Transport systems in plants

This version of the file contains very few pictures – less pretty but takes up MUCH less disc space and download time. To see the images in their full glory, make sure you come to the lecture!!

This lecture is about the transport systems that make the flows n this figure manageable.

Water has to flow around the plant, especially up from ground water to leaves.

Less obviously, sugars and nutrients flow down from leaves to storage sites (mainly roots). Sugars to ‘archive’ captured solar energy, nutrients rescued from senescing leaves.

The magnitude of the problem…

A single 15m high maple tree has been calculated to lose 220L of water per hour through its leaves. (A 50 gallon drum = 189 L). How does such a volume of water get supplied?

The tallest trees are c 110m high – how does water get so high up? If you try to suck a column of water up by mouth (or pump) it fails above c. 10m, leading the early scientists to propose that nature abhors a vacuum.

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

This cell was the tracheid, and it was characterised by a long thin shape with tough, often lignified cell walls (making it sclerenchyma). This had 2 effects:

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

2: It provides structural support, sometime that aquatic plants have never needed. Tracheids set the scene for the invasion of land by plants.

The original solution (tracheids)

Tracheids are stiff, water-conducting cells. They are covered in pits, holes, which permit water conductance. The cell contents are usually dead, having re-inforced the cell wall with spiral deposits of lignin to reinforce the ‘hozepipe’.

pits Lignin spirals(not DNA!!)

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

This allowed for the development of a branching, independent sporophyte phase – in non-vascular plants the sporophyte is smaller than the gametophyte and often nutritionally dependent on it.

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

Tracheids lie along side other tracheids, over-lapping extensively, so that water can flow out of the pits of one cell into an adjacent cell. This allows long range transfer of water and solutes, although (since the cells are dead) the flow has 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 from the suction pressure created by water being evaporated from leaves. Passive water flow in plants is upwards.

Tracheids are found in pteridophytes and gymnosperms.

Since they are the water-conducting vessels of the tallest trees in the world (the ancient conifers the redwoods, Sequoia), they must be a fundamentally good design!

Despite this, angiosperms (except Amborella) have evolved a modification of tracheids which appear to be better engineering, in as much as they approximate more closely to the human design of hosepipe! These are the vessel elements, which are also 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.

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

Tracheids

A perforated (scalariform) perforation plate

Vessel elements, idealised

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).

Xylem is composed of dead, hollow cells (sclerenchyma) and is passive – it carries the transpiration stream upwards from roots to leaves.

Phloem cells are living and are used to transport sugars amino-acids and 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.

The operation of XylemXylem 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 in the leaf surface called stomata.

Tension – the tensions that arise in the water columns of a leaf are transmitted down the water-bearing vessels of the whole tree. (This would fail if there were any large air bubbles, as the tension exerted may be many atmospheres). Dissolved gasses can come out of solution under this tension, and tree physiologists can put sensitive listening devices on tall trees in hot weather and hear tiny cracking noises as each microbubble appears).

Cohesion – the water column remains cohesive, transmitting the tension.

Pressures on xylem flow in a tree:

Roots push upwards. This can cause guttation (dripping of water from leaves resulting from active pumping), esp on damp mornings. The elevation attained is rarely more than a couple of metres. (Trees can drip sap in early spring).

Evaporation. This imparts a suction pressure than puts immense tension onto the water columns in a truck – trees shrink in diameter measurably on a hot sunny day.

Guttation – drops on leaf tips

Experiments on xylem flow

The German botanist Strasburger showed that 20m high trees if stood in tubs of poison (picric acid) would transport this to the tree top. Clearly this wasn’t 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 the twig the pressure in the sealed container = pressure in its xylem. Values are high enough to raise water 100m.

High pressure

sap

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/helical lignin thickening. Vessel elements are confined to angiosperms.

Tracheary elements (conducting sclerenchyma)

Tracheids vessel elementsShape: long, thin short, wide

End: pointed ends flat

Perforations: none one at each end

Softwoods (conifers) – tracheids only

Hardwoods – note the larger bore of the vessel elements

Water uptake by roots into xylem

Root hairs adhere strongly to fine soil particles and are hydrophilic, allowing water to enter root tissue by diffusion. It flows from the root hair through the root cortex both by apoplastic and symplastic routes. This means that water heading towards the plant’s main water conduits has 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 main xylem/phloem channels.

Symplastic vs Apoplastic flow

Big words, easy idea. This concerns the path taken by water when taken up from the soil. Does it go through cells’ cytoplasm, or in the spaces between cells? This turns out to matter: apoplastic means between cells, symplastic means going through cells’ plasma membranes.

Root hair

Apoplastic flow

Symplastic flow

Root cortex

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

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 into the main vessels symplastically. This band of corky tissue (suberin + lignin) is the casparian strip, and is present in the endodermis of the root systems of most vascular plants.

The casparian strip ensures that all water entering the stele of the root (thence up to the main stem) has passed through a plasma membrane so has been regulated by transport proteins.

Cortex

stele

Casparian strip

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

A simple if cruel experiment on this dates back to Malpighi around 1700. He girdled (ring-barked) trees and observed that the bark above the cut swelled while the bark below died (as eventually did the tree). This is because the cut stopped the downflow of metabolites in the outer regions of the bark (where trees’ phloem is found).

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

The model explaining how solutes are moved around the phloem is called the pressure flow model, and can be explained by consideration of osmosis, applied to solutions of two sugar solutions across a semipermeable membrane.

Water

Conc sugar(source end)

Dilute sugar(sink end)

Water

Water initially enters both ends by osmosis, but eventually the hydrostatic pressure on the semipermeable membrane offsets the osmotic 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

Initially influx at both ends

Membrane bulges, imposing hydrostatic pressure

Net flow of water and solutes along the tube

Phloem cells

Like xylem, phloem has 2 types of cells:

Sieve cells (elongate, sieve areas on all faces of cell)Sieve tube members (shorter, wider, stacked end-end with sieve areas aligned)

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

Called sieve elements because they are interconnected by many relatively large pores. Although alive they have the odd feature that the nuclei have degenerated, and instead nuclear control is supplied by adjacent cells called companion cells.

Collectively known as sieve elements

Leaf veins and their bundle sheath cells

There is one group of companion cells which will turn out to be very important 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 many chloroplasts.

Bundle sheath cells, with few chloroplasts.

Vascular bundle running along vein

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?

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. The majority of this happens rapidly in spring, leading to a thick band of spring wood then a thin band of harder summer wood. The new xylem cells lay down thick 2ndry walls, harden and die.

This process allows tree trunks to widen every year, and explains the growth rings in tree trunks.

One tends to think of meristems as being the tips of a plant – shoot and root 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 secondary thickening.

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 and roots.

Cork

Cork cambium

Secondary phloem

Vascular cambium

Xylem = wood

Bark is everything outside the vascular cambium, including phloem, cork cambium and cork.

C C C C C

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KeyCambial cell

Daughter cell

Phloem cell

Xylem cell

Production of 2ndry xylem and phloem by the vascular cambium.

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

Stem centre

PithPrimary xylem

Vascular cambium

Primary phloem

Primary xylem

Vascular cambiumSecondary xylem

Secondary Phloem

Periderm (corky outer layer)

The layers in secondary growth of a woody stem.