Distinct Adaptive Patterns in Root System Architecture of Synthetically Derived Wheat Lines under High-Air-Temperature Stress

High-temperature stress poses a major threat to wheat productivity across multiple developmental stages, including early seedling growth. Root system architecture (RSA) contributes to stress adaptation; however, its responses to high-temperature stress remain insufficiently characterized in genetically diverse wheat populations. In this study, RSA responses of representative genotypes from a Multiple Synthetic Derivative (MSD) wheat population were evaluated under control and high-air-temperature conditions using a time-resolved, two-dimensional phenotyping platform. High-air-temperature stress significantly affected most root traits, with traits associated with lateral root expansion, including the second-pair seminal root length, root system width, and convex hull area, being more responsive than vertical root traits. MSD417 and MSD034 maintained relatively higher root performance under high-temperature stress, whereas MSD392 showed pronounced sensitivity. In contrast, MSD054 exhibited relatively small changes in root traits but consistently low overall performance. Multivariate analyses and stress indices consistently differentiated tolerant, sensitive, and low-responsive genotypes. These findings highlight the importance of distinguishing active stress tolerance from passive stability and suggest that lateral-root-related traits may serve as useful targets for breeding heat-resilient wheat.