Introduction:
The medicinal plant Catharanthus roseus holds significant global economic and therapeutic value due to its production of indole-alkaloids, notably anti-cancer agents like vincristine and vinblastine. However, its cultivation and yield, particularly in arid and semi-arid regions, are severely hampered by drought stress. This abiotic stress induces oxidative damage, reduces photosynthetic pigments, and compromises cell membrane integrity. In response, plants activate antioxidant defense systems, including enzymes like catalase, and accumulate compatible solutes such as proline and soluble proteins. Arbuscular mycorrhizal fungi (AMF) offer a potent bio-solution by enhancing water and nutrient uptake (especially phosphorus) through extraradical hyphal networks, thereby improving host plant tolerance to drought. Beyond physiological and biochemical enhancements, AMF symbiosis stimulates secondary metabolite biosynthesis, leading to increased production of valuable alkaloids like cataranthine and vindoline in C. roseus under stress conditions. Consequently, this research aims to comprehensively investigate the physiological, biochemical, and phytochemical mechanisms by which AMF symbiosis augments the medicinal yield of C. roseus under drought stress.
Materials and Methods:
The experiment was conducted as a factorial in a completely randomized design with three replicates, comprising two arbuscular mycorrhizal fungi and three soil-moisture regimes (80%, 60%, and 40% of field capacity). Following seedling establishment in sterilized sandy loam soil, Catharanthus roseus plants were inoculated with Glomus deserticola or G. mosseae and exposed to water-deficit treatments for ten weeks. Soil moisture was maintained by pot weighing and monitored using a tensiometer, while plants were grown under controlled greenhouse conditions. Growth attributes, shoot and root dry weight, root length, mycorrhizal colonization, chlorophyll and carotenoid contents, catalase activity, and proline concentration were assessed. Leaf total alkaloid content was determined using acid–base extraction followed by perchloric acid titration, and data were analyzed using SAS software and Duncan’s multiple range test at the 5% significance level.

Results:
The interaction between drought stress and mycorrhizal inoculation significantly affected most growth and physiological traits of Catharanthus roseus. Increasing drought severity reduced shoot and root dry weight, plant height, root length, and total chlorophyll content. Inoculation with both fungal species, particularly Glomus mosseae, improved plant growth under water-deficit conditions, with the greatest effects observed at 40% field capacity. Root colonization increased under moderate drought stress but did not differ significantly under the most severe drought condition. Mycorrhizal inoculation maintained higher chlorophyll concentrations and increased carotenoid content, thereby supporting photosynthetic stability. The highest catalase activity occurred in plants inoculated with G. mosseae under 40% field capacity, whereas proline accumulation was higher in non-mycorrhizal plants. Total alkaloid content was also highest under severe drought in G. mosseae-inoculated plants, confirming its role in enhancing drought tolerance and secondary-metabolite production.
Conclusion:
Arbuscular mycorrhizal fungi can markedly alleviate the adverse effects of water-deficit stress in Madagascar periwinkle (Catharanthus roseus L.). Under drought conditions, mycorrhizal plants generally exhibit improved growth, enhanced water status, greater antioxidant activity, reduced oxidative damage, and increased production of valuable medicinal compounds. Therefore, the application of mycorrhizal fungi, particularly Glomus mosseae, represents a sustainable biological strategy for improving drought tolerance and enhancing the medicinal quality of this plant.