Mass transport in plants

Mass transport: two delivery systems for molecules

Unlike animals, plants do not have a single (circulatory) system for the transport of substances required by their cells. They do not have a heart to pump their internal liquids and dissolved substances, but they generate pressure so that the liquids move consistently when required.

They do have their own vascular tissues, which are composed of tubular structures that carry liquids and also give the plant some support. These tissues are known as xylem and phloem.

Each of these has a bulk flow system of its own, transporting water and different solutes around the plant.

Vascular bundles include both xylem and phloem, together with supporting and protective cells.

Between xylem and phloem there is a meristem - a layer of undifferentiated cells capable of cell division - called the vascular cambium. This produces more xylem and phloem cells, and is responsible for the thickening of tree trunks and formation of cork for the bark.

Substances transported by plants are moved from a point of origin: the source, to a destination: the sink. In other words, transport in plants is directional.

A plant stem in section

Xylem

The xylem transports water in the roots, stem and leaves of plants. It also carries dissolved inorganic ions (mineral salts). The direction of movement is upwards.

Cells involved

The principal cells in xylem are called vessel elements. They are actually dead remnants of cylindrical cells with thickened side walls, which have lost their ends and internal cytoplasm, leaving a hollow tube which contains an unbroken column of of watery liquid from the roots up the stem or trunk to the leaves. The cell walls are built up from cellulose, and in some plants - notably shrubs and trees - these are thickened with lignin, and this is the basis for wood.
Tracheids are also reinforced hollow tubular structures, and they are the only conducting cells in non-flowering plants.

Xylem tissue is concentrated in the centre of roots, but forms into (vertical) bundles at the edges of the stem and it branches into the network of 'veins' which spread out across the leaves.

Alternative xylem cells


Tracheids are lignified tubular structures that also conduct water and have no living tissue inside.

They are found closely packed in woody tissues in trees and shrubs and are the only conducting tissue in Gymnosperms (softwood).

These have pits for water to flow from one vessel element to the next.

The main driving force in the uptake of water is a pull from above

Water evaporates from mesophyll cells in the leaf, especially the spongy layer, and water vapour passes out of the (underside of the) leaves via the stomata. This lowers the water potential of the cytoplasm of the mesophyll cells, and causes water to move by osmosis from cell to cell across the leaf and from the liquid in the xylem in the vascular bundles of the leaf. As a result, more water is pulled up the column of water in the xylem in the stem and root.

Water is quite a special liquid in that its molecules are attracted to one another by hydrogen bonds, which explains why they form a continuous column, and they also bind to the walls of the xylem

This evaporation, replaced by water from below, is called transpiration. It causes a continuous movement of water called the transpiration stream. As far as the plant is concerned, it is a passive process, i.e. not requiring any (chemical, ATP-powered) energy from the plant. It is effectively powered by external physical conditions - temperature, humidity differences, wind movement. However, it may be controlled by the plant's closing of stomata in dark conditions or in the absence of water

As part of this process, inorganic ions (mineral salts) originating in the soil, e.g. nitrates, are delivered to photosynthesising cells. As a result, nitrogen (together with carbon-containing products of photosynthesis) can be incorporated into amino acids and used to make plant protein. Other inorganic ions, such as sulphates and phosphates from the soil can also be incorporated into organic molecules used by plants.

An oak tree may transpire 150,000 litres of water per year.

Roots give a small push from below

The uptake of water at the roots is by osmosis, as the root hair cells have a cytoplasm with a water potential lower (more negative) than that of soil water. Inorganic ions (mineral salts) in the soil water are absorbed and concentrated after crossing the outer cortex of the root, and deposited by active transport into the central xylem.

Within the root there is a ring of tissue known as the endodermis, the cells of which have a layer of waxy material (suberin) called the Casparian strip in their walls. This prevents the passage of water and solutes through spaces outside the cell and the cell wall itself ('the apoplast pathway'), and ensures that ions must pass in one direction: through the cell cytoplasm ('the symplast pathway'), to be pumped inwards towards the xylem by membrane-bound carriers.

This accumulation of inorganic ions lowers the water potential in the xylem vessels, and water moves by osmosis across the plasma membrane out of the cells of the surrounding root tissue as a result of a water potential gradient. It increases the volume of liquid in the xylem and increases the pressure, forcing the water and dissolved salts upwards.

This force is known as root pressure and it pushes the liquid up the xylem (in the stem) for a certain distance, but the majority of the vertical movement in a plant is caused by negative pressure ('suction') from above, rather than pressure from below.

Evidence for root pressure

Guttation is a process by which a plant loses water from the leaf edges, through special pores (hydathodes) as a consequence of high root pressure.

