Abstract
Recent studies have suggested that species-specific pit properties such as pit membrane thickness, pit membrane porosity, torus-to-aperture diameter ratio and pit chamber depth influence xylem vulnerability to cavitation. Despite the indisputable importance of using mean pit characteristics, considerable variability in pit structure within a single species or even within a single pit field should be acknowledged. According to the rare pit hypothesis, a single pit that is more air-permeable than many neighboring pits is sufficient to allow air-seeding. Therefore, any irregularities or morphological abnormalities in pit structure allowing air-seeding should be associated with increased vulnerability to cavitation. Considering the currently proposed models of air-seeding, pit features such as rare, large pores in the pit membrane, torus extensions, and plasmodesmatal pores in a torus can represent potential glitches. These aberrations in pit structure could either result from inherent developmental flaws, or from damage caused to the pit membrane by chemical and physical agents. This suggests the existence of interesting feedbacks between abiotic and biotic stresses in xylem physiology.
Highlights
Adjacent xylem conduits in both gymnosperms and angiosperms are joined by common endwalls
Depending on the thickness of the pit membrane and the amount of amorphous material left after the hydrolysis, some pit membranes show pores embedded within the fibrillar meshwork, whereas other membranes appear non-porous when observed with scanning electron www.frontiersin.org
Neglecting inherent variability in pit structure may lead to a biased view
Summary
Adjacent xylem conduits in both gymnosperms and angiosperms are joined by common endwalls. Some pits exhibit remarkable structural aberrations, including large pores independent of mean porosity in angiosperms (Figures 1A,C) and punctured or irregular tori in gymnosperms (Figures 1E–G). These pits, which represent the tails of, or even outliers in, the overall pit distribution, matter the most for the spread of embolism (Christman et al, 2009).the aim of this paper is (1) to highlight the existence of irregularities in pit structure that can have substantial influence on their permeability to air, and (2) to review possible mechanisms that can give rise to such irregularities. Depending on the thickness of the pit membrane and the amount of amorphous material left after the hydrolysis, some pit membranes show pores embedded within the fibrillar meshwork, whereas other membranes appear non-porous when observed with scanning electron www.frontiersin.org
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