User:Zoe Stanyon/Measure photosynthetic rate

This page focuses on the non-destructive measurement of photosynthetic rate in the field.

During photosynthesis, plants fix carbon dioxide and release oxygen while coping with the loss of water. Measurements of photosynthesis are used to investigate productivity (biomass accumulation) at the leaf, plant or community level as well as photosynthetic responses to environmental stresses.

most photosynthesis meters are designed to measure gas exchange -CO2 and H2O- by leaves. Since COSubscript text2 intake and H2O release share the same pathway -the stomata-, photosynthesis measurements commonly include the estimation of photosynthesis itself (assimilation or CO2 uptake), stomatal conductance, and transpiration.

Instruments for Photosynthesis Measurement
The term closed or open is used in the sense of whether or not the atmosphere of the leaf-enclosing chamber is renewed during the measurement. In a closed system design, a given leaf, plant or canopy is enclosed in a sealed chamber that is not resupplied with fresh air. The CO2 concentration in the chamber is decreased by leaf photosynthetic activity, while the H2O concentration increases. The change in CO2 and H2O concentrations per unit of time are correlated with net photosynthesis (assimilation) and transpiration, respectively.

In an open system design, an air stream that has a known CO2 concentration is constantly passed through the leaf chamber. The objective is to supply a steady state level of CO2. As a result of leaf activity, the air exiting the chamber (the "sample") will have a lower CO2 concentration as well as a higher H2O concentration than the air entering the chamber ("the reference"). A variation of the open system is the compensating system, where the CO2 removed by photosynthesis is compensated by CO2 injection until reaching equilibrium ("null balance"). At that equilibrium point, photosynthesis rate is equal to the CO2 injection.

Previous photosynthesis meters (such as the LICOR models 6000 and 6200) were of the closed-mode type. Newer models use an open-mode system.

Main Components of Gas Exchange Systems
The basic components of a photosynthesis measurement system are the gas exchange chamber, infrared gas analyzer, flow meters, gas lines, CO2 and water vapor filters,power batteries and a console with keyboard, display and memory. Leaf chamber architecture, aerodynamics and properties of building materials affect system performance, since temperature, CO2 concentration, humidity and light has to be controlled within htec chamber.

Modern systems measure the CO2 concentration with a non-dispersive infrared gas analyzer. This device includes an infrared source that is shined through a gas sampling chamber and then focused on a detector. The energy received at the detector is the total entering the system minus the energy absorbed by the CO2 in the sampling chamber. A major problem in IRGA performance is the discrimination between CO2 and water vapor, since both gasses absorb energy at similar wavelengths. To solve this problem, the gas sample is dried to a constant water content by means of a dessiccant before reaching the IRGA.

The incorporation of advanced computation programs allows the immediate access to data in the field and the possibility to detect errors during the measurement.

Formulas to calculate photosynthesis and related parameters
The usefulness of photosynthesis (A) measurements are enhanced by the simultaneous measurement of transpiration (E). Both A and E share the stomata as a port of control. Thus, leaf conductance to water vapor (glw) also determines the internal CO2 concentration (Ci). Ci represents an indicator of the availability of the primary substrate (CO2) for A. Furthermore, Ci is a meaningful parameter for balancing the biochemical and diffusion limitations to photosynthesis. The determination of conductance is thus an important aspect in both comparative and predictive photosynthesis studies. See formulas.