Water Potential Isotherms Reveal Different but Coordinated Responses at Leaf and Root Levels Under Controlled Drought

Understanding how leaves and roots modulate their water relations under drought is critical for predicting plant stress resistance/resilience. We compared water relations traits derived on water potential isotherms in 10 young woody and herbaceous species grown under the same soil and irrigation conditions (well-watered vs. mild drought). Measured traits included turgor loss point (Ψ_tlp), osmotic potential at full turgor (π_o), cell wall stiffness (ε), hydraulic capacitance, and structural traits such as dry matter content and saturated water content. Roots consistently showed higher Ψ_tlp and π_o, lower ε, and greater capacitance than leaves, highlighting organ-specific differences in water relation traits. Under drought, both organs primarily adjusted water relation traits via osmotic modification, with larger ε changes in leaves, indicating greater plasticity in this organ. Roots also showed increased dry matter content and reduced water storage capacity. Across species, leaf and root Ψ_tlp were positively coordinated, contrasting with previous studies reporting no clear relationship. Ψ_tlp correlated with π_o in roots, underscoring the dominant role of osmotic adjustment also in this organ. Moreover, species with lower leaf Ψ_tlp displayed greater divergence between leaf and root turgor thresholds, possibly revealing a pattern of hydraulic risk partitioning between organs. Overall, leaves and roots displayed complementary water relations trait strategies across species. Our study provides insights into organ-specific drought responses and emphasizes the role of roots in whole-plant water use strategies.