Abstract:

Xylem embolism refers to the interrupt of plant water transport function caused by external air bubbles entering the xylem water-filled conduits. Bordered pit membrane is the crucial location where xylem embolism, spread and fatigue occur. In recent years, the characteristics of bordered pit membrane have received increasing attention in the study of xylem embolism mechanisms. In order to have more comprehensive and clear understanding of the role of bordered pit membrane in the formation, spread and fatigue of xylem embolism, we firstly discussed the formation mechanism and diffusion pattern of xylem embolism in angiosperms and gymnosperms, based on relevant research on xylem embolism at home and abroad. We elucidated the "air seeding hypothesis" in xylem embolism formation and diffusion, and clarified the crucial role of the bordered pit membrane; we also reviewed the main characteristics of the structural, chemical, physical and micromechanical properties of the bordered pit membrane, as well as the interrelationships between the different characteristics. Then we summarized the important role of bordered pit membranes in different stages of xylem embolism, diffusion and fatigue. We also summarized the recent advances and problems of the key characterization method of bordered pit membrane, such as the construction of three-dimensional structure model, in-situ detection of chemical composition, and mechanical behavior of the bordered pit membrane, microflow quantification and analysis of the single pit membrane. Finally, we proposed three research topics which should be given priority in xylem embolism: 1) changes in the chemical composition, physical and mechanical properties of the bordered membrane under different moisture conditions; 2) the formation and diffusion of air bubbles at the water-solid interface between water and the three-dimensional porous structure of the bordered pit membrane; 3) structural changes of the bordered pit membrane under different moisture conditions and their effects on the efficiency of microflow. Meanwhile, it is urgent to break through the technical bottleneck of the accurate in-situ characterization of the three-dimensional structure of the bordered pit membrane, and the construction and optimization of biological models. The further investigation of the structural, chemical, physical, mechanical properties and microflow behavior of the bordered pit membrane to reveal the influences of the characteristics of the bordered pit membrane on xylem embolism would be helpful to understand the mechanisms of xylem embolism, to provide scientific basis for exploring the drought-resistant mechanisms of xylem and breeding excellent drought-tolerant germplasm resources.

Key words: xylem embolism, bordered pit membrane, structure, property, microflow behavior