Cortex death precedes stele failure in soybean fine roots during drought but does not prevent plant rehydration.

Fine roots regulate plant water uptake, but the dynamic cell-level hydraulic behavior of these organs remains poorly understood, particularly at the onset of drought. We investigated changes in fine root (<2-mm diameter) shrinkage, turgor loss, cell layer viability, and uptake during dehydration and rehydration in soybean (Glycine max) to identify critical physiological thresholds of water potentials experienced by plants exposed to experimental drought. Fine root diameter shrank by over 50% at the completion of drying, with 55.36% of this relative shrinkage occurring by -0.25 MPa, and most of that water volume loss was attributed to epidermal and cortex cells. Epidermal cells lost turgor at -0.5 MPa and cortex cells reached mortality by -1.0 MPa, prior to xylem embolism onset. Cells within the stele remained viable after cortex and epidermal mortality until -1.75 MPa, coinciding with the 50% loss of xylem conductivity through embolism (whole-plant P50). Drought recovery experiments revealed that cortical and epidermal cell mortality slowed but did not prevent rehydration of those same cell layers, or whole plant rehydration, prior to embolism. The rapid, dynamic changes in cortical and epidermal cells during the earliest stages of drought exposure and subsequent recovery are likely to act to physically decouple fine roots from the surrounding soil to limit plant dehydration, an effect likely accelerated by bare-root lab drying conditions. Movement of water through these dead cell layers allows rehydration of living stele tissue prior to embolism, supporting root growth and recovery post-drought.