Water potential tissue

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

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. 

Ability of tissues to increase their solute concentration (osmoregulate), conferring a temporary advantage by enabling the tissues to maintain turgor at low water potentials by decreasing their osmotic potentials. 

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. 

The two remaining of plant water status parameters (water and osmotic potential) reflect free water availability. These can be measured with a Dew Point Microvoltmeter (e.g., the WESCOR HR 33T [Logan, Utah]), but other methods and equipment can be applied. Osmotic potential can be determined from either fresh or frozen plant samples, but water potential requires fresh tissue. The water potential of leaf discs, strips or roots of the same size/weight can be measured with the WESCOR device (for detailed methods see http //www.wescor.com/environmental/index.phtml). Osmotic... 

ME systems intended for parenteral application have to be formulated using nontoxic and biocompatible ingredients. The o/w ME systems would be suitable to improve the solubility of poorly water soluble drug molecules whereas w/o ME systems would be best suited for optimizing the delivery of hydrophilic drug molecules that are susceptible to the harsh GI conditions. Moreover, w/o systems can serve to prolong the release and mask any potential tissue irritation and site toxicity that are caused by intramuscular (i.m.) administration of hydrophilic drug molecules. 

What causes the movement of solutes in the phloem This question proved difficult to answer, primarily because of observational problems. Another complication is that water may readily enter and leave the various types of cells in the phloem and the surrounding tissue. Therefore, the phloem cannot be viewed as an isolated independent system. For example, when the water potential in the xylem decreases, as occurs during rapid transpiration, solution in the phloem generally moves more slowly. Some water may move upward in the xylem and, later, downward in the phloem however, this is not the whole story because movement in the phloem can be in either direction. Moreover, the phloem can sometimes be the main supplier of water to certain regions of a plant, such as for fruits and various other organs when young (Nobel et al., 1994). 

Next we will estimate the various components of the water potential at the upper end of the phloem tissue under consideration (Fig. 9-18). The osmotic pressure in a sieve tube in the phloem of a leaf that is 10 m above the ground, nphloemi0m, might be due to the following solutes 0.5 m sucrose, 0.1 m other sugars, 0.05 m amino acids, and 0.05 m inorganic ions. Thus the... 

Consider a tree with a leaf area index of 6 and a crown diameter of 6 m. The trunk is 3 m tall, has a mean cross-sectional area of 0.10 m2 of which 5% is xylem tissue, and varies from an average water potential along its length of -0.1 MPa at dawn to -0.5 MPa in the steady state during the daytime. 

Smolenska-Sym, G., and Kacperska, A., 1996, Inositol 1,4,5-trisphosphate formation in leaves of water oilseed rape plants in response to freezing, tissue water potential and abscisic acid. Physiol. Plant 96 692-698. 

In the majority of crop plants that have been studied proline accumulation in leaf tissue occurs only in response to severe water stress and is usually accompanied by visible wilting. A reduction in leaf water potentials to —10... 

Furthermore, Liittge et al. (1975) showed malic acid synthesis by dark CO2 fixation and malic acid accumulation to occur in leaf slices of Kalanchoe daigre-montiana, provided the cells and external medium were approximately isotonic (i.e., when the turgor pressure of the cells was low). Moreover, the tissue slices acidified in a medium of proper water potential or deacidified if transferred into a medium of higher water potential. Hence, a rhythmic change in malic acid content of the cells could be achieved by varying the water potential of the external solution (i.e., indirectly the turgor of the cells). 

Osmotic pressures in the wall space are believed to explain the observation that nontranspiring plants have negative water potentials. Cosgrove and Cleland conclude that the internal gradient in water potential from the xylem to the epidermis, which sustains cell enlargement, is small. Auxin does not alter the hydraulic conductance of stem tissue either at the cellular or whole tissue level. How extracellular nitrogenous compounds contribute to or modify this response remains unknown. 

Water will, therefore, leave or enter the cell according as to whether the osmotic potential of the external solution is greater or less than the protoplast water potential ( Pp) (suction force). In land plants, the tissues are usually not fully turgid and the suction force of each separate tissue can be determined by finding the osmotic potential to which the tissue can be subjected without gain or loss in weight (or volume) (Fig. 2.5). Usually the value so determined cannot be regarded as the true suction force for the tissue in situ because within... 

Fig. 2.S. Relationship between change in tissue or cell volume or weight from its initial (natural) value and the osmotic potential of the solution in which it is immersed. The osmotic potential in which there is no change in volume or weight gives the (water potential of protoplast) of the cell or tissue. Pp is equal but opposite in sign to the DPD (diffusion pressure deficit) or SF (suction force). (From T. A. Rennet-Clark, in Plant Physiology, vol. 2, pp. 105-192, edited by F. C. Steward, Academic Press, New York, 1959.)...

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