Transport in Vascular Plants

LE 36-2_4 CO2 O2 Light H2O Sugar O2 H2O CO2 Minerals

Effects of Differences in Water Potential measurement that combines the effects of solute concentration and pressure determines the direction of movement of water Movement from high to lower water potential

Turgor Pressure

Bulk Flow in Long-Distance Transport xylem and phloem bulk flow movement of fluid in the is driven by pressure differences at opposite ends of the xylem vessels and sieve tubes

Animation: Transport in Roots Water and mineral salts from the soil through the epidermis of roots ultimately flow to the shoot system Animation: Transport in Roots

Absorption Root hairs Mutualism - Mycorrhizae Much of surface area of roots Mutualism - Mycorrhizae Plant roots and fungal hyphae facilitate absorption of water and minerals from the soil

The Endodermis endodermis waxy Casparian strip innermost layer of cells in the root cortex surrounds the vascular cylinder Is last checkpoint for selective passage of minerals from the cortex into the vascular tissue waxy Casparian strip Part of endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder

Transpirational Pull Water vapor in the airspaces of a leaf diffuses down its water potential gradient exits the leaf via stomata Transpiration produces negative pressure (tension) in the leaf exerts a pulling force on water in the xylem pulling water into the leaf

LE 36-12 Y = –0.15 MPa Y = –10.00 MPa Cell wall Air-water interface Airspace Low rate of transpiration High rate of transpiration Cuticle Upper epidermis Cytoplasm Evaporation Mesophyll Airspace Air space Cell wall Lower epidermis Evaporation Water film Vacuole Cuticle Stoma CO2 O2 CO2 O2 Xylem

Animation: Transpiration Transpirational pull facilitated by cohesion and adhesion Animation: Transpiration

Leaf (cell walls) = –1.0 MPa Xylem sap Outside air  = –100.0 MPa  Mesophyll cells Stoma Leaf (air spaces) = –7.0 MPa  Water molecule Transpiration Leaf (cell walls) = –1.0 MPa  Atmosphere Xylem cells Adhesion Cell wall Water potential gradient Trunk xylem = –0.8 Mpa  Cohesion, by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair Root xylem = –0.6 MPa  Soil particle Soil = –0.3 MPa  Water Water uptake from soil

LE 36-14 20 µm

Transpiration and evaporative cooling can lower the temperature of a leaf prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes

Cells turgid/Stoma open Cells flaccid/Stoma closed LE 36-15a Cells turgid/Stoma open Cells flaccid/Stoma closed Radially oriented cellulose microfibrils Cell wall Vacuole Guard cell Changes in guard cell shape and stomatal opening and closing (surface view)

Cells turgid/Stoma open Cells flaccid/Stoma closed LE 36-15b Cells turgid/Stoma open Cells flaccid/Stoma closed H2O H2O H2O H2O H2O K+ H2O H2O H2O H2O H2O Role of potassium in stomatal opening and closing

Xerophyte Adaptations That Reduce Transpiration Xerophytes plants adapted to arid climates leaf modifications stomata are concentrated on the lower leaf surface often in depressions that provide shelter from dry wind

LE 36-16 Cuticle Upper epidermal tissue Lower epidermal tissue Trichomes (“hairs”) Stomata 100 µm

Movement from Sugar Sources to Sugar Sinks Phloem sap aqueous solution mostly sucrose travels from a sugar source to a sugar sink A sugar source = organ that is net producer of sugar Ex. mature leaves A sugar sink = organ that is a net consumer or storer of sugar Ex. tuber or bulb

Pressure Flow: The Mechanism of Translocation in Angiosperms Movement of sap through a sieve tube by bulk flow driven by positive pressure Animation: Translocation of Phloem Sap in Summer Animation: Translocation of Phloem Sap in Spring