and these molecules function as signals to mutualistic soil microbes and/or phytoalexins against infecting pathogens (Dakora and Phillips, 1996). Calcium is involved in the closure of guard cells and also me- diates the response of plants to ethylene, a gaseous plant hormone that controls important physiological processes including seedling development, flowering, fruit ripening and senescence (Raz and Fluhr, 1992).