Trade-Off between Hydraulic Safety and Efficiency in Plant Xylem and Its Influencing Factors

Abstract

Forests are vital ecosystems that are increasingly threatened by environmental stress; exploring the possible trade-off between hydraulic safety and efficiency in plant xylem is crucial to understanding their environmental adaptation strategies. However, to date, there is no consensus whether such trade-offs exist among and within species. To better comprehend the mechanism of xylem water transport, in this review, we summarized previously published work on xylem hydraulic safety and efficiency trade-off from the inter-species, intra-species, and intra-tree perspectives and its influencing factors. We gathered data on xylem hydraulic safety and efficiency and their related anatomical traits, i.e., conduit diameter and inter-conduit pit membrane thickness, from a total of 653 plant species analyzed in 80 published papers. At the inter-species level, we confirmed that there is a weak hydraulic safety and efficiency trade-off. For gymnosperms and herbaceous species, the observed trade-off is stronger. At the intra-species level, the hydraulic safety and efficiency trade-off was found in individuals of the same species investigated in the literature. At the intra-tree level, there is a trade-off between hydraulic safety and efficiency for leaves, stems, and roots, and we confirmed the vessel widening hypothesis, i.e., vessel diameter in the outer wood increases from the top to the bottom of the tree. Additionally, pit membrane thickness increases as the tree height decreases, thus increasing the xylem hydraulic efficiency and affecting the trade-off. Finally, we discussed the environmental factors affecting the trade-off between hydraulic safety and efficiency in the xylem, such as plant habitats, temperature, rainfall, altitude, and soil. Further investigations of the bordered pit membrane from the three-dimensional perspective would be useful to understand the hydraulic safety and efficiency trade-off at the nanoscale.