The effect of spraying tryptophan amino acid on the physiological characteristics of the periwinkle plant (Catharanthus roseus L.) under drought stress

Background Periwinkle ( Catharanthus roseus L.) is a significant source of two valuable anticancer alkaloids, vincristine and vinblastine. Amino acids serve as precursors for alkaloid biosynthesis, and environmental stresses are known to induce an increase in the ratio of secondary metabolites in plants. This study was conducted using a completely randomized design with a factorial arrangement and three replications at the Khuzestan Agricultural Sciences and Natural Resources University under controlled greenhouse conditions. The experimental factors included foliar application of the amino acid tryptophan at varying concentrations (control, 50, 100, 150, 200, and 250 ppm) and drought stress levels (100%, 70%, and 40% of field capacity). Results Results from the mean comparisons indicated that increasing the concentration of tryptophan up to 250 mg per liter, combined with maintaining soil moisture at 40% of field capacity, exerted a positive and significant effect on biochemical and physiological parameters of both root and aerial organs. These parameters included dry root and plant weight, root volume, photosynthetic pigments, activities of catalase and peroxidase enzymes, total protein content, phenolic compounds, flavonoids, total amino acids, and the concentrations of vinblastine and vincristine alkaloids. Data analysis revealed that all measured traits improved significantly at high concentrations of tryptophan (above 200 ppm). Conversely, severe drought stress (40% field capacity) resulted in a significant reduction in dry plant weight, total protein, and photosynthetic pigments. Nonetheless, these conditions also contributed to an increase in the content of vinblastine and vincristine alkaloids, as well as levels of enzymatic and non-enzymatic antioxidants. Conclusions Ultimately, the application of tryptophan at concentrations of 200 and 250 ppm was effective in alleviating the adverse impacts of drought stress by enhancing dry weight, photosynthetic pigments, and antioxidant enzyme activities in the periwinkle plant, culminating in increases in vincristine and vinblastine levels by 230% and 488%, respectively, under severe drought stress (40% field capacity).