Morphological, microstructural and lignin-related responses of herbaceous peony stem to shading

The shading treatment which decreases light intensity affects the ornamental characteristics in herbaceous peony (Paeonia lactiflora Pall.), including flower color, leaf shape and stem straightness. However, the morphological, microstructural and lignin-related responses of its stems to shading have not been systematically and deeply studied. Here, we focused on these responses to shading treatment in herbaceous peony stems. Morphological results showed that shading treatment significantly bended stems, decreased stem diameters, lightened stem weights and reduced stem strength. Microstructural observations demonstrated that shading treatment decreased vascular bundle number, xylem area, xylem ratio and secondary cell wall (SCW) thickness. Lignin-related analysis revealed that shading treatment markedly decreased lignin content in cell walls especially S-lignin and G-lignin, deposition in SCWs and related enzyme activities. Furthermore, transcriptomic analysis showed that shading treatment suppressed lignin-related processes, lignin biosynthetic pathway and lignin-related transcriptional activators. Collectively, shading treatment suppressed lignin biosynthesis and reduced lignin deposition in SCW, leading to undeveloped secondary xylem and bender stems. These findings provide a fundamental understanding of the morphological, microstructural and lignin-related responses of herbaceous peony stem to shading.