1. Department of Biochemistry, Institute of Soil Science and Plant Cultivation, Czartoryskich 8, 24-100 Pulawy, Poland, Dipartimento di Chimica Organica, Università di Pavia, v.le Taramelli 10, 27100 Pavia, Italy, and Instituto Sperimentale per le Colture Foraggere, v.le Piacenza 29, 26900 Lodi, Italy