The PHOTOSYNTHESIS SYSTEM is a completely self-contained unit for measuring the CO2 assimilation (Photosynthesis and Respiration) and transpiration (water loss by evaporation of leaves of plants. These systems are designed specifically for use by students in Schools and Universities. It offers many of the facilities of instruments designed for research , but greatly simplifies the measurement procedure.
It operates on the Open System principle. The leaf is placed in a sealed enclosure with a window for illumination. This is referred to as the leaf cuvette. Through the cuvette is passed a measured flow of air. The CO2 / H2O concentrations of the air entering (reference air) and of the air leaving (analysis air) are measured. To measure the concentrations the PHOTOSYNTHESIS SYSTEM uses a single CO2 and H2O sensor and alternately switches the reference and analysis air. From the flow rate of air and the change in the concentration the assimilation and transpiration rates are calculated.
Though it is designed to supply ambient air to the cuvette, for the study of CO2 responses, it is possible to decrease the CO2 concentration in a series of steps. A similar provision is made for water vapour responses. It is supplied with a leaf cuvette which can be used for a wide variety of leaves. A light unit will be available shortly (LED) for use with the cuvette for manual control of cuvette light intensity.
CO2 /H2O ANALYSIS MEASUREMENT PRINCIPLE
Carbon Dioxide absorbs Infra-red radiation strongly at a wavelength of 4.26 microns. PHOTOSYNTHESIS SYSTEM uses this absorption to measure the CO2 concentration. The analyzer consists of a source of infra-red radiation (a small tungsten filament lamp) at one end of a highly polished, gold plated tube through which the air passes. At the other end of the tube is the infra-red detector which has a window through which only infra-red radiation at 4.26 microns can pass so that the responds only to the presence of CO2. The theoretical analysis range is from 0-100% CO2. However, because of the absorption characteristics of gases, the absorption path lengths, infrared source intensities, detector sensitivities and the S/N (Signal to Noise) ratio of the system define the effective range. The absorption path length of PHOTOSYNTHESIS SYSTEM is optimised for 2,000 volumes of CO2 per million volumes of air. This is correctly referred to as 2000 parts per million by volume or 2000 ppm. (Ambient air contains about 360 ppm.).
Temperature corrections are not required as the opto-electronics are thermostatted and the air is equilibrated to this temperature before entering the absorption cells. The built in transducer compensates for absolute pressure changes in the cell.
In part, the excellent stability of PHOTOSYNTHESIS SYSTEM is due to regular zeroing when CO2 free, air is passed through (referred to as ZERO). ZERO minimises the effects on span (gas sensitivity), of sample cell contamination, source ageing, and changes in detector sensitivity, amplifier gains, and reference voltages. It is done every approximately minute. The ZERO reading is used to compensate for changes in the signal level. From the relationship between absorptance and concentration, determined in the factory, and the current calibration factor, the sample concentration is determined.
Water vapor is measured using a high precision capacitive sensor. This consists of a small piece of glass coated first with a layer of metal, then with a polymer, followed by a second metal layer. Wires are soldered to the metal layers and the sensor is placed in a circuit that measures its electrical capacitance. The amount of water in the polymer depends on the water vapour content of the air and the electrical capacitance of the polymer depends on the water content. So with calibration, the water in the air can be measured. Water vapour concentration is again expressed as a volume/volume relationship but in parts per thousand, which is called millibars (mb)
Both CO2 and H2O measurements give the absolute concentrations for the reference air, and then the difference between the reference and the analysis concentration.
The complete PHOTOSYNTHESIS SYSTEM gas circuit with control valves is shown below.