TRANSPORT IN PLANTSFlowering plants (Angiosperms) have 2 transport systems

http://www.hort.purdue.edu/hort/courses/HORT301/MikesLectures/TranslPhloem.html© 2016 Paul Billiet ODWS

Xylem Made of dead cells

(xylem vessels) Uses physical

mechanisms to transport the fluid (the transpiration flow)

Transports water and mineral salts only

From the root to the leaves

www.skidmore.edu/academics/biology/plant_bio/...© 2016 Paul Billiet ODWS

Phloem Made of living cells (sieve

tubes and their companion cells)

Uses active transport to load the phloem and unload the phloem

Transports water, minerals and organic molecules

From shoot to roots and roots to shoot

www.skidmore.edu/academics/biology/plant_bio/...© 2016 Paul Billiet ODWS

Transport in the xylem

universe-review.ca/I10-22a-xylem.jpg

www.richmond.edu/.../KMO_stem_2nd_xylem_20Xs.JPG

© 2016 Paul Billiet ODWS

Movement of minerals to the root Diffusion Along fungal hyphae (mutualism) In mass flow of soil water

© 2016 Paul Billiet ODWS

Root uptake Roots provide a large

surface area for absorption

They are long, thin and highly branched

Near the tip the is a zone of root hairs

The soil solution can penetrate into the root up to the endodermis via the free space = the apoplasm

www.epa.state.il.us/.../images/root-system.gif

© 2016 Paul Billiet ODWS

Root uptake

sps.k12.ar.us/massengale/images/modroothairs.jpg © 2016 Paul Billiet ODWS

Root uptake

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html© 2016 Paul Billiet ODWS

Root uptake

At any point across the root the minerals can be taken up by a cell across a plasma membrane

This uses active transport Therefore movement into the root cell

cytoplasm is selective Once a mineral is in the cytoplasm of a cell

it can move from cell to cell via the plasmodesmata = the symplasm

© 2016 Paul Billiet ODWS

Root uptake Minerals cannot travel

in the apoplasm past the endodermis

The cell walls of the endodermis are water proofed by a Casparian strip

At the endodermis the plant can have control over what it absorbs

universe-review.ca/I10-22a-strip.jpg

© 2016 Paul Billiet ODWS

Xylem vessels Xylem is found in a vascular bundle in the middle of

the root As they mature their wall becomes impregnated with

lignin The tissue becomes wood This provides support to the plant When they are mature they lose the top and

bottom ends The xylem forms continuous columns of water up

the plant to the leaves

© 2016 Paul Billiet ODWS

The cohesion-tension theory The upward flow of sap is created by the

evaporation of water from the surface of the leaves (evapo-transpiration)

Most of the water evaporates through pores called stomata (sing. stoma) = transpiration

The water is pulled up as the water evaporates from the leaves = tension

BUT usually a column of water cannot be pumped up more than 10m without breaking

Some trees can grow to 100m (e.g. sequoia and eucalyptus)

© 2016 Paul Billiet ODWS

The cohesion-tension theory However when water travels

in thin tubes (like xylem vessels) there is a strong cohesion between the water molecules

A column of water in a xylem vessel has the same tensile strength as a thread of steel of the same diameter

The limit of this is reached at about 100m

lettres-histoire.ac-rouen.fr/histgeo/sequoia_...© 2016 Paul Billiet ODWS

Stomata

ABA = Abscissic acidwww.isv.cnrs-gif.fr/jg/images/stomata.jpg

© 2016 Paul Billiet ODWS

Stomata Stomata can open and close Controlled by abscisic acid (ABA) hormone Guard cells are found each side of the pore When the guard cells are turgid the pore opens When the guard cells are flaccid the pore closes When the plant looses more water than it can

absorb its cells become flaccid So a dehydrated plant closes its stomata When the stomata close the plant economises on

water

© 2016 Paul Billiet ODWS

Factors affecting the transpiration flowAnything that affects evaporation will affect

transpiration Humidity Temperature Wind speed But also lightWhen there is light the plant photosynthesizesWhen it is photosynthesizing it needs CO2

So when it is light it opens the stomata to absorb CO2 and it lets out more water

© 2016 Paul Billiet ODWS

Factors affecting the transpiration flow

biology.unlv.edu/.../Leaves/FicusStomata2.jpg Stomata of fig (Ficus)

© 2016 Paul Billiet ODWS

Translocation in the phloem

http://www.hort.purdue.edu/hort/courses/HORT301/MikesLectures/TranslPhloem.html© 2016 Paul Billiet ODWS

Sieve tubes Sieve tubes = long thin

cells joined end to end Each end has

perforated sieve plate They do not grow thick

lignified cell walls They keep their

cytoplasm but they lose their nucleus

They are living cells Difficult to experiment

with.

www.uic.edu/.../bios100/lectf03am/phloem.jpg© 2016 Paul Billiet ODWS

Using aphids to trace phloem flow Aphids are bugs

(hemiptera) Their mouth parts

are specialised for piercing plants and sucking sap from the phloem (stylet).

© P Billiet© 2016 Paul Billiet ODWS

Aphid stylets The phloem sap is

under pressure So the sap flows out

even when the aphid is removed

The stylet pierces a single phloem sieve tube

Transport in a single sieve can be traced.

Stylet (stained red) of the aphid Sitobion yakini terminating in a single sieve tube

© 2016 Paul Billiet ODWS

Using radioisotopes Leaves 4, 6 and 8 labelled with a 5 min pulse of

14CO2 in three plants, then left for 1 hour The radioisotope is traced to other leaves by

autoradiography.

© 2016 Paul Billiet ODWS

Tracing to other parts of the plant

Leaf 8 labelled using a 5 min pulse of 14CO2

Left for 1 hour Autoradiograph taken

for 13 days.

© 2016 Paul Billiet ODWS

Pressure flow hypothesis Movement in the phloem is still not completely

understood Movement seems to work on the principle of source

to sink Organic molecules (e.g. sugars) are loaded into the

sieve tubes where they are produced (e.g. photosynthesising leaves) = the source

This requires active transport This is followed by osmosis, water follows the

sugar into the cell The pressure rises and the liquid flows away

© 2016 Paul Billiet ODWS

Pressure flow hypothesis The sugars are removed from the sieve tubes where

they are used (e.g. respiring root cells) = the sink The water follows the sugars out of the sieve tube by

osmosis The pressure is lower in the sink than in the source

so the sap flows from source to sink In summer the photosynthesising leaves are the source

and root storage organs are sinks BUT after the winter in spring, the roots are sources

providing nutrients for the growing shoots So phloem transport is in two directions and it may

depend on the season© 2016 Paul Billiet ODWS

Pressure flow hypothesis

http://www.hort.purdue.edu/hort/courses/HORT301/MikesLectures/TranslPhloem.html © 2016 Paul Billiet ODWS

THE END!

© 2016 Paul Billiet ODWS