Herbaceous peony (Paeonia lactiflora Pall.) is an important ornamental plant used in urban green spaces,
but little is known about whether it can grow in a shaded environment or understory. In this study,
effects of shade on plant growth and flower quality in the herbaceous peony were investigated. The
results showed that P. lactiflora morphology parameters, including plant height, leaf number, stem
diameter, branch number, node number and plant crown width, were higher in plants grown with sun
exposure compared to those grown in shade; however, opposite trends were observed for the top and
middle leaf areas of the plant. Compared with sun exposure, shade decreased P. lactiflora photosynthetic
capacity, light saturation point (LSP) and light compensation point (LCP) and increased the apparent
quantum yield (AQY), mainly due to declined stomatal conduction (Gs). These decreases caused the
soluble sugar, soluble protein and malondialdehyde (MDA) contents to decline, which led to delayed
initial flowering date, prolonged flowering time, reduced flower fresh weight, increased flower diameter
and faded flower color. Through cloning and expression analysis of anthocyanin biosynthetic genes, we
determined that the fading of flower color was the result of reduced anthocyanin content, which was
caused by the combined activity of anthocyanin biosynthesis genes and, in particular, of the upstream
phenylalanine ammonialyase gene (PlPAL) and chalcone synthase gene (PlCHS). These results could
provide us with a theoretical basis for further application of P. lactiflora in the greening of urban spaces
and an understanding of the mechanisms behind the changes induced by shade.