Residual Mechanical Stiffening Is Associated With Radial Hydraulic Limitation Across the Turgor Loss Point in Opuntia...

Fine-root hydraulic failure under drought is often attributed to structural collapse or loss of soil-root contact; however, whether declines in radial hydraulic conductivity ( ${\mathrm{Lp}}_r$ ) reflect irreversible damage or coordinated biomechanical regulation remains unresolved. We tested the hypothesis that progressive soil drying induces a ${\pi }_{\mathrm{tlp}}$ -coordinated mechanical transition that is associated with radial hydraulic limitation in fine roots of Opuntia ficus-indica. Roots were exposed to 45, 120, and 180 days without irrigation, followed by short-term rehydration. We quantified pressure-volume (P-V) traits, root sap osmotic potential ( ${\pi }_r$ ), radial hydraulic conductivity ( ${\mathrm{Lp}}_r$ ), and anatomical restructuring. Drought progressively shifted the turgor loss point ( ${\pi }_{\mathrm{tlp}}$ ) to more negative values, increased the volumetric elastic modulus ( $\epsilon$ ), and reduced relative capacitance above ${\pi }_{\mathrm{tlp}}$ ( $C_{\mathrm{FT}}$ ). Pre- ${\pi }_{\mathrm{tlp}}$ P-V relationships remained strongly linear across treatments, indicating preserved hydrostatic buffering and stable ${\mathrm{Lp}}_r$ while tissues remained turgid. In contrast, post- ${\pi }_{\mathrm{tlp}}$ linearity progressively deteriorated with drought duration, coinciding with increased $\epsilon$ , reduced $C_{\mathrm{FT}}$ , cortical cell wall folding, lacuna formation, and sharp declines in ${\mathrm{Lp}}_r$ . Across treatments, changes in $\epsilon$ and $C_{\mathrm{FT}}$ were associated with reductions in ${\mathrm{Lp}}_r$ , consistent with a link between tissue stiffening and radial hydraulic limitation. Root osmotic potential ( ${\pi }_r$ ) became progressively more negative with drought severity; however, under extreme dehydration, divergence between measured ${\pi }_r$ and P-V-derived osmotic potential at full turgor ( ${\pi }_0$ ) indicated structural compartmentalization and departure from homogeneous tissue behavior. ${\mathrm{Lp}}_r$ remained stable under moderate drought, showed a non-significant trend toward reduction at 120 days, and declined sharply after 180 days of drought, with incomplete recovery following severe stress. Together, these results are consistent with concurrent shifts in hydraulic and mechanical traits across ${\pi }_{\mathrm{tlp}}$ . Above ${\pi }_{\mathrm{tlp}}$ , hydrostatic elastic buffering is associated with maintained radial hydraulic conductivity. Below ${\pi }_{\mathrm{tlp}}$ , increased stiffness and reduced relative capacitance constrain volumetric deformation and are associated with progressive radial hydraulic limitation. We propose a conceptual ${\pi }_{\mathrm{tlp}}$ -mediated hydraulic-mechanical safety margin as the hydration interval between the onset of post- ${\pi }_{\mathrm{tlp}}$ stiffening and the inflection point of ${\mathrm{Lp}}_r$ decline, which is associated with functional soil-root coupling under extreme soil drying.