Maintaining a constant soil moisture level can enhance the growth and phenolic content of sweet basil better than fluctuating irrigation

Although soil moisture sensor-based automated irrigation systems enable researchers to grow plants efficiently and quantify drought stress via constant control of volumetric water content (VWC), growers, in practice, typically irrigate plants using excessive amounts of water based on their own experience. Herein, we investigated the effects of four different irrigation strategies on the growth, physiological responses, and phenolic content of sweet basil (Ocimum basilicum). Three-week-old seedlings were grown using an automated irrigation system incorporating a soil moisture sensor and datalogger. Treatments comprised two irrigation strategies: (i) maintaining a constant VWC of 0.30, 0.45, or 0.60 m3·m-3 by applying small amounts of tap water over time; and (ii) providing a fluctuating VWC (0.30 F), in which a large amount of tap water was applied when the VWC decreased to below 0.30 m3·m-3. Growth parameters were measured at 0, 5, 10, and 15 days after reaching a set point (DAS), and the total irrigation amount was measured at the end of the experiment. Shoot growth differed significantly among treatments, with the highest values being recorded under the 0.60 treatment, followed by the 0.45, 0.30 F, and 0.30 treatments. Physiological responses decreased under the 0.30 treatment but recovered with increasing DAS. Higher constant VWC treatments (0.45 and 0.60) were conducive to higher phenolic content, with the lowest content obtained under the 0.30 F treatment. Although we detected no significant difference in average VWCs between the 0.45 and 0.30 F treatments, 0.30 F treatment showed lower growth, physiological responses, and phenolic content. Collectively, our findings indicate that basil plants with a higher yield and phenolic content can be obtained by maintaining a constantly high VWC (0.60 m3·m-3) than when they are provided with a lower or fluctuating VWC.