GCSE level School biology notes: Hormone control of plant growth, use of plant hormones

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Tropic responses in plants - gravitropism, phototropism, hydrotropism - hormone control of plant growth and use of plant hormones

Doc Brown's biology exam revision study notes:  This page will answer many questions e.g. How do hormones control plant functions What is phototropism? What is gravitropism or geotropism?  What controls the ripening of plants? How are plant hormones used commercially?  What are auxins? Keywords to understand: meristems, auxins. gibberellins ,ethene stimulant positive or negative phototropism, positive or negative gravitropism (geotropism) gravitropic growth, phototropic growth, hydrotropic growth

Sub-index for this page on plant hormones

(a) Introduction to meristems and hormone control in plants

(b) How plant growth hormone auxins works - phototropism, gravitropism, hydrotropism

(c) The commercial use of plant hormones

(d) Other stimulants used in controlling growing plants

For hormones and human biology see

Hormone system - Introduction to the endocrine system 

and Hormone Systems - menstrual cycle 

(a) Introduction to meristems and hormone control in plants

Plants grow and develop in three ways:

(i) Cell division  - mainly in the tips of roots and shoots in regions called meristems.

(ii) The growth in height of plants is due to plant cells expanding and elongating.

(iii) Plants usually grow continuously e.g. putting out new roots, branches and leaves - so plant cells can differentiate throughout their lifetime.

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

These effects are called tropisms and they are caused by an unequal distribution of plant hormones.

A definition of a tropism is to say it is a plant growth movement in a particular direction due to a directional stimulus.

The plant growth hormone auxins are unequally distributed in response to light, gravity and moisture (water).

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.

Auxins are produced in the tips of shoots and roots of plants.

Plants need to be able to detect and respond to stimuli from their environment in order to survive.

The plant hormone 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 or geotropism),

the tips of roots downwards with gravity (gravitropism or geotropism),

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 throughout the plant - a process of cell enlargement and cell 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 auxins can inhibit growth in the root to ensure it grows in the right direction - so things get a bit complicated.

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.


In plants, the only cells that divide by mitosis are found in plant tissues called meristems.

Meristem tissue is found in the tips of roots and shoots - the parts of plants that are growing (diagram on right).

Meristems make unspecialised cells that can divide and form any type of cell the plant needs.

These unspecialised or undifferentiated plant cells effectively acts the same as animal stem cells.

However, unlike human stem cells, they can differentiate to form any type of cell for the lifetime of the plant.

e.g. these unspecialised cells can form the specialised tissue cells of the xylem and phloem.

In the tips the meristems cells undergo cell division by mitosis and the daughter cells grow longer and longer to increase the length of the shoot or root and ultimately the full sized mature plant.

Plant hormones like auxins promote cell division in the meristems and the increase in size of the daughter cells.

As the cells get larger, so does the vacuole (not shown), filled with water to give physical support to the plant as a whole.

The cells at the end of the tips remain meristematic - that is they stay undifferentiated to provide any type of cell needed for new growth.

When cells from meristems are extracted you can them grow them in tissue culture.

This is a very useful procedure because tissue culture enables you to produce clones of plants quickly, that are genetically identical and can be grown economically by horticulturalists.

Section on plants in Cloning - tissue culture of plants 


(b) How the plant growth hormone auxins works!

Phototropism - shoots are positively phototropic - they grow towards light


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

Light is needed for photosynthesis, so plant need to know in which direction to point their shoots to ensure its food supply.

When light shines on a shoot, more auxin concentrates on the side that is in the shade (less light intensity side) giving an 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.

A simple experiment to demonstrate phototropism

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 inside walls 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.


Phototropism of fallen tree once horizontal branches growing vertically upwards towards the light gcse biology igcse

Phototropism and a fallen tree!

The tree had blown over in a gale, but sufficient roots survived, so that the once horizontal branches, are now growing vertically 'tree-like' towards the light in what is quite a densely wooded area.


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


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

Plants must sense gravity in order that shoots grow upwards in the right direction to access the maximum light intensity (and roots grow downwards for water and mineral ions).

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

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.

Shoots will still grow upwards eve in the dark - absence of light makes to difference in principle to negative gravitropism.


Gravitropism - roots are positively gravitropic - they grow down in the direction of the force of gravity


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

Plants must sense gravity in order that roots grow downwards in the right direction to absorb the maximum amounts of water and mineral ions (and shoots grow downwards for light powering photosynthesis).

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 tip 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.

Shoots will still grow upwards eve in the dark - absence of light makes to difference in principle to positive gravitropism.

A simple experiment to demonstrate gravitropism

You can take maize seeds, plant them, and allow them to germinate.

You then secure them in separate containers e.g. petri dishes, and set them up so the growing roots are horizontal.

You leave the roots to grow for several hours and note the length and shape of them.

It's probably best to take time lapse photographs e.g. every 30 mins, to monitor the growth progress.

You should find the ends of the roots will start to bend downwards and the tip eventually points in a vertical, but downward direction.

Phototropism - roots are negatively phototropic - grow away from light


Roots grow away from light if exposed to it on or near 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 tip, this high concentration of auxin inhibits growth on the underside i.e. inhibits 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.


Hydrotropism - 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.


(c) 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 controlling hormones can be used as selective weed killers to disrupt the growth of weeds but leave the crops unaffected. These

    • In high concentrations, auxins disrupt cell metabolism and kill plants.

    • Therefore you can use synthetic auxins as weedkillers or herbicides.

