Topic 6: Transport in Plants (Ch. 36)


I.             WATER (and mineral) TRANSPORT

A.      overview – How does water climb a 10-story tree?

1.       capillary action pulls water partway up tubes

·         thinner tube = greater height

·         but only about 1 meter for xylem width

2.       transpiration (evaporation of water, mainly from leaves)

·         continuous water column

·         remove water at leaf: negative pressure potential (vacuum)

·         tensile strength of water column = pull water up to replace

·         water potential concept

§  higher water potential at roots

§  movement is from high potential to low potential

3.       root pressure

·         active transport of ions into roots, leads to water coming in by osmosis

·         may see guttation from this

4.       net result

·         energy drives the movement of xylem sap (water and dissolved minerals)

·         energy enters system by evaporation (ultimately from sun)

·         energy used to do work of pulling water up against gravity

·         analogous to sucking water up a straw; the stem is the straw

·         energy also provided by plant to push water in (root pressure)

B.      absorption by roots

1.       most water enters through root hairs (large collective surface area)

2.       ions are actively pumped into root hairs

·         proton pumps in root hair plasma membrane

§  work against concentration gradient

§  use ATP for energy to do work

·         concentration higher than in surrounding soil

·         keeps root hairs turgid

·         supplies ions for transport in xylem (more on this later)

3.       osmosis – water moves into root to alleviate osmotic imbalance

·         enters through cells and intercellular spaces

·         provides positive root pressure that moves water through plant

§  works even without transpiration (even at 100% humidity)

§  can cause guttation through special cells in leaves

§  never enough to push water great distances

4.       two routes: apoplastic (outside cells) and symplastic (within cells)

5.       endodermal barrier

·         Casparian strips (suberized) block flow to inside, where xylem is

·         water and ions all must move to the symplastic route at endodermis to get to xylem

·         endodermal cells are selective, controlling what reaches xylem

C.      transpiration from leaves

1.       over 90% of water taken up through roots lost to evaporation (most of the rest used in photosynthesis)

2.       mostly, loss is as water vapor through stomata

3.       control of stomata can be critical

·         need water for metabolism and photosynthesis

·         need CO2 for photosynthesis

·         conflict: open stomata allow CO2 in, but water vapor out

4.       stomata are controlled by guard cells

·         shape changes used for control

·         thicker wall on inside than outside causes “bowing” when turgid

·         turgid when water accumulates, opening stoma

·         turgor maintained as follows:

§  active potassium ion uptake by ATP-powered ion channels

§  water enters due to osmotic imbalance

§  required energy typically provided by chloroplasts

·         no energy or wilted plant = loss of turgor = closed stomata

·         generally, stomata open in morning but not evening or night

(certainly not true for all plants; for example, desert plants)

5.       other factors can regulate transpiration

·         in some species, CO2 is used

§  high CO2 à loss of turgor à closed stomata (no need to open)

§  low CO2 à turgid à open stomata (used by plants like cacti)

·         high temperatures (30°C to 34°C) cause stomata to close

·         stomata closed during dormancy during dry times

·         thick, hard leaves with few stomata = less transpiration

·         trichomes – cooler and more humid surface = slower water loss

·         pits or crypts – water vapor content in them is higher, slows water loss

D.      mineral movement

1.       ionic minerals transported with water in xylem

2.       entry into xylem controlled through endodermal cells

3.       ionic minerals enter roots by an active process

·         roots need oxygen to be able to absorb ions

·         specific ion concentrations in plants higher than in surrounding soil

E.       flooding

1.       moving water generally not bad, constantly supplies oxygen

2.       still water presents problem: depletes oxygen in roots

·         loss of active pumping at root hairs

·         loss of ion entry

·         may dry out leaves (root pressure needed; endoderm greater barrier)

·         adaptations: aerenchyma, larger lenticels, adventitious roots, pneumatophores


II.           FOOD (carbohydrate) TRANSPORT

A.      translocation: movement of carbohydrates from where made or stored to where needed

B.      storage usually as starch, which must be converted to soluble molecules

C.      movement in phloem: how is it studied?

1.       radioactive tracers

2.       aphid stylets

·         aphids pierce into phloem to feed

·         cut off aphid, leaving stylet, and sample phloem

D.      movement in phloem: the facts

1.       contents: 10-25% dry matter, almost all sucrose (“syrup”)

2.       moves reasonably fast (up to 1 meter per hour)

E.       movement in phloem: how is it done?

1.       pressure flow hypothesis (AKA mass-flow or bulk flow)

2.       dissolved carbohydrate flow from source to sink

3.       source – place of dissolved carbohydrate production (leaves, storage organs)

4.       sink – place of usage (primarily growing areas – root and stem tips, fruits)

5.       phloem loading

·         carbohydrates enter sieve tubes at source by active transport

·         energy for transport comes from companion cells

·         sieve tube water potential lowered

·         creates water potential difference relative to nearby xylem

·         water enters sieve tubes by osmosis

·         increased turgor pressure in sieve tube pushes solution through them

6.       “unloading” at sink

·         removal of carbohydrates leads to drop in turgor pressure

·         drives flow from high water pressure at source to low pressure at sink

·         most of water at sink diffuses back into xylem