To examine the potential heterosis for water uptake by maize roots, the hydraulic properties of roots in the F1 hybrid (Hudan 4) were compared with those of its inbred parents (♂ 478 and ♀ Tian 4) at cellular, single-root and whole-root system levels under well-watered and water-deficit conditions. The cell hydraulic conductivity (Lpc) decreased under water deficit, but the Lp c of the F1 was higher than that of its inbred parents with or without stress from water deficit. Marked reductions in Lp c were observed following Hg2+ treatment. The hydrostatic hydraulic conductivity of single roots (hydrostatic Lp sr ) varied among genotypes under the two water treatments, with the highest in the F1 and the lowest in ♂ 478. Radial hydraulic conductivity (radial Lpsr ) and axial hydraulic conductance (Lax ) of the three genotypes varied similarly as Lpsr . The variations in hydraulic parameters were related to root anatomy. Radial Lp sr was negatively correlated with the ratio of cortex width to root diameter (R2=-0.77, P<0.01), whereas L ax was positively correlated with the diameter of the central xylem vessel (R2 =0.75, P<0.01) and the cross-sectional area of xylem vessels (R2 =0.93, P<0.01). Hydraulic conductivity (Lpwr ) and conductance (Lwr ) of the whole-root system followed the same trend under the two water treatments, with the highest values in the F1 . The results demonstrated that heterosis for water uptake by roots of the F1 occurred at cellular, single-root and whole-root system levels under well-watered and water-deficit conditions.
