Intraspecific variation in mycorrhiza-associated traits predicts plant growth and stress tolerance under resource limitation

While interspecific variation in arbuscular mycorrhizal fungi (AMF) colonization and associated root traits is relatively well documented, intraspecific variation in these traits and their role in plant performance under environmental stress remain poorly understood. To identify AMF root colonization and associated root traits linked to drought and nutrient limitation tolerance in forage legumes, we collected intact mature plants of alfalfa ( Medicago sativa ) and sainfoin ( Onobrychis viciifolia ) and subjected them to control, drought, and low-nutrient treatments. Drought induced the greatest intraspecific variation in plant performance and AMF-associated traits. In both species, pre-treatment levels of hyphal and arbuscular colonization were strongly associated with subsequent responses to drought stress. However, species-specific patterns emerged following stress. In alfalfa, lower shoot mortality was associated with greater vesicle colonization and increased average root diameter, indicating a shift toward carbon storage and altered root morphology during stress recovery. Although sainfoin exhibited overall higher arbuscular and hyphal abundance, variation in stress tolerance within the species was strongly linked to root tissue density. These findings highlight that intraspecific variation in the relative abundance of pre-established AMF structures is critical for predicting plant stress tolerance. Furthermore, interactions between AMF colonization patterns and root traits play a key role in shaping plant responses to environmental stress. Our results underscore the importance of incorporating intraspecific variability and trait-based approaches to better predict plant–mycorrhizal dynamics under changing environmental conditions.