Abstract:

The mechanism of heterosis in maize (Zea mays L.) has been studied mainly on kernel character to increase the growth and yield, but whether roots have heterosis on water uptake was not clear. Pressure probe techniques were extensively used to study water utiligation of plants, however, the feasibility of applying this technique to investigate the difference of water relation parameters among varieties of the same plant species has not been reported. In this study, the root pressure probe was employed to study hydraulic properties and interactions between water and solute flows of single primary root of three maize genotypes, 478, T4 and F₁ of hybrid HD4 (478×T4) under hydroponic conditions with two treatments (normal culture condition and water stress imitated by PEG-6000, ψs=－0.2 MPa). In the process of water flow across the single root, there are two different driving forces, hydrostatic pressure and osmotic pressure. Under hydrostatic pressure gradient, the pressure/time curves are monophasic, but in the presence of a permeable solution (50 mmol L^-1 NaCl), the curves are biphasic (water phase and solute phase). Under both normal and water stress conditions, hydrostatic hydraulic conductivity (Lp) (radial Lp and Lx) of single root was different among genotypes, F₁ was the highest and the male parent was the lowest. Osmotic Lp of single root in different genotypes showed the same trend with hydrostatic Lp and F₁ was significantly higher than the parents, but the difference between the two parents was indistinctively both under the two water levels. Compared with osmotic Lp, hydrostatic Lp was 30–53 times higher. Permeability coefficient (Ps) and reflection coefficient (σs) are important parameters in describing the interactions between water and solute flows. Ps means solute uptake ability by plant roots, and σs (0≤σs≤1) implies the passive selectivity of roots for given solute. Ps represented as F1>the female parent>the male parent, and σs showed the reverse trend. Water stress reduced both the hydrostatic Lp and the osmotic Lp, reduced the Ps indistinctively and enhanced the σs significantly. The results indicated that F₁ was prior to the parents to show the heterosis in water uptake ability by single root, and pressure probe techniques worked well in the study of genotype difference of water uptake by roots of plants. The above results also provide scientific references for breeding and selecting new genotypes of maize to improve the ability of water uptake by roots and drought-resistance.

Key words: Root pressure probe, Maize, Heterosis, Water uptake by roots