Received for publication January 31, 1973. Contribution from The Volcani Center, Agricultural Research Organization, Bet-Dagan, Israel 1972 Series, No. 106-E. This research was financed in part by a grant made by U.S. Department of Agriculture under P.L. 480.
The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot, Israel.
The authors wish to thank Drs. M. J. Bukovac and F. J. Dennis, Jr. for critically reviewing the manuscript, and Mrs. Elimelech, Mrs. N. Ronen and Mr. Z. Ben-Chetz, for skillful technical assistance.