Distinct metabolome changes during seed germination of lettuce (Lactuca sativa L.) in response to thermal stress as revealed by untargeted metabolomics analysis

It is well known that temperature strongly influences lettuce (Lactuca sativa L.) seed germination and that different lettuce genotypes respond differently to higher temperature or thermal stress. In this study, we first evaluated the germination performance of 304 lettuce accessions incubated at three temperature settings, 21°C, 28°C and 35°C, respectively, for 40 hours. At 21°C, seeds of all 304 accessions germinated very well with an average germination percentage of 87.72%; at 28°C, the average germination percentage dropped to 42.84% and at 35°C, the germination decreased to 1.01%. Then, we investigated the metabolic mechanism of lettuce seed response to thermal stress using an untargeted metabolomics approach. After profiling the primary and secondary metabolites in the dry seeds, germinated seeds at 21°C of two accessions (a thermosensitive variety (s13k0062) and a thermotolerant variety (s15k0106)), and the ungerminated and germinated seeds of the cultivar ‘s15k0106’ at 35 °C, we found a great variation in content of organic acids, amino acids, terpenoids, phenolic compounds, carbohydrates, and lipids among different cultivars. Results suggested that seeds of thermal-sensitive and thermal-tolerant cultivars employed different metabolic strategies in response to thermal stress during germination. This investigation paves the way to link the metabolomics to other external and internal factors affecting lettuce seed germination under thermal stress.