transport systems in vascular plants

8/8/2019 Transport Systems in Vascular Plants

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Transport systems in plants

This version of the file contains very few pictures less

pretty but takes up MUCH less disc space and downloadtime. To see the images in their full glory, make sure youcome to the lecture!!


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This lecture is about thetransport systems that

make the flows n thisfigure manageable.

Water has to flow aroundthe plant, especially up

from ground water toleaves.

Less obviously, sugarsand nutrients flow downfrom leaves to storagesites (mainly roots).Sugars to archivecaptured solar energy,

nutrients rescued fromsenescin leaves.


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The magnitude of the problem

A single 15m high maple tree has been calculated to lose220L of water per hour through its leaves. (A 50 gallon

drum = 189 L). How does such a volume of water getsupplied?

The tallest trees are c 110m high how does water get sohigh up? If you try to suck a column of water up by

mouth (or pump) it fails above c. 10m, leading the earlyscientists to propose that nature abhors a vacuum.


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Sometime in the Palaeozoic, before the Silurian (440MYBP) andextraordinary event took place. A new cell type appeared in anearly moss-like plant.

This cell was the tracheid, and it was characterised by a long thin

shape with tough, often lignified cell walls (making itsclerenchyma). This had 2 effects:

1: It allows for long-distance transport of water and nutrients, soroots and leaves can be well separated

2: It provides structural support, sometime that aquatic plants havenever needed. Tracheids set the scene for the invasion of land by

plants.

The original solution (tracheids)


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Tracheids are stiff, water-conducting cells.They are covered in pits, holes, which permitwater conductance. The cell contents are

usually dead, having re-inforced the cell wallwith spiral deposits of lignin to reinforce thehozepipe.

pits Lignin spirals(not DNA!!)


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Tracheids allowed plants to reach up towards the light,to form multi-layered canopies (for better lightutilisation).

This allowed for the development of a branching,independent sporophyte phase in non-vascular plants

the sporophyte is smaller than the gametophyte andoften nutritionally dependent on it.

Now for the 1 st time the sporophyte phase could produce tall, multiple spore-dispersing structures.Once the evolution had occurred, tracheophytes cameto dominate all but the wettest and most inhospitable

places.


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Tracheids lie along side other tracheids, over-lappingextensively, so that water can flow out of the pits of one cellinto an adjacent cell. This allows long range transfer of water and solutes, although (since the cells are dead) the flowhas to be passive, pulled by an external force.

Water Flow (passive flow)

The driving force for this flow is hydrostatic pressure, coming partly from root pressure (pushing up wards) but mainly fromthe suction pressure created by water being evaporated from

leaves. Passive water flow in plants is upwards.


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Tracheids are found in pteridophytes andgymnosperms.

Since they are the water-conducting vesselsof the tallest trees in the world (the ancientconifers the redwoods, Sequoia), they must

be a fundamentally good design!

Despite this, angiosperms (except Amborella ) have evolved amodification of tracheids which appear to be better engineering,in as much as they approximate more closely to the humandesign of hosepipe! These are the vessel elements , which arealso dead (sclerenchyma) lignin-reinforced cells, but instead of overlapping these lie head-to-toe, with water-tight walls and

permeable plates (perforation plates) at each end.


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V essel element, here with a open end (simple perforation plate).

Tracheids

A perforated(scalariform)

perforation plate

Vessel elements, idealised


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Phloem and XylemIf you understand these two transport systems you have got the

basics of fluid transport in plants. All vascular plants have both,though the cell form can vary (tracheids or vessel elements).

Xy lem is composed of dead, hollow cells (sclerenchyma) and is

passive it carries the transpiration stream upwards from roots toleaves.

Phloem cells are living and are used to transport sugars amino-acidsand relocated minerals, typically from leaf to roots. In other words

the opposite direction to xylem. Sugary solutions are actively pumped into the phloem from Source Cells and pumped out by Sink Cells.These are held side-by-side in linear complexes called vascular

bundles .


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The operation of Xylem

Xylem is dead hollow sclerenchyma cells which move water up the plant by transpiration tension cohesion.

Transpiration: loss of water from the leaves (mainly through holes inthe leaf surface called stomata.

Tension the tensions that arise in the water columns of a leaf aretransmitted down the water-bearing vessels of the whole tree. (Thiswould fail if there were any large air bubbles, as the tension exertedmay be many atmospheres). Dissolved gasses can come out of solutionunder this tension, and tree physiologists can put sensitive listeningdevices on tall trees in hot weather and hear tiny cracking noises aseach microbubble appears).

Cohesion the water column remains cohesive, transmitting thetension.


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Pressures on x ylemflow in a tree:

R oots push upwards. This cancause guttation (dripping of water

from leaves resulting from active pumping), esp on damp mornings.The elevation attained is rarelymore than a couple of metres.(Trees can drip sap in earlyspring).

E vaporation. This imparts a suction pressure than puts immense tensiononto the water columns in a truck

trees shrink in diameter measurablyon a hot sunny day.

Guttation drops onleaf tips


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Experiments on x ylem flow

The German botanist Strasburger showed that 20m high trees if

stood in tubs of poison (picric acid) would transport this to thetree top. Clearly this wasnt pumped!

Poison

Poison still sucked up to the top of the tree

We can measure the tension in a twig using a

pressure bomb: when sap is forced out of thetwig the pressure in the sealed container =

pressure in its xylem. V alues are high enough toraise water 100m.

High pressure

sap


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Xylem contains 2 types of cells (only 1 in gymnosperms):Tracheids (explained earlier the only tracheary tissue in

gymnosperms) and vessel elements . BOTH have annular/helicallignin thickening. V essel elements are confined to angiosperms.

