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Most plants secure the water and minerals they need from their roots.The path taken is: soil -> roots -> stems -> leaves The minerals (e.g., K+, Ca2+) travel dissolved in the water (often accompanied by various organic molecules supplied by root cells). Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth. Most of it is lost in transpiration.
|Link to Transpiration|
|Link to diagram of root structure.|
Once inside the stele, water is again free to move between cells as well as through them. In young roots, water enters directly into the xylem vessels and/or tracheids [link to views of the structure of vessels and tracheids]. These are nonliving conduits so are part of the apoplast.
Minerals enter the root by active transport into the symplast of epidermal cells and move toward and into the stele through the plasmodesmata connecting the cells.
They return to the apoplast from the cells of the pericycle (as well as of parenchyma cells surrounding the xylem) through specialized transmembrane channels.
Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessels and tracheids.
At any level, the water can leave the xylem and pass laterally to supply the needs of other tissues.
At the leaves, the xylem passes into the petiole and then into the veins of the leaf. Water leaves the finest veins and enters the cells of the spongy and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.
In 1895, the Irish plant physiologists H. H. Dixon and J. Joly proposed that water is pulled up the plant by tension (negative pressure) from above.
As we have seen, water is continually being lost from leaves by transpiration. Dixon and Joly believed that the loss of water in the leaves exerts a pull on the water in the xylem ducts and draws more water into the leaf.
But even the best vacuum pump can pull water up to a height of only 32 ft or so. This is because a column of water that high exerts a pressure (~15 lb/in2) just counterbalanced by the pressure of the atmosphere. How can water be drawn to the top of a sequoia or Douglas fir that may be 300 feet high? Taking all factors into account, a pull of at least 150 lb/in2 is probably needed.
The answer to the dilemma lies the cohesion of water molecules; that is the property of water molecules to cling to each through the hydrogen bonds they form.
|Link to discussion of hydrogen bonding in water.|
When water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water. Tensions as great as 3000 lb/in2 are needed to break the column, about the value needed to break steel wires of the same diameter. In a sense, the cohesion of water molecules gives them the physical properties of solid wires.
Because of the critical role of cohesion, the transpiration-pull theory is also called the cohesion theory.
The graph shows the results of obtained by D. T. MacDougall when he made continuous measurements of the diameter of a Monterey pine. The diameter fluctuated on a daily basis reaching its minimum when the rate of transpiration reached its maximum (around noon)
If forced to take water from a sealed container, the vine does so without any decrease in rate, even though the resulting vacuum becomes so great that the remaining water begins to boil spontaneously. (The boiling temperature of water decreases as the air pressure over the water decreases, which is why it takes longer to boil an egg in Denver than in New Orleans.)
When water is placed under a high vacuum, any dissolved gases come out of solution as bubbles (as we saw above with the rattan vine). This is called cavitation. Any impurities in the water enhance the process. So measurements showing the high tensile strength of water in capillaries require water of high purity.Under the negative pressures thought to exist in active xylem, might cavitation break the column of water and thus interrupt its flow? Probably not.
Recently two groups have demonstrated that water in the xylem avoids cavitation even at negative pressures exceeding 225 lb/in2 - tensions that would easily lift water to the top of the tallest trees. (Both groups made their measurements by spinning branches in a centrifuge.).
Root pressure is created by the osmotic pressure of xylem sap which is, in turn, created by dissolved
One important example is the sugar maple when, in very early spring, it hydrolyzes the starches stored in its roots into sugar. This causes water to pass by osmosis through the endodermis and into the xylem ducts. The continuous inflow forces the sap up the ducts.Although root pressure plays a role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport.
|Link to discussion of food transport in the phloem.|