularly at noon. Osmotic potential of stem xylem sap was lower than that of roots. Osmotic potential of root xylem sap was lowest at the predawn time. The osmotic substance inxylem sap from roots was mainly composed by mineral ions,but the role of soluble sugar couldnot be ignored too.[Conclusion] Root pressure of intact seedlings possesses a similar daily rhythm with that of decapitated roots, but the maximum value of intact root pressure is lower than that by the traditional destructive method, possibly due to the influence of transpiration.Root pressure can counter-balance the transpiration to a certain extent. Osmotic potential in root xylem sap was lowest at the predawn time,thereby providing the maximum driving force for upward water transport due to the existence of a osmotic gradient between stem and root. The osmotic substance in xylem sap from roots was mainly composed by mineral ions but the soluble sugars components can not be ignored.

Key words: intact seedling, root pressure, Populus alba×P. glandulosa, osmotic potential, ions, soluble sugar