Vegetation in geotechnical infrastructures is extensively used due to the stabilising effects by plant roots. While a large volume of research explicates the mechanical behaviour of plant roots in soil-root interaction, quantifying the hydraulic behaviour of roots by means of water potential (ψ) in a natural system is a stumbling block for researchers. Existing theoretical models still relies on the plant resistance parameter or sink term to compute the root hydraulics, in which the parameters are inputted either by simple empirical functions or by complex root parameters (such as root hydraulic conductivity) that are not easily measurable. Even if such parameters are obtained, they are limited to single excised roots and generally do not account for the natural changes of water uptake...[ Read more ]

Vegetation in geotechnical infrastructures is extensively used due to the stabilising effects by plant roots. While a large volume of research explicates the mechanical behaviour of plant roots in soil-root interaction, quantifying the hydraulic behaviour of roots by means of water potential (ψ) in a natural system is a stumbling block for researchers. Existing theoretical models still relies on the plant resistance parameter or sink term to compute the root hydraulics, in which the parameters are inputted either by simple empirical functions or by complex root parameters (such as root hydraulic conductivity) that are not easily measurable. Even if such parameters are obtained, they are limited to single excised roots and generally do not account for the natural changes of water uptake from soil. Therefore, a proper determination on root water potential (ψ_r) in heterogeneous soil water conditions and the ψ in complex plant system are still undetermined. The current study confronts these scientific challenges and put forwards objectives (1) to obtain a monotonic soil-root water potential correlation by applied water stress; and (2) to find ψ distribution in complex plant systems under constant soil water potential (ψ_s). The proposed objectives were achieved by two carefully designed test series.

In the first test series, soil-plant system was established in 1 m tall columns with a local soil, completely decomposed volcanic (CDV; silty clay) and vetiver grass (Chrysopogon zizanoides L.). The columns were water stressed after growing the grass for 3, 4 and 5-months. Along the water stress period at different stages of ψ_s, the columns were periodically experimented to measure ψ_r from the excavated roots using thermodynamically principled WP4C potentiometer, and also specific leaf area (SLA; leaf dry mass to dry weight) for selected leaves. The test series identified a unique logarithmic ψ_s – ψ_r correlation indicating the gradient involved in root water uptake. The significance of the obtained correlation was verified by ψ_r and SLA correlations, which showed linearity with water uptake by roots and water lost by leaves. The linearity was especially evident at the later stages of drought. Overall, the first test series completed the first intended objective by obtaining a monotonic ψ_s – ψ_r correlation with significant mirroring from hydraulic link in leaves.

The second test series used similar experimental method as the first series with certain changes based on the new knowledge gained from the first test series. Based on the root length observed, the planted columns tested in this series were shorter (0.6 m height). A ψ_s of -0.03 MPa near the wilting point was opted for constant ψ_s owing to evident linear correlation found between root and leaf hydraulics at later stages of drought. This test series adopted poorly graded sand amended with 5% (w/w) biochar as soil medium. In addition to ψ_r, leaf water potential (ψl), leaf area and root volume were measured in this test series to identify trait-based water potential distribution. The results showed that ψl reduced with an increase in leaf area, speculated to be because of maximum transpiration rate in larger leaf area and minimum transpiration rate in smaller leaf area. From the ψ_r distribution, the induced ψ was found to be higher in sandy soil compared to CDV soil from first test series, which could be associated with greater pore water availability for easier root water uptake in sandy soil. Contrary to the ψl response, the ψ_r was found to reduce with a reduction in root volumes. The lower ψ_r in smaller root volume could be due to minimum amount of water present in the root matrix and the higher ψ_r in larger root volume for its maximum amount of water. The obtained correlation fulfils the second objective to find ψ distribution in complex plant systems. Overall, the results obtained from both the studies addresses the hydraulic behaviour of roots under water stress (increase in soil suction) and the elemental water potential distribution in individual roots, that could be used to interpret the amount of root reinforcement provided to soil when the soil-root complex is subject to a varying hydraulic conditions that alter the water potential of the system.