Can Seed Priming with Seaweed Extract Neutralize the Effects of Salinity on New Red Fire Leafy Lettuce Characteristics?

In order to evaluate the influence of seed priming with extracts of different seaweeds—Ascophyllum nodosum (An), Sargassum angustifolium (Sa), Chlorella vulgaris (Cv), and Arthrospira platensis (Ap)—on characteristics of leafy lettuce cultivar New Red Fire under different levels of salt stress (irrigation with saline water with electrical conductivity levels of 1.37 as control, 2.5, 4, 5.5, and 7 dS m⁻¹), an experiment was conducted during the 2021–2022 season. The highest germination speed was recorded in seeds soaked in Cv. Vigor index decreased by 58.68% in non-primed seeds compared to Cv-primed seeds. Increasing the salinity level from 4 to 5.5 dS m⁻¹ led to a significant decrease (3.30%) in seedling plumule length. Salinity induction led to increased leaf taste index. The lowest leaf free proline content was recorded under a salinity level of 1.37 dS m⁻¹. Seed priming with seaweed extract led to a significant increase in leaf flavonoid content. The highest peroxidase enzyme activity was recorded in Cv-treated plants under salinity levels of 5.5 and 7 dS m⁻¹. Seed priming with seaweed extract led to an increase in antioxidant activities, osmolyte accumulation, photosynthetic pigment content, and salt tolerance of mature plants. Cv and Ap could be introduced as the best priming solutions to neutralize the negative effects of salinity. A salinity level of 4 dS m⁻¹ could be considered as a salt tolerance threshold for the New Red Fire cultivar. Cultivation of this cultivar under salinity levels of 5.5 and 7 dS m⁻¹ can seriously damage qualitative and quantitative lettuce parameters.