Water Potential and Plant Cells

In this section we will shift our emphasis from a consideration of the water relations of subcellular bodies to those of whole cells and extend the development to include the case of water fluxes. Whether water enters or leaves a plant cell, how much, and the rate of movement all depend on the water potential outside compared with that inside. The external water potential xf ° can often be varied experimentally, and the direction as well as the magnitude of the resulting water movement will give information about T1. Moreover, the equilibrium value of XP° can be used to estimate the internal osmotic pressure IT. We will also consider various ways of examining the relationships among VF1, IT, and P  

A loss of water from plant shoots—indeed, sometimes even an uptake — occurs at cell-air interfaces. As we would expect, the chemical potential of water in cells compared with that in the adjacent air determines the direction for net water movement at such locations. Thus we must obtain an expression for the water potential in a vapor phase and then relate this P to for the liquid phases in a cell. We will specifically consider the factors influencing the water potential at the plant cell-air interface, namely, in the cell wall. We will find that vFcel1 wal1 is dominated by a negative hydrostatic pressure resulting from surface tension effects in the cell wall pores. 

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Molz, F.S. Boyer, J.S. (1978). Growth induced water potentials in plant cells and tissues. Plant Physiology, 62, 423-9. 

Another contributor to chemical potential is gravity (Fig. 2-7). We can readily appreciate that position in a gravitational field affects fXj because work must be done to move a substance vertically upward. Although the gravitational term can be neglected for ion and water movements across plant cells and membranes, it is important for water movement in a tall tree and in the soil. 

Plant cells come into contact with air where the cell walls are adjacent to the intercellular air spaces (see Fig. 1-2). Thus, the water potential in the cell walls must be considered with respect to T 1W in the adjacent gas phase. The main contributing term for T in cell wall water is usually the negative hydrostatic pressure arising from surface tension at the numerous ail-liquid interfaces of the cell wall interstices near the cell surface. In turn, Z 11 wal1 can be related to the geometry of the cell wall pores and the contact angles. 

We next indicate how cell wall and membrane properties influence the kinetics of reversible swelling or shrinking of plant cells. When the water potential outside a cell or group of cells is changed, water movement will be induced. A useful expression describing the time constant for the resulting volume change is... 

Jones, H.G. (1983). Estimation of an effective soil water potential at the root surface of transpiring plants. Plant, Cell and Environment, 6, 671-4. 

Matthews, M.A., Van Volkenburgh, E. Boyer, J.S. (1984). Acclimation of leaf growth to low water potential in sunflower. Plant Cell and Environment, 7, 199-206. 

Cells exposed to excessive levels of salinity have to acquire essential nutrients from a milieu with a preponderance of ions that are potentially toxic and non-essential. In this ionic environment the success of a plant cell will require intracellular tolerance and/or specific acquisition of nutrients essential for normal metabolic functioning. The cell is also exposed to an unfavourable water balance with an absolute requirement to maintain an internal osmotic regulation that favours uptake of water into the cell (Stavarek Rains, 1984 ). 

Cell expansion rate, turgor, osmotic pressure, and water potential of leaf 6 and leaf 11 of wild type and invertase plants... 

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