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

Big Picture

Xylem venation brings water close to every cell in the leaf (and other living cells). Water Relations at 10 m Cell Xylem   S  =   P -0.9 = (-0.1) (-1.1) Hint: Think about the water relations of mesophyll cells, and don’t forget their function.

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. Water UseWater Loss

-  m = r -- T: surface tension of H 2 O, (7.28 x MPa m -1 ) r: radius Transpiration Matric potential...   P  =   S   m See Fig

Diffusion t c = 1/2 = 0.042s L (distance) = ~1 mm D s of water in air = 2.4 x m 2 s -1 t c = 1/2 = L2L2 DsDs

 across the plasma membrane and cell wall of the mesophyll cells produces tension that draws water through the xylem,  is driven by the  c wv (regulated by stomata), Note:  m is driven by the  c wv.

H 2 O versus CO 2 Flux …determined by pathway and diffusion coefficients, CO 2 pathway is longer, through cytoplasm, through two chloroplast membranes, Conductance for H 2 O is ~8-10x that for CO 2. …diffusion (concentration gradients), H 2 O representative concentrations: Inside: 1.27 mole m -3, Outside: 0.47 mole m -3,  = 0.8 mole m -3, CO 2 representative concentrations: Inside: ~ 0.0 mole m -3 (ideal), Outside: ~0.014 mole m -3,  = mole m -3, H 2 O gradient ~ 57x greater than CO 2. Flux = Conductance x Driving Force ~ ~600 H 2 O per every CO 2

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.

Guard Cell Function

One method of regulation…[sucrose].

Stomatal Function

Guard Cell Control Light, –blue light signal transduction, –ATP synthesis, –carbohydrates, CO 2 concentration, Circadian rhythms, Hormones.

Big Picture

Cavitation... you can hear plants “cry” H 2 O in the transpiration stream is in a physically metastable state, –experimental values for  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.

Cavitation “cures” Bubbles do not spread far because the they do not spread through the pores in the pit membranes, Reduction of tension  in 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.

Xylem Cells Bordered Pits Pits: microscopic regions where the secondary wall of a xylem cell is absent, and the primary wall is thin and porous.

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

Transport …molecular and ionic movement from one location to another, –H 2 O, –Sugars and other organics, –Ions, –Gases, –Proteins, RNA, Hormones, etc. Proteins/RNA/Hormones etc.+ Everything

Phloem Transport overview 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.

Phloem Cells review

Sieve Tube Elements Living cells, Plasma membrane, No nucleus, No tonoplast, vacuole, Some cytoplasm, Sieve plate, –no membrane between sieve tube members, i.e. does not divide! P-proteins, slime bodies, callous.

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

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

Pressure- Flow-Hypothesis Munch Hypothesis Source High concentration of sucrose, via photosynthesis, –  [sucrose] drives diffusion, Active H + -ATPase, –electrochemical gradient drives symporters, -  s builds, water enters the cell, +  p builds. Sink Low concentration of sucrose, –  [sucrose] drives diffusion, Active H + -ATPase, –electrochemical gradient drives antiporters, –-  s drops, water exits the cell,  p drops.

Pressure-Flow-Hypothesis  p Notice that the  s at 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.

Water Cycling

Control of Transport assimilation 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.

Phloem Loading symplastic and apoplastic symplastic: via diffusion apoplastic: via secondary active transport

Pressure- Flow-Hypothesis passive vs. active Driving Force Primarily Diffusion, –very low  p required, and phloem transport can occur at low temperatures, –and in the presence of H + -ATPase inhibitors.

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