Effect of MgSO₄ and K₂SO₄ on somatic embryo differentiation in Theobroma cacao L.

Somatic embryogenesis in cacao is difficult and this species is considered as recalcitrant. Therefore, reformulation of culture media might be a breakthrough to improve its somatic embryogenesis. In cacao, acquisition of somatic embryogenesis competence involves three main stages: induction of primary callus, induction of secondary callus and embryo development. Screening for MgSO4 and K2SO4 concentrations for somatic embryo differentiation was conducted on three genotypes (Sca6, IMC67 and C151-61) at the three stages. The effect of these two salts in culture media appears to be most efficient at the embryo development stage. At this stage, high MgSO4 (24 mM) and K2SO4 (71.568 mM) in the culture media induced direct somatic embryos on staminodes and petals of the Sca6 and IMC67 genotypes. Media supplemented with 6.0 mM and 12.0 mM MgSO4 enabled high responsive of explants and produced high proportion of embryos. The positive effect of MgSO4 and K2SO4 on the acquisition of embryogenesis competence was further tested on seven cacao genotypes reputed as non embryogenic: SNK12, ICS40, POR, IMC67, PA121, SNK64 and SNK10. All these genotypes were able to produce somatic embryos depending on the MgSO4 concentration. Thus, our results showed that the recalcitrance of cacao to somatic embryo differentiation can be overcome by screening for the suitable MgSO4 or K2SO4 concentration. Studies of the influence of different K+Mg2+ ratios (at normal sulphate concentration) on somatic embryo differentiation revealed that sulphate supply was the main factor promoting responsive explants and the proportion of embryos. Cysteine synthase isoforms showed patterns related to morphogenetic structures sustaining that sulphur supply and its assimilation improve somatic embryogenesis in cacao.