Long-Distance Transport in

Plants

Biology 1001

November 21, 2005

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!

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

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

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

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

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

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

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!