Unraveling the nutritional and bioactive constituents in baby-leaf lettuce for challenging climate conditions

•Healthier lettuces by applying this novel practice.•Determination and quantification of antioxidants in baby-leaf lettuce.•A significant reduction of the sodium content in lettuce with this novel practice.•Lower content of nitrates in lettuce after using this sustainable practice. The isolated effects of heat stress, fertilization and elevated CO2 on the content of several health-promoting compounds in plants have been quite studied. However, few studies have focused on two of these three factors together. This work provides information on how two different levels of CO2, four different NO3−/NH4+ ratios in the nutrient solution, and a short-term heat stress affect the biomass and nutritional quality of baby-leaf lettuce cv Derbi. Furthermore, the nutritional quality of the inner and outer leaves was also studied and compared. Results indicated that the strategy used led to a bigger and healthier baby-leaf lettuces. So, this lettuces contained a higher content of sugars, minerals and phenolic compounds and showed an enhanced antioxidant activity. On the other hand, results exhibited that whilst in inner leaves the biosynthesis of antioxidant compounds were favored, in outer leaves was favored the biosynthesis of sugars and mineral content.