Hormone control of plant growth and use of plant hormones

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page will answer many questions e.g.

 How do hormones control plant functions?

 What are auxins?

 What is phototropism?

 What is gravitropism or geotropism?

What controls the ripening of plants?

Hormone Control in plants

Know that plants also produce hormones and respond to external stimuli.

Know and understand that plants are sensitive to, and respond to, light, moisture and gravity:

Their shoots grow towards light and against the force of gravity,

Their roots grow towards moisture and in the direction of the force of gravity.

Know that plants produce hormones to coordinate and control the growth at the tips of shoots and roots.

These hormones, called auxins, are soluble in water and so can diffuse around to where they are needed.

Auxin is a plant hormone that controls and coordinates the growth of the tips of shoots and roots

Auxin acts by enabling the plant to respond to external stimuli e.g.

the tips of shoots to grow towards light, the effect is called phototropism,

the tips of shoots to grow upwards against gravity (gravitropism),

the tips of roots downwards with gravity (gravitropism),

the tips of roots grow to seek moisture in the soil (hydrotropism),

and note that auxin promotes growth in the shoots, but inhibits growth in the root.

Know that hormones called auxins controls phototropism and gravitropism (geotropism).

Auxin is produced in the tips of shoots and roots, being soluble, it moves back by diffusion to stimulate cell growth - a process of cell enlargement and elongation.

This process occurs in the cells immediately behind the tips of shoots and roots AND the change in growth direction is due to an unequal distribution of auxin.

If the tip of shoot is cut off, the shoot may stop growing because the auxin hormone is no longer available.

NOTE - auxin can promote growth in shoots but a high concentration of auxin can inhibit growth in the root to ensure it grows in the right direction.

The responses of plant roots and shoots to light, gravity and moisture are the result of unequal distribution of hormones like auxin, causing unequal growth rates and changes in growth direction - examples described in detail below.


How the plant growth hormone auxins works!


Shoots are positively phototropic - they grow towards light


Shoot tips growing towards light - positive phototropism (positively phototropic - shoots grow towards the light)

When light shines on a shoot, more auxin concentrates on the side that is in the shade (less light side) - unequal distribution of auxin.

This stimulates growth to elongate the cells more on the shaded side so the shoot bend upwards towards the light.

In bending towards the light the shoot can absorb more light for photosynthesis and hence plant growth.

Shoots that grown in poor light (very low light levels) grow tall and very thin (spindly) because the auxin in the tip makes the cells grow elongated quickly, but on all sides. A taller shoot has a better chance of finding light.

You can do a simple experiment with cress seeds on moist filter paper in a petri dish. You cover the petri dish with a box with black insides but with a hole cut in one side.

You then leave the box where it is exposed to bright light e.g. near a laboratory window with the hole pointing outwards.

leave it for a week and then carefully remove the box and you will find the cress shoots were growing towards the hole in the side of box - that is growing towards the light.


Shoots are negatively gravitropic - they grow up against the down force of gravity


Shoots growing up against gravity - negative gravitropism/geotropism (negatively gravitropic/geotropic)

If a shoot starts to grow sideways - at an angle or horizontally, gravity causes more auxin to concentrate on the lower side - unequal distribution of auxin.

Therefore the lower side cells are stimulated to grow faster causing the shoot to grow and bend upwards.

By growing upwards the shoots can better access the light for photosynthesis.


Roots are positively gravitropic - they down in the direction of the force of gravity


Roots growing down with gravity - positive gravitropism/geotropism (positively gravitropic/geotropic)

If a root is tending to grow sideways-horizontally, then, due to gravity it tends to have more auxin on its lower side - unequal distribution of auxin.

BUT, excess auxin in the root can inhibit growth and so the upper cells tend to elongate faster the lower side cells, causing the root to bend round downwards and become more firmly embedded in the soil.

By growing downwards the roots can better access the soil for minerals and water.


