New insights into bordered pit structure and cavitation resistance in angiosperms and conifers

摘要

The question of what structural features underlie differences in resistance to xylem cavitation is a long-standing issue fundamental to our understanding of water transport in plants. Plants routinely face xylem tensions great enough to cause cavitation and embolism, which may result in significantly increased hydraulic resistance, limitations on leaf gas exchange and ultimately carbon starvation and plant death (Tyree & Zimmermann, 2002; McDowell et al., 2008). The relative resistance of a plant toembolism is a major determinant of species distribution and the ability of plants to survive in the face of environmental stresses such as drought and freezing (Stuart et al., 2007). The xylem consists of a highly compartmentalized network of conduits inwhich emboli can be isolated while water transport continues in adjacent conduits. The continued function of this network depends to a large degree on the nano-porous primary cell walls (pit membranes) that separate conduits from one another. Pit membranes function as safety valves in the xylem, allowing the free passage of water between cells as it moves from the roots to the leaves, but limiting the spread of gas or pathogens. However, the fine porosity of pit membranes also results in significant hydraulic resistance, with pit hydraulic resistance accounting for a large proportion of total xylem hydraulic resistance (Zwieniecki et al., 2001; Choat et al., 2006). The structure and function of pit membranes is therefore of great importance in both the hydraulic efficiency of the xylem and cavitation resistance (Choat et al., 2008). Although there is a great breadth of diversity in bordered pit structure across higher plants, pit membranes can generally be divided into two major forms: homogeneous pit membranes, typical of angiosperm species; and margo–torus pit membranes of tracheid-bearing conifers (Fig. 1). In this issue of New Phytologist, two exciting studies extend our understanding of the relationship between xylem structure and resistance to cavitation: Christman et al. (2009, pp. 664–674) examine the anatomical underpinning of cavitation resistance in angiosperm species, while Hacke & Jansen (2009, pp. 675–686) report a detailed investigation of margo–torus pit structure and its influence on cavitation resistance in three conifer species.