Relative water content

Plant water status is affected by environmental pollution and consequently influences plant function at every level of biological organisation. It can be characterized by measurements of the relative water content (RWC), the water deficit, the water potential ( P ) and the osmotic potential ( Fq), along with transpiration rate and stomatal resistance. Since for the latter four parameters, tissue samples are removed from the plant, they are usually determined in the end of an experiment. If several sampling times are needed, then additional plants/replicates must be included. 

Figure 3. Relative water content in leaves of Arabidopsis thaliana L. plants exposed to Cd", and ions in hydroponics culture for 29 days. Data are expressed as % of control SE, n=5.

Stomatal closure was associated with a genetic factor in onion wherein the stomata of sensitive plants did not close. The effect of ozone and PAN on stomatal opening depends on many interacting factors those representing water stress appear to be the most important. Dean related stomatal density to the difference in sensitivity between two tobacco cultivars. Evans and Ting found that maximal sensitivity of bean primary leaves was not associated with changes in stomatal number or leaf resistance. Ozone exposure caused a decrease in relative water content, but no change in resistance. Bean leaf sensitivity seemed more a function of internal activities. 

The situation for hydrated Nafion in the acid form, or as containing aqueous acids or strong bases, is more complex because protons and defect protons (i.e., OH ions), migrate according to a somewhat different mechanism. Proton transfer in either case occurs throughout and between clusters of hydrogen bonded water molecules to a degree that depends on the relative water content. 

Note the relative water content of the various brands by comparing the depths of the water layers. 

Bearing in mind, from the discussion above, that °Brix really only allows estimation of relative water content in a series of related systems, this measure is used sufficiently frequently to require that the procedures be outlined. The sections below describe the use of the refrac-tometer and the hydrometer for determining water concentration. These instruments are readily available from a number of sources including Brannan and ERTCO Precision (for... 

Subsequent tests with velvetleaf, Kodkia, Jerusalem artichoke, and cocklebur showed that their allelopathic action altered water balance (55,94,95). Growth reductions in sorghum and soybean seedlings in nutrient solution amended with extracts from these weeds correlated with high diffusive resistances and low leaf water potentials. Stomatal closure occurred in plants treated with the more concentrated extracts. Depressions in water potential were due to a reduction in both turgor pressure and osmotic potential. A lower relative water content was also found in velvetleaf-treated plants. These impacts on water balance were not from osmotic factors. Allelochemicals from these weeds have not been thoroughly ascertained, but the present evidence shows that some contain phenolic inhibitors. Lodhi (96) reported that Kodkia contains ferulic acid, chlorogenic acid, caffeic acid, myricetin, and quercetin. As noted earlier, an effect on plant-water relationships is one mechanism associated with the action of ferulic acid. 

Figure 2-14. A Hbfler diagram showing the relationship between the water potential (T1), the hydrostatic pressure (P ), and the osmotic pressure (IT) in a plant cell for various protoplast volumes. Assuming that solutes do not enter or leave the cell, the internal osmotic pressure increases as the protoplast volume decreases. For a group of cells, relative water content is often used in such diagrams instead of relative protoplast volume. The nearly 10% decrease in volume from full turgor to incipient plasmolysis (at T1 = -1.0 MPa) is characteristic of many plant cells. Note that T), and 4 are defined by Equation 2.13b.

The photosynthetic efficiency mainly depends on the openness of stomata, particularly in C3 crops, while their closure tends to avoid excessive water loss. Abscisic acid (ABA) mediates water loss from the guardian cells of the stomata, which is triggered by a decrease in the water content of the leaf and inhibits leaf expansion. In muskmelon seedlings, ABA could improve the maintenance of the leaf water potential and relative water content, and reduce electrolyte leakage [55]. 

FIGURE 6.2 Formation of carnosic acid and a-tocopherol in rosemary during the Mediterranean summer (RWC = relative water content Ta= air temperature). Data taken from Munne-Bosch et al. (2000). 

