Present address: Dept. of Crop Science, Univ. of the Philippines, Los Banos, Laguna, Philippines.
Received for publication 21 July 1986. Scientific article no. A 4578, Contribution no. 7594 of the Maryland Agricultural Experiment Station. 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.