AP4 Porometer PRODUCT INFORMATION The AP4 Porometer measures 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. • Direct readout of conductance or resistance • Simple and rapid calibration in the field • Minimises leaf stress during measurement • Outstanding value and ease of use 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. Practical experience The theory behind the cycling porometer is well understood (ref. 1), and is backed by Delta-T’s considerable manufacturing experience of this type of instrument through four evolutionary stages. Over 1000 Delta-T porometers have been used worldwide since the first was devised by Dr. J. L. Monteith in 1974 (ref. 2). 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 temperature-compensated 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. A [HELP] button provides operating assistance, especially useful for field work, while a [SET] button conveniently recalls operating settings stored previously. 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-through procedures from menus displayed on the LCD guide the user through the processes of calibration, taking readings, and the review and output of stored data.

Light sensor Sensor head Function keys QWERTY keypad for notes Calibration plate 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...

Porometer Compact sensor head The lightweight, compact AP4 sensor head enables reliable and quick measurements, even in small or dense canopies, with minimum stress on the leaf. Made from low water absorption polypropylene, the head contains two cups (cuvettes): one slotted, the other circular, to suit different leaf shapes. A shuttered cover lets the user check leaf alignment. A silicone rubber seal presses gently onto the leaf surface, defining the sample area. Sensor Head Improvement The sensor head mouldings have been redesigned to incorporate several further mechanical enhancements, e.g •...

Porometer type AP4 Specifications Parameter [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...