Intervessel pits are considered to function as valves that avoid embolism spreading and optimize efficient transport of xylem sap across neighbouring vessels. Hydraulic transport between vessels would therefore follow a safety-efficiency trade-off, which is directly related to the total intervessel pit area (A_p), inversely related to the pit membrane thickness (T_PM), and driven by a pressure difference.

To test this hypothesis, we modelled the relative transport rate of gas (k_a) and water (Q) at the intervessel pit level for 23 angiosperm species, and correlated these parameters with the water potential at which 50% of embolism occurs (Ψ₅₀). We also measured k_a for 10 species using pneumatic measurements.

The pressure difference across adjacent vessels, and estimated values of k_a and Q were related to Ψ₅₀, following a convex safety-efficiency trade-off based on modelled and experimental data. Minor changes in T_PM and A_p exponentially affected the pressure difference and flow, respectively. 

Our results provide clear evidence that a xylem safety-efficiency trade-off is not linear, but convex due to flow across intervessel pit membranes, which represent mesoporous media within microporous conduits. Moreover, the convex nature of long-distance xylem transport may contribute to an adjustable fluid balance of plants, depending on environmental conditions. 

The measured and modelled results were processed using R programming (version 4.2.3; R Core Team, 2023) in RStudio environment (version 2023.03.0+386)