Litchi (Litchi chinensis Sonn.) is an important fruit cultivated in the subtropical regions. ‘Yu Her Pau’, the main commercial cultivar of litchi worldwide, is well known for its shriveled seed and high-quality taste, although it has an uneven fruit set from year to year (Chang and Lin, 2003, 2006; Chang et al., 2009; Jiang et al., 2012). Poor pollination and fertilization as well as undesirable photo-assimilate supply–demand between the sink and source organs result in high physiological fruit drop in litchi (Menzel, 1984; Mustard, 1960; Roe et al., 1997; Saúco and Menini, 1989; Yuan and Huang, 1988).
In response to recent climate change, particularly global warming, the increasing proportion of leafy inflorescences in ‘Yu Her Pau’ has occurred frequently, and the manual removal of leaves from leafy inflorescences has become a standard commercial practice, although it increases the costs of orchard management, while its benefits still need confirmation (Chang et al., 2009). Therefore, management of the leafy inflorescence has become the new issue of the litchi industry (Chang, 2017).
Vegetative and flowering shoots are homologous organs (Li, 2008). The litchi shoot apical meristem initiates inflorescence primordia, comprising leaf and floral primordia as a result of cool-temperature induction (Batten and McConchie, 1995; Li, 2008). The flowering shoot of litchi has a characteristic of flush cycles, and the process of inflorescence initiation and flower development in a flowering shoot is sensitive to temperature (Stern and Gazit, 2003), which may form two types of flowering shoots subsequently in terms of leafless (generative) and leafy inflorescences. The latter, by their leaves and inflorescence relative position, can be sorted into leafy and vegetative/generative transit shoots (Davenport and Stern, 2005). Rudimentary leaves within the inflorescence primordium shrivel or abscise after sensing adequate cool temperature, resulting in generation of leafless inflorescences. In contrast, rising temperatures during the initiation period of inflorescence primordium development will produce a leafy inflorescence with both rudimentary leaves and lateral inflorescences at the same node along the main inflorescence axis. In some instances, a vegetative/generative transit shoot will be produced if low temperatures followed by warm temperatures occur at the later stages of the inflorescence development (Chang, 1999; Chen, 1994; Chen et al., 2009; Lin, 1987; McConchie and Batten, 1991; Olesen et al., 2002; Zhou et al., 2008). The leafy flowering shoot has been the main leafy inflorescence type in Taiwan (Chang, 2017; Chang et al., 2009).
The leafy inflorescence in citrus has been well recognized to produce better fruit set and quality at harvest than the leafless inflorescence (Goldschmidt, 2013; Hansen, 1969; Hass, 1949; Moss, 1970); however, little is known about the subsequent performance of leafy inflorescence in terms of fruit set in litchi. It is generally assumed that the fruit set on leafless inflorescences is better than on leafy inflorescences (Chang et al., 2009; Davenport and Stern, 2005; Lee, 2008). Leaves and lateral inflorescences synchronize their development within leafy inflorescence (Huang, 2005; Lee, 2014). These young leaves have no net carbon assimilation ability at the initial growth stage of the inflorescence (Wang, 2014). In addition, the red and soft new leaves of the leafy inflorescence inhibit the differentiation of lateral inflorescences (Chen, 1990, 1994; Davenport, 2000), resulting in a decrease in the total number of flowers and female flowers, subsequently reducing fruit yield.
Few studies have been carefully employed on the relationship between fruit set and inflorescence types in litchi. Litchi has three types of flower: male flower (M1), hermaphrodite flower functioning as female (female flower, F), and hermaphrodite flower functioning as male (M2) (Chu et al., 2015; Wu et al., 2017). Kumar (2013) preliminarily indicated that neither female flower percentage nor fruit set rate was influenced by the appearance of leafy inflorescence in ‘Shahi’ litchi; however, in comparison with the leafless inflorescence, the 40% to 60% decrease in the total number of flowers and female flowers resulted in an ≈50% decrease in yield of the leafy inflorescence. In contrast, Chen et al. (2014) reported that the percentage of leafy inflorescence and number of fruit set of ‘Yu Her Pau’ litchi increased when 100 mg·L−1 GA3 was sprayed onto the foliage during quiescence period; however, the percentage of female flowers, cluster yield, fruit set rate, and the residual effects of GA3 on the leafy inflorescences and fruit were not documented in this study. Whether leafy inflorescences impose a major burden or reduction of the subsequent fruit set and fruit development normally in litchi was inconsistent from their results.
The purpose of this study was to determine whether the different inflorescence types affect flowering and fruiting performance in terms of flower and fruit number, yield, and quality at harvest in field-grown ‘Yu Her Pau’ litchi plants. The diameter and number of leaves of flowering shoots of two inflorescence types were measured to demonstrate whether the carbon assimilation supply potential was consistent. Next, the inflorescence quality was assessed in terms of the total number of flowers and female flowers and percentage female flowers within each inflorescence. Finally, to understand the influence of the leafy inflorescences on fruit production, the fruit number, fruit set rate, cluster yield, and fruit quality at harvest were also calculated. The results from our research may prompt changes in the management strategy of litchi orchards with regard to flowering and fruit production and thus provide information on defoliation of leafy inflorescences in the future.
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