Many life activities in cells occur at the metabolite level, such as cell signaling, energy transfer, and intercellular communication, which are regulated by metabolites (Judge & Dodd, 2020; Baker & Rutter, 2023). Intracellular metabolites changes are the result of the interaction of genes and environmental factors, which largely reflects the response and defense of cells to adversity and stress (Xu & Fu, 2022). In recent years, through the qualitative and quantitative analysis of all low molecular weight metabolites in plants under stress, metabolomics has become a powerful tool for studying the mechanism of the plant stress response and identifying plant stress tolerance (Carrera et al., 2021), such as cold stress (Benina et al., 2013; Yang et al., 2020), salinity stress (Jiao et al., 2018), drought stress (Yang & Lv, 2023) and other combined stresses (Obata et al., 2015). A diverse range of metabolites were found to be increased by metabolomics under cold stress. Benina et al. (2013) found that the cold adaptation mechanism of Haberlea rhodopensis was related to sugars, organic acids, and polyols, while the metabolic defense mechanisms of Arabidopsis thaliana and Thellungiella halophyla contributed to amino acids and amino acid derivatives by metabolic analysis. Compared to the sensitive genotype, the tolerant genotype of Miscanthus accumulates more proline, sucrose and maltose under cold treatment (Le Gall et al., 2017). Differentially metabolites enriched in glutathione metabolism were found between seedlings of Hordeum distichon L. exposed to -8℃ and those maintained at 24℃ (Yang et al., 2020a).
Euscaphis konishii and Euscaphis japonica belong to the shrubs or small trees of Staphyleaceae (Fang, 1981), which have important medicinal and ornamental value. Their fruits and roots are particularly rich in esters, triterpenoids, flavonoids, ellagic acids, organic acids, and steroids, with anti-inflammatory, anti-hepatic fibrosis, anti-cell proliferation and other pharmacological activities (Li et al., 2016; Huang et al., 2018; Man et al., 2019). Moreover, they are excellent ornamental fruit plants (Zheng et al., 2016; Sun et al., 2017; Zhang et al., 2019). In autumn, after follicle cracking, the peel rolls back, revealing a bright red endocarp, which is butterfly wing-shaped. Black seeds are stuck on the endocarp, similar to red flowers dotted with black pearls, which are very beautiful. The difference between the two is that E. konishii is an evergreen tree species with a long fruit-bearing period of more than 7 months, and E. japonica is deciduous with only a 3-month fruit-bearing period (Sun et al., 2017; Fang, 1981). In addition, E. konishii is a unique Chinese tree species, naturally distributed in southern provinces of China, such as Guangxi, Guangdong, Jiangxi and Fujian (Li, 2019). E. japonica is distributed in southern China, the Korean Peninsula and Japan (Zhang et al., 2020). As an ornamental tree species, the ornamental value of E. konishii is obviously better than that of E. japonica, but the suitable growth range is smaller than that of E. japonica.
Temperature is one of the primary environmental factors limiting the distribution of Euscaphis plants by affecting germination, plant photosynthesis, transpiration, respiration, reproduction, and growth. E. konishii is not distributed in areas with an average temperature of less than 12.1 ℃ (Zhang et al., 2020), while the average temperature of wild resources of E. japonica is not less than 16 ℃ (Li, 2019). Low temperature is indispensable for their seed dormancy release (Zhang et al., 2015; 2016). Extremely low temperatures can lead to defoliation, reduced results, and even frostbite twigs. In addition, the average annual temperature was also closely related to the content of triterpenes in leaves, peels and seeds of E. konishii (Zou et al., 2020). However, there are few studies on the cold tolerance of E. konishii and E. japonica, and the response mechanism to cold stress is not clear. Therefore, in the present study, the changes in the metabolites of E. konishii and E. japonica under cold stress were comprehensively compared and analyzed by a non-targeted metabolomics (GC‒MS) technique to reveal the cold-tolerant metabolic mechanisms of these two species.