Root-Zone Heating Boosts the Production of Mini Romaine Lettuce Grown in Nutrient Film Technique and Aeroponics Systems

Root-zone temperature is a critical environmental parameter affecting the development, physiology, and nutritional status of leafy vegetables in soilless systems such as the nutrient film technique (NFT) and aeroponics. In the present article, we report on responses of mini Romaine lettuce (Lactuca sativa L.) upon cultivation using heated nutrient solution targeting minimum temperatures of 14, 18, and 22 °C versus ambient (control; 11–12 °C), both in the NFT and in the aeroponics system. In both systems, the higher temperatures (i.e., 18 and 22 °C) led to considerably higher leaf mass per system area (127–232% in NFT; 54–75% in aeroponics) and leaf length (more than 21% in all cases). Root dry weight and total soluble solids were positively affected by increasing temperatures only in the NFT. Performance indices of the photosynthetic mechanism (PIabs and PItot) were increased in the lower temperatures in the NFT. Antioxidant activity and total phenolics were not affected in either soilless system. Total chlorophylls and carotenoids were enhanced by 18 and 22 °C in the NFT and aeroponics, respectively, while anthocyanins were also variably affected. Finally, nitrate content was significantly reduced (−42%) in 18 °C in the NFT. Sub-optimal root-zone temperatures constrained root development and biomass accumulation, indicating that growth limitation was mainly driven by sink-related processes rather than carbon assimilation. Overall, heating the nutrient solution to a minimum of 22 °C in low- and mid-tech greenhouses during cool months can increase the production efficiency of mini Romaine lettuce in the NFT and aeroponics.