Photosynthesis system
Photosynthesis systems are electronic scientific instruments designed for non-destructive measurement of photosynthetic rates in the field. Photosynthesis systems are commonly used in agronomic and environmental research, as well as studies of the global carbon cycle.
Photosynthesis systems function by measuring gas exchange of leaves. Atmospheric carbon dioxide is taken up by leaves in the process of photosynthesis, where CO2 is used to generate sugars in a molecular pathway known as the Calvin cycle. This draw-down of CO2 induces more atmospheric CO2 to diffuse through stomata into the air spaces of the leaf. While stoma are open, water vapor can easily diffuse out of plant tissues, a process known as transpiration. It is this exchange of CO2 and water vapor that is measured as a proxy of photosynthetic rate.
The basic components of a photosynthetic system are the leaf chamber, infrared gas analyzer (IRGA), batteries and a console with keyboard, display and memory. Modern 'open system' photosynthesis systems also incorporate miniature disposable compressed gas cylinder and gas supply pipes. This is because external air has natural fluctuations in CO2 and water vapor content, which can introduce measurement noise. Modern 'open system' photosynthesis systems remove the CO2 and water vapour by passage over soda lime and Drierite, then add CO2 at a controlled rate to give a stable CO2 concentration. Some systems are also equipped with temperature control and a removable light unit, so the effect of these environmental variables can also be measured.
The leaf to be analysed is placed in the leaf chamber. The CO2 concentrations is measured by the infrared gas analyzer. The IRGA shines infrared light through a gas sample onto a detector. CO2 in the sample absorbs energy, so the reduction in the level of energy that reaches the detector indicates the CO2 concentration. Modern IRGAs take account of the fact that H2O absorbs energy at similar wavelengths as CO2. Modern IRGAs may either dry the gas sample to a constant water content or incorporate both a CO2 and a water vapour IRGA to assess the difference in CO2 and water vapour concentrations in air between the chamber entrance and outlet.
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