Transport in Plants II (cont.) Water Balance of Plants

1. Transport in Plants II (cont.)Water Balance of Plants It is wise to bring some water, when one goes out to look for water. Arab Proverb

2. Big Picture

3. Y = YP + YS -0.9 = (-0.1) Xylem Cell -0.9 = (-1.1) Water Relations at 10 m -0.8 + 0.2 + Hint: Think about the water relations of mesophyll cells, and don’t forget their function. Xylem venation brings water close to every cell in the leaf (and other living cells).

4. Water Use Water Loss Water is pulled from the xylem into the cell walls of the mesophyll cells, and evaporates. Xylem venation brings water close to every cell in the leaf.

5. -2T -Ym = r • T: surface tension of H2O, • (7.28 x 10-8 MPa m-1) r: radius Transpiration Y = YS + YP + Ym Matric potential... See Fig. 36.11

6. Ds of water in air = 2.4 x 10-5 m2 s-1 L2 tc = 1/2 = tc = 1/2 = 0.042s Ds L (distance) = ~1 mm Diffusion

7. DY across the plasma membrane and cell wall of the mesophyll cells produces tension that draws water through the xylem, • DY is driven by the Dcwv (regulated by stomata), • Note: DYm is driven by the Dcwv.

8. …determined by pathway and diffusion coefficients, CO2 pathway is longer, through cytoplasm, through two chloroplast membranes, Conductance for H2O is ~8-10x that for CO2. …diffusion (concentration gradients), H2O representative concentrations: Inside: 1.27 mole m-3, Outside: 0.47 mole m-3, D = 0.8 mole m-3, CO2 representative concentrations: Inside: ~ 0.0 mole m-3 (ideal), Outside: ~0.014 mole m-3, D = 0.014 mole m-3, H2O gradient ~ 57x greater than CO2. H2O versus CO2 Flux Flux = Conductance x Driving Force ~ 450 - ~600 H2O per every CO2

9. Guard Cell Structure • Features • plasmodesmata between guard cells, but not between guard cells and other cells, • Chloroplasts, • typically the only epidermal cells with chloroplasts, • Highly flexible walls, • radially reinforced with cellulose microfibrils, • Pore that opens and closes.

10. Guard Cell Function

11. One method of regulation…[sucrose].

12. Stomatal Function

13. Guard Cell Control • Light, • blue light signal transduction, • ATP synthesis, • carbohydrates, • CO2 concentration, • Circadian rhythms, • Hormones.

14. Big Picture

15. Cavitation...you can hear plants “cry” • H2O in the transpiration stream is in a physically metastable state, • experimental values for DY required to break a pure water column in a capillary tube exceed -30 MPa, • -3 MPa exceeds physiolgical requirements of even the tallest trees, • As tension increases in the water, a higher probability of gas leaking into the system occurs (air seeding), • Gases do not resist tensile forces, thus the gas bubble expands (cavitation). • Gases also have reduced solubility in ice, thus freezing of the xylem sap also causes cavitation.

16. Bordered Pits Pits: microscopic regions where the secondary wall of a xylem cell is absent, and the primary wall is thin and porous. Cavitation“cures” • Bubbles do not spread far because the they do not spread through the pores in the pit membranes, • Reduction of tensionin times of limited transport might allow the bubble to go back into solution, • Root pressure increases can cause a reduction in tension as well, • Secondary cambium produces new xylem cells.

17. Transport in Plants IVPhloem There is no sugar cane that is sweet at both ends. …Chinese proverb

18. + Everything Proteins/RNA/Hormones etc. Transport …molecular and ionic movement from one location to another, • H2O, • Sugars and other organics, • Ions, • Gases, • Proteins, RNA, Hormones, etc.

19. Phloem Transportoverview • Long distance, bi-directional flow of sugars, • Sugar alcohols, • Organic acids, • Amino acids, • Hormones, • Source (phloem loading), • Sink (phloem unloading), • Pressure is manipulated at source and sink in order to create bulk flow in phloem conducting cells.

20. Phloem Cellsreview

21. Sieve Tube Elements • Living cells, • Plasma membrane, • No nucleus, • No tonoplast, vacuole, • Some cytoplasm, • Sieve plate, • no membrane between sieve tube members! • P-proteins, slime bodies, callous.

22. Slime bodies P Protein and CalloseSlime and wound repair • In Cucurbita… • Phloem Protein 1 (PP1): • Filament protein, • Phloem Protein 2 (PP2): • Lectin (plant defense proteins). Wound Callose (b-1,3 Glucan) is synthesized on the plasma membrane for long term solution.

23. Phloem Locationreview Root Stele Leaf Midrib Stem Vascular Bundle Phloem is always in close proximity to xylem.

24. Pressure- Flow-HypothesisMunch Hypothesis Source • High concentration of sucrose, via photosynthesis, • D[sucrose] drives diffusion, • Active H+-ATPase, • electrochemical gradient drives symporters, • -Ys builds, water enters the cell, + Yp builds. • Sink • Low concentration of sucrose, • D[sucrose] drives diffusion, • Active H+-ATPase, • electrochemical gradient drives antiporters, • -Ys drops, water exits the cell, Yp drops.

25. Pressure-Flow-Hypothesis • Yp • Notice that the Ywat the source is more negative than at the sink! • Why don’t we expect water to flow toward the source? • Water, along with solutes moves down the pressure gradient, not the water potential gradient.

26. Water Cycling

27. Control of Transportassimilation allocation and partitioning • Long distance, bi-directional flow of sugars, • source and sink relationships are reversible, and under environmental and developmental control, • Source: • sucrose synthesis balanced with starch synthesis, • Sink: • Thought to be controlled by sink strength (sugar demand), • Mechanisms for monitoring and switching unknown.

28. Phloem Loadingsymplastic and apoplastic symplastic: via diffusion apoplastic: via secondary active transport

29. Pressure- Flow-Hypothesispassive vs. active • Driving Force • Primarily Diffusion, • very low Yp required, and phloem transport can occur at low temperatures, • and in the presence of H+-ATPase inhibitors.

30. Long Distance Transport Sugars, Amino Acids Organics, Hormones, etc. Function Special Anatomy Requires Pressure-Flow-Hypothesis Companion Cells Sieve Tubes Sugar Loading Source Sugar Unloading Sink Membrane lined pipes Metabolic and genetic drivers of sieve tubes. creates creates DP drives bulk flow Sieve Plates High P Low P Pores, P protein, Callose Concept Map Phloem Transport

31. Exam I Friday • All class time allowed. • 80 pts Mult/Mult Choice (~18 questions). • 20 pts Essay Question, • 16 pts content, 4 pts essay quality, figures welcome.