Effective water potential

The water flux into a cell, and hence the volume increase, is driven by the effective water potential difference between the inside and the outside of the plasmalemma. In calculating an effective water potential difference it is necessary to take account of the reflection coefficient, a, a measure of the degree of semipermeability of the membrane. The volumetric increase in cell size with attendant water influx can be described by ... 

Many of the fixes or modifications necessary to make an effective water potential work can be traced back to the influence of polarization of the molecular charge distribution. Recent efforts in the development of water potentials have considered the explicit inclusion of a many-body polarizability term. The problem of including polarization is that it is not decomposable into pairwise additive terms. If one water molecule becomes polarized by an electric field generated by other surrounding water molecules, the extra induced moment will in turn affect the charge distribution of the surrounding water, which in turn will change the induced moment on the central water molecule, and so on. 

In view of the importance of water in chemistry and biology, there have been many attempts to construct simple yet effective intramolecular potentials for water molecules. Water monomers are traditionally left rigid. The early three-site model for water took positive charges on the hydrogens ( h) and a negative charge (qo = on the oxygen, and wrote the pair potential between two... 

Figure 2.3 Site-site interaetion for water 2.6.2 The Effective Pair Potential...

In the case of fluids which consist of simple non-polar particles, such as liquid argon, it is widely believed that Ui is nearly pairwise additive. In other words, the functions for n > 2 are small. Water fails to conform to this simplification, and if we truncate the series after the term, then we have to understand that the potential involved is an effective pair potential which takes into account the higher order-terms. 

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). 

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. 

Drought also has a profound effect on protein synthesis. In many plant tissues, a reduced water potential causes a reduction of total protein synthesis and a rapid dissociation of polyribosomes. The latter has been shown not to be the consequence of increase in ribonuclease activity (Hsiao, 1973 Dhindsa Bewley, 1976). For a specific protein, Jacobsen, Hanson Chandler (1986) have shown in barley leaves that water stress enhances the synthesis of one of the a-amylase isozymes. Using a cDNA probe they found that water-stressed leaves contained much more a-amylase mRNA than unstressed plants. 

The dry weights (104 C, 48 hr) of ten plants from each treatment group were taken at the termination of each experiment in order to compare growth effects with plant water status. Dry weight data were analyzed using analysis of variance (ANOVA) and Duncan s multiple-range test. Diffusive resistance and water potential were evaluated using the t-test. Each of these and subsequent experiments was replicated. 

Preparation of seedlings for treatments with extract-amended nutrient solution was similar to that described for testing the effects of phenolic acids, except 40 plants were used per treatment and no replacement of the nutrient solution was made during the treatment period. Data collection procedures were modified in that only ab-axial leaf resistance was obtained and water potential was determined from four plants each day. Prior work established that abaxial resistance provided an adequate indicator of stomatal effects. The data were analyzed as described in experiments with pCA and FA. 

The reported (14) mechanisms of action of allelochemlcals Include effects on root ultrastructure and subsequent Inhibition of Ion absorption and water uptake, effects on hormone-induced growth, alteration of membrane permeability, changes In lipid and organic acid metabolism, inhibition of protein synthesis and alteration of enzyme activity, and effects on stomatal opening and on photosynthesis. Reduced leaf water potential Is one result of treatment with ferulic and p-coumaric acids (15). Colton and Einhellig (16) found that aqueous extracts of velvetleaf (Abutllon theophrastl Medic.) Increased diffusive resistance In soybean fGlycine max. (L.) Merr.] leaves, probably as a result of stomatal closure. In addition, there was evidence of water stress and reduced quantities of chlorophyll In Inhibited plants. 

COARSE-GRAINED INTERMOLECULAR POTENTIALS DERIVED FROM THE EFFECTIVE FRAGMENT POTENTIAL APPLICATION TO WATER, BENZENE, AND CARBON TETRACHLORIDE... 

---