Foliar Application of Riboflavin (vitamin B2) Mitigates Drought-induced Stress in Geranium (Pelargonium graveolens L.)

Drought is a major abiotic stress factor that reduces crop yields and causes substantial economic losses. This study focuses on understanding the mitigating effect of riboflavin on geranium, a valuable medicinal and aromatic herb, under drought stress to improve crop resilience and productivity. Riboflavin application at 25, 50, 100 mg/L significantly stimulated geranium growth, photosynthetic activity, pigment biosynthesis, phytohormone concentrations, and nutrient accumulation at the second cut stage. The most significant biostimulatory improvements were observed at 50 mg/L, resulting in the highest oil yield of 41 L/hectare for the first cut and 34 L/hectare for the second cut. Riboflavin acts as an elicitor, modifying the essential oil profile under both normal and drought conditions. Riboflavin’s impact on oil profiles varies by concentration. While low concentrations (25 mg/L) maintain control-like oil profiles, moderate levels (50 mg/L) preserve citronellol and restore oil quality under drought. High concentrations (100 mg/L) shift the oil profile towards oxygenated monoterpenoids. Drought typically reduces monoterpenoids, primarily citronellol (4.8–fold), while increasing sesquiterpenoids and oxidized derivatives, thereby stimulating defensive compounds such as parthenolide and rose oxide. Drought reduced yield but increased the oil percentage by 1.4–fold (first cut) and 2.7–fold (second cut) compared to non-stressed conditions. Additionally, drought increased levels of carotenoids, ABA, oxidative enzymes, antioxidant enzyme (catalase), osmoprotectants, and malondialdehyde, while reducing nutrient content. Riboflavin alleviated drought by enhancing antioxidant capacity and osmoprotection, optimizing catalase activity, growth-promoting phytohormone levels, and reducing malondialdehyde, oxidative enzymes, and sodium levels. This ultimately improved photosynthetic efficiency, growth, nutrients, and oil yield, offering a cost-effective, eco-friendly drought-mitigation strategy.