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Horticulture 1000
HT1101 - Plant management 1 - Plant processes
Plant parts Plant training
2

Plant processes

Photosynthesis

Plants are producers. They make their own food by getting the material and energy they need from their environment. The process of converting sunlight into food is called photosynthesis.

This is how it works:

You can use this equation to show what happens in photosynthesis.

Photosynthesis equation

The plant uses the sugar immediately, or changes it into other substances that can be stored, such as starch or cellulose.

Leaves and photosynthesis

Hydrangea These hydrangea leaves are arranged so they get maximum sunlight and so they don't shade each other.

 

The external structure of the leaf is involved in photosynthesis.

Leaves are flat so they can catch a lot of sunlight, and very thin so that light can penetrate right through them.

 

The internal structures of the leaf are important in the photosynthesis process.

The internal structures of the leaf

Internal parts of a leaf and their functions:

  • Cuticle: waxy protective layer on top of the epidermis, which reduces the loss of water and stops the leaf from drying out.
  • Upper epidermis: outer protective layer. Carries out gas exchange. Has few stomata to help prevent water loss.
  • Palisade layer: contains cells with chloroplasts. Chloroplasts contain chlorophyll and are placed where they receive a lot of light for photosynthesis.
  • Vascular bundle (leaf vein): contains phloem and xylem cells. Xylem transports water and minerals to the leaf, and phloem takes sugar away from the leaf.
  • Spongy mesophyll: this is where gas exchange takes place. Carbon dioxide enters and oxygen is released.
  • Guard cells: these regulate the balance between water loss and carbon dioxide uptake.
  • Stomata: tiny openings bordered by guard cells in the epidermis of leaves and stems. Gases pass through stomata.
 

Increasing photosynthesis

The essential factors needed for photosynthesis are light, carbon dioxide, water and warmth.

Growers sometimes increase the level of carbon dioxide Growers sometimes increase the level of carbon dioxide in a greenhouse to increase the rate of photosynthesis.

Sometimes the rate of photosynthesis is increased artificially. You can:

  • use artificial light at night, dawn and dusk
  • increase the temperature
  • increase the level of carbon dioxide
  • provide the right amount of water.
 

These factors don't operate independently; they are affected by each other. When there are the right amounts of all four factors, growth rates can be very quick.

The worst conditions for photosynthesis are dimly lit places, especially if they are also cold. Few plants will grow in heavy shade.

Respiration

Respiration is the process where plant cells use the glucose made during photosynthesis for maintenance and growth.

This is how it works:

Sugar is broken down and energy, water and carbon dioxide is produced and released. Oxygen is needed for this process to take place.

You can use this equation to show what happens in respiration:

The Respiration process

Respiration is the reverse of photosynthesis. It can take place during day or night.

  

Photosynthesis  Respiration
builds  up food    breaks down food
takes place only in daylight    takes place day and night

Activity 2A Photosynthesis and respiration

Drag the words into the gaps to complete the equations for photosynthesis and respiration.

Activity 2BBelow are twelve sentences, six about photosynthesis, and six about respiration.

Drag each into the correct column in the table below, matching each sentence with its opposite.

 

Transpiration

Some of the water that is transported from the root to the leaves is used by the plant. The rest of it evaporates or is lost from the leaves. This is called transpiration.

Stomata

Water is lost through small pores on the leaves, called stomata Water is lost through small pores on the leaves, called stomata.

Water is lost through small pores on the leaves, called stomata. 'Stomata' means many pores. A single pore is one stoma. Most stomata are found on the underside of the leaf. You usually need a microscope to see them.

Stomata allow carbon dioxide gas into the leaf for photosynthesis. Water vapour and oxygen gas leave through the stomata. Oxygen is a waste product of photosynthesis.

 

The transpiration stream

Water loss or transpiration from the leaves causes more water to come up through the xylem, and more water flows into the roots.

It takes place when there is more water in the leaf than in the air.

The transpiration stream The transpiration stream.

Transpiration causes:

  • water to be drawn up through all the xylem cells for support
  • dissolved nutrients to reach the leaves
  • water to be drawn to the leaves for photosynthesis
  • leaves to be kept cool as water evaporates from them.

This stream of water going up through the plant, out through the stomata and into the air is called the transpiration stream.

 

Environment and transpiration

On cold, still, cloudy or humid days, when there is a lot of moisture in the air, water evaporates slowly. Transpiration also takes place slowly, so plants don't lose much water.

In hot, windy conditions there is very little moisture in the air and water evaporates quickly. Transpiration is fast, and plants lose a lot of water. If the water supply from the soil doesn't keep up with the transpiration rate, the plant will suffer water stress.

High light conditions will also increase transpiration levels.

This plant has wilted and if the cells lose further water it probably will die.

This plant has wilted and if the cells lose further water it probably will die.

Eventually, the cells lose so much water that the plant wilts and can die.

 

Plants control transpiration

Plants can take some measures to control their transpiration rate.

When a plant is well supplied with water, the guard cells swell. As they become fatter they open up more, like balloons pumped up with air. Water can escape more easily.

When a plant loses a lot of water, the guard cells lose water too. They become limp, like balloons losing air. This closes each stoma, and stops the plant losing so much water.

Decreasing transpiration rate

Various methods are used to decrease the transpiration rate of plants.

Grow shelter to protect plants from wind

Grow shelter to protect plants from wind.

 
When growing tropical plants, keep the inside of the greenhouse humid

When growing tropical plants, keep the inside of the greenhouse humid.

 

Mist pot plants to increase the humidity around their leaves.

Activity 2C Test your understanding of plant processes.

Activity 2D Under each of the following eight diagrams select whether the transpiration rate will be high or low.

Complete Activity 2E in your workbook

Key points   Key points

  • Plant leaves are designed to obtain maximum sunlight.
  • Plants are producers, making their own food by photosynthesis.
  • Photosynthesis can be explained by this equation: water + carbon dioxide + chlorophyll + sunlight = sugar + oxygen.
  • Plants use sugars and release energy in the process of respiration.
  • Respiration can be explained by this equation: sugar + oxygen = water + carbon dioxide + energy.
  • Transpiration is water loss from leaves. Stomata are small pores on the leaves that let water and oxygen out and carbon dioxide in. The guard cells help control water loss by transpiration. Transpiration is slow on cold, still, cloudy and humid days and  fast on hot windy days.

Water is:

  • needed for photosynthesis
  • produced during respiration.

Oxygen is:

  • needed for respiration in all living plant parts
  • produced during photosynthesis.

Carbon dioxide is:

  • needed for photosynthesis
  • produced during respiration.
 

What's next?

Go to: 3 Plant training.

Plant parts Plant training