Fine root shrinkage across intact root networks begins early during dehydration in diverse plants from conifers to angiosperms

Fine roots perform the bulk of plant water and nutrient uptake. During drought, roots shrink dramatically, theoretically decoupling the root vascular system from surrounding soil. The process of root shrinkage relative to hydraulic failure remains poorly understood. We used in situ imaging to measure continuous dehydration of intact bare roots (< 2 mm diameter) over xylem water potential decline until terminal embolism in nine vascular plant species, including a lycophyte, conifers, woody and herbaceous angiosperms, investigating interactions between anatomy and root shrinkage dynamics during decreasing water potential. Across all species, root shrinkage began rapidly, with 39% of relative root shrinkage occurring by the onset of xylem water potential decline (−0.05 MPa), 57.19% by −1.0 MPa, and 83.4% of final shrinkage occurring by water potentials known to cause 50% root network embolism (P50). Shrinkage patterns and cellular anatomy were highly variable across the root network, reflecting variation in uptake and transport function within fine roots. These data highlight fine root shrinkage as a dynamic, early-onset component of plant drying that precedes hydraulic failure. Significant variation in shrinkage patterns across root networks relating to age, function and placement may theoretically provide plants with adaptive heterogeneity and facilitate a gradient of dehydration patterns below-ground.