Nitric Oxide-Mediated Regulation of Cadmium Stress in Lettuce Varieties: Implications for Sustainable Food Safety and Crop Production

Nitric oxide (NO) is recognized as a key signaling molecule involved in plant tolerance to abiotic stress. Yet, its role in regulating cadmium (Cd) detoxification and ion homeostasis remains insufficiently understood across different lettuce genotypes. This study aimed to elucidate the NO-mediated mechanisms underlying Cd stress mitigation by focusing on oxidative regulation, ion balance, and Cd accumulation dynamics in lettuce. Three lettuce varieties (Lactuca sativa L.), namely curly (var. crispa), romaine (var. longifolia), and iceberg (var. capitata), were exposed to 100 and 500 µM Cd, with or without 200 µM sodium nitroprusside (SNP), under controlled greenhouse conditions in a modified Hoagland solution. Growth traits, antioxidant enzyme activities [catalase (CAT) and ascorbate peroxidase (APX)], oxidative stress markers [hydrogen peroxide (H2O2), malondialdehyde (MDA), membrane permeability (MP), and proline], ionic homeostasis [potassium (K), calcium (Ca), iron (Fe), zinc (Zn), copper (Cu), and manganese (Mn)], and Cd accumulation indices [bioconcentration factor (BCF), translocation factor (TF), total accumulation rate (TAR), and net accumulation via roots (NetAcc)] were evaluated. Cd exposure significantly reduced biomass production, photosynthetic pigment contents, and the accumulation of essential mineral nutrients, while markedly increasing oxidative stress indicators. Antioxidant responses varied among varieties, with Cd generally stimulating CAT activity but suppressing APX, indicating redox imbalance. SNP application partially restored antioxidant enzyme activities, reduced membrane damage, and alleviated oxidative stress in a genotype-dependent manner. Cd accumulation indices revealed substantial Cd uptake and translocation, particularly in curly and iceberg lettuce. SNP significantly reduced BCF, TF, TAR, and NetAcc values, suggesting NO-mediated restriction of Cd mobility, possibly through enhanced root sequestration and detoxification processes. Moreover, SNP improved K+, Ca2+, Fe2+, and Mn2+ homeostasis, highlighting its role in maintaining selective ion transport under Cd stress. Among the tested varieties, curly lettuce exhibited the highest NO-induced tolerance, followed by iceberg and romaine lettuce. Overall, the findings demonstrate that NO acts as an effective regulator of redox balance, ion homeostasis, and Cd detoxification, thereby enhancing physiological resilience and reducing Cd accumulation in lettuce exposed to Cd stress. From a sustainability perspective, these findings highlight the potential of NO application as an effective strategy to reduce Cd accumulation in leafy vegetables, thereby contributing to safer food production and more sustainable crop management under heavy metal-contaminated conditions.