The only suitable and accepted method is throughout an Infra-Red Gas Analyser (IrGA) that measures de CO2 and H2O fluxes in and out the leaves throughout infre-red detection.
Following on with Bernardo, get in touch with someone/lab with a LI-COR 6400 portable photosynthesis system ("http://www.licor.com/env/products/photosynthesis/")