Litchi (Litchi chinensis Sonn., Sapindaceae) is an important subtropical fruit tree that originated in China. Due to the delicate flavor, pleasant appearance, and fragrance, it has been known as “one of the top four fruits” along with banana, pineapple and longan in Southern China. In 2010, the cultivated area of litchi in China was about 0.57 million hectares with a total production of 1.7 million tons. The litchi industry has been developed in other countries, including Madagascar, India, South Africa, Thailand, America, Israel, Mauritius, and Mexico (Chen et al., 2013). Litchi is generally propagated by seeds, stem cuttings, grafting, marcottage, and stooling. However, new litchi cultivars with improved and desired characteristics can only be selected from seedlings (Menzel, 1985). The germination performance of seeds and the seed placement in sowing are important to achieve seedlings (Aou-ouad et al., 2014; de Andrade et al., 2004).
Litchi seeds are considered to be “recalcitrant,” due to their high sensitivity to desiccation (Roberts, 1973). They quickly lose their viability if allowed to dry out after extraction from the fruits. Litchi seeds completely lost viability when they were kept outdoors for 2 d or indoors for 6 d (Ray and Sharma, 1987). Xia et al. (1990) found that litchi seeds germinated fully when harvested at 10 d before fruit maturity or at fruit ripening time, but entirely lost seed viability after 6 d of natural drying. Ray and Sharma (1987) pointed out that seed germinability was positively correlated with seed moisture content above the critical limit of 20% moisture content. Therefore, the seed moisture content must be retained to ensure high seed germination rates.
Seed imbibition is an important period for successful germination (Woodstock, 1988). Sufficient water is necessary for rehydrating enzymes and their substrates in preparation for seed germination (Bewley and Black, 1982). Especially, the first phase of imbibition characterized by the rapid water uptake plays a crucial role in seed germination success (Harb, 2013). This suggests that if the seeds imbibe water slowly from the soil, or cannot uptake sufficient water for normal germination, early death of seeds is likely to occur. Soaking before sowing enables a more rapid imbibition than is usually the case in a nursery bed, resulting in more rapid seed germination (Schmidt, 2000). Though litchi seeds are vulnerable to drying in field, soaking has not yet been applied in practice.
Temperature also affects seed germination. Too high or too low temperatures could decrease seed viability (Corbineau et al., 1986; Eberle et al., 2014). Litchi fruits are mainly harvested in June and July in China when the soil temperature reaches ≈40 °C. Seeds of 2-7-6 litchi cultivar incubated at 40 °C for more than 2 d exhibited an inhibition in the growth of radicle and plumule. Long-term exposure to 40 °C for more than 4 d reduced germination percentage of seeds, and there was a reduction in germination rate with an increase in exposure time (Fu et al., 2014b). Fu et al. (2014a) pointed out that the upper limit for seed germination of ‘Qiyueshu’ litchi was 42 °C. Exposure to temperatures exceeding 42 °C for 24 h caused an inhibition in seed germination and seedling emergence. Therefore, temperature may be an important influence factor for germination of litchi seeds.
Seed orientation and sowing depth both play important roles in seed germination and seedling emergence (Aou-ouad et al., 2014; Huang et al., 2007; Thomas, 1978). Recalcitrant seeds of coconut sown in horizontal orientation exhibited much quicker germination and better growth of seedlings (Thomas, 1978). Planting seeds of Balanites aegyptiaca horizontally or vertically with stalk end downward showed better germination (Elfeel, 2012). The seedling emergence was slower when seeds were sown at deeper depth in Rhamnus alaternus and R. ludovici-salvatoris (Aou-ouad et al., 2014). Though litchi seeds are all direct-seeded after extraction from the fruits, little research about the seed orientation and sowing depth has been reported in litchi.
Litchi seeds were seldom fully germinated when sown in a humid sand bed under shade or in carbonized rice hull (de Andrade et al., 2004; Prasad et al., 1996). We also observed that the traditional method of planting litchi seeds without any pretreatment resulted in inferior seedling uniformity and less efficiency in the production methods. Therefore, studies are needed to improve litchi seed germination methods. In the present study, we added soaking and presprout steps during seed germination before sowing litchi seeds. Specifically, we investigated the relationship between soaking and seed germination, and determined the appropriate soaking time for seed germination in litchi. Furthermore, we examined the effects of short-term high temperatures on germination of litchi seeds. Finally, the influence of seed orientation and burial depth on litchi seedling recruitment was studied. The objectives of these experiments were to find fast and efficient ways to increase the percentage of seed germination and seedling emergence, which are important for field production.
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