Water transport within the soil-plant-atmosphere continuum

Abstract

Because water is generally free to move across the plant-soil, soil-atmosphere, and plant-atmosphere interfaces it is necessary and desirable to view the water transfer system in the three domains of soil, plant, and atmosphere as a whole. . . it must be pointed out that, as well as serving as a vehicle for water transfer, the SPAC is also a region of energy transfer. John R. Philip (1966) Closure of the water budget for an ecosystem requires that precipitation and flows of water from neighboring ecosystems be returned to the atmosphere through evapotranspiration, transferred to storage pools, or allowed to flow out of the system. Transfer and storage of water creates capacitance in the liquid phase of the water cycle and delays the inevitable return of water vapor to the atmosphere, but a globally balanced water cycle requires that the molar equivalent of precipitated water be accounted for in the fractions stored in surface and subsurface reservoirs, plus that evaporated back to the atmosphere. Recognizing that in terrestrial ecosystems a large fraction of precipitation is returned to the atmosphere through leaf transpiration, plants occur at an important interface between the liquid and vapor phases of the water cycle. Water moves from soil into plants through viscous flow in the liquid phase, as it is “pulled” by thermodynamic forces through roots, vascular tissues, and leaf mesophyll cells, following negative pressure (tension) gradients. Tension develops in the conduction tissues as water is evaporated faster from leaves than can be replaced by flow from the soil. Physical continuity within capillary “threads” of the ascending water column is maintained by cohesive and adhesive forces that are facilitated by the electrostatic polarity of water molecules. In the vicinity of stomata, water is evaporated to the atmosphere. In the atmosphere, water is carried in the vapor phase to and from leaf and soil surfaces through diffusion near the surfaces and turbulent air motions in the well-mixed atmosphere. Given the continuous nature of these water transfer paths, and their serial relation to one another, it was recognized early in the study of plant-water relations that the “whole plant” must be considered at the center of an integrated and articulated soil-plant-atmosphere continuum, or SPAC.