Received for publication 26 Jan. 1983. This work was supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison; Graduate Research Committee of the University of Wisconsin-Madison; and by the National Aeronautics and Space Administration Grant NSG-2405. The advice and facilities for the analytical procedures provided by G.E. Gerloff, Department of Botany, is gratefully acknowledged. 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.
Present address: National Vegetable Research Station, Wellesboume, Warwick, England CV35 9FF.