thanks for the clarification but ratio A/Gs or A/Ci what will be helpful to measure intrinsic water use efficiency in case of mandarin leaves.

Yes we have CIRAS 2 for photosynthetic measurements. Is it okay.

If you have a LI6400 and also it is calibrated for CO2 and water vapor, you can simply calculate the water use efficiency by dividing the net photosynthesis by transpiration value given by the meter. You will get the water use efficiency as the amount of net photosynthesis per unit of water transpired.

Here you have two interesting reading regarding WUE:

1. Pérez-Priego O., Testi L., Orgaz F., Villalobos F.J. (2010) A large closed canopy chamber for measuring CO2 and water vapour exchange of whole trees. Environmental and Experimental Botany 68 (2), pp. 131-138.

2. Villalobos F.J., Perez-Priego O., Testi L., Morales A., Orgaz F. (2012) Effects of water supply on carbon and water exchange of olive trees. European Journal of Agronomy 40, pp. 1-7.

You can calculate the instantaneous WUE as A/E, but for the values to be interpretable you need to do all measurements at the same controlled ambient conditions Ca, delta e, and irradiance, or at the actual ambient conditions. Which alternative to use will depend on what is your objective: the first alternative tells you only about the leaf itself, the second will be affected by things like changes in leaf position and actual weather conditions or canopy structure. The second alternative is in practice quite difficult as it requires very careful sampling, timing, and lots of replicates. To compensate to some degree for the artificial conditions in the cuvette one option is to measure quickly and use A/g to assess WUE. What is "quickly" enough will depend on the species and conditions, as stomatal response speed varies widely among species. As mentioned above, you will need a well calibrated system, and careful sampling and timing (time of day, and alternating measurements between treatments for each replicate). When measuring outdoors or in a greenhouse under changing weather one should be extremely careful about avoiding possible bias).