Induction of somatic embryos of recalcitrant genotypes of Theobroma cacao L.

Abstract

Objective: In cocoa tree (Theobroma cacao L.), some elite genotypes have shown, in standard study conditions, an absence or a very weak response to induction of somatic embryos. This is the case of the original C8  genotype Trinidad. This study aims to improve the production of somatic embryos in this genotype
Methodology and results: To do this, staminodes and petals excised from immature buds of genotypes C1, C8 and C14 were used. Genotypes C1 and C14 are embryogenic under standard conditions. These floral explants were cultured on induction media differing by the type and concentration of auxins. Callus induction obtained in the three studied genotypes ranged from 80% to 90% with the petal explants and from 70% to 80% with the staminodes regardless of the type and concentration of auxins. Transfer of callogenic explants to DKW (Driver & Kuniyuki, 1984) medium  supplemented with sucrose and glucose allowed the induction of somatic embryos at mean rates varying from 5% to 20% after only 84 days with petal explants and media supplemented with 2,4-dichlorophenoxyacetic acid (2,4-D) and 2, 4, 5-trichlorophenoxyacetic acid (2,4,5-T). No induction of somatic embryos was observed on the control medium with the C1, C8 and C14 genotypes. The C1 genotype induced somatic embryos in the presence of all concentrations of auxins. The highest rate of embryogenic callus and somatic embryo induction was obtained with 18 μM of 2, 4, 5-T in all C1 genotypes (39.29 ± 0.28; 17.98 ± 0.10), C8 (36.29% ± 0.26 and 15.01 ± 0.07) and C14 (33.92% ± 0.26 and 14.50 ± 0.16). The 2, 4, 5-trichlorophenoxyacetic acid (2, 4, 5-T). at 18 μM is the most appropriate auxin to remove recalcitrance in cocoa genotype C8. The use of higher 18 μM concentrations of 2, 4, 5-trichlorophenoxyacetic acid (2, 4, 5-T). and its application to other recalcitrant genotypes could confirm its beneficial effect on the removal of recalcitrance to somatic embryogenesis in cocoa.
Conclusion and application of results: 18 μM concentration of 2, 4, 5-trichlorophenoxyacetic acid (2, 4, 5-T) gave the best percentage of embryogenic explants and the highest average number of embryos in the three genotypes  tested. Therefore, this protocol using 18 μM of 2, 4, 5-trichlorophenoxyacetic acid (2, 4, 5-T) could be used to  overcome  recalcitrance to somatic embryogenesis in cocoa.

Keywords: recalcitrant, cocoa, auxins, somatic embryogenesis