Among gas exchange parameters indicating the efficiency of plant photosynthetic machinery; Pn, A and CER abbreviations are commonly used. Net assimilation rate (Pn) and photosynthetic rate (A) both refer to the same parameter indicated by umol carbon dioxide per m2 of the leaf area per second. The question is about CER (carbon dioxide exchange rate); does it refer to the same parameter, and is it logic to be measured with positive and negative values?
FVB for Farquhar von Caemmerer Berry biochemical model in Planta 1980 uses A for actual exchange rate in a gas exchange set up. Below compensation situations can produce negative A. CER is applied to community studies with intergration to day or season in a mixed reality with roots fruits and consumers Pn is an early indicator of some balance with photorespiration and was modelled by A.
Pn and A are the same. CER can also come from individual gas exchange measurements and can be negative if the leaf is respiring more than fixing C. CER can also be derived from eddy flux measurements and will be negative at time (for example, at night).
Thanks for both of you, Guido Bongi and Kurt Johnsen;
I completely agree with you Kurt Johnsen.
In an appendix included within the user manual of a portable photosynthesis measurement system, NCER (Net CO2 Exchange Rate) was referred to be equivalent to - A (Photosynthetic Rate but with a NEGATIVE sign).
While defining calculation protocol of some parameters measured by the device, it is mentioned that;
A = us Δc
and
NCER = us (−Δc)
where
us is the mass flow of air per unit area
Δc difference in CO2 concentration.
So, it's to somewhat confusing.
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