INTERVESSEL PIT MEMBRANE THICKNESS AS A KEY DETERMINANT OF EMBOLISM RESISTANCE IN ANGIOSPERM XYLEM

Pit membranes in bordered pits between neighbouring vessels play a major role in the entry of air-water menisci from an embolised vessel into a water-filled vessel (i.e., air-seeding). Here, we investigate intervessel pit membrane thickness (T-PM) and embolism resistance (P-50, i.e., the water potential corresponding to 50% loss of hydraulic conductivity) across a broad range of woody angiosperm species. Data on T-PM and double intervessel wall thickness (T-VW) were compiled based on electron and light microscopy. Fresh material that was directly fixated for transmission electron microscopy (TEM) was investigated for 71 species, while non-fresh samples were frozen, stored in alcohol, or air dried prior to TEM preparation for an additional 60 species. T-PM and P50 were based on novel observations and literature. A strong correlation between T-PM and P50 was found for measurements based on freshly fixated material (r = 0.78, P < 0.01, n = 37), and between T-PM and T-VW (r = 0.79, P < 0.01, n = 59), while a slightly weaker relationship occurred between T-VW and P50 (r = 0.40, P < 0.01, n = 34). However, non-fresh samples showed no correlation between T-PM and P50, and between T-PM and T-VW. Intervessel pit membranes in non-fresh samples were c. 28% thinner and more electron dense than fresh samples. Our findings demonstrate that T-PM measured on freshly fixated material provides one of the strongest wood anatomical correlates of drought-induced embolism resistance in angiosperms. Assuming that cellulose microfibrils show an equal spatial density, T-PM is suggested to affect the length and the shape of intervessel pit membrane pores, but not the actual pore size. Moreover, the shrinking effect observed for T-PM after dehydration and frost is associated with an increase in microfibril density and porosity, which may provide a functional explanation for embolism fatigue.