Received for publication February 25, 1974. Paper No. 4271 of the Journal Series of the North Carolina State University Agricultural Experiment Station,’ Raleigh. The authors express their thanks to John O. Rawlings for statistical assistance. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Experiment Station of the products named, nor criticism of similar ones not mentioned.
Associate Professor, Department of Horticultural Science.
Professor, Department of Horticultural Science and Research Plant Physiologist, Agricultural Research Service, U.S. Department of Agriculture.