Induction of tetraploidy in watermelon with oryzalin treatments

This study evaluated the effect of different concentrations and methods of oryzalin treatment on polyploidization in watermelon (Citrullus lanatus). The first method involved different concentration of oryzalin (0, 5, 10, 15, 20, and 25 mg∙L^-1) applied to the shoot apex of watermelon seedlings. Polyploidy was not induced by concentrations of 10 mg∙L^-1 or lower. In contrast, mixoploidy was observed with concentrations of 15 and 20 mg∙L^-1 and 84% of the plants were tetraploid in the 25 mg∙L^-1 concentration. The addition of the surfactant tween 20 in oryzalin was less effective for tetraploid induction than oryzalin alone. The second method involved soaking the seeds in different concentrations of oryzalin (0, 30, and 35 mg∙L^-1). With this method, the 30 mg∙L^-1 oryzalin concentration did not induce polyploidy and the 35 mg∙L^-1 concentration only induced tetraploidy in 25% of the plants. The morphological features of the second true leaf of a tetraploid plant showed a small, thick, and wrinkled leaf unlike that of the diploid plant. In addition, the number of stomata per unit area decreased and stomata doubled in size in the tetraploid plants. Tetraploid plants (2n = 4x = 44) were confirmed by counting the number of chromosomes of the treated plants. We concluded that the shoot apex treatment with 25 mg∙L^-1 oryzalin was the most effective method for inducing tetraploid watermelon.