Hydraulic conductivity of the xylem is computed as the quotient of mass flow rate and pressure gradient. Measurements on excised plant stems can be difficult to interpret because of time-dependent reductions in flow rate, and because of variable degrees of embolism. Using Acer saccharum Marsh. stems, we found that certain perfusing solutions including dilute fixatives (e.g. 0.05% formaldehyde) and acids with pH below 3 (e.g. 10 mol m−3 oxalic) prevent long-term decline in conductivity. Xylem embolism can be quantified by expressing the initial conductivity as a percentage of the maximum obtained after flow-impeding air emboli have been removed by repeated high-pressure (175 kPa) flushes. Correlation between microbial contamination and declining conductivity suggests that long-term (> 4h) declines are caused by microbial growth within the vessels. Unpredictable trends in short-term (< 4h) measurements may be caused by movements of air emboli in vessels and/or participate matter.

A method for measuring hydraulic conductivity and embolism in xylem

A root pressure probe was employed to measure hydraulic properties of primary roots of maize ( Zea mays L) The hydraulic conductivity ( Lp r ) of intact root segments was determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder In hydrostatic experiments, Lp r was constant along the segment except for an apical zone of approximately 20 millimeters in length which was hydraulically isolated due to a high axial resistance In osmotic experiments, Lp r decreased toward the base of the roots Lp r (osmotic) was significantly smaller than Lp r (hydrostatic) At various distances from the root tip, the axial hydraulic resistance per unit root length ( R x ) was measured either by perfusing excised root segments or was estimated according to Poiseuille9s law from cross-sections The calculated R x was smaller than the measured R x by a factor of 2 to 5 Axial resistance varied with the distance from the apex due to the differentiation of early metaxylem vessels Except for the apical 20 millimeters, radial water movement was limiting water uptake into the root This is important for the evaluation of Lp r of roots from root pressure relaxations Stationary water uptake into the roots was modeled using measured values of axial and radial hydraulic resistances in order to work out profiles of axial water flow and xylem water potentials

Axial and Radial Hydraulic Resistance to Roots of Maize (Zea mays L.)

Herein we review the current state-of-the-art of plant hydraulics in the context of plant physiology, ecology, and evolution, focusing on current and future research opportunities. We explain the physics of water transport in plants and the limits of this transport system, highlighting the relationships between xylem structure and function. We describe the great variety of techniques existing for evaluating xylem resistance to cavitation. We address several methodological issues and their connection with current debates on conduit refilling and exponentially shaped vulnerability curves. We analyze the trade-offs existing between water transport safety and efficiency. We also stress how little information is available on molecular biology of cavitation and the potential role of aquaporins in conduit refilling. Finally, we draw attention to how plant hydraulic traits can be used for modeling stomatal responses to environmental variables and climate change, including drought mortality.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/jipb.12534

Plant xylem hydraulics: What we understand, current research, and future challenges.

The ascent of sap in plants

When higher plants assimilate carbon dioxide they inevitably lose water. Evolution has provided these plants with facilities both to control and to make good this loss, so as to maintain adequate hydration of their tissues. The development of roots and a vascular system enabled plants to invade the land by ensuring a supply of water to their shoots; the development of a cuticle punctured with stomata enabled them to conserve water when the supply from their roots was outstripped by the evaporative demand.

Water in the Soil-Plant-Atmosphere Continuum

We measured water flow in simple xylem vessels of Sechium edule at various pressures. We found a disagreement with Poiseuille's law, which changes from vessel to vessel and becomes abruptly pronounced in some cases. We discuss our data in terms of an 'impediment coefficient,' K. It can be shown that the total flux in a stem, calculated as the sum of the flux in single vessels, agrees quite well with the data reported in literature.

R. Giordano

Papers

Flow in xylem vessels and Poiseuille's law