Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 38 LECTURE SLIDES

Transport Mechanisms Water first enters the roots Then moves to the xylem –Innermost vascular tissue Water rises through the xylem because of a combination of factors Most of that water exits through the stomata in the leaves 2

Water and dissolved minerals travel great distances in xylem –Some “pushing” comes from pressure of water entering roots –Most of the force is “pulling” created by transpiration Evaporation from thin films of water in the stomata Occurs due to cohesion (water molecules stick to each other) and adhesion (stick to walls) 3

4

5

Osmosis –If a single plant cell is placed into water Concentration of solutes inside cell greater than solution Water moves into cell by osmosis Cell expands and becomes turgid –If cell placed in high concentration of sucrose Water leaves cell Cell shrinks – plasmolysis 6

Water potential has 2 components 1.Physical forces such as plant cell wall or gravity Contribution of gravity usually not considered Turgor pressure resulting from pressure against cell wall Concentration of solute in each solution 7

Once thought water moved across plasma membranes only by osmosis through the lipid bilayer –Water moved more rapidly than predicted Osmosis is enhanced by membrane water channels called aquaporins –Speed up osmosis but do not change direction of water movement 8

Most of the water absorbed by the plant comes in through the region of the root with root hairs –Surface area further increased by mycorrhizal fungi Once absorbed through root hairs, water and minerals must move across cell layers until they reach the vascular tissues Water and dissolved ions then enter the xylem and move throughout the plant 9

Eventually on their journey inward, molecules reach the endodermis Any further passage through the cell walls is blocked by the Casparian strips Molecules must pass through the cell membranes and protoplasts of the endodermal cells to reach the xylem 10

Because the mineral ion concentration in the soil water is usually much lower than it is in the plant, an expenditure of energy (ATP) is required for these ions to accumulate in root cells Plasma membranes of endodermal cells contain a variety of protein transport channels, through which proton pumps transport specific ions against even larger concentration gradients 11

Xylem Transport Root pressure is caused by the continuous accumulation of ions in the roots –When transpiration from leaves is low or absent – at night Causes water to move into plant and up the xylem despite the absence of transpiration Guttation (production of dew) is loss of water from leaves when root pressure is high Root pressure alone, however, is insufficient to explain xylem transport –Transpiration provides the main force 12

Cavitation –An air bubble can break the tensile strength of a water column –A gas-filled bubble can expand and block the tracheid or vessel –Damage can be minimized by anatomical adaptations Presence of alternative pathways Pores smaller than air bubbles 13

14

Tracheids and vessels are essential for the bulk transport of minerals –Ultimately the minerals are relocated through the xylem from the roots to other metabolically active parts of the plant –Phosphorus, potassium, nitrogen, and sometimes iron –Calcium 15

Rate of Transpiration Over 90% of the water taken in by the plant’s roots is ultimately lost to the atmosphere At the same time, photosynthesis requires a CO 2 supply from the atmosphere Closing the stomata can control water loss on a short-term basis However, the stomata must be open at least part of the time to allow CO 2 entry 16

Guard cells –Only epidermal cells containing chloroplasts –Have thicker cell walls on the inside and thinner cell walls elsewhere Bulge and bow outward when they become turgid Causing the stomata to open –Turgor in guard cells results from the active uptake of potassium (K + ), chloride (Cl) –Water enters osmotically 17

18

19

Active pumping of sucrose out of guard cells in the evening leads to loss of turgor and closes the guard cell 20

Rate of Transpiration Transpiration rates increase with temperature and wind velocity because water molecules evaporate more quickly Several pathways regulate stomatal opening and closing –Abscisic acid (ABA) initiates a signaling pathway to close stomata in drought Opens K +, Cl – channels Water loss follows 21

Other pathways regulating stomata –Close when CO 2 concentrations are high –Close when temperature exceeds 30º–34ºC and water relations unfavorable Alternative photosynthetic pathways, such as Crassulacean acid metabolism (CAM), reduce transpiration 22

Water Stress Responses Many morphological adaptations allow plants to limit water loss in drought conditions –Dormancy –Loss of leaves – deciduous plants –Covering leaves with cuticle and wooly trichomes –Reducing the number of stomata –Having stomata in pits on the leaf surface 23

24

Plants have adapted to flooding conditions which deplete available oxygen –Flooding may lead to abnormal growth –Oxygen deprivation most significant problem –Plants have also adapted to life in fresh water Form aerenchyma, which is loose parenchymal tissue with large air spaces Collect oxygen and transport it to submerged parts of the plant 25

26

Plants such as mangroves grow in areas flooded with salt water –Must supply oxygen to submerged roots and control salt balance –Pneumatophores – long, spongy, air-filled roots that emerge above the mud Provide oxygen to submerged roots –Succulent leaves contain large amount of water to dilute salt –May secrete salt or block salt uptake 27

28

Halophytes –Plants that can tolerate soils with high salt concentrations –Some produce high concentrations of organic molecules in their roots This decreases the water potential enhancing water uptake from the soil 29

Phloem Transport Most carbohydrates produced in leaves are distributed through phloem to rest of plant Translocation provides building blocks for actively growing regions of the plant Also transports hormones, mRNA, and other molecules –Variety of sugars, amino acids, organic acids, proteins, and ions 30

Using aphids to obtain the critical samples and radioactive tracers to mark them, plant biologists have demonstrated that substances in phloem can move remarkably fast, as much as 50 to 100 cm/h 31 a. 400 µm b. Stylet Phloem 25 µm Phloem fluid a: © Andrew Syred/Photo Researchers, Inc.; b: © Bruce Iverson Photomicrography. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Phloem-loading occurs at the source –Carbohydrates enter the sieve tubes in the smallest veins at the source –Sieve cells must be alive to use active transport to load sucrose 32