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Received: 9 November 2017 / Accepted: 15 May 2018 / Published online: 4 June 2018 / Editor: John Finer
© The Author(s) 2018
Abstract
The establishment of cocoa embryogenic cell lines in liquid medium starting from high frequency somatic embryogenesis (HFSE) callus is described. The growth kinetics of the cultures during the multiplication and the expression steps conducted in 250 mL Erlenmeyer flasks were described for three genotypes selected for their agronomical traits (EET95, EET96, and EET103). The glucose and dissolved oxygen concentrations and the absorption of Murashige and Skoog medium macronutrients (nitrate, ammonium, potassium, sulfate, calcium, phosphorus, and magnesium) were monitored. The multiplication of the embryogenic calluses in a medium containing 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) at 1 mg L 1, initiated with an inoculation density of 20 g L-1 of callus, was achieved. The growth rate was characterized by two phases, with the second being concomitant with a depletion of phosphorus and magnesium, and a decrease in the embryogenic potential of the callus. The expression of the callus embryogenic capacity was conducted in an auxin-free medium. The embryo production starting from 1 and 5 g L-1 inoculation densities was compared. When placed in the optimal expression conditions in flasks, 1 g of callus produced 1000 to 1500 embryos within 5 to 7 wk. Finally, two paths for improving the plantlet regenerative capacities of cocoa SE produced in liquid medium were identified. Supplementing the expression medium with myo-inositol used as an osmotic agent at a concentration of 50 g L-1 increased the embryo-to-plantlet conversion rate from 13-16% to 40-48%. A 6-wk culture of the embryos on a maturation medium in Petri dishes optimized their subsequent development into plantlets.
Keywords Cocoa . Propagation . High frequency somatic embryogenesis callus . Embryo-to-plantlet conversion rate . Myo-inositol
Introduction
Theobroma cacao L. trees are grown in the humid tropics to produce cocoa beans. Four main countries produce 75% of the total cocoa beans: Ghana, Indonesia, Ivory Coast, and Nigeria (Food and Agriculture Organization, http://www.fao.org/ faostat/en/#data/QC). In producing countries, cocoa is considered as a cash crop, but this is a crucial crop for the confectionary business, as it provides the key raw material for chocolate-based products. Nestlé utilizes about 12% of the world cocoa production.
Cocoa production from T cacao...