When you go on web, you can find many methods how to extract metals from plant tissue. You must choose the methods which is the most suitable for your lab and plant materials to be analysed.
You could determine the concentration of heavy metals in the plant tissue by digesting 1 gram of plant dry powder with 10 ml of concentrated nitric acid and perchloric acid (1:1 v/v), then the digest samples were made up to 50 ml with distilled water (Jones, 1984). Then determining the metal concentration of samples using Inductively Coupled Plasma Spectrometry (ICP) or Atomic absorption spectrometry (AAS).