Technological overview of tipburn management for lettuce (Lactuca sativa) in vertical farming conditions

Tip-burn is an environmental nutrient disorder caused by calcium (Ca) deficiency in the plant. As Ca uptake is driven by mass flow, the incidence of tip-burn is often understood in plant water relations affected by many environmental factors. Plant transpiration, root pressure, and resulting xylem pressure take an important role determining the Ca transport to the growing shoot tip. However, plant water relations can only partially explain the incidence of tip-burn, as tip-burn appears when mass-flow-driven Ca does not meet the Ca demand for plant growth. Therefore, lettuce plants develop tip-burn under conditions that enhance growth rate (e.g., high light and high CO₂ concentration) but limit transpiration (e.g., low vapor pressure deficit (VPD) and limited air circulation). These conditions are commonly seen among indoor vertical farms. Effective mitigation measures include 1) general growth reduction by low light and/or temperature, 2) shortening the production cycle to avoid the final fast-growing phase to avoid the tipburn risk, 3) promotion of xylem mass flow by use of downward airflow fans over crops to promote transpiration rate, and 4) foliar Ca spray. Among them, promotion of transpiration using downward airflow fans is widely practiced in greenhouse hydroponic lettuce production. However, installation of fans to create uniform downward airflow is rather challenging in narrow headspaces of indoor vertical farms. Furthermore, the sole source electrical lighting lacks thermal near-infrared radiation (800-3000 nm) and does not drive plant transpiration as much as solar radiation does, when compared at the same photosynthetic photon flux density. Therefore, many growers select other mitigation methods including shorter production cycles to avoid tip-burn risk at final stage of production cycle, during which plant biomass (i.e., yield) increases rapidly. However, shortening production cycles may increase the production costs. Other mitigation measures include high night-time humidity (<0.1 kPa VPD) and foliar Ca spray, whose applications are limited. More research needs to be conducted for assessing tipburn risk of the lettuce growing environmental conditions and practical mitigation measures when a high risk is predicted.