The transpiration of water at negative pressures in a synthetic tree

Abstract

Plant scientists believe that transpiration-the motion of water from the soil, through a vascular plant, and into the air-occurs by a passive, wicking mechanism. This mechanism is described by the cohesion-tension theory: loss of water by evaporation reduces the pressure of the liquid water within the leaf relative to atmospheric pressure; this reduced pressure pulls liquid water out of the soil and up the xylem to maintain hydration. Strikingly, the absolute pressure of the water within the xylem is often negative, such that the liquid is under tension and is thermodynamically metastable with respect to the vapour phase. Qualitatively, this mechanism is the same as that which drives fluid through the synthetic wicks that are key elements in technologies for heat transfer, fuel cells and portable chemical systems. Quantitatively, the differences in pressure generated in plants to drive flow can be more than a hundredfold larger than those generated in synthetic wicks. Here we present the design and operation of a microfluidic system formed in a synthetic hydrogel. This synthetic 'tree' captures the main attributes of transpiration in plants: transduction of subsaturation in the vapour phase of water into negative pressures in the liquid phase, stabilization and flow of liquid water at large negative pressures (-1.0 MPa or lower), continuous heat transfer with the evaporation of liquid water at negative pressure, and continuous extraction of liquid water from subsaturated sources. This development opens the opportunity for technological uses of water under tension and for new experimental studies of the liquid state of water.

Tuning Transpiration by Interfacial Solar Absorber‐Leaf Engineering

“…Plant transpiration, a process of water movement through a plant and its evaporation from aerial parts especially leaves,3 consumes a large component of the total continental precipitation (≈48%)1, 2 and significantly influences global water distribution and climate 4. To date, various chemical and/or biological explorations have been made to suppress the transpiration but with uncertain environmental risks 5, 6, 7, 8, 9.…”

“…Transpiration is the process that water is taken up by plant and evaporated from aerial parts like leaves 3. Water in the mesophyll cells of leaves evaporates into air through the stomata on the underside of the leaves by photosynthesis or respiration process as follows31, 32 $\mathrm{Transpiration}:\mathrm{\hspace{0.17em}}\mathrm{Mesophyll}\mathrm{\hspace{0.17em}}\mathrm{cells}\mathrm{\hspace{0.17em}}\stackrel{\mathrm{Photosynthesis/Respiration}}{\to }\mathrm{\hspace{0.17em}}{\mathrm{normalH}}_{\mathrm{normal2}}\mathrm{O}\uparrow$ $\begin{array}{c}left\mathrm{Photosynthesis}:\\ \mathrm{\hspace{0.17em}}\mathrm{\hspace{0.17em}}\mathrm{\hspace{0.17em}}6{\mathrm{normalCO}}_{\mathrm{normal2}}\mathrm{\hspace{0.17em}}+\mathrm{\hspace{0.17em}}12{\mathrm{normalH}}_{\mathrm{normal2}}\mathrm{O}\mathrm{\hspace{0.17em}}\underset{\mathrm{Chlorophyll}}{\overset{\mathrm{Light}}{\to }}\mathrm{\hspace{0.17em}}{\mathrm{normalC}}_{\mathrm{normal6}}{\mathrm{normalH}}_{\mathrm{12}}{\mathrm{normalO}}_{\mathrm{normal6}}\mathrm{\hspace{0.17em}}+\mathrm{\hspace{0.17em}}6{\mathrm{normalO}}_{\mathrm{normal2}}\uparrow +\mathrm{\hspace{0.17em}}6{\mathrm{normalH}}_{\mathrm{normal2}}\mathrm{O}\uparrow \end{array}$ $\mathrm{Respiration}:{\mathrm{normalC}}_{\mathrm{normal6}}{\mathrm{normalH}}_{\mathrm{12}}{\mathrm{normalO}}_{\mathrm{normal6}}\mathrm{\hspace{0.17em}}+\mathrm{\hspace{0.17em}}6{\mathrm{normalO}}_{\mathrm{normal2}}\mathrm{\hspace{0.17em}}+\mathrm{\hspace{0.17em}}6{\mathrm{normalH}}_{\mathrm{normal2}}\mathrm{O}\mathrm{\hspace{0.17em}}\stackrel{\mathrm{Enzyme}}{\to }\mathrm{\hspace{0.17em}}6{\mathrm{CO}}_{\mathrm{normal2}}\uparrow \mathrm{\hspace{0.17em}}+\mathrm{\hspace{0.17em}}12{\mathrm{normalH}}_{\mathrm{normal2}}\mathrm{O}\uparrow$ …”

“…With increased water loss due to enhanced transpiration, the cells in the leaf are shriveled in volume. Meanwhile, the water supply (water uptake) of the plant will automatically increase to balance the osmotic pressure between the cells in the leaf and the air 3, 9…”

Osmocapillary phase separation

“…In the xylem of tall trees, liquid water is often under hydrostatic tensile stress in excess of 1 MPa [4]. Osmosis-induced high tensile stress in liquid water has been demonstrated in artificial trees on chips [5]. Biological and bioinspired actuators rely on swelling polymers in response to diverse stimuli [6,7].…”

Noncontact and noninvasive study of plant leaves using air-coupled ultrasounds

“…This is commonly described by the cohesion-tension theory, recently been tested in synthetic trees. 1 The knowledge of the way plant leaves operate, its mechanical and microstructural properties can be useful for the design of biologically inspired materials and is attracting a growing interest. [2][3][4][5] In the context of plant physiology, it is extremely interesting to develop noninvasive procedures to monitor the dynamic of water movement through the plant in the context of the plant-soil-atmosphere continuum model.…”