Received for publication 10 May 1984. Contribution from Volcani Center, Agricultural Research Organization, Bet Dagan, Israel, No. 988-E, 1984 series. This research was supported by a grant (338-80) from the US-Israel Binational Agricultural Research and Development Fund (BARD). The authors gratefully acknowledge the computer and statistical analysis assistance by A. Gnizi. 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.