Angiosperms follow a convex trade-off to optimize hydraulic safety and efficiency

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.