II. Water and nutrient acquisition by roots- Initial pathways (soil to endodermis)o Apoplastic: non-living space (cell walls)o Symplastic: living space (membrane)- Casparian strip: waxy layer in apoplast of endodermis that forces water to cross cell membrane before entering xylem- Once water and few nutrients in xylem in root, now ready for long distance transportClicker: the casparian strip allows the plant to A regulate solute transport into xylemB regulate amount of H2O transport into xylemC prevent solute transport into xylemD prevent H2O transport from soil into xylemE prevent H2O transport from xylem to soilIII. Transpiration and bulk flow through xylem- Transpiration: loss of water vapor by diffusion from leaves to the surrounding environment- What is actually driving water vapor loss? Concentration gradient (high  low)- Stomata: also necessary for gas exchange for photosynthesis; also loses water as consequenceTransport Process Driving force Where important in plantsOsmosis (liquid water) Water potential gradient Small-scale: cell membraneDiffusion (solutes dissolved in water)Concentration gradient Small-scale: within cellDiffusion (water vapor) Concentration gradient Inside  outside of leaf (transpiration)Pressure flow (bulk flow) Pressure gradient (Ψp) Xylem: less negative  more negative pressurePhloem: more positive  less positive pressure- Evaporation from water film in air space between cells- How is water lost by transpiration replaced?o Surface tension generated by “capillarity” of water in mesophyll cell walls (apoplast): -Ψp and thus Ψ (Ψ = Ψs + Ψp but Ψs is negligible)o This “pulls” water from living cells to cell wall, which then “pulls” water from xylem into living cells: osmosis due to Ψ gradient across each membrane- Trees lost all leaves; no evaporation & photosynthesis; tree cells would be in equilibrium with soil- Water leaving xylem in leaf creates tensions (negative Ψp) gradient in the “pipe” of dead xylem- Cohesion-tension theory of xylem transport: water is pulled up xylem, by process of bulk flow, driving force is pressure gradient (less to more negative Ψp)- Ring/spiral wall thickening of lignin protects against vessel collapse (sclerenchyma)- Xylem: vessels and tracheids- Clicker: why is the transport process in the xylem bulk flow and not osmosis?A living cell membranes being crossedB dead cell membranes being crossedC no cell membranes being crossedD no cell walls being crossedE no Ψ gradient in xylem- What type of cells form the functional xylem “pipeA parenchyma with lignified primary cell wallsB collengchyma with thickened piramary cell wallsC sclerencyhma with lignified secondary cell wallsD endodermal cells with suberized casparian stripE epidermal cells with waxy cuticle and stomata- Phloem: is not sclerenchyma- Leaf evaporative water loss (transpiration) sets up gradient that pulls water up through plant- Different transport processes dominate in different parts of pathway from soil to atmosphere- Tiny amounts of dissolved nutrients and hormones, are carried along by the bulk flow in the “xylem sap”How is transpirational water loss regulated?- Leaves have broad SA and high SA/V ratioo Increases potential for photosynthesiso Also increases potential water loss- But most leaf epidermal cells covered with cuticle that prevent water loss- Stomata, special epidermal cells, help regulated the rate of transpirationStomatal movement- Stomatal opening promoted by light and CO2 depletion, closure promoted by lack of light, dry hair, drought- K+ channels, aquaporins and radially oriented cellulose fibers play important roles- H+ pumped out K+ flow in  H2O flow in  stomata openo K+ goes up so Ψs goes down so osmosis occurs due to concentration and Ψ gradients- Open slowly but close quicklyHow much water is used?- 90% of water taken up is transpiredo But necessary for CO2 uptakeo Where does rest go?- Transpirationo Cools leafo Transport mineral from roots to shooto Peak velocity of xylem sap: 15-45 m/hr (fast)How do plants deal with water limitations?- Ecological time scale:o Reduce water loss by stomatal regulationo Increase water uptake with more/deeper roots- Osmotic adjustment: cells make compatible solutes to decrease Ψs (more -) and thus maintain turgor in living cells (+Ψp) as soil dries- Evolutionary time scale: plants adapted to dry habitatso Escape in time (desert animals)o Have inherently more conservative water use and water access strategyo Have more conservative physiology (ex: CAM)Examples adaptations to more sever water limitations- High root/shoot biomass ratio and deep roots (juniper in Texas)- Redwood trees, apparently near physical limit of “pulling” water from soil, also trap fog as a leaf water source- Ocotillo plant, SW US deserts, leaves are drought deciduous plant uses low rates of stem photosynthesis” to persist through leafless phase- “old man” cacti: leaf hairs to reflect sun and CAM photosynthesis so stomata open at night whencooler (trade-off: slow growth)IV. Phloem transport also via bulk flow, but in living cells and positive pressureTransport mostly sugars (in water) in highly specialized living parenchyma cells: sieve cells stacked in pipes and connected via sieve platesAccompanied by companion cells (has functional organelles)Phloem cells only have cytoplasm (exchange from one cell to the next)Direction of flow is sugar source (where it’s produced) to sugar sink (where it’s needed)Why is it bulk flow not osmosis? Big open pores so no crossing cell membranesHow do sugars get in and out of phloem?Chloroplast stroma  cytoplasm  apoplast or symplast pathway for loading  phloem (mesophyll cell ^)- But, what about within phloem?- Process for transport?o Bulk flow, higher to lower pressure potential- Driving force?o Pressure gradient, more positive to less