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Introduction: Water potential (?w) is an expression of the free energy of water. It is a measure of the driving force that causes water to move into or out of any system such as the soil, air and plant tissue. ?w is probably the most meaningful property that can be measured in the soil-plant-air system. Water potential is expressed by the following equation: ?w = ?p + ?? + ?m Where: ?w = water potential of a cell ?? = osmotic potential, the contribution of dissolved solutes to water potential, it is always negative ?p = pressure potential, (turgor pressure) – can be positive (turgid), zero (flaccid) or negative (tension). Negative pressure, or tension, is a very rare occurrence in a cell. ?m = matric potential, the contribution to water potential by water-binding colloids(e.g.cell walls). It is always negative and usually small enough to be ignored in the plant cell Activity 1: Determining the Water Potential of Potato Tuber Tissue In this experiment, we will measure the water potential (?w) of potato tubers and calculate the pressure potential (?p) based on an estimate of the osmotic potential (??). The water potential (?w)will be measured by placing potato tuber samples into a series of Sodium Chloride solutions which vary in their water potentials. Solutions with water potentials greater than the potato tissue will cause the sample to gain water (and size as cells swell). Solutions with potentials less than the potato will cause the samples to lose water, causing cells to shrink. Theoretically there should exist a solution whose water potential is exactly equal to that of the potato. By plotting the concentration of the salt solution (x axis) versus the change in length of the potato samples (y axis), one should be able to find the point at which no change in length will occur. This value will be the x-intercept of the graph after a linear regression is performed on the data. The x-intercept gives the theoretical value of the salt solution which has the same water potential of the potato tissue. To obtain the actual water potential, a table is used to convert the units of Molarity into Bars. The water potential is estimated using this method, assuming that all of the potato samples have been given enough time to equilibrate with their surrounding solutions. Thus, it is important that the samples are left in their solutions as long as possible before measuring for the second time (after about 8 hours). The osmotic potential (??) can be measured by determining the freezing point depression of the cell sap from homogenized tubers. This method will not be done during this activity, but an estimate of ? from the literature will be provided for purposes of calculations. For convenience, it will be assumed that the matric potential (?m) is very small, and thus after, rearranging the initial equation, the equation for calculating pressure potential (?p) is: ?p = ?w - ??

Introduction: Water potential (?w) is an expression of the free energy of water. It is a measure of the driving force that causes water to move into or out of any system such as the soil, air and plant tissue. ?w is probably the most meaningful property that can be measured in the soil-plant-air system. Water potential is expressed by the following equation:
?w = ?p + ?? + ?m
Where:
?w = water potential of a cell
?? = osmotic potential, the contribution of dissolved solutes to water potential, it is always negative
?p = pressure potential, (turgor pressure) – can be positive (turgid), zero (flaccid) or negative (tension). Negative pressure, or tension, is a very rare occurrence in a cell.
?m = matric potential, the contribution to water potential by water-binding colloids(e.g.cell walls). It is always negative and usually small enough to be ignored in the plant cell
Activity 1: Determining the Water Potential of Potato Tuber Tissue
In this experiment, we will measure the water potential (?w) of potato tubers and calculate the pressure potential (?p) based on an estimate of the osmotic potential (??).
The water potential (?w)will be measured by placing potato tuber samples into a series of Sodium Chloride solutions which vary in their water potentials. Solutions with water potentials greater than the potato tissue will cause the sample to gain water (and size as cells swell). Solutions with potentials less than the potato will cause the samples to lose water, causing cells to shrink.
Theoretically there should exist a solution whose water potential is exactly equal to that of the potato. By plotting the concentration of the salt solution (x axis) versus the change in length of the potato samples (y axis), one should be able to find the point at which no change in length will occur. This value will be the x-intercept of the graph after a linear regression is performed on the data. The x-intercept gives the theoretical value of the salt solution which has the same water potential of the potato tissue. To obtain the actual water potential, a table is used to convert the units of Molarity into Bars. The water potential is estimated using this method, assuming that all of the potato samples have been given enough time to equilibrate with their surrounding solutions. Thus, it is important that the samples are left in their solutions as long as possible before measuring for the second time (after about 8 hours).
The osmotic potential (??) can be measured by determining the freezing point depression of the cell sap from homogenized tubers. This method will not be done during this activity, but an estimate of ? from the literature will be provided for purposes of calculations.
For convenience, it will be assumed that the matric potential (?m) is very small, and thus after, rearranging the initial equation, the equation for calculating pressure potential (?p) is: ?p = ?w - ??

Added by Michael C.

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