Inhibition of Seed Germination of Lettuce (Lactuca sativa L.) by Salinity Stress: Insight of Enzymatic Activity

Abstract Salinity stress inhibits seed germination of lettuce (Lactuca sativa L.) due to osmotic stress and ion toxicity. Therefore, the objective of this study is to understand how salinity reduces seed germination of two lettuce cultivars, ‘Susan’ (the high salt-sensitive cultivar) and ‘Yafa’ (the low salt-sensitive cultivar), in relationship with the enzymatic activity of catalase and endo-β-mannanase during germination and seedling growth. The seeds of the two cultivars were sown in Petri dishes under low salinity (0, 0.1, 0.2, 0.3, 0.5, 1, 3, and 5 mM NaCl) in the first experiment and high salinity (0, 10, 20, 40, 80, 160, and 320 mM NaCl) in the second experiment for 8 days in a standard germination test to assess seed germination, mean time to germination, catalase, and endo-β-mannanase activities. Seedling emergence was also evaluated in different growing media of perlite, peatmoss, cocopeat, sand, and their combination. Salinity stress reduced seed germination percentage, seedling length, and increased mean time to germination in ‘Susan’ compared with ‘Yafa’. The highest seedling emergence was reported in perlite for ‘Susan’, while there was no difference in seedling emergence of ‘Yafa’ among growing media. Perlite had the lowest salinity (the lowest electrical conductivity values) among all growing media used in this study. The ‘Yafa’ had higher catalase and endo-β-mannanase activities during germination under salinity stress than the ‘Susan’ except for the 4th and 6th days post germination at the lower salinity stress. In conclusion, the high salt-sensitive cultivar of ‘Susan’ had lower seed germination and higher mean time to germination under salinity stress than the low salt-sensitive cultivar of ‘Yafa’, which was related to lower catalase and endo-β-mannanase activities during seed germination.