Phenotypic markers for tetraploid watermelon [Citrullus lanatus (Thunb.) Matsum. et Nakai] following parental exposure to colchicine in T0 generation

The goal of this investigation was to identify phenotypic traits that effectively distinguish tetraploid from diploid watermelon [Citrullus lanatus (Thunb.) Matsum. et Nakai) at various stages of growth. Assays for phenotypic markers were performed on five regular diploid cultivars (WC-8C-2C, WD-2-6K, GW11, GW7, and GW2) exposed to the alkaloid colchicine (0.1 or 0.2%) from January 2010 to August 2011 at Watermelon Experiment Station, Gochang, Jeonbuk, South Korea. Tetraploids were induced using treatment to one of three plant parts: directly on seed (DS), shoot apex (SA) or inverted hypocotyl (IH). Results showed that tetraploid plants exhibit shorter internode length, wider leaf width with high degree of blistering, light green fruit with greyish stripes, and thicker pericarp as compared to the diploid control. In tetraploids, the stamina of the male flower was erect to semi-erect with deep yellow to light green color. No major differences were observed in fruit size and weight between the tetraploid and diploid, and both characteristics were considered unreliable indicators of level of the ploidy during early growth stages of T0 plant. Length, width, and overall size of the seed were remarkably greater in tetraploids. Seeds of tetraploids had broad apical ends and fissures along the longitudinal axis. Number of seeds per fruit and 100 seeds weight of the tetraploid were 51.5 and 4.25 g, respectively. Results identified specific phenotypic traits that could be used to reliably identify tetraploid watermelon in T0 plants in the field. In the future, these traits could potentially be used to develop more efficient watermelon triploid breeding programs.