Optical reflectance and thermal radiation responses to combined drought and heat stress in apple trees (Malus domestica)

Climate change has increased and is expected to further increase the frequency and intensity of combined heat and drought stress in agriculture. In apple trees, these stresses can significantly impact fruit quality and yield. This study investigated the use of thermal infrared and optical reflectance measurements to detect and monitor combined heat and drought stress responses in apple trees under controlled conditions. Twelve two-year-old trees ( Malus domestica Borkh., cv. Bonita on M9 rootstock) were subjected to different combinations of watering regimes and a simulated heatwave. Well-watered trees showed resilience to heat stress, but drought-stressed trees exhibited pronounced and lasting symptoms. Thermal measurements, expressed as the Crop Water Stress Index (CWSI), effectively detected drought stress during the early stages of the heatwave. Under normal temperatures, however, accuracy of detection improved when CWSI data was combined with optical reflectance data. Furthermore, stem water potential was best estimated using optical reflectance data when transpiration was inhibited following the heatwave (R² = 0.94). Results suggest that combining these measurement techniques could enable early detection of vulnerable trees before and during heatwaves, allowing timely interventions to mitigate the impact of combined heat and drought stress.