Seed germination and initial seedling growth are two important phases in classical plant breeding programs based on crosses between existing cultivars such as in olive species (Olea europaea L.). Olive seed germination is slow, long-lasting (even up to three years from sowing), and staggered (Lagarda et al., 1983; Sotomayor-León and Caballero, 1990). Possible reasons include both the existence of dormancy and the influence of the parents, among other factors, and these must be taken into account in a breeding program.
Olive seeds show several types of dormancy with mechanical and embryo dormancy being the most important (Alvarado, 1994). Mechanical dormancy is related to the presence of the woody endocarp, which prevents the expansion of the embryo (Crisosto and Sutter, 1985; Hartmann et al., 2010). Even after removal of the endocarp, olive seed germination under suitable conditions is slow and erratic, indicating the existence of endogenous dormancy within the embryo (Voyiatzis, 1995).
Temperature seems to be the most important factor to release embryo dormancy in the olive as in other species (García-Gusano et al., 2004; Hartmann et al., 2010; Sharma and Singh, 1978). It affects the total percentage of germination as well as the speed of the process. Different authors proposed cold stratification in wet conditions to eliminate this type of dormancy (Crisosto and Sutter, 1985; Istanbouli, 1981; Lagarda et al., 1983). Alvarado (1994) and Santos-Antunes (1999) suggested stratification at 14 °C, always in darkness and during a variable time period, depending on the cultivar, although the time should not typically exceed 6 weeks. The standard protocol used in olive breeding programs in Spain reduces the time of seed stratification to 30 d (El Ryachi, 2007; Rallo et al., 2008). Nevertheless, El Ryachi (2007) did not find any differences when the time was reduced to 20 d. Later, Adakalic et al. (2008) indicated that cold stratification could be reduced from 30 to 18 d followed by a change in temperature to 25 °C for 12 d, which improves the growth of the radicle. Earlier studies by Voyiatzis and Porlingis (1987) also suggested that a change in the temperature during the stratification time improves olive seed germination. These data suggest the possibility to further reduce the time of stratification and improve the protocol in olive breeding programs, although adjusted according to the parent cultivars.
Limited references are concerned with the influence of the parents on olive seed traits, seed germination, and seedling emergence and growth. Cuevas and Oller (2002) and Farinelli et al. (2008) found that both the female parent and the pollinator had some influence on the number of fruits with double and empty seeds. Bini and Benilli (1975) and Santos-Antunes (1999) observed that the germination ability depends first on the female parent and to a lesser extent on the pollinator. The importance of the female parent was underlined later by Adakalic (2003). Santos-Antunes et al. (2005) observed that the length of the juvenile period of the seedlings was also influenced by the female parent. Furthermore, a seedling’s juvenile period length was correlated with the unproductive period of adult trees. Recently, Hammami et al. (2011) have shown the influence of the parents on the vigor and the growth habit of seedlings from different progenies obtained in an olive breeding program.
The aim of this work is to study the effect of different stratification treatments and table olive cultivars used as female parents on seed traits, seed germination, and early seedling growth. The results may not only help to advance knowledge of the olive species but also be of great interest in answering important questions arising in olive breeding programs such as the choice of parents, how to improve seed germination procedures, and seedling initial vigor, which is related to the length of the juvenile period. This is particularly interesting for table olive for which, although new cultivars adapted to new cropping systems demands (mechanized harvesting, irrigation expansion, susceptibility to pests and diseases, increasing demand for quality of consumer and industry, etc.) are necessary, there still is almost no information of the performance of present cultivars in breeding programs.
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