Cacao (Theobroma cacao) has great potential as a component of a small tropical farming system. It adapts to a wide range of soils, climatic conditions, grows well under minimum tillage, adapts to temporary intercropping, has the potential of being sold in local and export markets, and the pods are harvested year-round providing a ready source of cash income.
There is a growing demand for cacao products. The U.S. chocolate industry alone generated $19.5 billion in sales of chocolate products in 2012 (Lindell, 2012). However, it is estimated that diseases in cacao production cause losses of potential crop amounting to 43% in America, 20% in Africa, 13% in Oceania, and 9% in Asia (International Cocoa Organization, 2013; Willson, 1999). To satisfy future global demand for cacao products and reduce crop losses, research is needed to develop and/or identify superior cacao genotypes possessing disease tolerance and high yielding traits.
Most commercially grown cacao plantations are established from seed because this propagation method is easier and cheaper than others (Willson, 1999). However, cacao shows a high degree of segregation for many traits when propagated by seeds (Maximova et al., 2008). The production of controlled, pollinated seed using two or more parental clones may offer the opportunity to assemble into a single tree with useful traits from unrelated parents (Enriquez and Paredes, 1985; Enriquez and Soria, 1984). Others also consider the use of control-pollinated seed as the most useful means of increasing cacao production (Hunter, 1990). However, the data available to support the high yielding assumption attributed to control-pollinated seed are based only on the production obtained from a few unique segregating ‘F1’ trees. The percentage of high-yielding trees resulting from controlled-pollinated ‘F1’ progenies has been reported to be 8% to 14% (Batista, 1981; Esquivel and Soria, 1967). However, in a long-term experiment that included yield evaluation of five cacao interclonal families grown at three locations and harvesting periods lasting up to 8 years, the percentage of high-yielding trees was found to be only 3% (Irizarry and Rivera, 1998). Notably, this low percentage of high-yielding trees accounted for more than 60% of the total yield obtained from each family. Unfortunately, access to trained personnel and adequate germplasm is not always possible in cacao-producing regions to develop “hybrid seed.” Clonal propagation methods such as air layering, rooted cuttings, and grafting have been used for multiplication and preservation of superior cacao genotypes, but these methods are time-consuming, may create incompatibility issues between scion and rootstock, or successful rooting of cuttings may be highly variable among genotypes and locations. Additionally, there is a concern with the true yield potential of cacao selections propagated by grafting onto a common rootstock. The overall yield potential of 40 promising cacao clones grafted onto the rootstock ‘EET-400’ and planted in replicated plots was only 32% when compared with the yield obtained from ungrafted trees (Irizarry and Goenaga, 2000).
The use of in vitro propagation methods for cacao production could contribute to efforts to improve yield per area, germplasm conservation, and rapid distribution of high-yielding clones (Maximova et al., 2008). Somatic embryogenesis using floral explants has been the only method successfully developed for the de novo regeneration of cacao plants in vitro (Maximova et al., 2008).
Long-term field research is essential to demonstrate good yield performance of in vitro propagated cacao. Maximova et al. (2008) evaluated in St. Lucia nine cacao genotypes propagated in vitro by somatic embryogenesis for growth parameters for a period of 4.5 years. Their results indicated that somatic embryogenesis-derived plants showed normal phenotypes under field conditions and growth parameters similar to plants propagated by traditional methods such as rooted cuttings (Maximova et al., 2008). However, long-term yield data were not collected in that study.
The objective of this research was to evaluate yield performance of 12 cacao genotypes propagated by grafting and orthotropic rooted cuttings of somatic embryo-derived plants (SE-ORC) and grown under full sunlight and intensive management.
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