TRANSPORT IN PLANTS Flowering plants Angiosperms have 2

Xylem n n Made of dead cells (xylem vessels) Uses physical mechanisms to transport

Phloem n n Made of living cells (sieve tubes and their companion cells) Uses

Transport in the xylem www. richmond. edu/. . . /KMO_stem_2 nd_xylem_20 Xs. JPG ©

Movement of minerals to the root n n n Diffusion Along fungal hyphae (mutualism)

Root uptake n n Roots provide a large surface area for absorption They are

Root uptake © 2016 Paul Billiet ODWS sps. k 12. ar. us/massengale/images/modroothairs. jpg

Root uptake http: //users. rcn. com/jkimball. ma. ultranet/Biology. Pages/X/Xylem. html © 2016 Paul Billiet

Root uptake n n At any point across the root the minerals can be

Root uptake n n n Minerals cannot travel in the apoplasm past the endodermis

Xylem vessels n n n Xylem is found in a vascular bundle in the

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

The cohesion-tension theory n n n However when water travels in thin tubes (like

Stomata www. isv. cnrs-gif. fr/jg/images/stomata. jpg © 2016 Paul Billiet ODWS ABA = Abscissic

Stomata n n n n Stomata can open and close Controlled by abscisic acid

Factors affecting the transpiration flow Anything that affects evaporation will affect transpiration n Humidity

Factors affecting the transpiration flow biology. unlv. edu/. . . /Leaves/Ficus. Stomata 2. jpg

Translocation in the phloem http: //www. hort. purdue. edu/hort/courses/HORT 301/Mikes. Lectures/Transl. Phloem. html ©

Sieve tubes n n n Sieve tubes = long thin cells joined end to

Using aphids to trace phloem flow n n Aphids are bugs (hemiptera) Their mouth

Aphid stylets n n The phloem sap is under pressure So the sap flows

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

Tracing to other parts of the plant n n n Leaf 8 labelled using

Pressure flow hypothesis n n n Movement in the phloem is still not completely

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

Pressure flow hypothesis http: //www. hort. purdue. edu/hort/courses/HORT 301/Mikes. Lectures/Transl. Phloem. html © 2016

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

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

Root uptake n n 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 © 2016 Paul Billiet ODWS www. epa. state. il. us/. . . /images/root-system. gif

Root uptake n n 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 n n n 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 © 2016 Paul Billiet ODWS universe-review. ca/I 10 -22 a-strip. jpg

Xylem vessels n n n 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 n n n 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 10 m without breaking Some trees can grow to 100 m (e. g. sequoia and eucalyptus) © 2016 Paul Billiet ODWS

The cohesion-tension theory n n n 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 100 m © 2016 Paul Billiet ODWS lettres-histoire. ac-rouen. fr/histgeo/sequoia_. . .

Stomata n n n n 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 flow Anything that affects evaporation will affect transpiration n Humidity n Temperature n Wind speed n But also light When there is light the plant photosynthesizes When it is photosynthesizing it needs CO 2 So when it is light it opens the stomata to absorb CO 2 and it lets out more water © 2016 Paul Billiet ODWS

Sieve tubes n n n 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. © 2016 Paul Billiet ODWS www. uic. edu/. . . /bios 100/lectf 03 am/phloem. jpg

Aphid stylets n n 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. © 2016 Paul Billiet ODWS Stylet (stained red) of the aphid Sitobion yakini terminating in a single sieve tube

Using radioisotopes n n Leaves 4, 6 and 8 labelled with a 5 min pulse of 14 CO in three plants, then left for 1 hour 2 The radioisotope is traced to other leaves by autoradiography. © 2016 Paul Billiet ODWS

Pressure flow hypothesis n n n 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 n n n 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