Roots are negatively phototropic - grow away from light


Roots grow away from light if exposed to it on the surface of soil, so roots are negatively phototropic.

If roots are exposed to light on or near the soil surface more auxin concentrates on the more shaded underside of the root.

In the root, this high concentration of auxin inhibits growth on the underside i.e. cell elongation. This allows the greater elongation to occur on the upper side that is more exposed to the light.

Consequently, the upper greater cell elongation makes the root bend and grow downwards deeper into the soil.

All the roots deep in the ground tend to grow downwards due to positive gravitropism.

However, they will also grow towards a more concentrated area of water due to hydrotropism (described below).

By growing downwards the roots can better access the soil for minerals and water.


Roots are positively hydrotropic - grow towards moisture - the best source of water


Roots growing towards moisture - positive hydrotropism (positively hydrotropic)

If a root is exposed to an uneven distribution of moisture i.e. one side of the root is more moist than the other, more auxin concentrates on the side with the most moisture - unequal distribution of auxin.

Consequently the increased auxin level inhibits growth on the moist side and stimulates a greater growth rate on the least moist side to make the root bend towards the moisture.

By growing towards moisture the roots can better access the soil for water (and minerals too).


Summary of tropism advantages

Phototropism, gravitropism and hydrotropism all increase the chance of a plant's survival e.g.

shoots growing towards light increase the rate of photosynthesis - more light can be absorbed by chlorophyll,

the root growing downwards can find minerals and water in the soil AND be more firmly fixed in the soil too.

The commercial use of plant hormones

Plant growth hormones are used in agriculture and horticulture as weed killers and as rooting hormones and is a very important use of plant hormones like auxin derivatives and gibberellins.

  • Some plant growth hormones can be used as selective weed killers to disrupt the growth of weeds but leave the crops unaffected. These

    • Desired crops of grasses and cereals are narrow leafed plants but many weeds have broad leaves.

    • Selective weed killers have been developed from auxins which only affect broad-leaved plants.

    • Selective plant growth hormone based weed-killers have been developed that affect the growth development of broad-leafed weeds and eventually kill them, BUT, do not affect the grasses and cereals with narrow leaves.

  • A plant cutting is a part cut off e.g. the end of a branch with a few leaves on.

    • Some plant cuttings won't always readily grow when planted in soil or compost

    • By adding a rooting powder to the compost containing a plant hormone like auxin, the growth of roots and subsequent shoots are greatly encouraged so new plants grow more rapidly.

    • It then enables a flower grower or market gardener to rapidly produce lots of clones (exact copies) of a particular plant - ideally, of the best quality plants.

  • Plant hormones can be used to control the ripening of fruit or produce seedless fruit.

    • There are several techniques for controlling flower and fruit formation.

    • The ripening of fruits can be controlled while the fruit are still on the tree/bush or during transport to the warehouses/shops.

      • You can therefore pick fruit before it is ripe and still quite firm - which means the firmer unripe fruit is less easily damaged in transport.

      • You can then choose the time when the ripening hormone is added so the fruit is as fresh as it can be for you the consumer via the wholesaler, market vendor, small shop or giant supermarket!

      • The hydrocarbon gas ethene can act as a ripening hormone. If ethene is added to the crop it will ripen on the way to supermarket and be just ready for the shelves (more on ethene in last section).

    • Producing seedless fruit: Most fruit plants with seeds in the core, require pollination by insects, otherwise the fruits and seeds will not grow.

      • If the flowers don't get pollinated, the fruit and seeds can't grow.

      • By applying growth hormones like gibberellins to the unpollinated flowers of some fruit plants, the fruits grow BUT not the seeds!

      • Sometimes the plant hormones are applied after pollination, but still prevent the seeds developing.

      • A very handy way of producing common varieties of seedless fruits like watermelons, grapes, bananas and many seedless citrus fruits, such as oranges, lemons and limes.

  • Other uses of gibberellins

    • Gibberellin plant growth hormones are used to stimulate seed germination, stem growth (taller) and flowering.

