Needle Angle Dynamics as a Rapid Indicator of Drought Stress in Larix kaempferi (Lamb.) Carrière: Advancing Non-Destructive Imaging Techniques for Resilient Seedling Production

Ukhan Jeong^1,2Dohee Kim^1,2Sohyun Kim^1,2Seung Hyun Han^3,4*Eun Ju Cheong^1,2*

• ¹Kangwon National University, Chuncheon, Republic of Korea
• ²Plant Genetics and Breeding Lab, Department of Forest and Environment System, College of Forest and Environmental Science, Chuncheon, Republic of Korea
• ³Korea Forest Research Institute (KFRI), Seoul, Republic of Korea
• ⁴Forest Technology and Management Research Center, Pocheon, Republic of Korea

Larix kaempferi (Lamb.) Carrière, a valuable species for timber and reforestation, faces challenges in large-scale seedling production due to its slow growth cycle and susceptibility to environmental stresses. Early diagnosis of drought stress is critical for preparing seedlings for harsh field conditions and optimizing irrigation systems. The study explored early detection of drought stress in L. kaempferi seedlings by combining physiological traits and phenotypic measurements, with a focus on needle angle analysis under controlled drought and irrigation conditions. Apical needle wilting and recovery, along with seedling-level image analysis (parameter: Center of Mass(y)), exhibited a significant response to drought stress on Day 2. This provides a non-destructive method for early detection, occurring before physiological traits such as chlorophyll fluorescence and needle temperature parameters that responded to drought stress on Day 6, as well as seedling mortality. The multiple regression analysis results indicated that as drought stress persisted, solar radiation and thermal-related parameters (ΦNPQ and needle temperature parameters) were identified as key predictors of needle angle changes. Image-based methods, including RGB and thermal imaging, proved effective for real-time stress monitoring, highlighting their potential for practical nursery applications. These techniques enable dynamic irrigation management and improve seedling resilience against climate-induced stressors. This study provides a foundation for needle-based phenomic approaches using imaging in nursery systems, highlighting the need for future research to optimizing these methods for large-scale, cost-effective production of high-quality, stress-resilient L. kaempferi seedlings.