Flower color is an important characteristic of ornamental plants that results from substances in the petal cells, such as flavonoids, carotenoids, and alkaloids; and various intracellular environmental factors such as the pH value and metal ion content (Lapidot et al., 1999; Yoshida et al., 2009). There are three major groups of plant pigments—flavonoids, carotenoids, and alkaloids—and flavonoids are divided further into anthocyanins, flavonoids, and flavonols (Zhao et al., 2005). Studies on yellow camellia (section Chrysantha Chang), medicinal chrysanthemum (Chrysanthemum morifolium Ramat), beach rose (Rosa rugosa Thunb.), and tree peony (Paeonia suffruticosa Andr.) have shown that the content and components of anthocyanin are closely related to flower color (Jiang et al., 2019; Ma et al., 2016; Zhang et al., 2015b, 2018). The intermolecular cocoloration of anthocyanin and accessory pigments can also affect flower color; the chemical bonding energy between these factors can improve the stability of anthocyanin for changing flower color (Zhuang et al., 2018). Sugars are the main energy source and precursors for anthocyanin synthesis, which regulates the anthocyanin content (Zhang et al., 2015a), and proteins are the products of functional gene expression that control changes in the sugar content (He et al., 2010).
Herbaceous peony (Paeonia lactiflora) is a perennial herbaceous plant that exhibits varied flower types and colors. In Tai’an, China (lat. 117°06′N, long. 36°11′E), it blooms from late April to mid May. Studies on herbaceous peony flower color have focused mainly on the classification and identification of anthocyanin, and analyses of the anthocyanin synthesis pathway and anthocyanin stability (Jia et al., 2008; Zhong et al., 2012). The petal structure and cell contents of herbaceous peonies have never been investigated thoroughly.
In this study, we selected wild plants and eight representative cultivars of herbaceous peony with different colors. We documented the anatomic structure of the petals and determined the cell sap pH value, water content, Ct, soluble sugar and soluble protein contents, and pigment content and composition. We also analyzed the indexes influencing flower color formation to identify the key physiological factors underlying different colors and to elucidate the molecular mechanisms of herbaceous peony flower color, which will aid future breeding efforts of new cultivars.
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