A Photomixotrophic System to Improve the Growth of In Vitro-Cultured Seedlings of Coconut (Cocos nucifera L.)

Coconut (Cocos nucifera L.) is an important tropical palm globally, but due to declining production, it is failing to meet the rapidly increasing market demand. In vitro technologies, including embryo culture, cryopreservation, and clonal propagation, are being explored to increase productivity and to conserve unique varieties. Despite the development of these technologies, there is still a need to improve the acclimatization process used to produce the large numbers of high-quality coconut seedlings needed for planting. Enhancing seedling growth and development during acclimatization, using an elevated concentration of atmospheric carbon dioxide (CO2), and applying appropriate modifications to the light environment were the objectives of this study. A series of four semi-automated acclimatization chambers, which could control atmospheric CO2 concentration, relative humidity and temperature, and light intensity, quality, and photoperiod, were designed to undertake this study. An initial experiment demonstrated that a concentration of 1600 μmol mol−1 CO2 in combination with an elevated light intensity of 600 μmol m−2 s−1 could significantly enhance the growth and development of seedlings as compared to ambient CO2 concentrations and standard illumination conditions. Further experimentation demonstrated that continuous illumination with a combination of red and blue light (7:3) gave superior growth and development. This study found that the 6-month-old age is the ideal age for accelerating the acclimatization process using CO2. Overall, the maximum effectiveness of the photomixotrophic growth system was seen when CO2 (1600 μmol mol−1) was applied to 6- to 7-month-old seedlings under a constant (24/0 h photoperiod) but elevated light intensity (600 μmol m−2 s−1) of red and blue light in the ratio of 7:3.