Effects of Elevated Atmospheric Carbon Dioxide on the Growth, Physiology, and Secondary Metabolite Production of Ocimum gratissimum: A Controlled Environment Study

Rising atmospheric carbon dioxide concentrations are expected to alter plant growth physiology and secondary metabolite production, with implications for the medicinal quality of important herbal plants such as Ocimum gratissimum (African basil), widely used in South East Nigerian traditional medicine. This study investigated the effects of three elevated carbon dioxide concentrations (ambient 400 ppm, 600 ppm, and 800 ppm) on the growth, photosynthetic physiology, essential oil yield, and secondary metabolite profiles of Ocimum gratissimum grown in controlled environment growth chambers at the National Centre for Genetic Resources and Biotechnology, Ibadan, Oyo State. Plants were grown in identical potting media under controlled temperature (27 degrees Celsius) and 12-hour photoperiod. Growth parameters (plant height, leaf area, stem diameter, and fresh and dry weight) were measured fortnightly. Gas exchange measurements (net photosynthesis, transpiration, and water use efficiency) were taken using a Li-Cor Li-6400 portable photosynthesis system. Essential oil was extracted by hydrodistillation and characterised by GC-MS. Total phenolic content and flavonoid content were determined spectrophotometrically. Results showed that elevated CO2 at 600 ppm significantly increased plant biomass (25% above ambient), leaf area (31% above ambient), and water use efficiency (42% improvement). Essential oil yield increased significantly at 800 ppm. GC-MS analysis revealed that the proportion of the major constituent, eugenol, decreased at elevated CO2, while linalool increased. Total phenolic content declined significantly at 800 ppm. The study concludes that elevated CO2 alters the secondary metabolite profile of Ocimum gratissimum in ways that may reduce its medicinal quality, warranting further investigation.

Keywords: Ocimum gratissimum, elevated CO2, secondary metabolites, essential oil, medicinal plants