Elevated CO2 Influences the Growth, Root Morphology, and Leaf Photosynthesis of Cacao (Theobroma cacao L.) Seedlings

Growing quality seedlings is a challenge for sustainable cacao production as the survival rate of young seedlings is strongly influenced by environmental factors that affect the productivity of cacao farmers. In this study, cacao (Theobroma cacao L.) seedlings were cultivated in a nursery, and the effects of elevated CO2 concentrations (approximately 800 ppm) applied to cacao seedlings during daytime (6:00–17:59) on the root growth, morphology, and leaf photosynthetic capacity were examined. Treatment with elevated CO2 significantly improved root growth, dry matter weight, and root/shoot ratio. Three-dimensional imaging of roots showed that lateral roots grew longer horizontally, lateral roots and fine roots were distributed over a larger area, and root surface and root volume increased significantly under elevated CO2 treatment. Accurate quantification of root morphology using X-ray CT indicated that the treatment with elevated CO2 concentrations may significantly affect root quality during the seedling stage by expanding the distribution range of lateral and fine roots, which increases the ability of lateral roots to elongate and absorb water and nutrients from the superficial layers. The photosynthetic characteristics of the aboveground leaves of cacao seedlings exposed to elevated CO2 concentrations showed a tendency to adapt to elevated CO2 concentrations by increasing light-use efficiency and CO2-use efficiency. Therefore, the treatment of cacao seedlings with elevated CO2 concentrations improved the growth quality of seedlings due to the characteristics of the roots as large sinks.