Unveiling cellular-level mechanisms of shading-modulated stem mechanical strength in Paeonia lactiflora: Insights into fibre cell morphology and metabolism

Insufficient light intensity diminishes stem mechanical strength of Paeonia lactiflora , adversely affecting the quality of cut flowers. However, the underlying mechanisms remain poorly understood. Through anatomical analysis and matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) combined with LC-MS, the shading-induced changes of morphology and metabolism in fibre cells of P. lactiflora stems were comprehensively characterized for the first time. The results showed that shading inhibited secondary cell wall (SCW) thickening in fibre cells, leading to thinner SCWs and low stem mechanical strength. A total of 931 metabolites were detected in the full-sun and shade groups of P. lactiflora stem sections by MALDI MSI, and 25 significant differential metabolites were identified in the stem fibre cell zones by MS/MS fragmentations. The shading-induced differential metabolites in the fibre cell zones were primarily concentrated on various secondary metabolites with antioxidant defence properties, with their synthesis triggered by shade stress. It indicated that shading reduced P. lactiflora stem mechanical strength by inhibiting fibre SCW thickening, and this inhibition was closely correlated with an enhanced synthesis of defence metabolites in the fibre cells. This study lays a foundation for elucidating the cytological mechanism underlying shading that affects stem mechanical strength.