10.3 Water and Mineral Transport

10.3 Water and Mineral Transport

A tall angiosperm has no heart, yet leaves high in the crown still receive water. The lift is a physical pull called the transpiration-cohesion-tension mechanism. Minerals ride with that water, and the root controls much of what is allowed into the xylem. No metabolic pump lifts the whole water column.

Absorption in the root

Soil water enters root hairs and other epidermal cells, then moves across the cortex. Two routes are open there. In the apoplast, water travels through cell walls and water-filled spaces without crossing a plasma membrane at every cell. In the symplast, water crosses a membrane once and then moves from cell to cell through the cytoplasm, passing by way of plasmodesmata. Both routes operate in the cortex. The choice ends at the endodermis.

The Casparian strip

The Casparian strip is a waxy band of suberin in the radial walls of the endodermal cells. It blocks the apoplast. Water and dissolved solutes cannot slip between endodermal cells into the vascular cylinder. They must cross a cell membrane and enter the symplast before they can reach the xylem. Membrane proteins admit needed ions and limit others. That membrane step lets the plant select minerals. The strip is a checkpoint at the door of the stele. It is not a pump that hauls water to the leaves.

Mineral ions, including nitrate and potassium, are often taken up by active transport. The root spends metabolic energy and can move those ions against a concentration gradient. Many angiosperm roots also form mycorrhizae, whose hyphae extend past the root hairs and improve uptake of minerals, especially phosphate.

After ions reach the stele, water follows by osmosis. At night, when the leaves are barely pulling, that osmotic inflow can create a mild root pressure. Root pressure can force guttation, droplets pushed from hydathodes at the leaf edges. The push is real in short plants and in still, humid nights. It does not supply the crown of a tree on a sunny afternoon. No metabolic pump stationed along the trunk does that job either.

Hollow cells of the xylem

Two kinds of cells conduct the water. Tracheids are long and narrow, and pits in their walls let water pass from cell to cell. Vessel elements are wider, stack end to end into vessels, and have perforation plates at their ends. Vessel elements are typical of angiosperms and offer a wider pipe than a tracheid. Both kinds of cells are dead and hollow at maturity. Their living contents are gone, and lignified walls remain. Hollow tubes fit a passive stream. They are the wrong cells for active loading of sugar.

The sap in these tubes is mostly water with dissolved mineral ions. It is not a sugar solution. If an item claims that leaves load sucrose into vessel elements and sweeten the xylem, reject that picture. Sucrose belongs to phloem transport. Xylem sap is the dilute mineral water of the transpiration stream.

Transpiration, cohesion, and tension

The pull begins in the leaf. Water evaporates from the moist walls of mesophyll cells into the air spaces, then diffuses out through the stomata. That loss is transpiration. As water leaves the mesophyll, replacement water is drawn from the xylem of the vein. The draw puts the continuous water columns under tension, a negative pressure.

Cohesion keeps each column intact. Water molecules attract one another by hydrogen bonds, so a pull at the top is transmitted all the way to the root. Adhesion of water to the hydrophilic walls of the xylem helps the column resist gravity. Put the three words in order: transpiration at the leaves, cohesion by hydrogen bonds, and adhesion to the xylem walls. The root supplies the water. The stem does not contain a row of pumps.

If tension breaks one water column, pits can let water move sideways into a neighboring conduit.

Guard cells and the pore

Two guard cells flank each stoma. They are modified epidermal cells, and unlike ordinary epidermal cells they usually contain chloroplasts. When guard cells take up potassium ions, water follows by osmosis, the cells become turgid, and they bow apart. The pore opens. When the cells lose that water they become flaccid and the pore closes. An open stoma admits carbon dioxide for photosynthesis and lets water vapor escape. The plant is always trading those two flows.

What changes the rate

Transpiration rises when stomata are open and the gradient of water vapor from the moist leaf interior to the outside air is steep.

Bright light opens stomata in most angiosperms and warms the leaf, so water loss increases. Wind sweeps away the humid layer of air sitting on the leaf surface, so water loss increases. Dry air keeps the outside humidity low, so water vapor diffuses out faster. High humidity around the leaf shrinks that vapor gradient, so water loss falls. Humid air does not plug the stomata and then speed the stream. It slows transpiration. Darkness closes stomata in many species. Drought can trigger abscisic acid signaling, which also closes stomata and conserves water.

The xylem still carries water and minerals under all of those conditions.

The path is soil water, root hair, cortex, endodermis, xylem, leaf vein, mesophyll air space, stoma, and outside air.