Integrated Transcriptomic and Metabolomic Analysis Reveals Possible Molecular Mechanisms of Leaf Growth and Development in Disanthus cercidifolius var. longipes

<b>Background:</b><i>Disanthus cercidifolius</i> var. <i>longipes</i> is an ancient relic plant unique to China. However, the typical shade-loving plant is largely exposed to the sun, which poses a major challenge to its conservation. <b>Methods:</b> This study explored dynamic changes in primary and secondary metabolites in <i>D. cercidifolius</i> leaves at different stages of development, combining metabolomics and transcriptome analysis to discuss the differentially accumulated metabolites (DAMs) and differentially expressed genes (DEGs). <b>Results:</b> The DAMs and DEGs were enriched in pathways related to photosynthesis, carbon (C) metabolism, anthocyanin synthesis, plant hormone signal transduction, and flavonoid synthesis. At the initial stage of leaf development, many primary metabolites were synthesized in the leaves. Before leaf maturity, many primary metabolites were converted into secondary metabolites. Combined transcriptome and metabolome analysis showed that the metabolites and genes related to anthocyanin synthesis and flavonoid metabolism were upregulated. In contrast, the genes related to C metabolism and C fixation were downregulated. After leaf maturity, photosynthetic capacity increased, total flavonoid content peaked (implying the strongest photoprotection capacity), and the transformation of anthocyanins and flavonoids was weakened. <b>Conclusions:</b> Light intensity indirectly affects the accumulation of the primary and secondary metabolism of <i>D. cercidifolius</i>. With the enhancement of photoprotection, the photosynthetic energy capacity decreases. It is, therefore, inferable that <i>D. cercidifolius</i> has shading properties and achieves a stable nutrient supply during growth and development through these strategies. Thus, <i>D. cercidifolius</i> protection requires a shaded environment.