Potential water

From the definition of chemical potential (Eq. 2.4) and the formal expression for osmotic pressure (Eq. 2.7), we can express the chemical potential of water (pw) as 

A quantity proportional to pw — p w that is commonly used in studies of plant water relations is the water potential, P, defined as 

As mentioned when introducing osmotic pressure, a number of conventions are used to describe the osmotic and other water potential terms. One such convention is to define P as 

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Table A2.3.2 Halide-water, alkali metal cation-water and water-water potential parameters (SPC/E model). In the SPC/E model for water, the charges on H are at 1.000 A from the Lennard-Jones centre at O. The negative charge is at the O site and the HOH angle is 109.47°.

Water-retention curve Graph showing soil-water content as a function of increasingly negative soil water potential. 

Xerophile Organism adapted to grow at low water potential, i.e., very dry habitats. 

Hliiull. In v.tive B 5th ijifcni-.O pr< SM re ViilvL-. fail [jpai C)peration Excessive chlorine How to Tower Water Basin - high chlorine level to cooling water -potential for excessive corrosion in cooling water system Rotameter Relief valve an pressure check valve outlet in Nnilk-... 

Table 2.16 Potentials of metals in aerated moving sea water (Potentials are negative to the S.C.E., E = 0-246 V)...

The SAH water potential determines many aspects of their behavior in the soil. The processes of water redistribution in the soil, its transport to the plant roots, and assimilation follow the osmotic laws and are regulated by the thermodynamic potential. 

In many cases, where one is concerned with the effects of specific environmental factors it is appropriate to replace the general term stress by the appropriate quantitative measure (e.g. soil water content or water potential) together with an appropriate measure of the plant response (e.g. growth rate). 

Fig. 1. Rates of CO2 assimilation, A (/miol s ) leaf conductance, g (mol m s ) intercellular partial pressure of CO2, Pi (Pa) soil water potential and leaf water potential, xp (MPa) during gas-exchange measurements of a 30-day-old cotton plant, plotted against day after watering was withheld. Measurements were made with 2 mmol m sec" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure difference between leaf and air (S.C. Wong, unpublished data).

Fig. 2. Rates of CO2 assimilation,. 4, and leaf conductances, g, as functions of intercellular partial pressure of CO2, p in Zea mays on various days after withholding watering. Measurements made with 9.5,19.0,30.5, and 38.0 Pa ambient partial pressure of CO2, 2 mmol m" s" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure differences between leaf and air. Closed symbols represent measurements with 30.5 Pa ambient partial pressure of COj. Leaf water potentials were 0.05, - 0.2, - 0.5 and - 0.8 MPa on day 0, 4, 11 and 14, respectively (after Wong et al., 1985).

This point was made clearly by Wong, Cowan Farquhar (1985). They imposed slow water stress on 30-day-old Zea mays plants grown in 45 1 plastic bins during early spring where glasshouse vapour pressure deficit was generally about 1.0-1.5 kPa. During the 14 day period the predawn leaf water potential declined from - 0.5 to - 0.8 MPa. Rate of assimilation. 

Boyer, J.S. (1970). Differing sensitivity of photosynthesis to low leaf water potential in corn and soybean. Plant Physiology, 46, 236-9. 

Potter, J.R. Boyer, J.S. (1973). Chloroplast response to low leaf water potentials. II. Role of osmotic potential. Plant Physiology, 51, 993-7. 

Sinclair, T.R. Ludlow, M.M. (1985). Who taught plants thermodynamics The unfulfilled potential of plant water potential. Australian Journal of Plant Physiology, 12, 213-17. 

Cell enlargement occurs when a demand for water is created by relaxation of the cell walls under the influence of turgor pressure and wall-loosening factors. Water enters the cell down a water potential gradient, extending the cell walls (Lockhart, 1965 Boyer, 1985 Tomos, 1985). 

Fig. 1. Elongation rate of stem intemode 12 (A), silks (A), leaf 8 ( ), and nodal roots (O) of maize at various water potentials. Elongation rates are the average per hour for 24 h of growth in a controlled environment chamber. Water potentials were measured in the growing region of each organ in the same plants. Samples were taken immediately after the growth period when the plants had been in the dark for the last 10 h. Each point is from a single plant. Modified from Westgate Boyer (1985a).

Table 1. Primary root elongation rate of several species at various vermiculite water potentials... 

Seedlings were transplanted to the different water potentials 30 h after planting, and were grown in the dark at 29 °C and near saturation humidity. Elongation rates were constant when the measurements were made. Data of R.E. Sharp and G. Voetberg (unpublished). 

Fig. 9. Leaf water potential and turgor, abaxial stomatal conductance, and ABA content of abaxial epidermis of leaves of Commelina plants which were grown with their root systems divided between two pots. Water was either applied daily to both halves of the root system (A) or was withheld from one half of the root system after day 1 of the experimental period (A). Points for water relations and conductance are means s.e. Modified from Zhang, Schurr Davies (1987).

Fig. 10. Leaf water potential and abaxial stomatal conductance (upper figure), and water potential and turgor of secondary and tertiary root tips (lower figure) of maize plants growing in 1 m deep soil columns, watered daily (A) or not watered after day 0(A). The roots were sampled from the upper 20 cm of the soil column. Plants were 20 days old at the beginning of the experimental period. Points are means s.e. Modified from Zhang Davies (1989).

Boyer, J.S. (1968). Relationship of water potential to growth of leaves. Plant Physiology, 43, 1056-62. 

Bozarth, C.S., Mullet, J.E. Boyer, J.S. (1987). Cell wall proteins at low water potentials. Plant Physiology, 85, 261-7. 

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. 

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