BIO 131 Chapter 35: Water and Sugar Transport in Plants

35.1 - 35.4

Transpiration

* the evaporation of water from stomata of leaves
* occurs when stomata are open and the air surrounding the leaves is drier than the air inside leaves

Challenges of transpiration

* plants must maintain moisture levels in leaves

Benefits of transpiration

* creates a regular flow of water from roots to shoots, which is necessary to transport minerals required by the shoots to grow
* has a cooling effect

Water transport

* occurs because of differences in water potential
* ultimately driven by transpiration
* differences in water potential determine the direction of water movement—net movement is always from areas of higher water potential to areas of lower water potential

Water potential (Ψ/psi)

* the potential energy that water has in a particular environment compared with the potential energy it has at room temperature and atmospheric pressure
* measured in megapascals (MPa)
* Pure water at room temperature and atmospheric pressure has a water potential of 0 MPa
* Solutions with water and solutes have water potentials below 0 MPa
* ignoring gravity, water potential is defined by the equation Ψ = Ψs + Ψp

Solute potential (Ψs)

* the tendency of water to remove in response to differences in solute concentrations
* always negative relative to the solute potential of pure water
* solutions with high concentrations of solutes have low solute potentials (more negative)

Wall pressure

* the force exerted by the cell wall as a cell swells in response to incoming water

Turgor pressure

* pressure inside the cell (exerted by the cytoplasm)
* important because it counteracts/limits the movement of water into cells

Turgid

* describes cells that are firm and experience wall pressure

Flaccid

* describes cells with no turgor pressure (Ψp = 0MPa)

Pressure potential (Ψp)

* any kind of physical pressure on water
* in cells, pressure potential consists of turgor pressure and the opposing wall pressure
* may be negative (tension—force that draws liquid up a straw) or positive
* positive inside living cells due to turgor pressure, but may be negative in dead cells such as xylem

Water potential in moist soil

* water usually contains relatively few solutes and normally is under little pressure
* soil water potential tends to be high relative to the water potential in plants’ roots, which are higher in solutes (therefore, plant roots have a lower solute potential and a lower water potential)

Water potential in salty soils

* soils can have water potentials as low as -4MPa at ocean coastlines or irrigated fields, much lower than typical plant root water potential

Water potential in dry soils

* remaining water tends to adhere very tightly to soil particles, creating tension that lowers the water potential of soil

Salt-adapted species

* often respond to low soil water potentials by accumulating solutes in their root cells, lowering their own solute potential
* tend to have enzymes that increase the concentrations of certain organic molecules in the cytoplasm

Dry-adapted species

* tolerate lower water potentials in soil by adjusting their water potential to be lower in times with low precipitation

Water potential in air

* water exists in the atmosphere as water vapor (pure water with no solute potential)
* water potential depends on the pressure exerted, which depends on temperature and humidity
* the lower the pressure potential of the air, the faster water evaporates into the atmosphere (warm, dry air has the lowest potential, often approaching -100MPa)
* water potential of the atmosphere is normally lower than leaf air water potential, so water in leaves tends to evaporate quickly

Water potential gradient

* usually, water potential is highest in the soil, lower in the roots, even lower in the leaves, and the lowest in the atmosphere
* causes water to move constantly up through the plant

Structure of root cross-section (outside in)

* epidermis
* cortex
* endodermis
* pericycle
* vascular tissue

Epidermis

* a single layer of protective cells and root haris

Cortex

* ground tissue that stores carbohydrates

Endodermis

* a cylindrical layer of cells between the cortex and vascular tissue
* controls ion uptake and prevents ion leakage from the vascular tissue
* cells are tightly packed
* secretes the Casparian strip

Casparian strip

* narrow band of wax composed primarily of the compound Suberin
* forms a waterproof barrier where endodermal cells contact each other… like flextape?
* blocks the apoplastic route, allowing endodermal cells to act as gatekeepers to the vascular tissue by regulating what enters their cytoplasm and subsequently the xylem

Pericycle

* a layer of cells just interior to the endodermis
* forms the outer boundary of the vascular tissue
* capable of producing lateral roots

3 routes through cortex to xylem

* symplastic: water/ions travel inside the plasma membranes
* transmembrane: based on flow through aquaporin proteins across the membranes
* apoplastic: water/ions travel outside the plasma membranes—water moving along this route must eventually pass through the endoderm’s cytoplasm to enter the xylem

Symplast

* cytosol + continuous connections through cells via the plasmodesmata

Apoplast

* cell walls (which are porous) + the spaces that exist between cells

3 hypothesized mechanisms for water transport up the xylem

* root pressure
* capillary action
* cohesion-tension theory

Root pressure

* roots acquire ions as nutrients, which are then actively pumped into the xylem to lower its water potential
* water from surrounding endodermal cells (at higher water potential) flows into the xylem, generating a positive pressure at night that forces fluid up the xylem
* does not apply over long distances because the force of root pressure is insufficient to overcome the force of gravity on water in the xylem of tall stems/trunks
* not entirely true—cut stems with no roots are still able to transport water to leaves

Guttation

* phenomenon based on root pressure
* water droplets can be forced out of leaves due to root pressure generated

Water movement via capillary action

* based on capillary action (upward force resulting from adhesion, cohesion, and surface tension)
* transports water only a limited distance—can only raise water in the xylem of a vertical stem about one meter
* used to move water along mosses and other low-growing nonvascular plants

Adhesion (capillary action)

* molecular attraction among unlike molecules
* water is pulled upward as molecules adhere to the walls of the capillary tube

