Plants are able to modify their root hydraulics to maintain water status and strategically use soil water, according to a new study published today. When soils are drying, plants can decrease water use from topsoil whilst 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 focussed 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, according to a press release.
“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 refer to this concept as “root economy.” Topsoil water, although prone to evaporation, is rich in nutrients, prompting plants to preferentially absorb it when available, while relying on deeper subsoil water as a reserve.
The exact mechanism behind this process remains unclear. Our study suggests that plants may adjust the hydraulic permeability of their roots at different depths through aquaporins — proteins that form pores in cell membranes, controlling cell-to-cell water flow. However, the trigger for this regulation is still unknown.
The research team examined changes in water uptake, water potential, and hydraulic permeability of roots in both a wheat field and a permanent grass field over three months. A subtle difference was observed between the two plant systems: the grass system demonstrated greater tolerance to water stress in the topsoil and was more efficient in utilizing subsoil water.
These findings offer valuable insights into how plants manage periodic water stress. They also highlight the importance of phenotyping adaptive changes in root hydraulic conductivity, alongside assessing root morphology and the rhizosphere, for developing and screening drought-tolerant crops.
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