Transport in Vascular Plants

Overview of Transport MechanismsAbsorption of Water and

Minerals by RootsTransport of Xylem SapThe Control of TranspirationTranslocation of Phloem Sap

Overview

At the cellular level: active and passive transport

Proton pumps-move H+ out of cell, creates electrochemical gradient and pulls + ions into the cell

Role of water potential and osmosis Role of aquaporins Symplast and apoplast pathways

Absorption of Water and Minerals SoilEpidermisRoot CortexXylem Soil epidermis-root hairs, mycorrhizae Epidermisto root cortex-combination of

apoplastic and symplastic routes Root cortex xylem-apoplastic route

blocked by Casparian strip surrounding endodermis, minerals and ions must go through symplastic route

Transport of Xylem Sap

Depends upon transpiration and physical properties of water

15 m/hr or faster Root pressure pushes, limited TACT forces (transpiration, adhesion,

cohesion and tension) Bulk flow, movement of fluid due to

pressure differences at opposite ends of the conduit

The Control of Transpiration• Role of Guard Cells-regulate

transpiration rate• Benefits of transpiration-mineral

transfer, evaporative cooling• Problems created by transpiration-

excess water loss• Regulation of guard cells balances

the benefits and problems

Guard Cell Regulation

Size of cells change in response to water flow in or out

Osmotic changes initiated by manipulating K+ concentrations. K+ in water comes in, increased turgor, cell size increase, K+ out….

[K+] probably regulated by proton pumps responding to light and blue light receptors

Abcissic acid (plant hormone) can override system when excess transpiration occurs

Xerophytes-Special Adaptations• “desert” plants• Thick cuticle• Small leaves, sometimes needles• Store water• Stomates in pits• Fewer stomates• CAM plants• C4 plants

Phloem Function

Translocation Sugar produced at source (leaf), unloaded into

sink (roots, stems, fruit) Active transport moves sugar into system by

proton pumps creating an H+ gradient and subsequent cotransport of sucrose.

Utilizes both symplastic and apoplastic routes One meter/hour Pressure flow moves material through the tubes

(hydrostatic pressure) Aphids have been used to study this process?????