Lemon seedlings treated with 1 mu M CaCl2 increased phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) activity 1 hr earlier than when treated with fungal elicitors from Alternaria alternata, or when wounded. The calcium dependent response was suppressed by EGTA and Verapamil, and was mimicked by the calcium ionophore A23187.