Crossability studies and zygotic embryo culture in cassava (manihot esculenta crantz)

Cassava (Manihot esculenta Crantz) germplasm in Ghana is mostly uncharacterized and includes a large collection of landraces variously suitable for specific end-uses at different locations across the country. None of the existing released varieties meets the requirements of an emerging local industry in starch production. In the absence of an active molecular genetic research group in the country to facilitate the incorporation of desired genes for high yield, high starch content and disease resistance into a single genotype, intra-specific hybridization remains a viable option in creating variability from which new varieties with a combination of the desired characteristics may be selected. Following a study of their phenological and reproductive characteristics, crosses were carried out among nine accessions of cassava (Megyewontem, Bamboo Akwetey, Ankra, BNARI Selection-1, Afisiafi, Security, Larbi, Asare and HO-008, abbreviated as ME, BA, AN, BS-1, AF, SE, LA, AS and HO-008 respectively). Flowering and fruiting characteristics differed significantly among the accessions. Percent crossability ranged from 0% (in AN x HO-008, AF x ME and LA x HO-008 crosses) to 88% (in AS x AF crosses). No clear relationship existed between seed set and embryo formation among the accessions. Fruit drop rate ranged from 11.7% to 83.3%. Zygotic embryos were harvested prior to seed maturity and cultured in vitro on phytohormone-free Murashige and Skoog medium to raise a collection of F1 base population lines. In vitro germination rates of the hybrid embryos harvested at 45DAP ranged from 32.14% to 100%. Ex vitro acclimatization of 237 plantlets recovered from zygotic embryo cultures resulted in the survival of 35 hybrid progenies. These were grown for six months in a plant barn. Preliminary characterization of the hybrids with reference to above- and below-ground morphological traits, using IBPGR descriptors, revealed that they are generally similar in terms of pubescence of young leaves, leaf vein colour, colour of mature stems, colour of root pulp and number of leaf lobes. They however, differ with respect to colour of unexpanded apical and first fully expanded leaves, shape of central leaf lobe and colour of phellem and phellogen of the roots (au).