Transpiration is also crucial in maintaining water pressure within cells, keeping them
rigid so they can support the plant.The water
pressure inside plant cells is called turgor
pressure, and it is maintained by a process called osmosis.
Technically speaking, osmosis is the movement of water across a differentially
permeable membrane from a place where water concentration is higher to one where the
concentration is lower.
Fluids like to reach a state of equilibrium. If I pour
milk into my coffee, it doesn’t all stay in one place but diffuses throughout the
cup. If you put a drop of food coloring in a basin of water, it diffuses until all the
water is tinted.
Plant cells maintain a delicate balance of water and
various dissolved salts and sugars. If the fluid inside the plant cell is
"saltier" than the surrounding fluid, water molecules move in to try to reach
equilibrium. If there were no cell membrane, then at the same time the salty water would
diffuse out, until the salt concentrations inside and outside the cell were equal.
BUT: the cell membrane is "differentially
permeable," meaning that water molecules can enter, but the salt molecules are too
large to escape. The result is that water pressure builds inside the cell, causing the
cell membrane to exert pressure on the cell wall—in much the same way a balloon
inflated inside a box would exert pressure on all sides of the box.
Osmosis
These rigid, stacked "boxes" keep the plant
upright. If the "balloons" deflate, then the boxes collapse. Plants must
maintain their internal water pressure, or turgor pressure, to keep stems rigid and leaves
expanded to the sunlight. This means that water must be available to the plant whenever it
needs it. If water isn’t available, cells collapse and the plant wilts. So, next time
you have a dry spell, and are commiserating with your gardening friends, you might just
say, "Boy, the soil is dry as a bone. All my beautiful plants are losing turgor
pressure!"
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Salt of the Earth
We’ve mentioned that plants must maintain a delicate balance
of water and various dissolved salts in order to sustain the proper turgor pressure and
keep the cells rigid. What are these salts, and where do they come from?
Though we commonly think of "salt" as table salt, or sodium chloride,
chemically-speaking there are many different types of salts. A salt is, by definition,
"a substance formed from an acid when all or part of its hydrogen is replaced by a
metal or metallic radical." For example, if you take hydrochloric acid, HCl, and
replace the hydrogen (H) with sodium (Na), you get NaCl, sodium chloride, or table salt.
Other salts include calcium chloride and magnesium sulfate (also called Epsom salts).
Most salts dissolve readily in water. And many fertilizers contain nutrients in
the form of salts, since in this form they will dissolve in water and quickly become
available to the plants.
More Isn’t Always Better
You may have heard warnings that you can
"burn" your plants’ roots if you overfertilize them. But just what is
burning?
If you water a plant with a concentrated fertilizer
solution (which contains high levels of dissolved salts) you can create a condition in
which the water in the soil is "saltier" than the water in the plant. We’ve
discussed the tendency of liquids to move toward a state of equilibrium. In this case,
water will begin to flow out of the plant cells and into the "saltier"
soil. Though you may think you are providing water to your plant, you are actually causing
water loss.
To prevent fertilizer burn, always measure and mix
fertilizers carefully, following the manufacturers’ instructions—never feed
plants with a concentrated solution of fertilizer that hasn’t been properly diluted
with water. Or use organic or slow-release fertilizers— the nutrients in these are in
a less soluble form and become available over time rather than all at once. |
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