Applications

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 measurement, 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.

Easy and convenient use

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 temperaturecompensated readings, and straightforward calibration. New users can quickly make effective use of the AP4, without a deep knowledge of all the options available to the more experienced user. In use, the AP4 is carried on a shoulder strap in its own protective carrying case, with the keyboard, control buttons and display at a convenient working level. The sensor head, on a flexible cable, is simply clipped on to the leaf to be measured. Once the instrument has been initialised, a READ button on the head allows singlehanded operation. Successive readings can be taken, evaluated and stored just by pressing this one button. Stable readings are audibly indicated by a ‘double beep’. Step-by-step procedures from menus displayed on the LCD screen guide the user through the processes of calibration, taking readings, and the review and output of stored data. A SET button conveniently recalls operating settings stored previously.

Direct calibration

The direct calibration technique used in the AP4 gives much greater confidence in the absolute accuracy of its readings than is possible in practice with other systems. The AP4 is supplied with a moulded polypropylene calibration plate with six groups of holes; the rate of diffusion of water vapour through these holes has been carefully verified. Water vapour is provided by backing the plate with dampened paper. The sensor head is clipped onto the calibration plate, and readings are stored from each of the six standard calibration positions. Calibration is a simple process, and easily undertaken in the field. It should be carried out at the start of a measurement session, and when necessitated by a change in temperature, or by moving to a new RH cycling level, or by changing to the alternative cup on the sensor head.