Drought and Elevated Carbon Dioxide Impact the Morphophysiological Profile of Basil (Ocimum basilicum L.)

Treating plants with elevated carbon dioxide (eCO₂) can increase their drought tolerance. Increased atmospheric CO₂, a fundamental factor in climate change, may compensate for the drought-induced reduction in crop growth and yield. Basil, being moderately sensitive to drought stress (DS), experiences several morphological changes under DS. Thus, we designed an experiment that addresses how DS and different levels of CO₂ affect the overall morphological growth patterns during basil’s early and late-season growth. The experiment was conducted under four different growth conditions: two water treatments, (1) a full-strength Hoagland’s solution was added to the basil plants at 120% of the evapotranspiration each day, and (2) 50% of the full-strength Hoagland’s solution was added to basil plants for the drought treatment, alongside two levels of CO₂ application [ambient 420 ppm (aCO₂) and elevated 720 ppm (eCO₂)]). The DS had a severe impact on the morphological traits of the shoot and root systems. Compared to control, DS reduced the marketable fresh mass (FM) by 31.6% and 55.2% in the early and late stages of growth. FM was highest under control + eCO₂ (94.4–613.7 g) and lowest under DS + aCO₂ (67.9–275.5 g). Plant height under DS + aCO₂ and DS + eCO₂ reduced by 16.8% and 10.6% during the late season. On the other hand, dry mass percent (DM%) increased by 31.6% and 55.2% under DS + eCO₂ compared to control in the early and late stages of growth, respectively. This study suggested that eCO₂ during DS significantly impacts basil morphological traits compared to aCO₂. Besides, anthocyanin decreased by 10% in DS + aCO₂ and increased by 12.6% in DS + aCO₂ compared to control. Similarly, nitrogen balance index, a ratio of chlorophyll and flavonoids, was recorded to be the highest in DS + aCO₂ (40.8) compared to any other treatments. Overall, this study indicates that the suppression of basil’s morphophysiological traits by DS is more prominent in its later growth stage than in the earlier stages, and eCO₂ played an important role in alleviating the negative effect of DS by increasing the DM% by 55%.