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.

Porometry is the study of gas diffusion through pores, particularly through leaf stomata. Since plant transpiration is mainly controlled by the opening and closing of stomata, the use of porometers is vital to many areas of plant research.

The AP4 Porometer measures stomatal aperture in terms of leaf conductance to water vapour