Terminology:
Absolute, Differential and Continuous Mode
The CI-340 can perform measurements in absolute, differential or continuous mode. In
absolute mode, the absolute CO
2
/H
2
O concentration from a single source, the inlet of the CI-340, is
measured. This is compared against an absolute CO
2
calibration. Differential IRGA mode compares the
measurement of CO
2
and H
2
O before the leaf chamber to the measurement of CO
2
and H
2
O after the leaf
chamber. In differential mode, the CI-340 acquires absolute measurements both from the chamber
environment and the instrument intake. Almost all commercial plant IRGA systems are of the
differential type, because they provide more reliable measurements. The CI-340 is capable of obtaining
absolute (single channel absolute, S) and differential (photosynthesis, P) measurements, as well as
continuous measurements (continuous photosynthesis, C). In continuous mode, the gas concentration
is measured once from the inlet and then continuously measured from the outlet of the leaf chamber.
In order to accurately use continuous mode, the initial measurement from the inlet needs to be very
stable, but this mode allows for very quick, repeated measurements.
Photosynthesis
Photosynthesis is the process by which plants (and photoautotrophs) generate carbohydrates
and O
2
from CO
2
, H
2
0 (water) and sunlight or light energy (Figure 2). This process occurs in the
chloroplasts or chlorophyll-containing tissues of the plant leaves and stem. The rate at which
photosynthesis is occurring is determined by measuring the rate at which a known leaf area assimilates
CO
2
over a given period of time.
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