Growth characteristics and bioactive compounds of dropwort subjected to high CO2 concentrations and water deficit

Here, we determined the effects of mild water deficit (WD) and high CO₂ concentration (HC) on the growth and bioactive compounds of dropwort and proposed culture conditions for producing high-quality plants. Plantlets with two to three offshoots were transplanted to a controlled environment room with artificial light and cultivated for 6 weeks. To investigate the effects of HC, plantlets were grown under relatively low CO₂ concentration (LC; 600 μmol mol⁻¹ CO₂) or HC (1000 μmol mol⁻¹ CO₂) conditions for 6 weeks (HC₆) or grown under standard conditions for 3 weeks and transferred to HC conditions for the remaining 3 weeks (HC₃). To investigate the effects of WD, control plantlets were subirrigated by keeping the pots in a tray of nutrient solution, whereas WD-treated plants were allowed to absorb water through wicks extending from the pot bottoms to a nutrient solution below. Leaf water potential of WD-treated plantlets was significantly lower than that of controls. Both WD and HC significantly decreased leaf area but did not significantly affect shoot fresh and dry weights. The photosynthetic rates of HC₆- and HC₃-treated plantlets were 53% and 64% greater, respectively, than that of LC; the stomatal conductance and transpiration rate exhibited opposite trends. WD significantly decreased net photosynthetic rate, transpiration rate, and stomatal conductance. The total nonstructural carbohydrate content of HC₆- and HC₃-treated plantlets was 8% and 14% greater, respectively, than that of LC. Total phenolic content and antioxidant capacity of WD plantlets were 17% and 23% greater, respectively, than those of controls. In controls, total phenolic content of HC₆- and HC₃-treated plantlets was increased significantly (by 24% and 34%, respectively) than that of LC plantlets. Phenylalanine ammonia-lyase (PAL) activity of WD-treated plantlets was 14% higher than that of controls; in the controls, PAL activity of HC-treated plantlets increased significantly (by 19%) compared to that of LC plantlets. However, HC did not affect PAL activity under WD. Cyanidin content was increased by both WD and HC treatments. These results indicate that reddish small dropwort (WD treated plantlets) is more useful than greenish dropwort as a functional food and can be easily produced through HC when grown in a closed environment (e.g., a greenhouse or plant factory). Finally, the cultural practice of HC can improve the industrial value of small dropwort grown for food processing.