Water Relations. Steady- State. Surface. Area. “Tho’ vegetables have not an engine, which, by its alternate dilatations and contractions, does in animals forcibly drive the blood through the arteries and veins; yet has nature wonderfully contrived other means, most powerfully,
by its alternate dilatations and contractions,
does in animals forcibly drive the blood through
the arteries and veins; yet has nature wonderfully
contrived other means, most powerfully,
to raise and keep in motion the sap.
I shall begin with an experiment upon roots,
which nature has providently taken care to
cover with a fine thick strainer; that nothing
shall be admitted into them, but what can
readily be carried off by perspiration,
vegetables having no other provision for
discharging their recrement.”
Stephan Hales, Vegetable Staticks, 1727
Filter out the unwanted
Communicate with shoot
Growing root tips
growth is critical
to root function.
Root Cross Section
Endodermis is location of filter
Longest roots recorded are for mesquite, Prosopis glandulosa (80’)
Primary roots of an adult rye plant Secale cereale were measured and found to be 380 miles in length.
Note that even at near 100% RH, air still more negative than leaf
Thus: water flows from leaf to air
However, even at air RH 100%, the slightest air movement across the leaf lowers air to less than in leaf so water flows from leaf to air
During all this pulling, hydrogen bonds hold water molecules together in columns inside xylem
tubes = cohesion
The very negative
of the air tugs on
the water column,
causing the H2O
move up through
(Water molecules, not Disney symbols)
Cohesion/tension explains how water can travel upwards against gravity in a plant.
Transpiration at leaves
Water molecules pulled up stem to replace molecules lost to air
Tension on water in xylem
Water pulled into roots
Water into the Root against gravity in a plant.
Roots have evolved to increase water absorption area by formation of root hairs.
New root hairs have to be constantly produced to have water uptake.
Damaged or diseased roots do not produce root hairs, severely limiting their ability to take up water.
Many fungal or bacterial pathogens cause diseases with a characteristic symptom of wilt. The wilting comes because the pathogen enters the vascular tissue and as it grows, it clogs the water-conducting vessels.
Cutting a stem and seeing discolored vascular tissue is a good “clue” that helps diagnose disease.
In herbaceous stems a vertical cut is made just under the epidermis of the stem. If there is an infection, you can see a “streaking” in the vascular tissue.
Disease-clogged xylem against gravity in a plant.
Air bubble (vapor lock) in the xylem, break in the water chain NOT GOOD - stops water flow through that column in its tracks and often forever
Cut flowers often can’t take up water because of cavitation at cut ends of xylem - leads to the idea of cutting stems underwater.