Abstract
Introduction
Catharanthus roseus is a medicinal plant rich in indole terpenoid alkaloids, including the anticancer compounds vinblastine and vincristine. However, these valuable metabolites occur at low concentrations in vegetative tissues, necessitating effective strategies to enhance their production. Gamma irradiation can act as a physical mutagen and abiotic elicitor, affecting plant growth and secondary metabolism through genetic and physiological changes. Nevertheless, its dose-dependent effects on growth and phytochemical traits of C. roseus ‘Alba’ remain unclear. Therefore, this study investigated the effects of different gamma-ray doses on growth, photosynthetic pigments, antioxidant traits, and vincristine content under greenhouse conditions.


Materials and Methods
The experiment was conducted as a completely randomized design with seven gamma-irradiation treatments (0, 10, 20, 30, 40, 50, and 60 Gy) and three replications under greenhouse conditions at Bu-Ali Sina University, Hamadan, Iran, during the 2022–2023 growing season. Seeds of C. roseus ‘Alba’ were irradiated using a Cs-137 gamma source at a dose rate of 10 Gy min-1. Morphological traits, photosynthetic pigments, total phenolics, total flavonoids, antioxidant capacity (DPPH), and vincristine content were then measured using standard analytical methods. Relationships among the measured traits were also evaluated using Pearson correlation and path analysis

Results and Discussion
Gamma irradiation affected the growth and phytochemical traits of C. roseus ‘Alba’ in a dose-dependent manner, showing a dose-dependent response. The 60 Gy treatment produced the greatest vegetative growth, increasing plant height, fresh leaf weight, and fresh root weight compared with the control. This suggests that an appropriate gamma dose can stimulate biomass accumulation and overall plant performance under greenhouse conditions. In addition to growth, gamma irradiation improved photosynthetic pigments, phenolic compounds, flavonoids, and antioxidant capacity, especially at 50 and 60 Gy. These changes may be associated with enhanced pigment accumulation and activation of antioxidant defense mechanisms. Vincristine content was highest at 50 Gy, indicating that the optimal dose for secondary metabolite production may differ from the dose that maximizes biomass. The 60 Gy treatment significantly enhanced photosynthetic pigment levels and morphological traits, increasing plant height, leaf weight, and root weight by 34, 93, and 83%, respectively, compared with the control, thereby indicating an improved growth potential. Conversely, the maximum biosynthesis of secondary metabolites was achieved at 50 Gy, where the highest total phenolic and flavonoid contents (31.29 and 2.34 mg g-1 DW, respectively) and the target alkaloid vincristine (299.09 µg g-1) were recorded. Notably, the highest antioxidant capacity, with 92.30% DPPH radical scavenging activity, was observed at 60 Gy. The different responses at 50 and 60 Gy show that plant growth and medicinal compound accumulation are not always maximized by the same treatment. Therefore, dose selection should depend on the main production goal, whether it is biomass improvement or vincristine enhancement.


Conclusion
Gamma irradiation had a significant and dose-dependent effect on the growth and phytochemical traits of C. roseus ‘Alba’. The 60 Gy treatment was most effective for enhancing vegetative growth, whereas the highest vincristine content was recorded at 50 Gy. Both treatments also improved photosynthetic pigments, phenolic compounds, flavonoids, and antioxidant capacity. These results indicate that gamma irradiation can act as an abiotic elicitor and stimulate both biomass accumulation and secondary metabolite production. The highest-performing dose among those tested depends on the desired objective, with 50 Gy being more suitable for vincristine production and 60 Gy for growth enhancement. In all, under the specific greenhouse conditions evaluated in this study, gamma irradiation in the range of 50-60 Gy showed the most pronounced positive effects on vegetative biomass and phytochemical traits in C. roseus ‘Alba’. However, multi-year and multi-location field trials are necessary to validate these responses before proposing gamma irradiation as a broad-scale agronomic strategy.