Botany - 2f: Movement of Water & Solutes

transpiration stream

the movement of water through a plant from the roots to stems to leaves, where it is lost by evaporation

assimilate stream

the flow of sucrose/carbs. produced by a plant during photosynthesis within the phloem

- translocation of photosynthate from source (photosynthesis-usually leaf) to sink (storage- usually stem or root)

radioactive tracer study

evidence for stream separation

transpiration stream pathway

a) Apoplastic - water

b) Symplastic - water and minerals

c) Transcellular - water

stomata

regulate transpiration

- guard cells regulate stomata

what regulates the transpiration stream?

stomata

what regulates stomata?

guard cells

turgid guard cells

stoma open

- active transport K+ (pr sucrose)

- malate 2- and Cl- and K+ flowing in to guard cell

ABA

abscisic acid (plant hormone)

flaccid guard cells

stoma closed

- K+ (or sucrose), Cl-, and malate 2- flow out of guard cell

stomal aperature

highest (≈ 23 µm) between 1:00 and 3:00 pm at ≈ 23 µm

- lowest (≈ 8-9 µm around 8:00 am and 10-11:00 pm)

K+ content

highest around 10:00 am at ≈ 41% area

- lowest (≈ 4% area) around 9:00 pm

sucrose content

highest around 4:00 pm at ≈ 1.75 pmol/guard cell pair

- lowest (≈ 0.5 pmol/guard cell pair) around 8:00 am

what factors regulate guard cells and transpiration?

environmental factors

- water

- light

- [CO2] in leaf

- temperature

how water regulates guard cells & transpiration

if abundant --> stomata open

if scarce --> stomata closed

how light regulates guard cells & transpiration

daytime --> stomata open

nighttime --> stomata closed

how [CO2] in leaf regulates guard cells & transpiration

lower concentration --> stomata open

higher [ ] --> stomata closed

how temperature regulates guard cells & transpiration

lower --> stomata open

higher --> stomata closed

why do guard cells have chloroplasts?

to create ATP from light for active transport

water scarcity leads to

ABA (abscisic acid) - opens leak channel

what inhibits the K+ pump?

CO2

higher temperature leads to...

more water loss --> ABA (abscisic acid)

more CO2 from respiration

mechanisms for movement in transpiration stream

push and pull

push mechanism for movement in transpiration

root pressure (osmotic pressure)

- ion active transport into xylem

- water potential now greater in nearby cells

- water flows into xylem generating pressure

- guttation: safety valve to protect leaf from too much water pressure from roots

- problems...

problems of push mechanism for movement (transpiration)

1. no root pressure in many plants

2. tallest trees over 350 ft; root pressure only good up to 100-170 ft

3. water still moves up a severed stem with leaves (so no roots)

pull mechanism for movement in transpiration

due to transpiration or water use by the leaves

- water pulled into root because of H+ bonding

- cohesion-tension

- cohesion-adhesion-tension theory

- problems....

guttation

Due to root pressure, droplets of water appear in the morning on the leaf tips of some herbaceous plants (different from dew)

- safety valve to protect leave from too much water pressure from roots

problems of pull mechanism for movement (transpiration)

- cavitation and embolism in a vessel (formation of empty space filled with water vapor)

- drought conditions --> high vulnerability

- theoretical max tree ≈ 425 ft

assimilate stream pathway =

translocation

- pathway = source to sink

assimilate stream movement mechanisms

the cells are alive

- early work- diffusion and cytoplasmic streaming

- Aphid research with isotopes like 14C showed phloem sap is under pressure and so diffusion is too slow

- pressure-flow hypothesis/theory: osmotic pressure drives translocation and the movement of sucrose is passive

pressure-flow hypothesis

hypothesis that explains the method by which phloem sap is transported through the plant from a sugar "source" to a sugar "sink"

- osmotic pressure drives translocation

- movement of sucrose is passive

problem of assimilate stream- translocation

rupture or penetration of a sieve tube

- solution: P-protein and callose (a polysaccharide)