Abstract
Pit membranes in between neighboring conduits of xylem play a crucial role in plant water transport. In this review, the morphological characteristics, chemical composition and mechanical properties of bordered pit membranes were summarized and linked with their functional roles in xylem hydraulics. The trade-off between xylem hydraulic efficiency and safety was closely related with morphology and properties of pit membranes, and xylem embolism resistance was also determined by the pit membrane morphology and properties. Besides, to further investigate the effects of bordered pit membranes morphology and properties on plant xylem hydraulic functions, here we modelled three-dimensional structure of bordered pit membranes by applying a deposition technique. Based on reconstructed 3D pit membrane structures, a virtual fibril network was generated to model the microflow pattern across inter-vessel pit membranes. Moreover, the mechanical behavior of intervessel pit membranes was estimated from a single microfibril’s mechanical property. Pit membranes morphology varied among different angiosperm and gymnosperm species. Our modelling work suggested that larger pores of pit membranes do not necessarily contribute to major flow rate across pit membranes; instead, the obstructed degree of flow pathway across the pit membranes plays a more important role. Our work provides useful information for studying the mechanism of microfluid flow transport across pit membranes and also sheds light on investigating the response of pit membranes both at normal and stressed conditions, thus improving our understanding on functional roles of pit membranes in xylem hydraulic function. Further work could be done to study the morphological and mechanical response of bordered pit membranes under different dehydrated conditions, as well as the related microflow behavior, based on our constructed model.
Highlights
Xylem sap is transported under negative pressure mainly driven by surface tension generated from transpiration at cell wall surfaces of leaf stomata [1]
The air-seeding hypothesis has been generally accepted as the principle mechanism for embolism formation [10,11], i.e., a tiny air bubble could pass through inter-conduit pit membranes from an embolized conduit into a neighboring functional conduit under certain pressure gradient [12]
Increased pit membrane porosity in principle should make xylem more susceptible to embolism [68]; it did not get experimental support [69].Even though, pit aperture diameter must be taken into account when considering the whole-pit hydraulic resistance, if the pore diameter is much larger than the pit aperture, as much of the resistance would come from the pit aperture [70]
Summary
Xylem sap is transported under negative pressure (tension) mainly driven by surface tension generated from transpiration at cell wall surfaces of leaf stomata [1]. Our knowledge on the pit membrane properties, such as its morphological characteristics especially in three-dimensional perspective, chemical compositions as well as mechanical properties is incomplete, which partly impedes our understanding about its functional role in plant hydraulics, as well as its response under certain environmental stress conditions. A summary of pit membrane morphology and properties, their roles in plant hydraulics as well as the response of pit membranes under environmental challenges was demonstrated, which could potentially provide more detailed information for studying xylem hydraulics from individual perspective in microscopic and ultrastructural scale. Our work might provide useful further information for studying water transport mechanism from pit membrane level, as well as the response of pit membranes both in normal condition and under certain stress levels
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