Morpho-physiological, leaf hyperspectral, and nutritional responses of lettuce to salt stress

Abstract Lettuce is a cool-season leafy green commonly grown in hydroponics. Romaine lettuce is economically expensive, yet it has more desirable health benefits and market demand than other lettuce types. However, high electrical conductivity (> 2.0 dS m −1 ) of water impacts nutrient supply and growth, thus affecting its economic viability. Despite negative impact of salt stress on hydroponically grown lettuce, its effects on the plant’s morphology, physiology, biochemistry, and nutritional content remains unexplored. In this study, two romaine lettuce genotypes, Green Forest (GF) and Burgundy Delight (BD), were subjected to two sodium chloride treatments (NaCl, 0 and 100 mM) at the late-rosette stage for 9 days to assess the difference in salt tolerance. Salt stress reduced GF’s fresh biomass by 57.3%, while BD displayed a 20% lesser reduction. BD had higher potassium retention even under higher sodium accumulation than GF. The photosynthetic efficiency of GF decreased by 19.3% with a 45.4% transpiration reduction under salt stress, while both traits in BD remained constant. Chlorophyll fluorescence (steady-state fluorescence) decreased on average by 8.5%, with a more significant reduction for GF (10%). Salt stress reduced glucose in GF by 31.5% and increased by 20% in BD. Under salt stress, BD accumulated higher phenolics and flavonoids than GF. Hyperspectral signatures exhibited reduced reflectance in the green and water band regions under salt stress. BD showed improved osmoregulation, potassium retention, and hyperaccumulation of iron and zinc, making it a salt-tolerant donor for developing improved salt-tolerant romaine genotypes.