Induction of tetraploids in ‘Red Flash’ caladium using colchicine and oryzalin: Morphological, cytological, photosynthetic and chilling tolerance analysis

Abstract

In vitro pre-cultured leaf segments of ‘Red Flash’ Caladium (Caladium × hortulanum Birdsey) were treated with 0.1%, 0.2% and 0.3% (w/v) colchicine, or 0.001%, 0.002% and 0.003% (w/v) oryzalin for 2, 4 and 6 days, respectively, with the aim to develop an efficient polyploid induction protocol for caladium, and to identify promising caladium variants for cultivar development and chilling tolerance breeding. A total of 206 out of 723 plants were found to exhibit stable and remarkable morphological changes, and were grouped into 10 variant types based on differences in leaf shape, color, and/or coloration. As many as 93 plants were identified preliminary as tetraploids by flow cytometry, and the most efficient way for chromosome doubling seemed to be exposed to 0.002% oryzalin for 6 days. Chromosome counting were performed to further determine the ploidy level of the variants as extensive variation of mean fluorescence intensity (MFI) were recorded among them, and results showed that chromosome gains or losses occurred frequently in the established variants. As compared to the wild type, tetraploidization resulted in plants with rounder and thicker leaves, larger petiole diameter, higher plant height, lower leaf number per plant, and lower stomatal density, and significantly increased the net photosynthesis rate, transpiration rate, and stomatal conductance. Furthermore, enhanced chilling tolerance were observed in the tetraploids (T1), as evidenced by their higher superoxide dismutase (SOD) activity, peroxidase (POD) activity and proline (Pro) content, and a lower relative electrical conductivity (REC) and malondialdehyde (MDA) content in the leaves compared with those of the diploid counterparts and the diploid aneuploids (SVT1) during chilling stress. The variants associated with valuable phenotypic traits including the tetraploids, diploid variants, diploid aneuploids, and tetraploid aneuploids hold considerable potential for cultivar development, genetic study and chromosome engineering in caladium.