Pectin removal in <i>Acer rubrum</i> increases pit membrane compliance and embolism propagation

Xylem pit membranes are discrete regions of primary cell wall that are permeable to water but prevent the spread of air embolism. Here we investigate how pectin, a cell wall hydrogel with a kPa-scale modulus, affects the functioning of intervessel pit membranes in Acer rubrum (red maple). We show that enzymatically digesting pectin significantly lowers the resistance against embolism spread, confirming earlier reports in other species and providing evidence that pectin is present in at least some portions of A. rubrum's intervessel pit membranes. Removing calcium with a chelating agent had a smaller effect that became non-significant after accounting for radial flows. Vulnerability curves of control and calcium removal stems measured with and without the addition of a surfactant (0.1% w/v Triton-X) exhibited large differences in P50 as expected for the change in surface tension and implying an invariant effective Laplace radius. In contrast, pectin removal stems did not exhibit a significant change in P50 when vulnerability curves were measured with and without surfactant. Instead, the difference in surface tension implied large changes in the effective Laplace radius with increasing xylem tension, suggesting that pectin removal increases the local compliance of pit membranes. We hypothesize that pectin stabilizes the spacing of cellulose microfibrils, thereby contributing to the transport of water under tension.