Measurements of water uptake of maize roots: the key function of lateral roots

Abstract:

Maize (Zea mays L.) is one of the most important crop worldwide. Despite its importance, there is limited information on the function of different root segments and root types of maize in extracting water from soils. Therefore, the aim of this study was to investigate locations of root water uptake in maize.

We used neutron radiography to: 1) image the spatial distribution of maize roots in soil and 2) trace the transport of injected deuterated water (D₂O) in soil and roots. Maizes were grown in aluminum containers (40×38×1 cm) filled with a sandy soil. When the plants were 16 days old, we injected D₂O into selected soil regions containing primary, seminal and lateral roots. The experiments were performed during the day (transpiring plants) and night (not transpiring plants). The transport of D₂O into roots was simulated using a new convection–diffusion numerical model of D₂O transport into roots. By fitting the observed D₂O transport we quantified the diffusional permeability and the water uptake of the different root segments.

The maize root architecture consisted of a primary root, 4-5 seminal roots and many lateral roots connected to the primary and seminal roots. Laterals emerged from the proximal 15 cm of the primary and seminal roots. Water uptake occurred primarily in lateral roots. Lateral roots had the highest diffusional permeability (9.4×10^-7), which was around six times higher that the diffusional permeability of the old seminal segments (1.4×10^-7), and two times higher than the diffusional permeability of the young seminal segments (4.7×10^-7). The radial flow of D₂O into the lateral (6.7×10^-5 ) was much higher than in the young seminal roots (1.1×10^-12). The radial flow of D₂O into the old seminal was negligible.

We concluded that the function of the primary and seminal roots was to collect water from the lateral roots and transport it to the shoot. A maize root system with lateral roots branching from deep primary and seminal roots would be efficient in extracting water from the subsoil and better tolerate periods of water shortage. However, in this case the xylem axial resistance could be the limiting factor for the uptake of water.