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.
Aou-ouad, H.E., Medrano, H., Lamarti, A. & Gulías, J. 2014 Seed germination at different temperatures and seedling emergence at different depths of Rhamnus spp Cent. Eur. J. Biol. 9 569 578
Bewley, J.D. & Black, M. 1982 Physiology and biochemistry of seeds in relation to germination. Springer-Verlag, Berlin
Bhanuprakash, K., Yogeesha, H.S., Vasugi, C., Arun, M.N. & Naik, L.B. 2008 Effect of pre-soaking treatments and temperature on seed germination of guava (Psidium guajava L.) Seed Sci. Technol. 36 792 794
Chen, H.B., Zhuang, L.J., Huang, X.M. & Su, Z.X. 2013 Present situation and development prospect for litchi and longan industry China Tropical Agriculture 2 12 18 (In Chinese)
de Andrade, R.A., Martins, A.B.G. & Oliveira, I.V.M. 2004 Influence of the substrate in germination of lychee seeds Rev. Bras. Frutic. 26 375 376
de Macedo, M.C., Scalon, S.P.Q., Sari, A.P., Scalon Filho, H., Rosa, Y.B.C.J. & Robaina, A.D. 2009 Biometry of fruit and seeds and germination of Magonia pubescens ST.Hil (Sapindaceae) Rev. Bras. Sementes 31 202 211
Eberle, C.A., Forcella, F., Gesch, R., Peterson, D. & Eklund, J. 2014 Seed germination of calendula in response to temperature Ind. Crops Prod. 52 199 204
Elfeel, A.A. 2012 Effect of seed pre-treatment and sowing orientation on germination of Balanites aegyptiaca (L.) Del. seeds Am-Euras. J. Agr. and Environ. Sci. 12 897 900
Fu, D.W., Wang, L.M., Ou, L.X., Chen, J.Z., Cai, C.H. & Zhang, C.Y. 2014a Effect of high temperature on seed vigor of Litchi Guangdong Agr. Sci. 5 89 91 (In Chinese)
Fu, D.W., Wu, J.F., Wang, L.M., Chen, J.Z., Cai, C.H., Zhang, C.Y. & Ou, L.X. 2014b Effect of high-temperature germination time on litchi seed vigor Guangdong Agr. Sci. 16 33 36 (In Chinese)
Harb, A.M. 2013 Reserve mobilization, total sugars and proteins in germinating seeds of durum wheat (Triticum durum Desf.) under water deficit after short period of imbibition Jordan J. Biol. Sci. 6 67 72
Hartmann, H.T. & Kester, D.E. 1979 Plant propagation: Principles and practices. 4th ed. Prentice Hall, New Delhi
Prasad, J.S., Kumar, R., Mishra, M., Kumar, R., Singh, A.K. & Prasad, U.S. 1996 Characteristics of litchi seed germination HorScience 31 1187 1189
Schmidt, L.H. 2000 Guide to handling of tropical and subtropical forest seed. Danida For. Seed Ctr., Humlebaek, DK
Thomas, K.M. 1973 Influence of certain physical and chemical treatments on the germination and subsequent growth of coconut Cocos nucifera L. seedlings. a preliminary study Ceylon Cocon. Q. 24 85 90
Thomas, K.M. 1978 Influence of seed size and planting orientation on the germination and growth of coconut seedlings in the nursery Indian J. Agr. Sci. 48 63 67
Zhao, C.X., Wang, X.Y., Li, H.B. & He, F. 2005 Preliminary study on germination characteristics of Hylocereus undatus seed Seed 24 37 40 (In Chinese)