Root Hydraulic Adjustment Contributes to Water Balance in Arabidopsis With Increased Stomatal Density

Understanding the coordination between root and shoot hydraulics is fundamental for improving plant performance under water stress. We used the Arabidopsis thaliana double mutant epf1 epf2, which displays elevated stomatal density and transpiration rate, to investigate how root hydraulics is influenced by enhanced transpiration. Mutant and wild-type (Col-0) plants were grown hydroponically under control and osmotic stress conditions, imposed by adding 2% polyethylene glycol to the nutrient solution. Both genotypes were compared for stomatal traits, water relations and aquaporin expression. Relative to Col-0, the epf1 epf2 mutant displayed ~150% higher stomatal density, but stomatal conductance and rosette water loss increased by only ~30%. Despite greater water loss and a more negative leaf osmotic potential, the mutant maintained leaf relative water content, while root osmotic potential remained similar between genotypes. Under control conditions, epf1 epf2 exhibited lower root hydraulic conductivity (Lp_r) than Col-0. However, aquaporin transcript levels and the relative contribution of aquaporins to root water transport did not differ between genotypes. Under osmotic stress, the pattern reversed: Col-0 showed lower Lp_r than epf1 epf2, without detectable differences in aquaporin expression or relative contribution. These results are consistent with an active role of the root in modulating whole-plant hydraulic balance. Across the tested conditions, both stomatal aperture and Lp_r declined under scenarios expected to challenge plant water balance. We propose that enhanced transpiration may increase xylem tension, which could act as a long-distance signal coordinating reductions in stomatal aperture and root hydraulic conductivity, constraining water flux and stabilizing plant water status.