Herbaceous peony (Paeonia lactiflora Pall.) is an important ornamental plant which contains different flower colors. In this paper, eight genes encoding phenylalanine ammonialyase (PAL), chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), UDP-glucose: flavonoid 3-o-glucosyltransferase (UF3GT) were isolated. Moreover, the expression patterns of these eight genes and UF5GT in the flowers were investigated in three cultivars, that is, 'Hongyanzhenghui', 'Yulouhongxing' and 'Huangjinlun' with purplish-red, white and yellow flower respectively. Furthermore, flavonoid accumulation in the flowers was also analyzed. The results showed that in different organs, most of genes expressed higher in flowers than in other organs. During the development of flowers, all genes could be divided into four groups. The first group (PlPAL) was highly expressed in S1 and S4. The second group (PlCHS and PlCHI) was at a high expression level throughout the whole developmental stages. The third group (PlF3H, PlF3'H, PlDFR, PlANS and PlUF5GT) gradually decreased with the development of flowers. The fourth group (PlUF3GT) gradually increased during the flower development. In addition, anthoxanthins and anthocyanins were detected in 'Hongyanzhenghui' and 'Yulouhongxing', chalcones and anthoxanthins were found in 'Huangjinlun'. When different color flowers were concerned, low expression level of PlCHI induced most of the substrate accumulation in the form of chalcones and displaying yellow, changing a small part of substrates to anthoxanthins, and there was no anthocyanin synthesis in 'Huangjinlun' because of low expression level of DFR. In 'Yulouhongxing', massive expressions of upstream genes and low expression of DFR caused synthesis of a great deal of anthoxanthins and a small amount of colorless anthocyanins. In 'Hongyanzhenghui', a large number of colored anthocyanins were changed from anthoxanthins because of PlDFR, PlANS and PlUF3GT high expressions. These results would provide us a theoretical basis to understand the formation of P. lactiflora flower colors.

Calcium is an essential element and imparts significant structural rigidity to the plant cell walls, which provide the main mechanical support to the entire plant. In order to increase the mechanical strength of the inflorescence stems of herbaceous peony, the stems are treated with calcium chloride. The results shows that preharvest sprays with 4% (w/v) calcium chloride three times after bud emergence are the best at strengthening “Da Fugui” peonies’ stems. Calcium sprays increased the concentrations of endogenous calcium, total pectin content as well as cell wall fractions in herbaceous peonies stems, and significantly increased the contents of them in the top segment. Correlation analysis showed that the breaking force of the top segment of peonies’ stems was positively correlated with the ratio of water insoluble pectin to water soluble pectin (R = 0.673) as well as lignin contents (R = 0.926) after calcium applications.

Herbaceous peony (Paeonia lactiflora Pall.) is an important ornamental plant that has different flower types. However, the molecular mechanism underlying its floral organ development has not been fully investigated. This study isolated six floral organ development-related genes in P. lactiflora, namely, APETALA1 (PlAP1), APETALA2 (PlAP2), APETALA3-1 (PlAP3-1), APETALA3-2 (PlAP3-2), PISTILLATA (PlPI) and SEPALLATA3 (PlSEP3). The expression patterns of these genes were also investigated in the three cultivars 'Hangshao', 'Xiangyangqihua' and 'Dafugui'. Furthermore, gene expression during floral development was also analyzed in different organs. The results showed that PlAP1 was mainly expressed in the sepals, and PlAP2 was mainly expressed in the carpels and sepals. PlAP3-2 and PlPI had the highest expression levels in the stamens, followed by the petals. The expression levels of PlAP3-1 (from highest to lowest) were in the following order: petals, stamens, carpels and sepals. PlSEP3 was mainly expressed in sepals and carpels. With the depth of stamen petaloidy, the expression levels of PlAP1, PlAP2 and PlSEP3 increased, whereas those of PlAP3-1, PlAP3-2 and PlPI decreased, which showed that PlAP1 mainly determined sepals and petals of P. lactiflora. The PlAP2 not only determined the sepals and petals, and it participated in carpel formation. PlAP3-1, PlAP3-2 and PlPI mainly determined stamens and petals. PlSEP3 determined the identities of sepals and petals. This study would help determine the molecular mechanism underlying floral organ development in P. lactiflora.