The role of plant water status on source water uptake
All terrestrial plants transpire water, but where exactly does that water come from within the soil? This seemingly simple research question has been the subject of studies for many years. Stable isotopes of oxygen and hydrogen “tracers” have proven useful for shedding light on patterns of plant water use. However, the use of such tracers alone limits our understanding of patterns of source uptake and the mechanisms behind tracer observations. To go beyond this black box approach, we integrated measurements of stable isotope composition in xylem and soil water. We measured tree water status, fine root distribution, and soil matric potential, to investigate the mechanism(s) driving soil water source partitioning. We used a large soil column planted with a small willow tree (Salix viminalis) to conduct a high-resolution experiment on the campus of the École Polytechnique Fédérale de Lausanne in Switzerland. By carefully monitoring tree and soil conditions and controlling moisture regimes, our experiment showed that patterns of tree water use were driven by the tree’s water status, and not by patterns of fine root distribution. The proportions of water taken up by roots across different depths in the soil profile and held under different tension within the soil matrix was regulated by plant hydraulic response to changing atmospheric demand and soil water availability. Source water partitioning followed patterns of soil water availability during periods of tree water deficit. In the absence of a water deficit, tree water uptake was proportional across all available soil sources. Contrary to steady-state assumptions in previous tree water source investigations, the willow showed a dynamic water uptake driven by tree water deficit. Our results also indicated that tree water deficit is a more integrative measure of plant water status when compared to sap flow rates. Sap flow rates only showed response to atmospheric demands, without any indication of water stress. Our findings suggest a new research focus for future plant water investigations. Tree water status needs to be incorporated with tracer observations to provide a clearer understanding of plant water uptake patterns.
Magali F. Nehemy, Global Institute for Water Security, School of Environment and Sustainability, University of Saskatchewan, 11 Innovation Boulevard, Saskatoon, SK S7N 3H5, Canada, e-mail: firstname.lastname@example.org
The article is available in Chronica Horticulturae