Plants maximize the conductive efficiency of the xylem

In vast areas of the world, the growth of forests and vegetation is water-limited and plant survivaldepends on the ability to avoid catastrophic hydraulic failure. Therefore, it is surprising that plantstake high hydraulic risks by operating at water potentials (ψ) that induce partial failure of thewater conduits (xylem) 1, and which makes them susceptible to drought mortality under climatechange 2. Here we present an eco-evolutionary optimality principle for xylem design that explainsthis phenomenon - xylem is adapted to maximize its effective conductivity. A simple relationshipemerges between the xylem intrinsic tolerance to high negative water potential (ψ50) and theenvironmentally dependent minimum ψ, which explains observed patterns across and withinspecies. The theory provides a fundamental conduit-level principle that complements previouslydescribed principles at higher organizational levels, such as hydraulic-network scaling and droughtavoidancebehavior. The new optimality principle may be universally valid, from within-individualsto across-species, and thus improve our basic understanding of drought tolerance of plants andforests globally.