Dear Ligia,

I think that conversion would be correct as long as the Photosynthetic Quotient (PQ) of your studied organism is 1. PQ is a ratio of the rate of O2 evolution by a cell or organism capable of photosynthesis to that of CO2 uptake. I don't know what type of photosynthetic organisms or cell you are working with, but for example, in macroalgae PQ values below 1 are possible (Rosenberg et al 1995).

Kind regards

Article Primary Production and Photosynthetic Quotients of Seaweeds ...

Dear Raquel,

Thank you for the answer. I am looking for the better way to calculate the amount of O2 released from the photosynthesis in  urban trees.

The oxygen released by evolving complex at PS1 has a different path from CO2 absorbed in the stroma by Rubisco. Rubisco in ambient air reacts too with oxygen. If the trees are C3 at first approximation their sunlit area fraction by light saturated CO2 net exchange rate is an indicator of potential oxygen release. with a 50% potential error. There are however other types of informations needed for tree evaluation in towns . The Molina report and VOC literature can add complexity.http://wiki.esipfed.org/images/2/2c/MegaCityPollutionMolina.pdf

Thank you for the answer. Indeed I'm interested in in trees(C3) evaluation of O2 released from the photosynthesis and how can I do it with fewer errors?In urban areas at coniferous and deciduous trees.

A few words of comment about your aim: O2 variation is usually referred to N2 as a Ratio called APO in air.  There is a link on global scale and at  sites far from fuel burning, between CO2 and APO. Giving to these trends a specific signature in a town is far from  being  actuated. A couple of ants respire what was made by aquite big leaf. Trees are active on temperature by their shade and transpiration; it is possible to measure fine particolate deposition by measuring powders at different places. Trees also emit VOC and this can be harmful for Ozone levels. A basic reading for APO and relative problems is in the fundamental thesis of Ralf F Keeling in 1988 at Harvard University (develoment of an interferomeric analyzer...). 

The principles, equipment and procedures for measuring leaf and canopy gas exchange have been described previously as has chlorophyll ¯uorescence.

Simultaneous measurement of the responses of leaf gas exchange and modulated chlorophyll fuorescence to light and CO2 concentration now provide

a means to determine a wide range of key biochemical and biophysical limitations on photosynthesis in vivo. Here the mathematical frameworks and practical procedures for determining these parameters in vivo are consolidated. Leaf CO2 uptake (A) versus intercellular CO2 concentration (Ci) curves may now be routinely obtained from commercial gas exchange systems. The potential pitfalls, and means to avoid these, are examined. Calculation of in vivo maximum rates of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) carboxylation

(Vc,max), electron transport driving regeneration of RuBP (Jmax), and triose-phosphate utilization (VTPU)are explained; these three parameters are now widely

assumed to represent the major limitations to lightsaturated photosynthesis. Precision in determining these in intact leaves is improved by the simultaneous

measurement of electron transport via modulated chlorophyll ¯uorescence. The A/Ci response also provides a simple practical method for quantifying the limitation that stomata impose on CO2 assimilation.Determining the rate of photorespiratory

release of oxygen (Rl) has previously only been possible by isotopic methods, now, by combining gas exchange and ¯uorescence measurements, Rl may

be determined simply and routinely in the ®eld. The physical diffusion of CO2 from the intercellular air space to the site of Rubisco in C3 leaves has long been suspected of being a limitation on photosynthesis,but it has commonly been ignored because of the lack of a practical method for its determination.

Again combining gas exchange and ¯uorescence provides a means to determine mesophyll conductance. This method is described and provides insights into the magnitude and basis of this limitation.

The question problem is at tree scale for O2 emission or rate. A stoichiometry of 1 or less to CO2 is questionable. We can measure CO2 rates (like12 ppm on a base of 300) but O2 is a thin variation on 20.8% (10ppm on a base of 208000). O2 is evolved by water splitting at ps1and reabsorbed by rubisco oxygenase in a chloroplast (with a variable O2/CO2 ratio linked to Ci or CO2 concentration), but in open air of a town a lot of O2 is consumed by combustion and respiration. Further wind boundary layer conduction and advection makes equal O2 in Antarctica and New York. The advantage of trees is that at night some of carbon made is fixed in the wood and this fraction is kept away from respiration, until tree is living. An equivalent CO2 sink can be calculated but the corresponding O2 is a matter of guess.

Dear Guido and Heshmat, thank  you both for the answers and interesting debate about O2 releasing and CO2 absorbing in tree. I'am thinking to take in account the A/Ci ratio for photosynthesis determination and for O2 releasing the fraction of CO2 which are fixed in the wood in the night. It exist some mathematical relation between rate of assimilation and carbon sequestration ?

Dear Thomas, thank you for the answer but I am not familiar with AQ or RQ and I ask you to give me more details about it.

In an 'ideal' system, amount of CO2 consumed can be equated to amount of O2 released. However, under any sort of stress (be it be high light, low temperature, salt, drought, heavy metal etc.) this is not true. Electrons drawn from H2O by photosystem II need not always be used for CO2 fixation. These electrons can be used for reduction of nitrate, nitrite, suphate etc. They can also find their way to O2 as well, leading to the generation of O2.- and H2O2. ATP produced during photophosphorylation can be used to tackle stress. Therefore, your assumption will depend on the conditions under which you are performing the experiments.

Thank you all for the answers. The analysis  will be conducted in the field, in situ,  to be more specific is about different tree species in a park from a town.  It wants an estimation of   CO2 captured and  O2 released.

The IRGA has to be calibrated and O2 evolved can be measured in either in ppm or umole per leaf area

I think that the equivalence of CO2 to O2 is possible but not experimental, the instrumentation for O2 is a huge interferometer, because O2 is not IR absorbing. The global value for exchange is about 1.1 in molar units because O2 reacts also with N2, but in a town there would be mixing with fuel combustion exausts and you will observe the timing of traffic with O2 records. Then the solution is using the wood stock mass and take an equivalence of 1.1 in APO units as a potential O2 source. Please read carefully http://cdiac.ornl.gov/trends/oxygen/modern_records.html

Hi, i want to ask if any of you guys know a computer model to calculate carbon dioxide uptake by a tree. I know that there are some that take in account some meassurments of the tree to predict its gas exchange, but I can't find any. So if anyone knows it will be really helpfull. Thanks.

It is just a metter of abacus math, even if carbon storage may be varying with years and growth curves are not linear but Gompertz like. https://www.broward.org/NaturalResources/ClimateChange/Documents/Calculating%20CO2%20Sequestration%20by%20Trees.pdf

Hi, it certainly depends on Molecular weight of co2 and o2. The amount of carbon fixation or o2 emission is'nt equal.