Received for publication 20 Oct. 1988. This work was supported in part by the California Prune Board. We acknowledge R.M. Carlson, T.M. DeJong, I. Klein, and C. Rosen for critical reviews of this manuscript. 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.