Relationship between Vapor Pressure Deficit Condition and Water-stress Status in Strawberry

To develop a novel highly robust humidification technology that requires minimal adjustment of operational methods, we investigated the vapor pressure deficit (VPD) conditions that cause water stress in strawberry (Fragaria × ananassa Duch.) plants. The relationship between the rate of relative change in fruit-stalk diameter (RCFSDt) and VPD conditions, including VPD and rate of change in VPD (VPD′; hPa·min−1) were analyzed by performing multiple regression. A negative RCFSDt was defined as an indicator that the plant was experiencing water stress. Regression of RCFSDt on VPD and VPD′ revealed that VPD′ greater than 0.069 hPa·min−1 led to negative RCFSDt when VPD was 8.0 hPa or higher. In addition, we investigated whether the rate of change in VPD′ (VPD″; hPa ·min−2) could be used as an indicator to cease humidification, employing both VPD and VPD′. Analysis of the relationship between RCFSDt and VPD″ revealed that even if VPD and VPD′ exceeded 8.0 hPa and 0.069 hPa·min−1, respectively, RCFSDt could not be negative when VPD″ was less than −0.00426 hPa·min−2. We measured the relative change in fruit-stalk diameter (CFSDt) of strawberry under the conditions generated by a novel humidification system. The novel humidification system humidified the air when VPD, VPD′, and VPD″ exceeded each threshold for avoiding the condition, where VPD, VPD′, and VPD″ exceeded 8.0 hPa, 0.069 hPa·min−1, and −0.00426 hPa·min−2, respectively. In practice, there were periods when humidification control was not executed because while VPD exceeded its control threshold, VPD′ and VPD″ were below their respective control thresholds. In addition, the novel humidification system prevented a decrease in CFSDt during the daytime under the non-humidification treatment.