    • 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 compared to narrow 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 desired grass and cereal crops with narrow leaves.

    • This enables farmers to kill broad leafed weeds without harming fields of crops of barley or wheat.

    • Gardeners can use these selective weedkillers to remove dandelions and other plants regarded as weeds - I don't mind the dandelions myself!

    • 2,4-D (2,4-Dichlorophenoxyacetic acid) is a synthetic auxin, which a type of plant growth regulators.

      • 2,4-D is absorbed through the leaves and is translocated to the meristems of the plant.

      • This causes uncontrolled, unsustainable growth resulting in stem curl-over, leaf withering, and eventual plant death.

  • A plant cutting is a part cut off from a plant 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 without any extra help

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

  • Use of issue culture

    • You can use tissue culture to grow lots of clones of a plant from just a few of its cells.

    • In order for the technique to work, auxin hormones are added to the tissue growth medium, as well as the nutrients any plant needs to grow.

    • The hormone auxin stimulates cell division to promote growth and form roots and shoots.

  • Plant hormones like gibberellins 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).

      • You can increase the size of some fruits by spraying them with gibberellin 4 to 6 weeks before they are due to be harvested - you can reduce flower formation, fruit grows when they are pollinated - so instead of having lots of small fruits, you can have a smaller number of larger fruits of superior quality.

    • 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!

        • This is how you grow seedless citrus fruits.- 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.

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

      • Some seedless varieties like grapes do not grow as large as seeded varieties, but application of gibberellins will cause them to grow to their normal larger size.

  • 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 lack of stimulation due to dryness or a period of coldness due to low ambient temperatures.

      • Its good if you can control this dormancy and gibberellins can stimulate germination..

      • 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 seed batches 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 - called induced flowering.

        • Some plants need certain specific conditions to flower e.g. longer summer days or lower temperatures in spring or autumn.

        • You can treat these plants with gibberellins to make them flower without changing their environmental conditions.

        • Also, if a variety of a dwarf plant is treated with gibberellin, it can be made to grow much bigger flowers or a taller variety!

        • It is possible to use gibberellins to promote the flowering of some apple varieties in a poor year due to poor weather.

      • Gibberellins are used in the brewing industry - production of alcoholic drinks like beer

      • Barley supplies the necessary sugars and amino acids that yeast needs for fermentation - converting sugars to 'alcohol' (ethanol) and carbon dioxide.

      • The barley must first germinate to do this and spraying it with water containing gibberellin speeds up the germination.

      • Gibberellins are naturally produced by seeds like barley, but enough for commercial speed!

    • Complex growth control

      • 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.


(d) Other stimulants used in controlling growing plants

alkenes structure and naming (c) doc bEthene - promotes the chemistry of ripening

You normally come across ethene when studying alkene hydrocarbons and the oil industry.

The hydrocarbon gas ethene is produced by aging parts of plants e.g. 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.

However, because ethene is a gas, its difficult to apply to fruit crops to ripen them.

However, compounds have been developed that can release ethene and a product called Ethrel can be used to ripen fruit in the field.

Ethene can act as a ripening hormone

Ethene affects the growth of plants in several ways, including controlling cell division.

Ethene stimulates the enzymes that cause fruit to ripen.

Bananas will ripen, even when enclosed in a plastic bag.

They, like many other fruits, release ethene and increase the rate of respiration to ripen.

If you put a banana in a bag of another fruit e.g. tomatoes, they will ripen more quickly as the banana releases its ethene!

Some fruit only ripen on the tree, but many other fruits only ripen after they are harvested.

These include apples, bananas and tomatoes.

Therefore you can use ethene to control the time when you want the fruit to ripen - like speed things up!

You can do this when the fruits are still on the plant OR during transportation to the depot, store or supermarket - so some fruits can be picked unripened and transported to ripen later under controlled conditions.

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 'ripened' condition at the shop or supermarket.

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!

The fruit can be stored in special rooms where ethene concentration, humidity and temperature can be carefully monitored and controlled.

You can also, if required, delay the ripening while the fruit is being stored before distribution to shops and supermarkets.

You do this by chilling or spraying with a chemical inhibitor that block's ethene production during transportation and storage - so you can work the effect of ethene both ways!

Health and safety note:

Ethene is a very flammable gas and mixed with air can explode, so I presume great care is taken in using it and with low levels in the atmosphere around fruit I presume!

AND see ALKENES - unsaturated hydrocarbons for the 'usual' GCSE chemistry of ethene!

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,

For hormones and human biology see

Hormone system - Introduction to the endocrine system 

and Hormone Systems - menstrual cycle 

Summary of learning objectives, keywords and phrases about plant hormones

Know the function of meristems in plants.

Be able to describe how hormones control plant functions, particularly growth.

Know what controls the ripening of plants?

Know that auxins are plant hormones.

Know the use of ethene as a stimulant and gibberellins which can have a positive or negative stimulating effect on plant growth.

Know what is meant by phototropism (phototropic effect),  gravitropism (gravitropic effect) and geotropism (geotropic effect) in plant growth as extra cells are formed.


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General PLANT BIOLOGY revision notes

Photosynthesis, importance explained, limiting factors affecting rate, leaf adaptations

Plant cells, transport, gas exchange in plants, transpiration, absorption of nutrients, leaf & root structure

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

Respiration - aerobic and anaerobic in plants (and animals) 

Plant diseases and defences against pathogens and pests 

See also Adaptations in plants - emphasis on extreme environments 

and a section on Stem cells and uses - meristems in plants

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