Unveiling xylem structure with MicroCT and investigating hydraulic conductivity’s impact on leaf morphology and photosynthesis in Populus euphratica Oliv.

Populus euphratica Oliv. is a poplar with heteromorphic leaves. Leaves in the upper part of the crown are mostly dentate oval, whereas those in the lower part of the crown or on saplings are mostly lanceolate. To further explore the reasons of P. euphratica at different heights exhibit different leaf shapes, here, the morphology and the anatomy of the upper and the lower branches and leaves were observed based on optical microscopy in the first. In addition, we have established a detection method for branch xylem based on micro-computed tomography (MicroCT) and we found that mean vessel diameter, mean vessel wall thickness, hydraulic diameter, and mean vessel density showed significant differences between upper and lower branches based on this method. Furthermore, hydraulic conductivity of upper branches was higher than that of lower branches so that water can be transported to the top of the canopy, and the photosynthesis rate of the upper leaves was higher than that of the lower leaves. Finally, we investigated there was a significant positive correlation between water use efficiency (WUE) of the upper leaves, the xylem vessel diameter, and the hydraulic diameter, whereas there was a significant negative correlation between the WUE and the vessel wall thickness. These results indicated that the water status of the branches and the leaves of P. euphratica may be one of the physiological mechanisms causing morphological differences in leaves, and this research provided a new method and insights into the further study of heterophylly in plants.