Litchi (Litchi chinensis Sonn.) originated in southern China and northern Vietnam but has now spread to countries with subtropical climates (Menzel and Waite, 2005). Litchi is not only an important fruit in China, India, Vietnam, and Thailand but is also cultivated in Australia, South Africa, the United States, and South America. A major factor affecting the production of this subtropical fruit in many countries is its low yield (Yu et al., 2015). The trees may fail to flower some seasons or fail to set and carry a reasonable crop (Menzel, 2001). Aizen and Harder (2007) considered pollen limitation as the combined effect of quantity and quality limitation, indicating that quantity limitation is less frequent than was inferred from pollen supplementation experiments. Moreover, their results suggested that an expanded broad perspective that recognized the fecundity consequences of pollination with poor-quality pollen would improve the ecological understanding of pollen limitation.
The litchi flower is a type of unisexual flower for which self-pollination is not common (Xiang et al., 2001). Litchi is a gynaecandrous and monoecious plant, yet gynaecandrous pollination or geitonogamy does not commonly occur because male and female flowers do not appear simultaneously. Moreover, litchi is a typical cross-pollinated plant; the productive cultivars (except very few varieties or individual plants with natural parthenocarpic ability) do not bear fruit unless they are pollinated and fertilized. However, the viability of pollen usually affects pollination, fertilization, and fruit set rate, so it is necessary to study the characteristics of litchi pollen for cultivation and breeding.
For entomophilous flowers, the amount of pollen grains deposited on the stigma depends on the number of pollinator visits received by a floret during a certain period (visit rate) and the proportion of viable pollen in the pollen loads carried by the pollinators (Dafni et al., 2005). Litchi is a typical out-pollinated plant, and fruit set can occur only if pollen from male flowers is transferred to the stigma of the female flowers (Stern and Gazit, 1996). Insufficient pollination is an important factor responsible for low yields in litchi. Litchi pollination requires large amounts of pollen grains during the flowering period. However, not much information is available about the relative levels of the quality and number of pollen grains among different litchi cultivars. Moreover, a better understanding of the relationship between the number and quality of pollen grains among cultivars may help in choosing cultivars for litchi orchards to improve the fruit set.
The study concerning the characteristics of litchi pollen has drawn increasing attention in recent years. Previous studies have indicated that there is a great difference in pollen viability among varieties (Ou et al., 2010). Moreover, the same variety may show distinctly different pollen viabilities in different years; flowering may be advanced or delayed significantly by weather at the development stage of anthers in the same year (Xiang et al., 1994), and flowering may also be affected by air temperature at the stage of germination (Xu et al., 1982). Litchi pollen germination rate and pollen amount have been reported in many studies (Jiang et al., 2012; Ou et al., 2010; Stern and Gazit 1998; Xiang et al., 2000); however, systematic studies on the change of pollen germination rate and pollen amount from variety to variety at different flowering stages are still lacking. In this study, the changes in the germination rate of pollen at different development stages were studied. In 2009 and 2010, the viability and number of pollen grains per anther were investigated in 65 litchi cultivars. More precise information is needed in several major areas, including the selection of a parent, the collection of pollen, and the determination of hybridization time in the crossbreeding of litchi. The objective of this study was to evaluate the number of pollen grains per anther and the pollen germination capacity of different cultivars of litchi trees.
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