Photoautotrophic culture with CO₂ enrichment for improving micropropagation of Coffea arabusta using somatic embryos

Coffea arabusta somatic embryos were cultured photoautotrophically (without sugar in the medium) at different CO₂ concentrations, and conversion of the somatic embryos was compared to that cultured conventionally (with sugar in the medium). Torpedo-shaped and cotyledonary embryos were cultured photoautotrophically in a medium with a vermiculite-based supporting material in vessels with 0.8 h^-1 air exchanges under 100 μmol m^-2 s^-1 photosynthetic photon flux (PPF) at one of three CO₂ concentrations [400 (ambient), 1500 and 5000 μmol mol^-1], or conventionally in an agar-gelled medium with 20 g/L sucrose in vessels with 0.1 h^-1 air exchanges under 30 μmol m^-2 s^-1 PPF at 400 μmol mol^-1 CO₂ for 61 days. When cultured photoautotrophically at 1500 or 5000 μmol mol^-1 CO₂, percent conversion of cotyledonary embryos to plantlets attained 55-60%, a significantly higher percentage than that when cultured conventionally (17%). The conversion at 400 μmol mol^-1 CO₂ under photoautotrophic conditions was not significantly different from that under the conventional conditions. The percent conversion of torpedo-shaped to cotyledonary embryos was 90-100% when cultured conventionally or photoautotrophically at 5000 μmol mol^-1 CO₂, while it was 20-60% when cultured photoautotrophically at the lower CO₂ concentrations, where 15-50% torpedo-shaped embryos died. Cotyledonary embryos exhibited greater fresh and dry mass at 5000 μmol mol-1 CO₂ under photoautotrophic conditions than under conventional conditions. When cultured photoautotrophically at 5000 μmol mol-1 CO₂, torpedo-shaped embryos attained fresh and dry mass comparable to those under conventional conditions. Photoautotrophic culture was applicable to Coffea arabusta for improving micropropagation using somatic embryos at torpedo and cotyledonary stages provided that the CO₂ concentration was properly controlled.