Root-applied glycinebetaine controls nitrate content and edible quality of lettuce in continuous commercial hydroponic greenhouse production

Lettuce (Lactuca sativa L.) may accumulate high amounts of nitrate (NO3-) in hydroponic cultivation, such as in the Nutrient Film Technique (NFT) system. The NFT system is a commonly used hydroponic cultivation method in which a thin layer of nutrient solution flows continuously over the roots and is then recirculated. Since NO3- is known to be harmful to human health, lettuce producers need methods for controlling it and improving the quality of lettuce. Here, we investigated whether root-applied glycinebetaine (GB) can be used to control the NO3- content in commercial-scale greenhouse production. Three commercial-scale experiments were conducted with different GB application rates (0, 4, 7.5, and 10+10mM GB). Glycinebetaine was applied to the nutrient solution, and lettuce was exposed to the GB treatment during the last 6 growing days before the harvest. Even though the root-applied GB slightly reduced the fresh weight of harvested lettuce and some minerals in lettuce dry matter, it significantly reduced the lettuce NO3- content (over 29%) and increased the contents of some amino acids in comparison to untreated plants. Thus, the edible quality of lettuce was improved. Furthermore, the tested GB application method proved to be functional on a commercial scale. Based on our study, adding GB to the nutrient solution can be considered a practical method for controlling lettuce NO3- content.