Besides normal seed, litchi (Litchi chinensis Sonn.) fruit may contain shriveled (chicken-tongue) seeds with aborted embryos (Stern and Gazit, 2003; Yen et al., 1984). Fruit with shriveled seeds is a character favored by consumers for its large edible part (Menzel and Simpson, 1990; Stern and Gazit, 2003). The shriveled seeds are caused by stenospermocarpy resulting from embryo abortion after fertilization before fruit formation (Huang, 2001, 2005; Huang and Qiu, 1987).
‘73-S-20’ litchi, discovered in central Taiwan by Yen et al. (1984), is considered to have the best palatability among all other litchi commercial cultivars and has a high shriveled seed ratio (greater than 90%) (Menzel and Simpson, 1990; Yen et al., 1984). ‘73-S-20’ has become one of the main cultivars in Taiwan both for local and export markets since the 1990s (Chang, 2004; Chang et al., 2009, 2012). However, its yield is irregular, large variation in shriveled seed ratio (26.6% to 87.1% over 3 years) occurs, and small, seedless fruits have occasionally occurred in recent years (Fig. 1) (Chang, 2004, 2008), limiting the development of the litchi industry in Taiwan (Chang et al., 2009, 2012). On the other hand, normal- and shriveled seeded and small seedless fruit may result from double fertilization, stenospermocarpy, and parthenocarpy simultaneously within different fruits of a cluster, but the causative factors for the various seed in this cultivar are still poorly understood (Chang, 2004; Chang et al., 2009).
Menzel et al. (1986) and Huang (2005) reported that the shriveled seed ratio in litchi may change with year, cultivar, locality, and weather. The effects of pollen source on seed development have been studied in litchi but with inconsistent results (Stern and Gazit, 2003). Litchi has been regarded as a cross-pollinated crop with self-compatibility (Galán Saúco and Menini, 1989). Three distinct flowering waves can usually be found in each litchi inflorescence with each wave comprising flowers of the same type. The first wave consists of male (M1), the second of female (hermaphrodite function as female, F), and the third of male (hermaphrodite function as male, M2) flowers, which bloom sequentially with partially overlapping bloom periods (Galán Saúco and Menini, 1989; Liu, 1954; Menzel, 1984; Mustard et al., 1953; Stern and Gazit, 2003). The outcrossed fruits of ‘Floridian’ with pollen from ‘Mauritius’ showed higher yield, heavier fruit, and greater seed weight than selfed fruits, reflecting xenia and metaxenia (Degani et al., 1995; Stern et al., 1993). However, the responses of the fruit characteristics of ‘Bengal’ and the shriveled seed ratio of ‘Nuo Mi Chi’ and ‘Gui Wei’ from the outcrossed fruit were variable, indicating that pollen parent is important in fruiting and seed development but that exotic pollen is not always more efficient than self-pollen (McConchie et al., 1991; Stern and Gazit, 2003; Xiang et al., 2001); pollen parent effects may greatly differ among litchi cultivars.
In Taiwan, middle-maturing ‘Haak Yip’ and late-maturing‘73-S-20’ are the main cultivars cultured in the central region and are usually grown in the same orchards. Because of fluctuating climate, their bloom periods have been closer or even simultaneous in recent years (Chang, 2008; Chang et al., 2009). ‘Haak Yip’ has a high yield whose normal seed weighs ≈2.6 g (Yen et al., 1984). We accordingly hypothesized that the peak bloom of F flowers in ‘73-S-20’, with synchrony and crossing with the M2 flowers of ‘Haak Yip’, is one of the causes of erratic fruit set and the shriveled seed ratio of ‘73-S-20’ (Chang, 2004; Chang et al., 2009).
The goals of this study were to document whether pollen parents are required for fruiting and their effects on fruit set and seed development per cluster in field-grown ‘73-S-20’ trees. To identify the induction of parthenocarpic fruit, only F flowers were retained and bagged to prevent pollination before anthesis. To evaluate the pollen effects, selfing and outcrossing with ‘Haak Yip’ by hand pollination was conducted. To compare results obtained from crossing trials by hand, OP fruits derived from F flowers at peak bloom of ‘73-S-20’ synchronized with the second wave of M2 flowers in an adjacent ‘Haak Yip’ orchard were examined. To determine whether fruit set was influenced by shriveled seed/seed weight from outcrossing or selfing, the correlation between the percentage of fruit and shriveled seed/seed weight was calculated. The results may be useful for clarifying the fruiting mechanism and for developing a strategy for the optimal arrangement of different cultivars in ‘73-S-20’ orchards for fruit production.
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