Temperature strongly influences lettuce (Lactuca sativa L.) seed germination. Different lettuce genotypes respond differently to higher temperatures or thermal stress. In this study, we evaluated the germination performance of 304 lettuce accessions incubated at three temperature settings, 21 °C, 28 °C and 35 °C, respectively, for 40 h. At 21 °C, seeds of all 304 accessions germinated with very well 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 changes in metabolic profiles of lettuce seed response to thermal stress using an untargeted metabolomics approach. Results suggested that seeds of thermal-sensitive and thermal-tolerant cultivars employed different metabolic strategies in response to thermal stress during germination. Thermal-sensitive buds accumulated more significant amounts of organic acids, amino acids, sugars, sterols, phenolic compounds and terpenoids compared to thermal-tolerant buds at 21 °C. Thermal-tolerant lettuce cultivar accumulated higher concentrations of amino acids, organic acids, sugars, sesquiterpene lactones, sterols, and fatty acids derivatives during the germination at 35 °C compared to germinated at 21 °C. This investigation paves the way to link the metabolomics to other external and internal factors affecting lettuce seed germination under thermal stress.

Lettuce is a commonly consumed vegetable worldwide, due to its crisp edible organs and rich nutrition. Heat stress is known to seriously threaten its growth and development. However, research on lettuce response to heat stress focusing on molecular aspects is limited. Therefore, we aimed to understand the complex molecular mechanisms underlying the regulation of lettuce plant response to heat stress (37 °C), by transcriptomic analysis of a heat-resistant lettuce variety (K482) following heat stress treatment for 1.5 and 3 days. A total of 3798 and 4121 differentially expressed genes (DEGs) were identified at 1.5 and 3 days vs. the control after heat treatment, respectively; they were mostly enriched in protein processing in the systemic lupus erythematosus, alcoholism, neurotrophin signaling pathways, and the cell cycle. Most DEGs were NACs, HSPs, MYBs, bHLHs, and WRKYs. Transcriptome analysis showed that the NAC domain transcription factor LsNAC28 , which is associated with heat response, is up-regulated during heat stress and LsNAC28 inhibited yeast growth under heat stress (37 °C). These findings provide a basis to comprehensively understand the complex molecular mechanism of lettuce response to heat stress.