I think the answer depends on how you would like to process the data gaining from your ACi curves. Which parameter is the most important one in your experiment?

If you want to measure more samples in wheat and make sure that circadian rhythm doesn't influence your measurements, a recent publication (Driever et al. Journal of Experimental Botany, Vol. 65, No. 17, pp. 4959–4973, 2014) provides a good alternative (see the section A/Ci photosynthetic gas-exchange measurements).

Dear Alain,

Several references are available dealing with A/Ci and A/Cc curves and methods for parameter estimations.

I would suggest:

Long and Bernacchi 2003. J Exp Bot 54: 2393

Sharkey et al. 2007 Plant Cell Envir 30: 1035

Pons et al 2009 J Exp Bot 60: 2217

Regards

SG Nebauer