This water - which is sometimes seen as "dew on the grass" - comes from the xylem of the plant and thus mainly consists of water.

It usually occurs as a consequence of a combination of high root pressure and a low evaporation rate caused by high humidity. This often occurs just after sunrise when the plant becomes active and the humidity is high.

Cohesion-tension - not just a theory

The movement of water up a tree, or a smaller plant, relies on water molecules clinging together as a result of hydrogen bonds. This is called cohesion.

Because of the pulling power from the evaporation of water above, the column of water is under tension.

Water molecules also cling to the walls of xylem vessels - this is called adhesion. When a plant is transpiring, there is an inward force, making xylem vessels narrower, and collectively tending to reduce the diameter of the stem or tree-trunk.

The structure of xylem vessel elements and tracheids (thick walls, reinforced with lignin) means that plant stem tissue resists this, but the reduction in diameter can be shown using a sensitive measuring device called a dendrometer.

The fact that the diameter of a tree is less during the day, when the tree is transpiring, than it is at night, supports the cohesion-tension theory, and shows that root pressure has a lesser effect.

However, it is said that the presence of an air bubble in a xylem vessel in the stem may block the movement of water through that vessel because the air bubble prevents cohesion and breaks the water column. In extremely drying environmental conditions, cavitation can occur in the xylem of some plants when the tension of water within the xylem becomes so high that dissolved gas within the water leaves solution and expands to fill the vessels or the tracheids of that section.

Evidence for cohesion-tension

These graphs show the daily changes in light intensity and environmental temperature, together with changes in the diameter of the trunk of a pine tree.


Explain why the diameter of the trunk is smallest at midday.

> This is the brightest and warmest time of day
> so there will be more stomata open, and more water evaporating/transpiring
> There is cohesion between water molecules
> caused by hydrogen bonding
> Transpiration from leaves above causes reduced pressure in the xylem
> so the upward movement of water is caused by evaporation
> This (greater movement) results in tension
> pulling water up the plant
> and adhesion (between water and walls) results in xylem being pulled inwards

Phloem

The phloem transports organic compounds (sugars e.g. sucrose) from the leaves of plants to the growing tips of shoots and roots, as well as to storage organs, flowers and fruits. It is also involved in the 'rising of the sap' which is seen at the end of winter when trees mobilise carbohydrate stored in roots and other regions to provide sugary liquid to power the growth of leaves from buds at the top of the trees.

This transport process is called translocation. The direction of movement may be either upwards or downwards.

Cells involved

The principal cells in phloem are called sieve tube elements, but each of these has an associated companion cell.

Sieve tube elements are cylindrical and have end walls with perforations through which pass strands of cytoplasm. There is a large central space in them rather like a vacuole, and a thin layer of cytoplasm on the outside edges. They do not have a nucleus, but they are still considered to be living. Most of the activity appears to be controlled by the companion cells (which are active cells) located alongside, and there are cytoplasmic connections (plasmodesmata) between them.

There are no membranes at the sieve plates between each sieve tube element, so osmosis does not have a part to play in the transfer between each of these subsections, and the whole of the phloem is full of a column of water (like the xylem).

Xylem and phloem are often found alongside one another, in vascular bundles. Phloem is found underneath xylem in leaves, and on the outside of vascular bundles in stems. In the roots, phloem is found in the gaps between the star-shaped bundles of xylem.

The long, wide cell in the center of the micrograph (marked by arrows) is a sieve tube element. Both of its end walls - sieve plates - (arrowed) are slightly tilted.

Translocation of sucrose by mass flow under pressure

Movement in the phloem is mass flow of water and dissolved solutes generated by (positive) hydrostatic pressure, and there is a pressure gradient from source to sinks. There are effectively three regions, the loading zone, the (fairly long) transport zone and the unloading zone at the other end.

The source is usually the leaves which are photosynthesising, but it may be an organ of perennation, such as a tuber, mobilising stored reserves at the beginning of the growing season. The sinks are any areas of growing tissue where cells are dividing, so they could be the tips of roots or shoots, flowers or fruits, which may be above or below the leaves. So transport in phloem is (potentially) bidirectional.

The product of photosynthesis is glucose, but it is converted to sucrose. In some plants this is converted into other oligosaccharides such as raffinose, stachyose, and verbascose - all of which consist of sucrose with extra galactose. Sucrose is actively transported, in conjunction with hydrogen ions (H⁺), into the phloem by a co-transporter system operating in the companion cells. A separate ATP-powered H⁺ pump establishes a concentration gradient to power this loading process.

