Herbaceous peony (Paeonia lactiflora Pall., Paeoniaceae) is a popular ornamental plant that grows well in temperate regions, blooming from May to July in the Northern hemisphere (Zhao et al., 2012). P. lactiflora has a long-standing reputation as the “garden queen” in many Western countries, whereas in China it is a symbol of prosperity and wealth, and has enjoyed a good reputation as a famous traditional flower for more than 3000 years (Kamenetsky and Dole, 2012; Qin, 2004). P. lactiflora has a large germplasm, with more than 600 cultivars worldwide whose flower-related characteristics vary widely among cultivars (Zhao et al., 2014). Most varieties are highly valued commodities in the horticultural industry.
Flower bud development is a highly complex process that is characterized by two distinct physiological phases: floral initiation and floral bud development (Fan et al., 2015; Koutinas et al., 2010). The formation of peony flower buds and their related developmental events are controlled by multiple genetic factors (Ge et al., 2014), and these are influenced by seasonal changes in daylength and temperature (Zhang et al., 2015). The differentiation of floral buds during autumn until the following spring must go through a period of bud dormancy in cold winter, indicating that plants cease meristem activity and are insensitive to growth-promoting signals for some time before they can resume growth (Rohde and Bhalerao, 2007). Temperature is strictly correlated with flowering in the spring, as well as the geographic distribution of plants (Hall et al., 2007; Walton et al., 2007). The floral transition of plants and other developmental processes depend on changes in photoperiod and low temperature (Park et al., 2015; Zhao et al., 2012). Prolonged chilling before forcing at warm temperatures promotes the release of dormancy, but it is commonly accepted that low temperature is sensed by the shoot apical meristem only after the bud reaches a certain stage of development (Barzilay et al., 2002). Thus, the synchronization of environmental conditions and bud differentiation is critical for the flowering time of plants, and is of great importance to sustainable cut flower or potted plant production (Koutinas et al., 2010).
Over the past few decades, great attention has been paid to the study of peony flower types, which can be classified into six types, including single, Japanese, anemone, semidouble, bomb, and double types, in Western countries (Kamenetsky and Dole, 2012). The process of flower bud differentiation of some of these types of flowers has also been studied. Flower bud differentiation of single-type P. lactiflora ‘Fenyunu’ was investigated by stereomicroscopy (Wang and Zhang, 1991) and also under an optical microscope using paraffin sections (He et al., 2014). Anemone (P. lactiflora ‘Yulinghong’) and semidouble (P. lactiflora ‘Zijinlong’) types were observed by cryosections under an optical microscope (He et al., 2007). Ai (2016) investigated the differentiation of anemone-type P. lactiflora ‘Liantai’ flowers by using paraffin sections under an optical microscope. These studies showed that flower bud differentiation was basically similar among different cultivars, and can be divided into several successive stages of differentiation: formation of leaf, bract, sepal, petal, stamen, and pistil primordia, whereas the flowers are organized into concentric whorls of bracts, sepals, petals, stamens and carpels (He et al., 2007; Huang et al., 2009; Wang and Zhang, 1991). Compared with the single type, other flower types often arise when some or all of the stamens in a flower develop various types of abnormal petals after bud dormancy release in early spring (Ai, 2016; He et al., 2014).
In China, proliferated peonies are often called “Taige” by horticulturists, indicating that a flower with many more than the normal number of petals often contains a flower within a flower, which can be categorized as double-flowered in the Western classification system (Ai, 2016; Lian et al., 2004; Meyerowitz et al., 1989; Qin and Li, 1990). This flower shape is common in many ornamental plants, and a large number of double-flowered varieties are found in Camellia japonica (Zhang, 2006), Nelumbo nucifera (Jiang et al., 2009), Amygdalus persica (Wang and Zhang, 2010), Osmanthus fragrans (Zhu et al., 2012), and other plants. The double type of peony has the most popular varieties in the commercial cut flower market as they tend to have a longer postharvest shelf life (Cheng et al., 2009; Lian et al., 2004; Meyerowitz et al., 1989). However, there are fewer studies on flower bud differentiation of double-flowered peony than of other kinds of flower types. Barzilay et al. (2002) observed flower bud development of double-flowered P. lactiflora ‘Sarah Bernhardt’ by scanning electron microscopy, but focused mainly on the life cycle and morphogenesis of the floral shoot and failed to fully understand the complete process of bud differentiation, which is crucial for understanding the cause of flower type formation, and also the basis for molecular genetic research. Moreover, the description and classification of double-flowered or other types are based primarily on morphological structures of flowers during the blooming period, but do not include bud differentiation. Until recently, such studies in herbaceous peony did not receive much attention from scientists because of the complicated structure of flower buds of double-flowered types and the lack of rapid and accurate methods to assess flower bud differentiation.
The objectives of this research were to study flower bud differentiation and the developmental stages of a double-flowered herbaceous peony cultivar (Paeonia lactiflora ‘Dafugui’, a traditional and popular variety with a long history of cultivation and applications, including use in city landscaping in northern China), and to find effective methods to assess P. lactiflora flower bud differentiation.
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