Biomass Production and Volatile Oil Accumulation of Ocimum Species Subjected to Drought Stress

Ocimum, commonly known as basil, is a group of aromatic plants extensively cultivated for their aromatic leaves, flavorful seeds, and essential oils, finding applications in food and herbal medicine. Drought stress is a crucial environmental factor that has a considerable impact on basil growth and the accumulation of bioactive compounds. This study aims to evaluate how drought stress affects biomass production and volatile oil accumulation in selected Ocimum species over two consecutive years in an open-field cultivation setting. Five distinct basil genotypes, O. basilicum L. ‘Ohře,’ O. basilicum L. ‘Genovese,’ O. × africanum Lour., O. americanum L., and O. sanctum L., were evaluated under two levels of water supply, with one group receiving irrigation as a control and the other exposed to non-irrigated conditions to induce drought stress. Consistent negative impacts of drought stress on biomass production were observed in both years. The reduction in fresh herb yield varied from 16.5 g plant−1 (10.3%) for O. sanctum to 118 g plant−1 (41.7%) for O. basilicum ‘Ohre.’ Across the study years, drought stress slightly increased the essential oil content of O. × africanum and O. basilicum ‘Genovese’ by 9.8% and 26%, respectively. The essential oil composition varied considerably among the different Ocimum species and cultivars. Cultivars Ohře and Genovese had linalool as a major component, exceeding 40%. O. americanum was rich in citral compounds—neral and geranial—accounting for 26–37%, which contribute to its strong lemon-like fragrance. The hybrid O. × africanum contained high proportions of 1,8-cineole (32–38%) and limonene (14–16%), while O. sanctum was characterized by its elevated levels of eugenol (36.4–50.3%) and β-caryophyllene (26.4–38.5%). The influence of water availability on essential oil content and composition was inconsistent across species. Similarly, variations were observed in total phenolic content (TPC) and antioxidant capacity (AOC) depending on both species and growing year. Notably, the highest TPC (341.4 mg GAE g−1 DM) and AOC (122.9 mg AAE g−1 DM) were recorded for O. sanctum grown under drought stress during the first experimental year. In conclusion, it is recommended to irrigate the studied basil species at least twice a week under open-field conditions to minimize the negative effects of drought stress.