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Transport of Water and Minerals in Plants

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

However, transpiration does serve two useful functions:

Water and minerals enter the root by separate paths which eventually converge in the stele.

Link to diagram of root structure.

The Pathway of Water

Soil water enters the root through its epidermis. It appears that water then travels in both

However, the inner boundary of the cortex, the endodermis, is impervious to water because of a band of lignified matrix called the casparian strip. Therefore, to enter the stele, apoplastic water must enter the symplasm of the endodermal cells. From here it can pass by plasmodesmata into the cells of the stele.

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.

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.

The Pathway of Minerals

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 enter the water in the xylem from the cells of the pericycle (as well as of parenchyma cells surrounding the xylem) through specialized transmembrane channels.

What Forces Water Through the Xylem?

Observations

Transpiration-Pull

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 34 ft (10.4 m) or so. This is because a column of water that high exerts a pressure of ~15 lb/in2 (103 kilopascals, kPa) just counterbalanced by the pressure of the atmosphere. How can water be drawn to the top of a sequoia (the tallest is 370 feet [113meters] high)? Taking all factors into account, a pull of at least 270 lb/in2 (~1.9 x 103 kPa) 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 ultrapure water is confined to tubes of very small bore, the force of cohesion between water molecules imparts great strength to the column of water. It has been reported that tensions as great as 3000 lb/in2 (21 x 103 kPa) 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.

Some support for the theory

Problems with the theory

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 — not the case for sap in the xylem.

So might cavitation break the column of water in the xylem and thus interrupt its flow? Probably not so long as the tension does not greatly exceed 270 lb/in2 (~1.9 x 103 kPa).

By spinning branches in a centrifuge, it has been shown that water in the xylem avoids cavitation at negative pressures exceeding 225 lb/in2 (~1.6 x 103 kPa). And the fact that sequoias can successfully lift water 358 ft (109 m) — which would require a tension of 270 lb/in2 (~1.9 x 103 kPa) — indicates that cavitation is avoided even at that value.

However, such heights may be approaching the limit for xylem transport. Measurements close to the top of the tallest living sequoia (370 ft [=113 m] high) show that the high tensions needed to get water up there have resulted in: (Reported by Koch, G. W. et al., in Nature, 22 April 2004.)

So the limits on water transport limit the ultimate height which trees can reach. The tallest tree ever measured, a Douglas fir, was 413 ft. (125.9 meters) high.

Root Pressure

When a tomato plant is carefully severed close to the base of the stem, sap oozes from the stump. The fluid comes out under pressure which is called root pressure.

Root pressure is created by the osmotic pressure of xylem sap which is, in turn, created by dissolved

that have been actively transported into the apoplast of the stele.

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.

So although root pressure may play a significant role in water transport in certain species (e.g., the coconut palm) or at certain times, most plants meet their needs by transpiration-pull.

Link to discussion of food transport in the phloem.
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19 March 2014