Leaf wilting as a phenotypic indicator of heat and drought stress in crops: an overview of physiological mechanisms and machine learning applications

Crop production is often affected by the co-occurrence of heat and drought, which significantly impacts physiology, growth, development, and yield. Leaf wilting is a phenotype that can provide important insights into how plants respond to combined stressors, linking visible morphological changes to internal physiological changes and water transport. This review emphasizes the importance of leaf wilting as a visual indicator of stress under water-limited conditions. It discusses (i) the water transport processes within plants, (ii) canopy responses to heat and drought stress such as paraheliotropism, leaf rolling, and leaf wilting, (iii) wilting dynamics in plants, in response to heat and drought stress, and (iv) highlights the importance, opportunities, and challenges of emerging technologies, i.e., the potential of unmanned aerial vehicles and machine learning, to enable efficient and large-scale high-throughput phenotyping of heat and drought stress-induced leaf wilting in crop production. A deeper understanding of leaf wilting mechanisms under water-limited conditions is essential for optimizing the use of advanced technologies for crop stress monitoring, thereby improving crop resilience and global food security.