Cohesion (capillary action)

* molecular attraction among like molecules
* transmits the upward pull by adhesion and surface tension to the rest of the water column

Surface tension (capillary action)

* results from the force among water molecules at the air-water interface
* tendency of water to minimize surface area to maximize hydrogen bonding
* results in upward pull across the surface

Meniscus

* results from the upward pull of cohesion and adhesion along the sides of the capillary and the downward force of gravity in the middle of the column

Cohesion-tension theory

* leading hypothesis to explain the long-distance movement of water
* states that water is pulled from the roots to the tree tops along a water potential gradient via forces generated by transpiration at leaf surfaces
* centered around the key concept that the negative force or pull (tension) generated at the air-water interfaces in the leaves is transmitted through the plant, drawing water up

Cohesion-tension transport mechanism

* water vapor diffuses out of the leaf due to the differences in the water potential in the air inside and outside the leaf
* water evaporates inside the leaf to replace the diffused vapor
* water is pulled out of the xylem by the tension created at the menisci—as water evaporates from the water around the uneven parenchyma cell walls, the menisci become steeper and the total area of the air-water interface increases, creating tension
* water is pulled up the xylem as the tension is transmitted by cohesion from the water in the leaf xylem through the stem xylem and all the way to the root xylem
* water is pulled out of the root cortex
* water is pulled from the soil into the root due to active ion uptake into the root cells (occurs independently of transpiration)

Bulk flow

* the mass movement of molecules along a pressure gradient
* movement in the xylem is entirely driven by differences in pressure potential

Evidence for the cohesion-tension theory

* xylem sap withdraws from surface (back toward the inside of the leaf) when cut, indicating tension
* supported by experiment using xylem pressure probe (as light intensity and therefore transpiration increased, the pressure probe also documented increased tension as expected)

Features for limiting water loss

* particularly thick cuticles covering adaxial leaf surfaces to minimize transpiration
* thick, multilayered epidermis
* stomata located on abaxial surface in deep pits within the epidermis, which are shielded by trichomes
* needle-like leaves which reduce surface area

Trichomes (water loss)

* hypothesized to slow water loss by creating a layer of still air around the stomata
* creates another step in the gradient and reduces the difference in pressure potential

Translocation

* the movement of sugars by bulk flow in multiple directions throughout a plant (particularly from sources to sinks)
* can occur very rapidly (50-100 cm/hr)
* there is a strong correspondence in physical location of sources and sinks (sources send sugars to tissues on the same side of the plant)

Source

* a tissue where sugar enters the phloem
* includes actively photosynthesizing mature leaves/stems which are producing sugar in excess of their own needs (during the growing season) or storage cells in roots and stems (early in the growing season)

Sink

* a tissue where sugar exits the phloem
* includes tissues where sugar use is high and production is low
* includes apical/lateral meristems, developing leaves/flowers/seeds/fruits, and storage cells in roots

Anatomy of phloem

* sieve-tube elements
* companion cells
* continuous transport system for phloem sap
* phloem in each vascular bundle is independent of phloem in other bundles (explains correspondence in sources’ and sinks’ physical locations)

Sieve-tube elements

* cells which lack nuclei and most organelles
* connected end-to-end by perforated sieve plates
* connected to the cytoplasm of different cells directly through pores (enlarged plasmodesmata)

Companion cells

* have nuclei and a rich assortment of organelles
* located adjacent to sieve-tube elements
* function as “support staff“

Phloem sap

* dominated by sucrose
* may also contain small amounts of minerals, amino acids, mRNAs, hormones, and other compounds

Pressure-flow hypothesis

* proposed by Ernst Munch, 1926
* states that events at source and sink tissues create a pressure potential gradient in the phloem down which water and dissolved molecules are carried by bulk flow
* the necessary force is generated by differences between turgor pressure in the phloem near source and sink tissues, which usually require an energy expenditure to generate (phloem loading/unloading)
* supported by observation of aphids

Pressure-flow hypothesis mechanism

* sink cells remove sucrose from the phloem sap (passive or active transport)
* resulting loss of solutes creates an increase in the water potential of sieve-tube elements
* water in the phloem flows into the xylem along a water potential gradient
* turgor pressure in the sink tissue sieve-tube elements drops

Phloem loading

* process which establishes a high pressure potential in the sieve-tube elements near source cells
* large amounts of sugars are transported into the phloem sap (may be active or passive transport, though will likely usually be active)
* strong pH differences between the interior and exterior of phloem cells suggests that sucrose may enter companion cells by secondary active transport with a proton pump and proton-sucrose symporter

Phloem loading model mechanism

* proton pumps in the companion cell membranes create a strong electrochemical gradient favoring the flow of protons into companion cells
* symporters in the membranes of companion cells use the established proton gradient to bring sucrose into companion cells from the source cells
* once inside companion cells, sucrose moves to sieve-tube elements via plasmodesmata

Phloem unloading

* membrane proteins and mechanisms vary for different sinks
* may be active or passive

Phloem unloading mechanism in growing sugar beet leaves

* sucrose is unloaded along a concentration gradient by simple diffusion, since sucrose is rapidly used up inside cells of young leaves to provide energy for ATP synthesis and carbon for the synthesis of other organic molecules required by growing cells

Phloem unloading mechanism in sugar beet roots

* sugar beet roots have cells containing a large vacuole that stores sucrose
* tonoplast: the membrane surrounding the large central vacuole; contains a proton pump and a proton-sucrose antiporter
* active transport of sucrose into the vacuole allows sucrose to move passively from the phloem into the storage cells