Received for publication 24 July 1984. The authors wish to thank G. Palma (Istituto di Radiobiochimica ed Ecofisiologia Vegetali), A. Stazzi, E. Gerani, and M. Giacometti (Servizio Giardini-Comune di Roma) for their technical assistance, and Servizio Microanalisi (Area della Ricerca di Roma-C.N.R.) for N and C analysis. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.