Photosynthetic Validation of Decision Tree-based Water Stress Thresholds in Strawberry under Varying Vapor Pressure Deficit and CO₂ Conditions

Abstract

　Water stress strongly constrains photosynthetic performance and yield in greenhouse-grown strawberry, making appropriate vapor pressure deficit (VPD) control essential for stable production. Recently, decision tree-based analyses have been proposed to identify VPD thresholds for water stress detection; however, the physiological validity of statistically derived thresholds remains unclear. Here, we evaluated whether water stress responses identified by decision tree analysis are insightful from a photosynthetic perspective. Leaf gas exchange measurements were conducted on strawberry plants under varying VPD conditions (0.8, 1.0, 2.0, and 3.0 kPa) at ambient and elevated CO₂ concentrations (400 and 800 μmol mol^-1). Across both CO₂ conditions, stomatal conductance (g_sw) and net photosynthetic rate (P_n) peaked at VPD = 1.0 kPa. Compared to ambient CO₂, under elevated CO₂, P_n was enhanced, whereas g_sw was reduced and showed lower sensitivity to increasing VPD. Intercellular CO₂ concentration was higher at elevated CO₂ and followed VPD-dependent patterns similar to those of g_sw. These results indicate that the VPD threshold identified by decision tree analysis (～1.0 kPa) reflects photosynthetic responses irrespective of CO₂ enrichment. These findings provide physiological support for decision tree-based water stress assessment in greenhouse-grown strawberries.