Thanks for the informations.
I gave a call to the Inrae laboratory I mentioned earlier about their grafting technique.
As lauta suggested, they said that they grow their rootstocks in the dark in order to have elongation and to graft under the cotyledon.
They added that they graft like that only for research purpose citrus. It is not a good way to graft if we want to keep the trees for a long time, because they are very sensible.