The presence of the sucrose lowers the water potential in the sieve tube element, and water flows in by osmosis from surrounding tissue and indirectly from the nearby xylem.

This extra volume produces higher pressure in the liquid, causing it to flow towards the sink. It is probably worth mentioning that water is resistant to compression forces, so it transmits this force without losing volume. The molecules are already close together, unlike gases.

Of course there is usually more than one sink, and so the liquid can flow both upwards and downwards from the source.

It is said that during transport some organic substances are lost from the phloem to surrounding cells which use it for their own purposes, e.g. respiration and secondary growth. Conceivably companion cells along the length of the transport section work on retrieving sucrose that has 'leaked out' and replacing it in the phloem.

At its destination, cells remove the sucrose from the phloem and absorb it, for respiration or storage. The sucrose is said to be unloaded. In storage organs it is usually converted into other, less soluble carbohydrates, reducing osmotic effects and maintaining a concentration gradient which assists in unloading sucrose from the phloem.

Name a carbohydrate that may be built up in storage organs.
> starch or other polysaccharides e.g.inulin

How does this assist in maintaining a concentration gradient?
> sucrose is removed as it is converted into other compounds (in the storage organs), so it is at a lower concentration here than in the phloem and so it is more likely to move from higher to lower concentration.

Heat and chemical treatments

Movement of fluid in the phloem may be slowed down by cooling or stopped by extreme (higher) heat treatment. This presumably affects active transport in the companion cells or the cytoplasm in the sieve tube elements.

Active transport is affected by respiratory inhibitors. Sodium azide and cyanide cause great reduction in the tranlocation of sucrose.

The compound PCMPS (p-chloromercuriphenylsulphonic acid) acts as an inhibitor to sucrose transporters so it practically prevents the loading of sucrose into the phloem. This movement may take place via the companion and other cells (symplastic pathway) or through the interstitial spaces/cell walls (apoplastic pathway).

Typical contents of phloem
/ mg cm^-3
Non-reducing sugars / ∽ 80-120
(There is very little reducing sugar - glucose etc - transported)
Amino acids / ∽ 5
Proteins / ∽ 2
Organic acids / ∽ 3
Inorganic ions / 5

This shows that phloem also transports other organic compounds than sucrose, as well as inorganic ions (recycled from the leaves, after removal of nitrates etc for production of proteins etc).

A classic ringing experiment

Single leaf labelling with CO₂ radioisotope to trace translocation of photosynthate

This diagram shows how the products of photosynthesis are translocated in the plant.

A mature leaf from the plant (numbered 14 on the diagram, seen from above) was supplied with carbon dioxide containing radioactive carbon (¹⁴CO₂).

The red shading indicates how much radioactivity is found in the other leaves of the plant, a week later.

What can you conclude from this observation?

> CO₂ is converted ('fixed') into organic compounds (sugars)
> that are moved up the plant
> to younger leaves that are immediately above
> but not on the other side


Suggest what you would see if you measured the radioactivity in the roots of the plant by spreading them out over photographic film for some time (then developing it).
This is called an autoradiogram

> Film would be exposed where radioactive isotope moves to
> Roots would show radioactivity (on the same side as the leaves)
> but not on the other side

Summary of differences

Tissue	XYLEM	PHLOEM
Direction of movement	Upwards	Upwards or downwards
Source	Roots	Leaves (maybe storage organs)
Sink	Leaves	Growing tips of roots and shoots, flowers, fruits, storage organs
Powered by	Evaporation from leaves (mesophyll) - also osmosis (root pressure)	Active transport from leaves (mesophyll) and osmosis
Pressure	Predominantly Negative	Positive
Substances transported	Water and inorganic ions (mineral salts)	Water and organic compounds (sugars)
Cells involved	Vessel elements and tracheids [dead, hollow]	Sieve tube elements [living but reduced components] and companion cells [live and active]
Secondary functions	Support	Transport of inorganic ions recycled from leaves

Insects getting stuck in

Suggest some reasons why aphids (greenfly) are usually found on the underside of leaves (rather than the top surface).

> The phloem is on the underside of vascular bundles
> the lower epidermis is thinner
> so it is easier to get sugars using biting mouthparts
> and they get better protection from sun and predators, including gardeners!

In fact, aphids tapping into a supply of sugars in the phloem are usually provided with more than they can consume, and at a pressure which is higher than expected. They therefore release the excess sugary liquid from their rear end. This 'honeydew' is often used by other insects, especially ants.

In woodland in the summertime it may drip onto objects below, such as cars parked in the shade of trees. Not only does this sometimes make them sticky, but it often encourages the growth of fungi which produce dark coloured spores.