Abstract: Tetraploid watermelons were successfully induced in vitro by treating cotyledons of diploid watermelon lines Z-A and Z-R in liquid MS medium with three dinitroanilines oryzalin, pendimethalin and trifluralin in different concentrations, colchicine or no inductor used as control. The results indicated that the effects of tetraploid induction were better by using three dinitroanilines than those of colchicine. The induced tetraploid rate of oryzalin, pendimethalin,trifluralin and colchicine was 40.0%，33.3%，26.7% and 13.3% respectively. The induction rate of direct treatment in which the cut cotyledons were directly planted intoinductive medium was higher than that of indirect treatment in which the cotyledons were pre-cultured on differentiation medium (MS+BA 2.0 mg·L^-1 ) for 7 d before planted into inductive medium. The tetraploid of watermelon plantlets was determined in the early stage with FDA fluorescence method in which the chloroplasts of epidermal guard cells were counted under a microscope and UV light. The detective rate was above 90%. The confirmed tetraploid plantlets were transplanted into a greenhouse and self-pollinated and the seeds of tetraploids were obtained.

Key words: watermelon, tetraploid, dinitroanilines, in vitro induction, ploidy