‘Smart’ roots modify hydraulics to make strategic use of water, field study confirms

Plants are able to modify their root hydraulics to maintain water status and strategically use soil water, according to a new study published today in the New Phytologist. When soils are drying, plants can decrease water use from topsoil while boosting uptake at greater depths. Conversely, when topsoil is rewetted, the plants can instantly rearrange their water uptake to be more energy efficient by making maximum use of water near the surface and reducing uptake lower down.

The study focused on wheat and permanent grass fields and found that water uptake of deep roots was controlled by topsoil water, and thus the topsoil water serves not only as a resource but also as a cue coordinating optimal use of soil water in different depths.

“These findings have important implications for improving understanding of the mechanisms plants use to cope with periodic water stress and screening drought-tolerant varieties,” said the corresponding author for the study Dr. Xiaoxian X Zhang. “We believe this is the first time this has been demonstrated in field conditions.”

In the field, plants experience periodic water stresses, and their roots penetrate much deeper than in pot-based studies. To date, the strategies plants use in fields to cope with such stresses are poorly understood because of the difficulty of in situ measurements.

“Our findings suggest that topsoil water not only serves as a resource but also acts as a regulatory trigger coordinating root water uptake in the whole soil profile,” said Zhang. “When topsoil dries, plants increased their water uptake from the subsoil, whereas when the topsoil was rewetted by rainfall, the plants promptly reduced their water uptake from the subsoil while increasing water uptake from the topsoil. Such adaptive changes in root water uptake from different soil layers may arise because absorbing topsoil water is more energy-efficient for plants.”

Plant scientists call this concept “root economy.” The topsoil water is prone to evaporation but rich in nutrients, so plants preferentially adsorb it whenever possible, and use the banked subsoil water as a reserve.

The mechanism for this process remains elusive. The study suggests that the plants appear to adjust the hydraulic permeability of their roots at different depths using aquaporins (proteins that form pores in cell membranes) that control the cell-to-cell pathways, but the actual triggering mechanism for this regulation is unknown.

The research team looked at changes in water uptake, water potential and hydraulic permeability of roots in a wheat field and a permanent grass field for three months. There was a subtle difference in these strategies between the two plant systems: the grass system is more tolerant to water stress in the topsoil and more efficient in using the subsoil water.

These findings have significant implications for understanding the mechanisms plants use to cope with periodic water stress. They also indicate that, in addition to assessing root morphology and the rhizosphere, phenotyping the adaptive changes in root hydraulic conductivity is crucial for developing and screening drought-tolerant crops.