you can download some of my articles about electrical potential measuring in fruit trees. In matherial and methods is very well described how you can measure those signals. Basically you need Ag/AgCl electrodes that you can insert into the stem to reach xylem (that is for variation potential measurements). If you want to measure in phloem (por action potential for example) you will need and "aphid microelectrode" or surface electrodes. You also need an amplifier of course. There are many articles about this (search the works that belong to Dr. mancuso, Dr. Volkov, Dr. Fromm, Dr. Gurovich, etc).