Trachear y elements (conducting sclerench yma)

Tracheids vessel elementsShape: long, thin short, wide

End: pointed ends flat

Perforations: none one at each end


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Softwoods (conifers) tracheids only

Hardwoods note the larger bore of the vessel elements


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Water uptake by roots into xylemR oot hairs adhere strongly to fine soil particles and are hydrophilic,allowing water to enter root tissue by diffusion. It flows from the roothair through the root cortex both by apoplastic and symplastic routes.This means that water heading towards the plants main water conduitshas undergone very little quality control by living plant tissue.

Evolution has put a barrier in the way, ensuring that only purified water

(that which has passed through a symplastic route) enters the mainxylem/phloem channels.


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Symplastic vs Apoplastic flowBig words, easy idea. This concerns the path taken by water when

taken up from the soil. Does it go through cells cytoplasm, or inthe spaces between cells? This turns out to matter: apoplasticmeans between cells, symplastic means going through cells

plasma membranes.

R oot hair

Apoplastic flow

Symplastic flow

R oot cortex

This matters since the trans-membrane transport gives the plantthe ability to filter and regulate the composition of its fluids.


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The flow of water into roots is controlled by a band of corky, water-impermeable cells lining the root cortex which force water to flow intothe main vessels symplastically. This band of corky tissue (suberin +

lignin) is the casparian strip , and is present in the endodermis of theroot systems of most vascular plants.

The casparian strip ensuresthat all water entering the

stele of the root (thence up tothe main stem) has passedthrough a plasma membraneso has been regulated bytransport proteins.

Cortex

stele

Casparianstrip


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PhloemPhloem cells are involved in the active transport of sugars, aminoacids and other metabolites around the plant. Their operation isvery different to xylem: the main flow is downwards, and isinhibited by metabolic poisons.

A simple if cruel experiment on this dates back to Malpighi around1700. He girdled (ring-barked) trees and observed that the bark abovethe cut swelled while the bark below died (as eventually did the tree).This is because the cut stopped the downflow of metabolites in theouter regions of the bark (where trees phloem is found).

We now know that phloem sap moves as fast as 1m per hour too fastfor diffusion. Instead a form of bulk flow must be involved.


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The modelexplaining howsolutes are movedaround the phloemis called thepressure flowmodel , and can beexplained byconsideration of

osmosis, applied tosolutions of twosugar solutionsacross asemipermeablemembrane.


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Water

Conc sugar (source end)

Dilute sugar (sink end)

Water

Water initially enters both ends by osmosis, but eventually thehydrostatic pressure on the semipermeable membrane offsets theosmotic pressure, stopping influx at the dilute (sink) end. The

pressure is greater at the top end (where conc is higher), effectively pushing water into the conc end and out of the dilute end

Water

Water

Initiallyinflux at

both endsMembrane

bulges,imposing

hydrostatic pressure

Net flowof water andsolutes

alongthe tube


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Phloem cells

Like xylem, phloem has 2 types of cells:Si eve cells (elongate, sieve areas on all faces of cell)Si eve tube members (shorter, wider, stacked end-endwith sieve areas aligned)

Unlike xylem they remain alive to conduct water and pump metabolites around the plant.

Called sieve elements because they areinterconnected by many relatively large pores.Although alive they have the odd feature that thenuclei have degenerated, and instead nuclear controlis supplied by adjacent cells called compan ion cells .

Collectively knownas sieve elements


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Leaf veins and their bundlesheath cells

There is one group of companion cells which will turn out to be veryimportant for understanding the slightly odd photosynthesis route used

by C4 plants (more later). These surround the veins in a plant leaf and are called bundle sheath cells.

Upper epidermis

Lower epidermis

Mesophyll cells, with manychloroplasts.

Bundle sheath cells, with fewchloroplasts.

V ascular bundle running along vein


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To study the physiology of phloem one needs to extract liquids only

from the phloem tubes (the sieve tubes). How to do this?

It turns out to be almost impossible for humans to do, but easy for aphids (greenflies), sap-sucking insects that unerringly insert needle-like mouthparts (a stylet) into sieve tubes. To sample phloem, let an

aphid start to feed than cut its head off!

Stylet

Why do aphids select phloem instead of xylem?


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Secondary thickening

In a typical tree trunk the phloem lies outside the woody core, in the bark. Inside that is a layer of Collenchyma, then dead xylem tissue.Once a year the collenchyma (which is a meristematic layer)

differentiates, producing a new layer of phloem on the outside and of xylem on the inside. The xylem is laid down as new wood. Themajority of this happens rapidly in spring, leading to a thick band of spring wood then a thin band of harder summer wood. The new xylemcells lay down thick 2ndry walls, harden and die.

This process allows tree trunks towiden every year, and explains thegrowth rings in tree trunks.


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One tends to think of meristems as being the tips of a plant shoot androot tips but there are also sheathing (lateral) meristematic tissues,circling stems and roots. Most dicots, all gymnosperms, but very few

monocots undergo a widening process each year this is secondarythickening.

There are 2 lateral meristems involved: the vascular cambium ( which produces new xylem = wood and secondary phloem) and the cork

cambium which makes a thick tough protective covering for stems androots.

Cork

Cork cambium

Secondary phloemV ascular cambium

Xylem = wood

Bark is everything outsidethe vascular cambium,including phloem, cork cambium and cork.


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C C C C C

C

D

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D

P P P

X

C

P

X

D

C

P

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K eyCambial cellDaughter cell

Phloem cell

Xylem cell

Production of 2ndry xylemand phloem by the vascular cambium.

Xylem is laid down on the inside, hence phloem always remains just under the bark.

Stem centre


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PithPrimary xylem

V ascular cambium

Primary phloem

Primary xylem

V ascular cambiumSecondary xylem

The la yers in secondar y growth of a wood y stem.

transport systems in vascular plants

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