Last week’s leaf about humidity and evaporation rates mentioned partial pressure as a measure of water vapour air. This week is about “osmotic pressure”, otherwise know as waterpotential (Ψw), that is a major topic in studying plant physiology.
By conversion we assignpure water a Ψw of zero. If we dissolve anything in the water, the Ψw becomes negative, e.g. seawater with 3% saltcontent has Ψw≈-27 atm. This means that to desalinate seawater by squeezing itthrough a special membrane used for the purpose 27 atm pressure; it is anenergy intensive process.
Similar rules apply to botanical plants. Soil Ψw is always negative, so plants need to “suck” water up to the leaves. This means that Ψw inside leaf cells needs to be more negative that soil Ψw.
A plant cells issurrounded by a cell membrane that keeps the molecules it needs inside. Thecell swells when water enters but constrained by a rigid cell wall on theoutside. Thus, a positive hydrostatic pressure develops. We call this turgorpressure, and it keeps the leaves and stems stiff. A good analogy is inflatinga bicycle tyre innertube inside its stiffer outer casing. But when cells losewater, the cell membrane shrinks away from the cell wall, a state calledplasmolysis, causing loss of turgor pressure and wilting of leaves.
The two circles in thecover illustration show views of cells in states of turgor and plasmolysis, ademonstration that students of plant physiology would have seen in the teachinglaboratory. The demonstration uses a layer of plant cells (e.g. epidermispeeled from onion skins), a low-power microscope and a stain (usually pink) tovisualize the cell contents.
When the tissue pieces are place in a sugar solution (Ψw less than Ψw of the cells), the stained cellcontents shrink away from the boxlike cell walls to a state of plasmolysis (upper circle in illustration). When the tissue pieces are then transferred to water, they swell back to full turgor (lower circle), Ψw reaches zero (Ψw of water)and the negative Ψw of cell contents is balanced by the positive turgor pressure. The process is reversible.
