Supplementary UV-B Radiation Effects on Photosynthetic Characteristics and Important Secondary Metabolites in Eucommia ulmoides Leaves

To explore the effects of ultraviolet light supplementation on the photosynthetic characteristics and content of secondary metabolites in the leaves of Eucommia ulmoides Oliver (E. ulmoides), the effects of supplementary UV-B (sUV-B) radiation on the medicinally active components of E. ulmoides were comprehensively evaluated. In our study, we selected leaves of five-year-old E. ulmoides seedlings as experimental materials and studied the effect of supplemental ultraviolet-B (sUV-B) radiation on growth, photosynthetic parameters, photosynthetic pigments, fluorescence parameters, and secondary metabolites of E. ulmoides using multivariate analysis. The results showed that the leaf area and the number of branches increased after sUV-B radiation, which indicated that sUV-B radiation was beneficial to the growth of E. ulmoides. The contents of chlorophyll a and chlorophyll b increased by 2.25% and 4.25%, respectively; the net photosynthetic rate increased by 5.17%; the transpiration rate decreased by 35.32%; the actual photosynthetic efficiency increased by 10.64%; the content of the secondary metabolite genipin increased by 12.9%; and the content of chlorogenic acid increased by 75.03%. To identify the genes that may be related to the effects of sUV-B radiation on the growth and development of E. ulmoides leaves and important secondary metabolites, six cDNA libraries were prepared from natural sunlight radiation and sUV-B radiation in E. ulmoides leaves. Comparative analysis of both transcriptome databases revealed a total of 3698 differential expression genes (DEGs), including 1826 up-regulated and 1872 down-regulated genes. According to the KOG database, the up-regulated unigenes were mainly involved in signal transduction mechanisms [T] and cell wall/membrane biogenesis [M]. It is also involved in plant hormone signal transduction and phenylpropanoid biosynthesis metabolic pathways by the KEGG pathway, which might further affect the physiological indices and the content of chlorogenic acid, a secondary metabolite of E. ulmoides. Furthermore, 10 candidate unigenes were randomly selected to examine gene expression using qRT-PCR, and the six libraries exhibited differential expression and were identical to those obtained by sequencing. Thus, the data in this study were helpful in clarifying the reasons for leaf growth after sUV-B radiation. And it was beneficial to improve the active components and utilization rate of E. ulmoides after sUV-B radiation.

Automated summary

This study comprehensively evaluated the effects of supplementary UV-B (sUV-B) radiation on the photosynthetic characteristics and content of secondary metabolites in Eucommia ulmoides Oliver leaves. The results showed that sUV-B radiation promoted leaf growth and increased the content of chlorophylls and secondary metabolites. Transcriptome analysis revealed differential expression of genes related to signal transduction and metabolic pathways, which might contribute to the physiological changes and increased content of chlorogenic acid in E. ulmoides. This study provides valuable insights into the effects of sUV-B radiation on leaf growth and secondary metabolites in E. ulmoides.