The AP4 Porometer measures the stomatal aperture in terms of leaf conductance to water vapour. This is a major determinant of water loss from plant leaves and of CO₂ uptake in photosynthesis.

Loss of water vapour through plant leaf stomata is one of the critical factors linking transpiration with ambient temperature, pressure, humidity, and wind speed. The stomata are sensitive to light, relative humidity (RH), carbon dioxide (CO₂), water stress, pathogens, and pollutants. The AP4 Porometer makes accurate, repeatable, and convenient field measurements of stomatal conductance using the cycling diffusion principle. When combined with leaf area and leaf temperature measurements, the instrument enables water loss from a whole plant or crop canopy to be estimated. The AP4 is therefore an invaluable tool for quantifying the effect of various influences on stomatal behaviour. It also has an important role to play in comparing the performance of different crop varieties in response to environmental variations and stresses.

The innovative design of the AP4 provides users with logical, convenient operation and a comprehensive range of features and functions. In particular, the AP4 offers an automated, rapid cycle that gives temperature-compensated readings, and straightforward calibration.

Dynamic diffusion porometers, such as the AP4, are also known as stomatal diffusion porometers.

Parameter	Reading range	Resolution^[1]	Accuracy^[2]	Test conditions^[5] Reading range
Conductance	5.0-1200 mmol m^-2 s^-1	0.1 – 10	± 10 % ± 20 %	5 – 800 mmol m^-2 s^-1 800 – 1200 mmol m^-2 s^-1
Conductance	0.25 – 30.0 mm s^-1	0.01 – 0.1	± 10 % ± 20 %	0.25 – 20 mm s^-1 20-30 mm s^-1
Resistance	0.2 – 40 s cm^-1	0.01 – 0.1	± 10 % ± 0.2 s cm^-1	0.5 – 40 s cm^-1 0.2 – 0.5 s cm^-1
RH	0 – 100%	0.1	± 4%
Cup temp	-5 – +55 °C	0.1	± 0.7 °C
Cup-leaf temp	-5 – +5 °C	0.1	± 0.2 °C
PAR flux^[3]	0 – 2500 µmol m^-2 s^-1	10	± 15%
Pressure^[4]	600 – 1200 hPa, settable in steps of 5 hPa
RH cycle level	20 – 80 %RH, settable in steps of 5%

[1] Resolution varies with the magnitude of the value obtained. The range shown corresponds to the reading range. In relative terms, the resolution is better than 2%, but at least the smallest amount shown.
[2] The stated accuracy applies over the range of the calibration plate and for optimum cup conditions, i.e. from +10 to -5 °C difference between actual cup temp. and that existing at calibration, and for +2.5 to -2.5 °C difference between leaf and cup temperatures.
[3] Spectral and cosine responses are approximate only.
[4] Ambient pressure may be read from a Wristwatch Barometer type PBR1.
[5] Cycling at extreme combinations of temperature, conductance and RH level may not always be possible.

AP4 Porometer Data Sheet

AP4 Porometer User Manual

AP4 Porometer Technical Manual

AP4 Porometer EC Declaration of Conformity

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