Changes in electrical conductivity and moisture content of substrate and their subsequent effects on transpiration rate, water use efficiency, and plant growth in the soilless culture of paprika (Capsicum annuum L.)

The moisture content (MC) and electrical conductivity (EC) in substrates are major root-zone environmental factors that affect the transpiration rate and subsequent plant growth in soilless culture. For maintaining optimum root-zone environments, efficient real-time irrigation control is required based on the substrate EC, substrate MC, and transpiration. The objectives of this study were to clarify the relationship between substrate MC and EC and analyze the changes in substrate EC, plant growth, and water use efficiency under different moisture control regimes. Irrigation systems maintaining three regimes of substrate MC (70–85, 60–85, and 50–85%) were set as treatments, and a conventional irrigation using accumulated radiation served as a control. Subsequent changes in the substrate EC and transpiration rate were continuously measured at different substrate MCs, and the relationships between these variables were derived. The transpiration rate was most sensitive to substrate EC at general cultivation conditions of a substrate EC of 2.5 to 4.5 dS·m⁻¹ and a substrate MC of 60 to 85%. The transpiration rate tended to decrease with increasing substrate EC and decreasing substrate MC. More water was consumed in a higher substrate MC, which was controlled within a narrow range of MC. However, substrate EC was well-controlled below 4.5 dS·m⁻¹ in a substrate MC of 70 to 85%. The relationship between the range of substrate MC and the increase in substrate EC was obtained using equations. Although more water was supplied for the control with a substrate MC of 70 to 85%, fruit productivity tended to increase compared to the other substrate MC treatments (60–85 and 50–85%). From the results, it is hypothesized that precise control of root-zone environments can be used to increase fruit productivity and water use efficiency and to minimize plant water stress as well.