PLANT BIOPHYSICS - Y. I. Posudin - 2004

II. TRANSPORT PROCESSES IN THE SOIL-PLANT-ATMOSPHERE SYSTEM

6. MASS TRANSPORT

One of the primary processes occurring within the three-component soil-plant-surface atmosphere system is mass transport (mass exchange) between its individual components, which takes the form of Water, solute, and gas transfer [Vershinin et al., 1959; Chudnovsky, 1963; Nerpin and Chudnovsky, 1975; Monteith and Unsworth, 1990; Campbell and Norman, 1998; Guyot, 1998; Jones, 2000]. Water transport encompasses moisture influx into the soil and plants via precipitation, as well as vertical liquid-phase moisture movement; solute exchange primarily involves the interchange of nutrients between the soil and the plant ROOT system, followed by their internal transport throughout the plant; whereas gas exchange is associated with Plant Respiration (specifically oxygen uptake and carbon dioxide release), the evaporation of water into the atmosphere during Transpiration, and Condensation processes.

6.1. MECHANISMS OF WATER TRANSPORT

Water is continuously transported from the soil through the roots and stems to the leaves, and subsequently into the ambient air via the Stomata. The primary Mechanisms of water transport are molecular diffusion and bulk flow. The rate of molecular diffusion is defined by Fick's first law (see Section 2.2):

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where J is the flux density, representing The amount of substance transferred per unit cross-sectional area per unit time, S is the cross-sectional area through which the substance diffuses, D is the diffusion coefficient, and is the concentration gradient (the spatial rate of change in particle concentration).

The time required for substance transport via diffusion is proportional to the square of the distance, t ≈ х_e²/D. Consequently, as demonstrated in the chapter "Molecular Processes in Cells," diffusion is a sufficiently rapid process on intracellular scales, yet exceedingly slow over large distances. For instance, the time required for a glucose molecule to diffuse (diffusion coefficient D = 10^-9м²·с^-1) across a 50 μm Cell wall is t = (50·10^-6 м)²/10^-9 м²·с^-1 = 2.5 s, whereas transporting the same substance over a distance of 1 m would require t = 1 м²/10^-9 м²·с^-1 = 32 years. Thus, diffusion serves as a significant transport mechanism within The Cell or during water evaporation from leaves, given that the diffusion coefficient of molecules in air (≈ 10^-4 м²·с^-1) is considerably larger than that In aqueous solutions (≈ 10^-9 м²·с^-1).

Over long distances, water transport is governed by bulk flow, which represents the mass movement of molecules driven, for example, by a pressure gradient.