User:Wcornwell/Sandbox

Summary

Definition

Hydraulic conductance is a measure of the efficiency of bulk flow through a material, and defined as the flow rate per unit pressure driving force.

Terminology and equations

Measurement of xylem hydraulic properties falls into two main categories. The first involves measuring flow rate for a given driving force, with the resulting conductance (F/D_P) being normalized by the length of the segment and referenced either to the cross sectional area of the xylem (K_s: xylem-specific conductivity) or to the leaf area supported by the xylem (K_L: leaf-specific conductivity): K_s = F L / (D_P A_sw) K_L = F L/(D_P A_L) where F is the recorded flow rate, L is the length of the measured segment, D_P is the pressure drop across the segment, A_sw is the cross sectional area of the conducting sapwood, and A_L is the total leaf area supplied by the measured segment. These two conductivities are inter-related by the ratio of xylem cross-sectional area to supplied leaf area, a parameter known as the ‘Huber value’, H_v: K_L= H_vK_s.

Measurement approaches

For a stem segment, this can be measured by multiple methods, in the field or lab, for stem segments in vivo, or for excised stem segments

In vivo methods

1. Sapflow and water potential driving force

2. Transpiration and water potential driving force

When using the sapflow method for hydraulic conductance the K_s can be determined as one of several other water use and water relations traits (e.g., whole plant hydraulic conductance, whole plant water use, etc).

The transpiration method is relatively simpler, as depends on leaf-level gas exchange measurements rather than more logistically intense sapflow work.

Excised segment methods

1. Pipette method

2. Balance

3. Flowmeter

The balance method allows for continuous logging of flow rate and is best suited for a controlled laboratory environment. The pipette and flow meter methods are more portable and suitable for field based measurements but do not allow for continuous monitoring of flow rate. The driving force for flow is typically provided by a reservoir raised by some height above the balance but can also be generate by a pressurized water source (tank) or vacuum pump.

Ranges of values

K_s: xx-xxx

K_L: xx-xxx

k_s: xx-xxx

k_L: xxx-xxx

Safety considerations

Care in handling pressurized gas and acidic flow solutions.

Hazardous materials and disposal

Pressurized gas (MSDS sheet)

Protocols

Pipette method

Gold leaf PROTOCOLS

other protocols

Balance method

Gold leaf PROTOCOLS

other protocols

Flow meter method

Gold leaf PROTOCOLS

other protocols

Related techniques

[Leaf hydraulic conductance]

Hydraulic vulnerability

Leaf water potential