Heterosis for water uptake by maize (Zea mays L.) roots under water deficit: responses at cellular, single-root and whole-root system levels

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 Lpc of the F1 was higher than that of its inbred parents with or without stress from water deficit. Marked reductions in Lpc were observed following Hg2+ treatment. The hydrostatic hydraulic conductivity of single roots (hydrostatic Lpsr) 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 Lpsr was negatively correlated with the ratio of cortex width to root diameter (R2=−0.77, P<0.01), whereas Lax 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.