Linking Leaf Angle to Physiological Responses for Drought Stress Detection in <em>Quercus acutissima</em> Seedlings in Forest Nursery

Due to climate change, seedling damage caused by drought stress is expected to increase in both afforestation sites and nurseries. Therefore, to ensure stable seedling production under high-temperature conditions and to cultivate seedlings with enhanced drought tolerance through hardening treatments, the development of an effective irrigation system is required. Conventional physiological methods for non-destructive drought detection, such as chlorophyll fluorescence and leaf temperature measurements, require expensive and manual operation, thereby limiting their real-time applicability in forest nurseries. This study evaluated the feasibility of using image-based leaf angle measurements for drought stress detection in Quercus acutissima seedlings. One-year-old seedlings were grown under two water regimes—well-watered (control; CT) and unwatered (drought; DT)—and physiological parameters, including chlorophyll fluorescence, vapor pressure deficit, and leaf angle, were measured. Statistical analyses (RMANOVA) revealed that changes in the leaf angle parameter (PMD–MD: difference between the previous measurement day and the current measurement day) exhibited temporal patterns similar to physiological responses to drought on day 6 (D6), but not earlier than physiological changes. Multiple regression models identified air temperature, soil moisture, Fm′, and VPD as the main factors influencing leaf angle variation. These findings demonstrate that leaf angle monitoring can serve as an effective surrogate indicator of physiological drought stress responses in forest nursery seedlings, with potential for integration into automated irrigation systems for drought hardening management, thus enabling enhanced real-time applicability over conventional methods constrained by costly equipment and manual handling.