Induction, regeneration and characterization of tetraploids and variants in ‘Tapestry’ caladium

Abstract

Caladium (Caladium × hortulanum Birdsey) is an important ornamental aroid that is valued for its long-lasting colorful foliage. Genetic improvement and cultivar development in caladium have been primarily achieved through sexual hybridization, and there is a strong need to explore in vitro-based approaches to generate novel phenotypic variations in caladium. This study was conducted to develop an efficient in vitro chromosome doubling technique for caladium, to assess the effects of chromosome doubling on caladium leaf morphology, and to identify promising caladium tetraploids for breeding. Leaf segments of ‘Tapestry’ caladium were pre-cultured in vitro, treated with colchicine, and then cultured to regenerate tetraploid plants. Out of 501 established plants, 56 showed considerable variation from the wildtype in leaf shape, color, coloration pattern, and/or thickness. Ploidy analysis showed that 48 appeared to be tetraploids, and the remaining eight appeared to be diploids. In two treatments with 0.1 or 0.2 % (w/v) colchicine for 4 days, 13.74 and 24.14 % of the regenerated plants were tetraploids. Caladium tetraploids exhibited round and thick leaves with thick petioles. These morphological changes were a reliable morphological indicator for visual screening for tetraploids in caladium. Based on observed morphological changes, the eight diploid variants were categorized into five variant types (VT1–VT5), and the 48 tetraploid variants were categorized into another five variant types (VT6–VT10). Most caladium plants in VT1, VT2, VT3, VT6, and VT7 showed much improved ornamental values and held potential as promising new selections for container and landscape performance trials and tuber yield tests. Nuclear DNA content and chromosome number analysis were conducted on 17 representative variants; results revealed the two diploid variants had gained one or two additional chromosomes, one tetraploid variant gained four additional chromosomes, and two tetraploid variants lost two or four chromosomes. These results demonstrate for the first time in caladium that in vitro induction of tetraploids is a very powerful tool for generating novel phenotypes for genetic improvement and breeding and that chromosome gain or loss is a common type of cytological variation in caladium.