Many thanks for your kind reply.I have a glance on your paper I just did not have any idea how you use glass capilary tube on plant( do you have any photo of this method) and how many replicates do you have to collect enought xylem sap for analyziz and finally howyou calculate Sap flow rate.
Sorry, it's not my paper. I haven't done this kind of analysis. Just trying to be helpful! Maybe you could contact the authors for more details.
I would guess that under pressure the sap would form a droplet at the cut end. You could probably just touch the end of the capillary tube to the droplet and the sap would wick up into the tube via capillary action.
I would also guess that you could calculate flow rate by volume/time - you know the volume, because you collected the sap in the capillary tube (of known volume). Again, I'm guessing here. Hopefully an expert will chime in.