Transport in Plants

1. Transport in Plants Chapter 37

2. Outline • Overview of Water and Mineral Movement • Water Potential • Water and Mineral Absorption • Water and Mineral Movement • Bidirectional Phloem Transport

3. 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

4. Water Movement Through A Plant

5. 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.

6. Mineral Transport in Roots

7. 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

8. 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.

9. Transpiration

10. 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.

11. 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

12. Water and Mineral Movement • Other factors regulating transpiration • carbon dioxide • light • temperature • Water loss regulation • dormancy • deciduous leaves • thick, hard leaves • trichomes

13. 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

14. Aerenchyma

15. Plant Response to Flooding • Salt water • Salt must be excluded, actively secreted, or diluted as it enters. • Mangroves have pneumatophores.

16. 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.

17. 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