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

Transport in Plants Chapter 38

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

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)

Movement of water at cellular level Water can diffuse through plasma membranes Other substances depend on protein transporters Facilitated diffusion or active transport ATP-dependent hydrogen ion pumps often fuel active transport Unequal concentrations of solutes drive osmosis

Water Potential Potentials are a way to represent free energy Water potential (yw) is used to predict which way water will move Water moves from higher to lower yw Measured in units of pressure called megapascals (MPa)

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

Water potential has 2 components Physical forces such as plant cell wall or gravity Contribution of gravity usually not considered Turgor pressure resulting from pressure against cell wall is pressure potential yp As turgor pressure increases, yp increases Concentration of solute in each solution

Solutions that are not contained within a vessel or membrane cannot have turgor pressure, and they always have a Ψp of 0 MPa

Concentration of solute in each solution Solute potential (ys) – smallest amount of pressure needed to stop osmosis Pure water has a ys of 0 As solute concentration increases, ys decreases (< 0 MPa) A solution with a higher solute concentration has a more negative ys

yw = yp + ys The total water potential of a plant cell is the sum of its pressure potential (yp) and solute potential (ys) Represents the total potential energy of the water in the cell When the yw inside the cell equals that of the solution, there is no net movement of water

c. Water Potential ψ = ψs + ψp ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Water Potential ψ = ψs + ψp ψcell = – 0.7 MPa + 0.5 MPa = – 0.2 MPa ψsolution = –0.2 MPa (solution has no pressure potential) c.

Osmosis is enhanced by membrane water channels called aquaporins 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

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

Symporters contribute to the yw gradient that determines the Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Air Water enters plant through roots. Decreasing water potential Plant Soil H2O Soil Cytosol Symporters contribute to the yw gradient that determines the directional flow of water. H+ Symporter Mineral ions Soil Water -0.5 -1.0 -100 ψw Water potential (MPa)

result in higher rate of transpiration than smooth cell surfaces. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Air Water exits plant through stomata. Smooth surface Smooth surface Rippled surface Rippled surface H2O The water film that coats mesophyll cell walls evaporates. Rippled cell surfaces result in higher rate of transpiration than smooth cell surfaces. Decreasing water potential Plant Water moves up plant through xylem. Adhesion due to polarity of water molecules Cohesion by hydrogen bonding between water molecules Soil -0.5 -1.0 -100 ψw Water potential (MPa)

Three transport routes exist through cells Apoplast route – movement through the cell walls and the space between cells Avoids membrane transport Symplast route – cytoplasm continuum between cells connected by plasmodesmata Transmembrane route – membrane transport between cells and across the membranes of vacuoles within cells Permits the greatest control

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

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

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

Water potential regulates the movement of water through a whole plant Water moves from the soil into the plant only if water potential of the soil is greater than in the root Water in a plant moves along a yw gradient from the soil to successively more negative water potentials in the roots, stems, leaves, and atmosphere

Water has an inherent tensile strength that arises from the cohesion of its molecules The tensile strength of a water column varies inversely with its diameter Because tracheids and vessels are tiny in diameter, they have strong cohesive water forces The long column of water is further stabilized by adhesive forces

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

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, an essential nutrient, cannot be transported elsewhere once it has been deposited in a particular plant part

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 CO2 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 CO2 entry

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–), and malate Water enters osmotically

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

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–, and malate channels Water loss follows

Other pathways regulating stomata Close when CO2 concentrations are high Open when blue wavelengths of light promote uptake of K+ by the guard cells Close when temperature exceeds 30º–34ºC and water relations unfavorable Alternative photosynthetic pathways, such as Crassulacean acid metabolism (CAM), reduce transpiration

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

Plants have adapted to flooding conditions which deplete available oxygen Flooding may lead to abnormal growth Oxygen deprivation most significant problem Form larger lenticels and adventitious roots 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

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

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

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

Copyright © The McGraw-Hill Companies, Inc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phloem fluid Stylet Phloem a. 400 µm b. 25 µm a: © Andrew Syred/Photo Researchers, Inc.; b: © Bruce Iverson Photomicrography. 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

Pressure-flow theory is a model describing the movement of carbohydrates in phloem Dissolved carbohydrates flow from a source and are released at a sink Sources include photosynthetic tissues Food-storage tissue can be sources or sinks Sinks include growing root and stem tips as well as developing fruits

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 Water flows into sieve tubes by osmosis Turgor pressure drives fluid throughout plant At sink, sucrose actively removed and water follows by osmosis Water my be recirculated in xylem or lost