From these results, it may be concluded that the degree of refining has a relatively small effect on the bulk conductivity of paper. A small change in the electrical properties is also observed for different pulp types in paper. These variations are not significant when compared with the variations resulting from a change in relative water content. For cellulose the conductivity increases with water content by a factor of 1014 from 0% to 20% water content 8). 

An important decrease of the water content is obtained with the low temperature isotherms (fig. 5). At saturation the relative water contents at different temperatures are given in the following table ... 

Numerous investigations in nanoparticle formation through W/O microemulsions have used extensive ranges of relative water content (indicated by the ratio w = [water]/[surfactant]) not only to obtain optimized conditions of synthesis, but also to examine the effect of relative water content on the size of the aqueous droplets and the particles synthesized therefrom. Such control parameters are not linked with merely the water content perse. It has been shown (as discussed below) that the structural arrangements and chemical bondings of the water molecules in the droplets of W/O microemulsion can vary with water content. 

For relative water content (RWC) measurements fresh leaf discs were weighded before and after 3h period in destilled water. After, the discs were overdried at 80 C for two days. RWC was obtained by RWC= ( (FW-DW) /FWmax-DW) ) xlOO. 

RWC (Relative Water Content). The values shown are means (SE) of 4 replicates... 

Fig. 1 Schematic presentation of photosynthesis of spinach leaves in air or in 155 CO as a function of the relative water content. Leaves or leaf discs were wilted at dim room light in air until the desired water content was reached. Photosynthesis in air was measured as C0 gas exchange (data from ref78), or as Op evolution in a leaf oxygen electrode chamber flushed with 15% CO2 (data from ref.9,10).

Data of the Water Use Efficiency and Relative Water Content of the leaves studied (Fig. 3 and 4) show improved water relations of the resprouts. The stability of their water content, especially in hot months, may account for the observed photosynthesis enhancement in these leaves (Radosevich and Conard, 1980). 

Fig. 3 Water Use Efficdency and Fig. 4 Relative Water Content of Arbutus unedo ( O) and Coriaria myrtifolia ( ) during the first post-fire year. ( — resprouts, - - - control leaves WI= winter, SP= spring, SU= summer, AU= autumn).

Water potential(y ), osmotic potential( 3)/ relative water content(RWC) All measurements were made at predawn time. for Fw measurements leaf discs of maize and bean from 4 replicates... 

Fig. 1. Relative water content,RWC(A) and osmotic potential, ys (B) In two maize lines (..) 2PBL 1304, and (—) ZPL 389, exposed to drought. Each point Is the mean of 3-5 measurements.

The fluorescence quenching of two maize lines leaves was measured under steady state Illumination at different relative water content of leaves. With decreasing relative water content the saturation pulse fluorescence Induction pattern Is markedly affected especially In the susceptible line. At 90X RWC Q-quenchIng decreases to become more pronaunced with further dehydration whereas the relaxation of E-quenchIng Is strongly suppressed staying at the same level until 50X RWC, whereupon It rises with the RWC drop to 45 (Fig. 2.A.). In the... 

The tin(IV)oxide powder was either calcined for one hour at different temperatures, or for different times at 350 °C, the calcination temperature during catalyst preparation. Both the variation of the surface area and the remsuning relative water content have been determined. The total amount of water was quantified by the weight loss during calcination at 1200 C. 

Fig. 3.20. (A) The respiration rate of pea cotyledons during the final phases of seed development plotted against the relative water content of the cotyledons. Figures in brackets represent water content and fresh weight (mg), respectively, per cotyledon. From these data the relative water content upper row) was calculated. (B) The activity of the succinate oxidase system, and the succinate dehydrogenase activity of mitochondria from pea cotyledons during the final phases of seed development. ADP/0 ratios are averages of data from several experiments (<- ). Vertical arrows ( ) mark the average relative water content of the cotyledons used in these experiments. After Kolloffel, 1970 [96]...

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