## 4 Discussion  
Due to the degradation of the stratospheric ozone, the amount of UV-B radiation received by earth and plant surfaces has increased [39]. In addition, in terms of growth, development, and morphology, UV-B strongly affects plants [40; 41; 42]. In light of this, the effect of UV-B stress on _R. chrysanthum_ deserves further study. In this experiment, domesticated _R. chrysanthum_ was found to be more resistant to UV-B radiation [18]. In this study, it was observed that the biomarker of resistance to potential UV-B stress in _R. chrysanthum_'s UV-B response was Glyceric acid, and the UV-B-responsive metabolites affecting the UV-B response of _R. chrysanthum_ were explored (Figures 5 and 6). Regarding the changes in the content of metabolites of both forms under UV-B stress, especially the dynamics of amino acids and carbohydrates, osmoregulation is a form of regulation when plants are under stress [43; 44]. Under unfavorable conditions, to reduce the cytosol's osmotic potential and prevent excessive water loss, solutes are actively accumulated in the cell. The accumulation of sugars plays a critical role in the adaptation of _R. chrysanthum_ to UV-B stress, both from a growth and tolerance perspective. Glucose can act as a signal during plant stress [45; 46; 47]. Two different pathways are involved in amylose metabolism and sucrose metabolism in _R. chrysanthum_[48; 49]. It is starch synthesis that takes place first in chloroplasts. As a result of the Calvin cycle, fructose 6-phosphate is converted into glucose 6-phosphate and starch follows. In the second pathway, the cytoplasm serves as a site for the synthesis and breakdown of sucrose [50]. The reduction in sugar transport and starch accumulation is likely to be related to the reduction in carbon flux for leaf sucrose synthesis [51]. The catabolism of starch can mitigate the effects of environmental factors on carbohydrates [52]. Therefore, _R. chrysanthum_ can respond to UV-B through starch accumulation and catabolism [52]. Starch is one of the main nutritional components of _R. chrysanthum_. The growth, development, and physiological state of chrysanthemum are greatly affected by the changes in starch metabolites, which affect the quality of _R. chrysanthum_. In abiotic stress situations, glycolysis increases ATP production, which helps plants adapt to their environment [53]. When in an environment is unfavorable to the plant itself, the plant keeps the carbohydrate biosynthetic pathway from being disturbed through the TCA cycle [54]. Thus, the acquisition of greater UV-B resistance in domesticated _R. chrysanthum_ may result from the activation of sucrose and starch metabolism. Amino acids are a vital metabolite in plants, capable of functioning as intermediates in the metabolic process [55; 56; 57]. In previous studies, plants treated with UV-B radiation showed a significant increase in the expression of many amino acids internally [27]. In the present experiment, it was also found that most of the amino acid contents increased due to UV-B radiation, indicating that _R. chrysanthum_ accumulated amino acids (Figure 4C). The experimental results show that most of the identified UV-B-responsive metabolites were amino acids (Table 1). In this study, prolonging the stress time significantly increased the Serine content in the experimental material. Serine is essential for photorespiration, and the accumulation of Serine suggests that _R. chrysanthum_ can enhance photorespiration under UV-B stress [58]. Succinic acid, as an important intermediate in the TCA cycle, showed higher accumulation under UV-B stress, indicating that UV-B stress promoted the TCA cycle. Moreover, the simplified metabolic model showed that plants may indirectly promote the expression of fumarate, an important intermediate of the TCA cycle, through the high expression of N-Acetyl-L-asparatic acid and L-Asparaginede (Figure 7D). Thus, the whole TCA cycle is promoted and the normal process of carbohydrate biosynthesis is guaranteed.