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Transportation in plants

A properly functioning transportation system in the plant creates a healthy, strong plant. Sugars from the leaves, water and nutrients from the roots and plant hormones from all cells have to be transported to the places they’re needed. Transportation in plants takes place in xylems and phloems.

Xylem

Transportation in plants

Water inside the plant is mainly transported by xylems. The evaporation from the leaves is the motor of water absorption. The water that is absorbed by the roots evaporates through the stomata in the leaves. This creates a flow of water from the roots to the leaves.

The water is pulled upwards in the plant. The vessels run from right above the root tip until just under the top of the stem and inside every leaf. The xylems aren’t active and there’s only one possible direction: upwards. So it’s a passive system that works a bit like a straw, in which suction comes from the evaporation of water in the stomata of the leaves.

The intake of water and nutrients mainly takes place in the root hairs. Then it’s on to the xylems, for which it can follow three routes:

• Between the cells. The plant has little influence on this
• From cell to cell, but not through the cell fluid
• From cell to cell, via the cell fluid and the vacuole. The plant can steer this process

All these routes lead to a suberized layers of cells around a xylem. This is where all the water needs to pass through the cells. The plant therefore has some control over the amount that passes through. After this, water and nutrients move upwards in the plant through the xylems. Evaporation plays a big role in this, so the parts that evaporate the highest quantities will receive the most water and nutrients.

Phloems

Phloem sieve tube
Cross section of some phloem cells

Photosynthesis takes place in the leaves. The sugars that are produced in this process have to move to other parts of the plant to be used there. The transportation system of phloems takes care of this. In addition to sugars, phloems also transport other substances, such as amino acids and water. Non-evaporating parts such as flowers get most of their water through phloems.

Phloem consists of living cells and transportation can take place in all directions. The terms ‘source’ and ‘sink’ are used to indicate where the transportation is going to. Sources are the production centers of photosynthesis, so mainly the adult leaves at the top of the plant that catch a lot of light. All organs in the plant work as a sink the sugars flow to. The flowers, roots and meristems all draw in sugars.

The joint pulling force is usually much larger than the production of sugars. The part that pulls the hardest will therefore get the most sugars.

A young leaf doesn’t produce enough itself to be able to grow out properly. So it attracts sugars and is therefore a sink. As the young leaf grows, it requires fewer sugars from elsewhere and produces more itself, thereby becoming a source. The transition point occurs when the leaf reaches about half of its final size.

Loading and unloading phloem

Loading and unloading of the phloem
Transport in Phloem

In order to transport produced sugars from the leaves to elsewhere, they have to be loaded onto the phloem. The sucrose can’t pass the membrane of the cell itself: the molecule is too big and therefore has to be let into the cell through special systems. The sugars are actively pumped into the phloem by the cells. This takes place as follows: the phloem cells first emit hydrogen ions (H+) in an active process that requires energy.

This creates a gradient: there are now many more hydrogen ions outside the phloem than inside. The H+ wants to go back, but can’t, because the membrane is closed. They can only go back along with a transportation protein in the membrane, so the H+ ion attaches itself to it. But it doesn’t flip over to the inner side of the membrane until a sugar molecule is attached to the same protein. That’s how hydrogen ions and the sugar molecule end up in the phloem. The protein that’s responsible for this is called H+ sucrosepump.

This process keeps increasing the concentration of sucrose in phloems. And a liquid with a higher concentration of dissolved substances attracts water through the membrane from neighboring cells, through the principle of osmosis. This creates a hydrostatic pressure, automatically starting the transportation of water and sugars. When arriving at a sink, the exact opposite process takes place through the H+ sucrosepump.

The sugars are ‘unloaded’. The plant made sure the rate of unloading is higher than the rate of loading, so problems with unloading can never lead to an inability to load. The loading rate determines the total rate of the system.

Sending sugars to the right location

Send sugar to the right location
Sugar transport in plant

There’s a strong competition in the plant to attract the produced sugars. The part of the plant that pulls the hardest (strongest sink), gets the most sugars. But breeders want to influence this to make the buds of the weed plants nice and heavy. The temperature of the environment barely has any influence on photosynthesis and the loading onto and unloading from the phloem, but it does influence how fast the cells can convert sugars into new cells.

In the shoot tips, the sugars have to be converted into proteins, lipids and other parts of the new cells. This requires various enzymes and enzymatic processes occur much faster at higher temperatures. If the growing processes are very slow because of a low temperature, the unloading of sugars from the phloem is slowed down. The sink already received many sugars that aren’t processed yet, so it’s hard to attract any more sugars.

Since photosynthesis continues (not as sensitive to temperature), more and more sugars are being added. The concentration of sugars in the producing cells in the leaf increases, because the sugars have nowhere to go. The cells decide to just store the sugars in the chloroplasts in the form of starch. When this happens a lot, the accumulation of starch blocks the light and photosynthesis won’t go as well. In weed plants, the leaf will first get darker (accumulation of starch) when the temperature is too low, and will then turn yellow (degradation of chlorophyll).

There should therefore always be an equilibrium between source and sink. If you increase the temperature, you disturb this balance, because it increases the power of the sink. If you give more light or CO2, you promote the source power. So in order to restore the balance you have to increase the temperature when you’re giving more light or more CO2.

Storage of sugars

Storage of sugars
Store sugars in the stems and roots

The plant has many ways to get rid of its photosynthesis products, since the sink power is usually much stronger than the source power. If the plant has few flowers, it will send excess sugars in the direction of the stems or roots. These will become visibly thicker.

The plant can also store sugars in the stem and roots to temporarily park these as starch. They’re stored in the form of starch, because starch doesn’t attract moisture. Sugars do attract moisture, and have a strong osmotic value. This would lead to a lot of moisture and pressure in the storage cells.

These stocks can be accessed at a later point in time through the phloem, if photosynthesis isn’t working optimally. The starch will then be broken down into sugars, which will be transported to the sinks in the plant.

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