The effect of spraying tryptophan 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 ppm, in conjunction with maintaining soil moisture at 40% of field capacity, exerted a positive and significant effect on the biochemical and physiological parameters of both root and aerial organs. Data analysis revealed that all measured traits exhibited significant enhancements at elevated tryptophan concentrations (250 ppm), culminating in an 87% increase in plant dry weight, a 105% in root dry weight, and a 66% in total protein compared to the control group. The activity of enzymatic antioxidants, including catalase (CAT) and peroxidase (POX), in tryptophan-inoculated plants subjected to severe drought stress demonstrated increases of 844% and 650%, respectively, relative to control plants. Conversely, the imposition of severe drought stress (40% field capacity) resulted in a substantial decrease in plant dry weight, total protein, and photosynthetic pigments. However, these conditions also facilitated an increase in the concentrations of the alkaloids vinblastine and vincristine, as well as the levels of non-enzymatic antioxidants. Collectively, these results highlight the potential of using the amino acid tryptophan as a sustainable and environmentally friendly approach to enhance drought tolerance in periwinkle, offering promising implications for agricultural practices in arid and semi-arid regions. CONCLUSIONS: Ultimately, the application of tryptophan at concentration 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 193% and 138%, respectively, under severe drought stress (40% field capacity).