Analysis of Water-stress Responses to Vapor-pressure Deficit Conditions in June-bearing Strawberries (<i>Fragaria</i> × <i>ananassa</i> Duch.)

Effective humidity control is essential to optimize strawberry production in greenhouses, where an excessive vapor-pressure deficit (VPD; kPa) can induce water stress. This study investigated water-stress responses in June-bearing strawberries (Fragaria × ananassa Duch.) using decision-tree analysis. Three cultivars—‘Koiminori’, ‘Kaorino’, and ‘Sanukihime’—were grown without humidity control. Fruit-stalk diameter (FSD; mm) was measured at 1-min intervals, and relative changes in FSD (CFSD; mm‧mm−1) and their rates (RCFSD; mm‧mm−1‧min−1) were calculated. VPD and its rate of change (dVPD; kPa‧min−1) were also recorded. Water-stress status was categorized into three classes: non-shrinkage, shrinkage with radial expansion, and shrinkage with further contraction. Decision-tree models (DTMs) were constructed using VPD and dVPD as explanatory variables. The models revealed cultivar-specific thresholds for strong water stress, defined by the condition “shrinkage with further contraction”: for ‘Koiminori’, VPDt ≥ 1.29 kPa; for ‘Kaorino’, 1.70 ≤ VPDt < 2.09 kPa and the dVPDt ≥ 0.014 kPa·min−1; and for ‘Sanukihime’, VPDt ≥ 1.21 kPa and the dVPDt ≥ −0.012 kPa·min−1. In ‘Sanukihime’, dVPD was a key factor in determining whether the FSD contraction progressed, even under decreasing VPD conditions. The DTMs achieved moderate classification accuracy based on training data (e.g., 0.71 for ‘Koiminori’). However, in ‘Kaorino’, the recall for the severe stress class (“shrinkage with further contraction”) was notably lower due to frequent misclassification, indicating limited sensitivity to detect strong stress in this cultivar. Decision-tree analysis was employed to identify environmental thresholds for strawberry water stress. Incorporating dVPD into humidity control strategies may improve efficiency by preventing unnecessary humidification during recovery phases when plants transition from contraction to expansion. These findings support the development of advanced control systems tailored to cultivar-specific responses, contributing to improved yield and reduced physiological disorders in greenhouse strawberry cultivation.