Nickel uptake, partitioning and physiological responses of lettuce grown in a closed-cycle hydroponic system

Nickel (Ni) is a ubiquitous metal that is essential for plants at trace concentrations but becomes phytotoxic at relatively low levels, representing a growing concern for food safety in leafy vegetables. This study evaluated the effects of increasing Ni concentrations in the nutrient solution (NS) on growth, physiology, Ni uptake and partitioning, and key biochemical responses of lettuce (Lactuca sativa L.) grown in a closed-cycle nutrient film technique (NFT) system. Two commercial cultivars (green- and red-leaf) were cultivated under greenhouse conditions and fertigated with three Ni addition in the NS [(CH3COO)2Ni·4H₂O: 0, 75, and 150 µM]. Growth and yield traits, water and NS use efficiency, chlorophyll fluorescence, chlorophyll content, Ni accumulation in leaves and roots, and the activity of antioxidant (CAT, POD, SOD) and nitrogen-related (NR, NiR) enzymes were assessed during the crop cycle. Ni exposure caused a marked, dose- and time-dependent reduction in growth and yield under elevated Ni exposure conditions, with yield decreasing by 48.9% and 72.1% at 75 and 150 µM Ni, respectively, compared with the control. Ni uptake was rapid in both cultivars and predominantly retained in roots, confirming root sequestration as the main detoxification mechanism; however, prolonged exposure increased Ni translocation to leaves. At 30 DAT, leaf Ni concentration increased from 0.21 mg kg−1 FW in the control to 11.24 and 16.05 mg kg−1 FW at 75 and 150 µM Ni, respectively. Overall, the results demonstrate that closed-cycle NFT systems are highly sensitive to Ni contamination and can rapidly lead to critical accumulation in edible tissues. This study provides novel insights into Ni uptake, partitioning, and physiological responses in lettuce grown under closed-cycle NFT conditions. The results highlight the importance of continuous NS monitoring and preventive mitigation strategies to support sustainable hydroponic production, food safety, and compliance with current European regulations.