Phenanthrene-, potassium-, or phenanthrene plus potassium-induced membrane potential changes. Impalements were made in basal solution (5 mM MES, 0.5 mM CaCl2, 0.05 mM NaCl, pH 5.5) with or without 5.62 μM phenanthrene, potassium, or 5.62 μM phenanthrene plus potassium. Wheat roots grew for 15 d. Arrows indicate times at which addition and removal of phenanthrene, K+, or phenanthrene and K+. MES, 2-(N-morpholino)ethanesulfonic acid. PHE, Phenanthrene.