Leaf Porometer: Theory of Operation
The Leaf Porometer measures the stomatal conductance of leaves using the steady state technique. The steady state technique measures the vapor flux from the leaf surface to the atmosphere. A fixed diffusion path is clamped to the surface of the leaf, and the vapor flux is determined from the vapor pressure gradient in the diffusion path and the known vapor conductance through the fixed path. If the vapor flux and the conductance in the diffusion path are known, then the stomatal conductance can be easily calculated.
Other Types of Porometers:
There are 4 methods for measuring stomatal conductance:
1. Mass Flow (air under pressure forced through leaf)
2. Dynamic (rate of change of vapor pressure in cup attatched to leaf)
In this type, a small chamber containing a fast response humidity sensor is sealed to the leaf surface. Dry air is
pumped through the chamber to achieve a pre-set humidity, then the time required for the chamber humidity to rise to
some other preset value is measured.
3. Null Balance (adjust flow of dry air to maintain cuvette vapor pressure constant)
In this sytem a chamber is attached to a leaf (or a leaf is enclosed in a chamber) and the flow of dry air through the
chamber is controlled such that the humidity in the chamber remains constant. The change in air flow is measured to
calculate conductance.
4. Steady State (measure the vapor flux and gradient near a leaf)
There are a variety of ways to define "steady state". The Decagon porometer measures diffusion through a pathway.
Other porometers that are "steady state" monitor the time versus the reading until nothing is changing or is "Steady".
This method is more similar to the null balance porometer.
Further Reading:
1. Parkinson, K.J. 1985. Porometry. In B. Marshall and F.I. Woodward (eds.) Instrumentation for Environmental
Physiology. Cambridge University Press, Cambridge, UK. pp. 171-186
2. Pearcy, R.W., E.-D. Schultz, and R. Zimmerman. 1991. Measurement of Transpiration and Leaf Conductance.
In R.W.
Pearcy, J. Ehleringer, H.A. Mooney, and P.W. Rundel (eds.) Plant Physiological Ecology. Chapman
and Hall, New York. pp. 137-160