IN VITRO STUDIES OF ASEXUAL EMBRYOGENESIS IN THEOBROMA CACAO L. (CALLUS, PROPAGATION)
Abstract
Embryogenically-competent callus of Theobroma cacao L. (cacao) occurred spontaneously from three clones of asexual embryos proliferated in vitro by direct embryogenesis via hypocotylary budding. Sporophytic tissues such as flower parts and seedling leaves produced only non-embryogenic callus. Asexual embryogenesis via callus occurred in 3 clones at low frequency on a hormone-free basal medium but was increased by media modification. Three clones of embryogenic callus (BC 5, 36, and 46) showed significant differences in their capacities to form embryos with various nutrient factors and growth regulators. Auxin is not essential for the induction of embryos from callus but low concentrations (10('-5) 10('-4) mg/liter) of the auxin 2,4-D or short exposure to a relatively high concentration (1.0 mg/liter) enhanced embryogenesis in clone BC 36 in liquid media. In semisolid media, however, concentration in the order of 10('-2) and 10('-1) mg/liter was optimum for BC 36 but reduced embryogenesis in BC 5. Response to proline, NH(,4)('+), adenine sulfate, GA(,3) and GA(,3)-synthesis inhibitors (AMO 1618 and daminozide), and coconut water depended on clone and media state. Embryo initiation from callus occurred with either sucrose, glucose, or fructose as the carbon source but the response was affected by clone, carbon source and concentration. Embryogenesis was higher with glucose or fructose than with sucrose but the optimum concentration of each sugar differed by clone. Embryogenesis in all clones was reduced at sugar concentrations above 0.26 M. The presence of coconut water (CW) reduced the concentration of sucrose required for optimum embryogenesis. The number of cotyledonary embryos increased with 10-20% CW depending on clone. Germination of asexual embryos as measured by radicle extrusion and epicotyl growth was greatest with the combination of embryo leaching, half-strength salts, and the removal of casein hydrolysate. A cytological study of embryogenic callus revealed both aneuploid and euploid variation but the predominant chromosome number was diploid (2n = 20).
Degree
Ph.D.
Subject Area
Plant propagation
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