1. Professor, Dept. of Agricultural and Forest Sciences, Università degli Studi di Palermo, Viale delle Scienze 12 Ed. 3, 90128 Palermo, Italy (corresponding author).
2. Professor, Politecnical Univ. of Madrid (UPM), Agricultural Engineering School, Ciudad Universitaria, Avenida Computense 3, 28040 Madrid, Spain.