Use of Ipomoea trifida (HBK.) G. Don germ plasm for sweet potato improvement. 1. Development of synthetic hexaploids of I. trifida by ploidy-level manipulations

Crosses were made between 21 tetraploid accessions and 41 diploid accessions of Ipomoea trifida, obtaining a total of 9185 triploid seeds from 215 different cross combinations. Doubling the somatic chromosome number of the triploids was attempted by colchicine treatment on young seedlings to obtain synthetic hexaploids. A total of 787 axillary buds of 316 triploid plants were treated with a 0.5% colchicine solution for 24 h, applied to cotton plugs surrounding the buds. The survival rate of the treated buds was, on average, 41%. The ploidy level in germ-layer L2 was determined in 258 clones by evaluating a pollen sample from each clone. Fifty-five clones were selected for high stainability and pollen size. The selected genotypes were meiotically analyzed. Twenty-two of them were identified as hexaploid and 33 as triploid with 2n pollen production. The present study is the first report on 2n pollen production in triploid plants of Ipomoea species. The use of these triploid clones with 2n pollen production and synthetic hexaploid clones for sweet-potato breeding is discussed.Key words: Ipomoea batatas, Ipomoea trifida, colchicine, triploids, 2n pollen.