    • Some seeds will not germinate unless they have experienced certain conditions such as a dormant period or dryness or a period of coldness.

      • Gibberellins can break this period of dormancy, allowing the seeds to germinate.

    • If you treat these seeds with gibberellins you can make them germinate at any time in the year when they would not normally germinate. This is a technique used by commercial plant growers.

      • This technique has the added commercial advantage of making all the seeds germinate and the same time.

      • Germination is no longer dependent on a 'dry' or 'cold' period.

      • You can produce a variety of flower all the year round.

      • Gibberellins are also used to reduce flower formation to improve fruit quality.

        • Fruit grow on pollinated flowers and some fruit trees produce too many flowers.

        • This causes too many fruits to form which the tree struggles to support nutritionally and they tend to grow too small.

        • Using hormone control you can reduce the number of flowers and enable larger better quality fruit to grow.

      • Gibberellins also trigger a plant response known as 'bolting' in response to a cold spell or lack of water - both of the latter slow-inhibit plant growth.

        • 'Bolting' is when a plant produces lots of flowers in an attempt to reproduce before it dies.

        • Commercial plant growers use gibberellins to make plants flower earlier than they would normally do, OR, under conditions where they wouldn't usually flower.

    • If a variety of a dwarf plant is treated with Gibberellin, it can grow as high as a tall variety!

    • Sometimes combinations of hormones e.g. auxins plus gibberellins can be used in conjunction with each other to have greatly enhanced effect e.g. producing very tall plants.

Other stimulants used in controlling growing plants

alkenes structure and naming (c) doc bEthene

The hydrocarbon gas ethene is produced by aging leaves. Ethene stimulates expansion of the cells that connect the leaf to the stem or branches. This expansion breaks the cell walls and causes the leaves to fall off the plant.

Therefore, commercial growers can, if so required, use ethene to remove leaves from a crop e.g. in farming cotton, treat with ethene before its harvested.

You treat fruit bushes in the same way with ethene to make the collection of fruits or seeds easier i.e. without the waste leaves.

Ethene can act as a ripening hormone.

Ethene stimulates the enzymes that cause fruit to ripen.

Therefore you can use ethene to control the time when you want the fruit to open.

You can do this when the fruits are still on the plant OR, while the fruits are still on the plant, during transport to the depot, store or supermarket.

This offers a way of picking unripe fruit which will be firmer and less easily damaged in transportation.

You then add the ethene to ripen the fruit on the way to the consumer so it arrives in perfect condition.

So it seems somewhat unnatural, but very useful, to add ethene to an unripe crop and ripen on the way to the supermarket and be just ready for the shelves and you!




Practical investigation you may have experienced

  • You are expected to use appropriate skills, knowledge and understanding to:

  • Any practical work and investigations you did should also be revised  (which should also be revised, helps in understanding 'how science works' and context examination questions):

    • evaluate the use of plant hormones in horticulture as weedkillers and to encourage the rooting of plant cuttings.

    • plant growth investigation e.g. ...

      • ... the effect of light on the growth of seedlings,

      • ... the effect of gravity on growth in germinating seedlings,

      • ... the effect of water on the growth of seedlings,

      • ... using a motion sensor to measure the growth of plants and seedlings,

      • ... the effect of rooting compounds and weed killers on the growth of plants,


 General PLANT BIOLOGY revision notes

See also cell biology section

Photosynthesis, importance explained, limiting factors affecting rate, leaf adaptations  gcse biology revision notes

Transport and gas exchange in plants, transpiration, absorption of nutrients, leaf and root structure gcse biology revision notes

See also Diffusion, osmosis, active transport, exchange of substances - examples fully explained

Respiration - aerobic and anaerobic in plants  gcse biology revision notes

Hormone control of plant growth and uses of plant hormones  gcse biology revision notes

Plant diseases and defences against pathogens and pests  gcse biology revision notes

See also Adaptations, lots explained including plant examples  gcse biology revision notes

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