Received for publication 5 Apr. 1982. Contribution from the Agricultural Research Organization. The Volcani Center, Bet Dagan, Israel, 1982 Series, No. 379-E. The authors gratefully acknowledge the technical assistance of D. Reinhartz and liana Kobiler. 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.