Plasmodesmatal pores in the torus of bordered pit membranes affect cavitation resistance of conifer xylem

The pit membrane in bordered pits of conifer tracheids is characterized by a porous margo and central thickening (torus), which is traditionally considered to function as an impermeable safety valve against air-seeding. However, electron microscopy based on 33 conifer species, including five families and 19 genera, reveals that pores occur in the torus of 13 of the species studied. The pores have a plasmodesmatal origin with an average diameter of 51 nm and grouped arrangement. Evidence for embolism spreading via pores in tori is supported by the pore sizes, which correspond relatively well with the pressure inducing cavitation. Predictions based on earlier correlations between pit structure and cavitation resistance were only weakly supported for species with punctured tori. Moreover, species with punctured tori are significantly less resistant to cavitation than species with non-punctured tori. Nevertheless, absolute pore diameters must be treated with caution and correlations between theoretical and measured air-seeding pressures are weak. Because most pores appear not to traverse the torus but are limited to one torus pad, only complete pores would trigger air-seeding. Embolism spreading through a leaky torus is not universal across gymnosperms and unlikely to represent the only air-seeding mechanism.