Catharanthus roseus seedling was treated with different concentrations (1.5, 3.16, 15, and 30 mmol) and forms (K<sub>2</sub>SO<sub>4</sub> and KNO<sub>3</sub>) of potassium (K<sup>+</sup>) via Hoagland’s nutrient solution. Ascorbic acid (AsA) was sprayed twice (plant days 68 and 78) with different concentrations (750 and 1 500 mg/L) on the leaves. Vinblastine, vincristine, tryptophan contents, D4H and DAT genes expression, peroxidase activity, and H<sub>2</sub>O<sub>2</sub> content of leaves were measured. Potassium in KNO<sub>3</sub> form increased vinblastine (60%) and vincristine (50%), compared to 30% and 20% using K<sub>2</sub>SO<sub>4</sub>. Vinblastine and vincristine inhibit microtubule assembly and ultimately metaphase-arrested caused by the polymerisation. The genes expression was higher 3 times in KNO<sub>3</sub> and 2.5 times in K<sub>2</sub>SO<sub>4</sub> in excess of K<sup>+</sup>. Foliar application of 750 mg/L AsA led to an increase in vinblastine (20%) and vincristine (16%). Both concentrations of AsA had the same additional effect on the expression of D4H and DAT about 30% and 60%, respectively, compared to the control plant. Tryptophan decreased 2.5 times in excess of K<sup>+</sup> and 35% due to the exterior of AsA. H<sub>2</sub>O<sub>2</sub> decreased while peroxidase activity increased along with AsA treatment. A positive interaction existed between the K<sup>+</sup> and AsA on the amount of vinblastine, vincristine, tryptophan, and gene expression.