Monoterpenes as biostimulants: dose-dependent effects on metabolic and nutritional traits of lettuce ( Lactuca sativa L.)

Plant-derived substances, particularly monoterpenes, offer eco-friendly biostimulant potential to enhance crop yield and nutritional quality, yet their individual effects remain poorly explored. This study investigated the physiological and metabolic responses of lettuce (Lactuca sativa L.) to weekly foliar applications of cineole, carvacrol, and thymol at 500 and 1000 ppm under controlled conditions. Biomass accumulation, photosynthetic efficiency (chlorophyll a fluorescence), and nutritional profiles (primary metabolites, antioxidants, and minerals) were assessed. All treatments maintained leaf fresh weight, indicating no phytotoxicity, while inducing dose-dependent metabolic shifts. At 500 ppm, carvacrol and thymol acted as metabolic enhancers, increasing starch accumulation by up to 264% and raising total phenolic content by 24% (cineole). At 1000 ppm, carvacrol induced a dose-dependent reversal from stimulation to inhibition in chlorophyll content and total phenolic content, alongside a dose-dependent decline in photosynthetic performance and stimulation of defense metabolism. This resulted in a 44% increase in vitamin C and significant biofortification with phosphorus (+156%), potassium (+61%), and iron (+89%). These results demonstrate that monoterpenes can act as tunable biostimulants, with low doses promoting carbon storage and higher doses enhancing antioxidant activity and mineral enrichment. This study supports the use of monoterpenes for targeted modulation of crop functional quality in an eco-friendly and sustainable approach.