I do not see big limitation, most use the method by Harley et al (1992). it estimates using the model gm = An/(Ci -(Γ*.(Jflu + 8.(An + Rd)))/(Jflu - 4.(An + Rd))), since you get An and Ci from gas exchange measurement. I recommend you to read all papers from Jaume Flexas since he did amazing job on mesophyll conductance. I was in one of his lectures and he is real scientist.

I agree with Eshetu, also you can check the work of Evans (http://www.plantphysiol.org/content/72/2/297.full.pdf+html). He worked with a "blade resistance" in wheat based in An:Cc.

Nevertheless, as Flexas states, drought conditions (if it's your case) may change not only mesophyll conductance but also Jmax, and Vcmax. Changing these parameters may affect An, thus decoupling An from gs and, in this case also gm, so you must be careful.

In what species and under what conditions do you want to study gm, it could be interesting...

Also be careful, if you want to publish or look for more information you have to use Cc (Co2 concentration in the carboxilation site) rather than Ci, when trying to assess gm.

Good Luck!!!

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2486.2009.02017.x/pdf

http://www.sciencedirect.com/science/article/pii/S0168192310000110

both by Trevor Keenan, you can check them together with the work of Flexas

This methods review by Pons could be of helpful

Estimating mesophyll conductance to CO2: methodology, potential errors, and ecommendations.

Journal of Experimental Botany, Vol. 60, No. 8, pp. 2217–2234, 2009

Hi Lahcen and all. Eshetu, many thanks for your compliments about my work. I agree with Miquel that you all should revise the paper by Pons et al., all the methods are critically reviewed there with all their limitations highlighted. There is also a more recent work on this (Flexas et al. 2012), but this is on a book I edited recently for Cambridge with a few colleagues (Terretrial photosynthesis in a changing environment - Cambridge University Press) for which it may be more difficult to access. Anyway, if you want to check it, just write me at my personal e-mail ([email protected]) and I can send you the manuscript in Word format.

Now, back to specifically the curve fitting method, I have to say that Gilbert Ethier (who is usually quite prone to collaborate with people sending him questions) and Tom Sharkey (who has a PCE paper which includes an Excel application to apply this method easily) are much better experts than myself on this.

Still, from my experience - as a user, not an expert - of the curve-fitting method, I would summarize its limitations as follows:

1) In principle, it is designed to estimate a sinlge gm value along the A-Ci curve (although it is possible to estimate it separatedly for different mechanism-limited parts of the curves, see for this our Flexas et al. 2007 PCE paper), while using other methods it has been shown that gm changes strongly along a Ci gradient (although this is controversial, results from the ANU group in Canberra shows the opposite)

2) It requires exeptionally 'perfect' A-Ci curves to retrieve reliable values. You can sometimes achieve this by calibrating very perfectly your IRGA and using very new components of the equipment (i.e. filters, dessicant, gaskets, etc ..), using very low flow rates to maximize the deltas between entering and exiting chamber CO2 and/or taking a high number of different Ca values, which would extend greatky the time needed to perform a sinbgle curve. This is - I guess - because the method essentially attempts to retrieve 3 or more unknowns (gm, Rd, Vcmax, Jmax ....) from a single curve-fitting - i.e. from a single equation somehow -, for which the degrees of freedom are largely reduced.

Still, the method is useful but, as highlighted by Pons et al., it has to be combined with a second alternative method (either chlorophyll fluorescence or isotope discrimination) to confirm its reliability.

I hope this is useful for you all

Best

Jaume

Hi Jaume and all

Thank you for all this useful clarifications.

I'm currently in the forest and I'll be back probably with other question on my return.

For information, I'm Workin on young white spruce plantation in boreal forest of Canada. i'm working on geographical variation in phenology and photosynthesis parameters in the context of climate change and proactives management strategies (Assisted migration).

thanks to all.