Thermoinhibition refers to the inability of seeds to germinate when inhibited by high temperatures, but when environmental conditions return to normal, the seeds are able to germinate rapidly again, which is different from thermodormancy. Meanwhile, with global warming, the effect of the thermoinhibition phenomenon on the yield and quality of crops in agricultural production is becoming common. Lettuce, as a horticultural crop sensitive to high temperature, is particularly susceptible to the effects of thermoinhibition, resulting in yield reduction. Therefore, it is crucial to elucidate the intrinsic mechanism of action of thermoinhibition in lettuce seeds. This review mainly outlines several factors affecting thermoinhibition of lettuce seed germination, including endosperm hardening, alteration of endogenous or exogenous phytohormone concentrations, action of photosensitizing pigments, production and inhibition of metabolites, maternal effects, genetic expression, and other physical and chemical factors. Finally, we also discuss the challenges and potential of lettuce seed germination thermoinhibition research. The purpose of this study is to provide theoretical support for future research on lettuce seed germination thermoinhibition, and with the aim of revealing the mechanisms and effects behind lettuce seed thermoinhibition. This will enable the identification of more methods to alleviate seed thermoinhibition or the development of superior heat-tolerant lettuce seeds.

Seed germination requires the relaxation of endosperm cap and radicle cell walls, with cell wall hydrolases playing a significant role in this process. Our study revealed that a type of cell wall hydrolase, xyloglucan endotransglucosylase, may significantly contribute to endosperm weakening during lettuce seed germination. Through bioinformatics analysis, the XTH gene family in lettuce was divided into five subfamilies localized on nine chromosomes. Notably, there were significant differences in gene structure among the members of the LsXTHs family containing 1–4 exons and 20 conserved motifs. Among these motifs, motif1, motif2, and motif3 encoded the XTH structural domain. The promoter regions of LsXTHs contained a large number of cis-acting elements responsive to various abiotic stresses, such as drought, anaerobiosis, low temperature, high temperature, and salt stress. Germination experiments showed that seeds imbibed in water and 5 μmol/L abscisic acid (ABA) were able to achieve typical germination with radicle protrusion from the endosperm cap, achieving germination of 100% and 36%, respectively. Conversely, in 0.3% sodium dichloroisocyanurate (SDIC), the swollen seeds were unable to germinate or complete atypical germination, resulting in a germination rate of 30%. Compared to the control, the mechanical strength of the endosperm cap of seeds imbibed in 0.3% SDIC for 8 h increased by 14%, indicating that SDIC may inhibit seed germination by enhancing the mechanical strength of the endosperm cap. Enzyme activity analysis revealed that during lettuce seed germination, XTH enzyme activity in the endosperm cap was significantly higher than in other tissues and increased gradually with imbibition. Transcriptome analysis of the endosperm cap detected the expression of 10 LsXTH genes. Among these, LsXTH43 exhibited the highest expression during germination and was significantly upregulated two-fold by high temperatures, suggesting a potential role in the high-temperature germination of lettuce seeds. Additionally, SDIC downregulated the expression of LsXTHs to varying degrees, with the expression of LsXTH15 reduced to only 6% of its original level. Low temperature, high temperature, drought, and salt stress all reduced the expression of most LsXTHs to different degrees; when seeds germinated under waterlogging and cadmium stress, LsXTH6, LsXTH7, LsXTH8, LsXTH32, and LsXTH33 were all upregulated to some extent.

Accurate identification of high-quality seeds is crucial for maintaining superior crop traits. Lettuce is widely consumed vegetable with diverse varieties, however, the traditionally identification methods are both time-consuming and labor-intensive. This study explores feasibility of rapid, non-destructive identification of different lettuce varieties using multispectral imaging combined with machine learning. We firstly collected seed morphological and spectral data from 15 lettuce varieties using multispectral imaging. Then we applied Support Vector Machine (SVM), Random Forest (RF), and Back-Propagation Neural Network (BP), Linear Discriminant Analysis (LDA) for variety identification. The results demonstrated that multispectral imaging combined with machine learning models, effectively distinguished different lettuce seed varieties. The LDA model based on morphological and spectral fusion feature data performed best, and the average classification accuracy was 92.7 %. In the batch validation, the LDA model achieved an accuracy of 93.2 %.This method reduces cost and improves efficiency, showing great potential for seed identification in other crops.

Isodon serra is a medicinal plant distributed in the Lingnan region of China. However, I. serra seeds are sensitive to heat, limiting germination in Lingnan’s climate. Seed priming technology provides an effective means to address this problem. Meanwhile, karrikin (KAR) enhances plant growth and stress tolerance, but its mechanism in I. serra remains unclear. Therefore, in this study, different KAR concentrations were selected for priming to examine the phenotypic and physiological responses, and multi-omics analysis was used to explore the molecular mechanism of promoted germination under heat stress. The optimal concentration of 0.05 µmol/L KAR raised the seed germination rate under heat stress. This priming treatment reduced membrane damage, indicated by lower malondialdehyde (MDA) levels and relative electrical conductivity (REC) and oxidative stress, by regulating antioxidant enzymes and increasing glutathione (GSH) content. Multispectral phenotyping revealed distinct spectral features at 880 nm and 970 nm in primed seeds, correlating with enhanced vigor. Integrated multi-omics analysis demonstrated that the alleviation of heat stress was primarily mediated through the glutathione metabolism pathway, with key roles played by genes such as cluster-23509.3, cluster-15076.0, and cluster-22471.0. In summary, KAR priming enhances osmotic adjustment, improves reactive oxygen species (ROS)-scavenging enzyme activities, and regulates glutathione metabolism to mitigate heat stress in I. serra. These findings provide valuable insights for optimizing the cultivation and management of I. serra under heat stress conditions.