Transport of Water

Water and dissolved minerals enter the roots through root hairs by osmosis. There are two pathways by which the water moves toward the center of the root, as follows.

1. Water moves through cell walls from one cell to another without ever entering the cells.

This pathway is called the apoplast and consists of the “nonliving” portion of cells.

2. Water moves from one cell to another through the symplast, or “living” portion of cells. In this pathway, it moves from the cytoplasm of one cell to the cytoplasm of the next through plasmodesmata, small tubes that connect the cytoplasm of adjacent cells. When water reaches the endodermis, it can continue into the vascular cylinder only through the symplast pathway. The apoplast pathway is blocked by the suberin that permeates the casparian strip. The endodermal cells allow water to enter the stele (vascular cylinder) but are selective as to which minerals are allowed to enter. For example, potassium (K+), an essential mineral, is allowed to pass, while sodium (Na+), common in soils but unused in plants, is blocked. Once through the endodermis, water and minerals continue by the apoplast pathway to the xylem.

The xylem tissue, consisting of tracheids and vessels, is the major conducting mechanism of the plant. Three mechanisms are involved in the movement of water and dissolved minerals in plants.

These mechanisms are described in the following points:

1. Osmosis.Water moves from the soil through the root and into xylem cells by osmosis. A concentration gradient between the soil and the root is maintained in two ways—by the continuous movement of water out of the root by xylem and by the higher mineral concentration inside the stele maintained by the selective passage of ions through the endodermis. To a certain extent, the movement of water into the root by this concentration gradient forces water up the xylem. This osmotic force, called root pressure, can be seen as guttation, the formation of small droplets of sap (water and minerals) on the ends of leaves of grasses and small herbs in the early morning. Under most environmental conditions, however, the forces generated by root pressure are too small to have a major effect on the movement of water in plants, especially large plants such as trees.

2. Capillary action. Capillary action or capillarity, is the rise of liquids in narrow tubes. It also contributes to the movement of water up xylem. Capillary action results from the forces of adhesion (molecular attraction between unlike substances) between the water and the capillary tube (a tube with a narrow bore). These forces combine to pull water up the sides of the tube. As a result, a meniscus, or crescent-shaped surface, forms at the top of the water column. In active xylem cells, however, water forms a continuous column without menisci. Thus, the effect of capillary action is minimal, confined to minute cavities in the cellulose microfibrils of the cell wall.

3. Cohesion-tension theory. Although root pressure and capillary action may make minor contributions to water movement under special conditions, most water movement through xylem is explained by cohesion-tension theory. The major concepts of this theory are as follows:

• Transpiration, the evaporation of water from plants, removes water from leaves, causing a negative pressure, or tension, to develop within the leaves and xylem tissue.

• Cohesion between water molecules produces a single, polymerlike column of water from roots to leaves. Cohesion is the molecular attraction between like substances. In water, cohesion results from the polarity of water molecules, which causes hydrogen bonding to occur between adjacent water molecules. As a result, the water molecules within a series of xylem cells (vessels or tracheids) behave as a single, polymerlike molecule.

• Bulk flow of water through xylem cells occurs as water molecules evaporate from the leaf surface. When a water molecule is lost from a leaf by transpiration, it pulls up behind it an entire column of water molecules. In this way, water moves by bulk flow through the xylem by a pulling action generated by transpiration. Since transpiration is caused by the heating action of the sun, the sun, then, is the driving force for the ascent of sap through plants.