The Dynamic Pipeline: Homeostatic Mechanisms that Maintain the Integrity of Xylem Water Transport from Roots to Leaves

Over the last decade, it has become increasingly apparent that the properties of the root-to-leaf hydraulic pathway in trees can quickly acclimate over short timescales. In this context, the term hydraulic architecture takes on a broader meaning, making it essential to understand how static and, above all, dynamic hydraulic properties are integrated at the organismal level. Here we discuss some key processes operating in roots, stems and leaves that act in a coordinated manner to stabilize plant hydraulic function under both non-extreme and extreme conditions such as prolonged drought. Hydraulic redistribution (HR) is often manifested as reverse flow of water in shallow roots via transport of water from deeper soil layers. By partially uncoupling root water potential from that of the surrounding dry soil and slowing the rate of soil drying, HR serves to mitigate seasonal drought-induced hydraulic dysfunction in roots and to maintain nutrient uptake, thereby extending their lifespan. Stems of many species show a high capacity for refilling of embolized xylem conduits. This is an important component of their apparent hydraulic safety margins that is likely to be related to xylem structural features that confer resistance to tension-induced embolism. Hydraulic capacitance in stems contributes to hydraulic safety margins by dampening fluctuations in xylem tension that might otherwise result in excessive embolism. Embolism-induced loss of stem xylem conductivity can be partially compensated by rapid increases in the ionic concentration of xylem sap in remaining functional conduits. In many species, leaves lose and recover a substantial fraction of their hydraulic conductance daily, suggestive of a hydraulic circuit breaker function that promotes stomatal closure to avoid excessive embolism in stems upstream. The functioning of these homeostatic processes is likely to be influenced by the drier climates that are predicted for much of the globe.