Variations in Arbuscular Mycorrhizal Colonization Associated with Root Diameter and Hypodermis Passages Cells across Temperate and Tropical Woody Species

Root hypodermis passage cells (PCs) lack suberin and lignin deposition, responsible for nutrient absorption and arbuscular mycorrhizal colonization, which are crucial for root resource acquisition. Nevertheless, their quantitative variability across diverse woody species and their relationships with root morphology and anatomy, as well as arbuscular mycorrhizal colonization, are still not well understood. Herein, the number and proportion of PCs in the root cross-section, root morphology, anatomy, and arbuscular mycorrhizal colonization rate were quantified across 10 temperate and 12 tropical woody species. The objectives of this study were to determine how PCs vary across contrasting environmental conditions and to explore their relationships with arbuscular mycorrhizal colonization rate and other root functional traits. The results showed that tropical species possessed 56% more PCs than temperate species; by contrast, they had similar proportions of PCs. In both biomes, the number of PCs had a tightly positive correlation with arbuscular mycorrhizal colonization rate (R-2 = 0.35-0.87), root diameter (R-2 = 0.84-0.93), and cortex thickness (R-2 = 0.87-0.89), but the proportion of PCs was mostly independent of root morphological and anatomical traits. Our results suggest that variation in passage cells could well explain the tight linkage between arbuscular mycorrhizal colonization and root diameter across species and biomes, which provides insight into the collaboration gradient between plant roots and mycorrhizal fungi that dominates the root economics space.