1. Long-Distance Transport in Plants Biology 1001 November 21, 2005

2. 1. Introduction  Roots absorb water and minerals from the soil, and shoots absorb light and carbon dioxide for photosynthesis  Vascular plants use xylem to transport water and minerals from roots to shoots and phloem to transport sugars from where they are produced or stored to where they are needed for growth and metabolism  Long-distance transport occurs by bulk flow, the pressure-driven movement of a fluid  We consider four transport functions in plants Absorption of water and minerals by roots Ascent of water and minerals in the xylem The control of transpiration Translocation of sugars in the phloem Figure 36.2!

3. 2. Absorption of Water and Minerals by Roots  Short-distance or lateral transport from the soil, through the epidermis and cortex, to the xylem Precedes ascent via bulk flow up the tracheids & vessels of the xylem  There are two routes of cell-to-cell transport in plants: a living route called the symplast through the protoplasts, and a non- living route called the apoplast through the cell walls and intercellular spaces

4. Lateral Transport of Minerals and Water in Roots – Figure 36.9!,!!

5. Lateral Transport in Roots  Water & minerals in soil solution must first cross the epidermis of the root, usually near its tip Root hairs greatly increase the surface area for uptake of soil solution Uptake of soil solution by the hydrophilic walls of epidermal cells provide entry into the apoplast Soil solution travels via the apoplast towards the stele, where it can divert into the symplast at any time by crossing plasma membranes of cortical cells  Cortical cells increase the surface area for exposure to soil solution and concentration of minerals in the symplast Alternatively, some soil solution crosses the plasma membrane and directly enter the symplast at the root hair Mycorrhizae also greatly increase the surface area of roots exposed to soil solution

6. The Role of the Endodermis  All materials travelling through the cortex of the root must enter the xylem before being transmitted to the rest of the plant  The endodermis, the innermost layer of cells in the root cortex, acts as a checkpoint for selective passage of minerals from cortex to vascular tissue Minerals in the symplast continue through the plasmodesmata of the endodermal cells into the vascular tissue Minerals that reach the endodermis via the apoplast encounter a waxy suberin belt called the Casparian Strip which forces them to cross a plasma membrane before entering the endodermis In this way all minerals passing into the vascular tissue are screened by a selectively permeable plasma membrane  To enter the xylem water & minerals re-enter the apoplast, as xylem cells lack protoplasts

7. 3. The Ascent of Water & Minerals in the Xylem  Water & minerals ascend from roots to shoots through the xylem  The mechanism is a type of bulk flow caused by transpiration, the loss of water vapour from the leaves and other aerial parts of the plant Causes xylem sap to rise as much as 100 m in certain trees A single tree can transpire 200L of water per hour in the summer  Transpiration generates a negative pressure (tension) from above Best explained by the Transpiration-Cohesion-Tension Mechanism  A small amount of xylem sap is pushed upward during the night by root pressure, resulting in guttation

8. The Transpiration-Cohesion-Tension Mechanism  The Transpiration-Cohesion-Tension Mechanism starts with transpiration pull generated in a leaf, Figure 36.12! Water vapour in the air spaces between mesophyll cells diffuses through the stomata because the air outside the leaf is drier This water is replaced from the water lining the surface of mesophyll cells in the air spaces, creating surface tension (negative pressure) This tension pulls water from the xylem – relies on adhesion of water molecules to hydrophilic cell wall components and cohesion to each other

9. Cohesion and Adhesion in the Ascent of Xylem Sap  The transpirational pull generated in the leaf is transmitted along the water column in the xylem, all the way to the roots and into the soil solution, by cohesive and adhesive forces Cohesion is the attraction of water molecules to each other by hydrogen bonding Adhesion is the adherence of water molecules to the hydrophilic components of cells such as xylem  Note that the bulk flow caused by the negative pressure of transpiration moves the water and the minerals together up the xylem, from the roots to the shoots  Xylem sap ascent is ultimately solar powered! Figure 36.13!