Plant-Transport

Transport in Plants
Chapter 37
Outline
Overview of Water and Mineral Movement
Water Potential
Water and Mineral Absorption
Water and Mineral Movement
Bidirectional Phloem Transport

Overview of Water and Mineral Movement
Most nutrients and water enter plant through roots, and move upward in xylem.
Water moves through spaces between cell protoplasts, plasmodesmata, cell membranes and xylem.
Osmosis is enhanced by aquaporins (water channels) in vacuole and plasma membranes.
Transpiration “pulls” water through stomata.
cohesion
adhesion
Water Movement Through A Plant
Water and Mineral Absorption
Most water absorbed through root hairs
solute potential greater than surrounding soil
energy expenditure required to accumulate ions in root cells
proton pumps
Water and ions must pass through plasma membranes and protoplasts of endodermal cells to reach the xylem.
Mineral Transport in Roots
Water and Mineral Absorption
Root pressure - movement of water into the plant and up the xylem columns despite absence of transpiration
Active transport increases solute potential of roots.
High root pressure may lead to guttation.
occurs through groups of cells located near edge of leaf

Water and Mineral Movement
Evaporation from leaves produces a tension on entire water column extending down to the roots.
Water has inherent tensile strength that varies inversely with the diameter of the column.
Air bubbles cause xylem to fail.
Plants must admit carbon dioxide, but must control water loss.
Transpiration
Water and Mineral Movement
Regulation of transpiration
Stomata open and close due to changes in turgor pressure of guard cells.
Turgor results from active uptake of potassium (K+) ions.
Increase in K+ concentration creates a water potential that causes water to enter osmotically, guard cells to become turgid, and stomata to open.
Some other facts about the stomata
Open in the day and closed at night - need carbon dioxide in the daylight for photosynthesis
When water is scarce, plant wilts and guard cells become flacid
Abscisic acid - plant hormone that causes K+ to pass out of cells and guard cells become flacid
High levels of CO2 cause guard cells to become flacid
Leaves lost when water is scarce
Water and Mineral Movement
Other factors regulating transpiration
carbon dioxide
light
temperature
Water loss regulation
dormancy
deciduous leaves
thick, hard leaves
trichomes
Water and Mineral Movement
Plant response to flooding – depletes oxygen
fresh water
aerenchyma - loose parenchyma tissue with large air spaces
oxygen transported from above-water to below-water structures

Aerenchyma
Plant Response to Flooding
Salt water
Salt must be excluded, actively secreted, or diluted as it enters.
Mangroves have pneumatophores.
Phloem Transport is Bidirectional
Translocation - distribution of carbohydrates manufactured in leaves to rest of the plant

Energy requirements for phloem transport
mass-flow hypotheses
Dissolved carbohydrates flow from a source and are released at a sink.


Translocation - Distribution of nutrients, especially carbohydrates, through the phloem – “source and sink”
Osmosis plays an important role
Phloem loading - Sucrose is actively loaded into phloem tubes
Water moves in and carries sucrose along passively
No energy required for this process - but
Loading and unloading sucrose in phloem tubes does require energy
Companion cells of phloem provide the ATP