An Analysis of the Mechanism About CO 2 Enrichment Promoting Carbohydrate Metabolism in Cucumber ( Cucumis sativus L.) Leaves.

Elevated CO ₂ can affect the synthesis and distribution of photosynthetic assimilates. However, the carbohydrate metabolism molecular mechanism of cucumber leaves in response to CO ₂ enrichment is unclear. Therefore, it is of great significance to investigate the key functional regulatory genes in cucumber. In this study, the growth of cucumber leaves under different CO ₂ conditions was compared. The results showed that under CO ₂ enrichment, leaf area increased, the number of mesophyll cells increased, stomata enlarged, and more starch grains accumulated in the chloroplasts. Compared with the control, the starch and soluble sugar content of leaves were maximally increased by 194.1% and 55.94%, respectively; the activities of fructose-1,6-bisphosphatase (FBPase), ADPG pyrophosphorylase (AGPase), starch synthase (SSS), sucrose phosphate synthase (SPS), sucrose synthase (SS) and invertase (Inv) were maximally increased by 36.91%, 66.13%, 33.18%, 21.7%, 54.11%, and 46.01%, respectively. Through transcriptome analysis, a total of 1,582 differential expressed genes (DEGs) were identified, in which the starch and sucrose metabolism pathway was significantly enriched, and 23 genes of carbon metabolism were screened. Through metabolome analysis, a total of 22 differential accumulation metabolites (DAMs) were identified. Moreover, D-glucose and D(+)-glucose were significantly accumulated, showing upregulation 2.4-fold and 2.6-fold, respectively. Through combined analysis of transcriptome and metabolome, it was revealed that seven genes were highly related to D-glucose, and Csa6G153460 (AGPase), Csa5G612840 (β-glucosidase), and Csa4G420150 (4-α-glucanotransferase) were significantly correlated to the carbohydrate regulatory network. Furthermore, the mechanism of CO ₂ enrichment that promotes carbohydrate metabolism in leaves at the molecular level was revealed. This mechanism advances the development of the cell wall and leaf morphology by activating the expression of key genes and improving enzyme activity.