Biology › Hormonal communication, plant responses and homeostasis › Plant responses: what the classic experiments actually show
Plant responses: what the classic experiments actually show
A plant cannot move away from anything, so its response to the world is to grow differently. A handful of substances made in one part of the plant and moved to another decide which cells elongate, and the whole of tropism follows from that.
Before this Water potential and turgor in a plant cell · Transcription factors and the control of gene expression
Before you start
Light destroys the auxin on the lit side of a shoot, so there is less of it there and that side grows less. Textbooks and revision sites still say it, and the experiments usually printed alongside cannot tell it apart from the alternative — that the auxin is moved sideways rather than broken down. The experiment that settled it collected auxin in agar from the lit and the shaded halves of illuminated coleoptiles and measured both. The total was unchanged by the light; what had changed was where it was. Insert a barrier down the middle and the asymmetry disappears. So the evidence supports lateral movement, not destruction, and an answer built on destruction is arguing against the data.
What you should be able to do
- Define a tropism and name the common ones with their stimulus and direction.
- State what Darwin's, Boysen-Jensen's and Went's experiments each establish, and what they do not.
- Explain how auxin causes a cell to elongate, using the cell wall.
- Explain why the same auxin distribution bends a shoot upwards and a root downwards.
- Describe the effects of gibberellins and of abscisic acid.
- Give a commercial use of a plant growth substance and explain how it works.
Growing in a direction, on purpose
A tropism is a growth response of a plant in which the direction of growth is determined by the direction of the stimulus. Growth towards the stimulus is positive and growth away from it is negative, and the stimulus gives the tropism its name: phototropism for light, gravitropism for gravity, hydrotropism for water, thigmotropism for touch, chemotropism for a chemical.
Shoots are positively phototropic and negatively gravitropic; roots are the other way round on both counts, and positively hydrotropic besides. Each of those is worth stating as an advantage, since questions often ask for one. A shoot growing towards light reaches the light its leaves need to photosynthesise; a root growing downwards and towards water finds both anchorage and the soil water and minerals it must absorb. A pollen tube growing down a style towards chemicals released by the ovule is chemotropism doing the same job for reproduction.
Naming matters here. The substances involved are made in several tissues rather than in glands, they often act on the cells that made them or on cells millimetres away, and they are not carried in a circulatory system, so plant growth substance or plant growth regulator is a better description than 'hormone'. Boards accept 'plant hormone', and CAIE and OCR A both use it, but the differences are real and they explain why plant responses are slow and local rather than fast and general.
The classic experiments, and the limits of each
Almost every question on this topic is built on four experiments done on coleoptiles — the protective sheath around the shoot of a germinating cereal, which is straight, fast-growing and easy to cut. The experiments are worth knowing individually, because each supports one conclusion and no more.
Charles and Francis Darwin, 1880. A coleoptile lit from one side bends towards the light. Cover the tip with an opaque cap and it does not bend, although it goes on growing; cover a band further down and it still bends. So the tip is where the light is detected. The Darwins showed nothing at all about what travels from the tip, or whether anything does.
Boysen-Jensen, 1913. Cut the tip off, replace it with a thin block of gelatine between tip and stump, and bending still happens. Replace the gelatine with mica, which nothing can diffuse through, and bending stops. So whatever passes from the tip to the growing region is a chemical that diffuses, rather than an electrical signal or a mechanical pull. Still no molecule is identified.
Went, 1926. Stand cut tips on agar blocks so that the substance diffuses into the agar, then place a block on one side of a decapitated stump in complete darkness. The stump bends away from the side the block sits on. That result does two things at once: it shows the substance alone causes bending with no light involved, and it makes the degree of curvature a way of measuring how much substance a block holds. Went called the substance auxin. He had still not identified it chemically — indole-3-acetic acid was isolated later — and his own explanation of phototropism went beyond what these experiments could show.
The habit to build is separating a result from its interpretation. The lateral redistribution model, which every board teaches, is well supported in coleoptiles and is the answer to give; in other organs the details of how the growth difference arises are still argued over. An exam answer should describe what the experiment did, state the result and then draw only the conclusion the result supports.
How auxin makes a cell longer
Auxin — in practice indole-3-acetic acid, IAA — is made in the tip of a shoot and in young leaves, and is transported away from the tip through the plant, including sideways by carrier proteins in the cell membranes. The elongation it causes is a physical change in the cell wall.
Auxin binds to receptors in the growing cells and stimulates the proton pumps in their cell surface membranes, which actively transport hydrogen ions out into the cell wall. The pH of the wall falls to about 5, and at that pH enzymes called expansins become active and loosen the bonds between cellulose microfibrils. A loosened wall is more plastic, so the turgor pressure that was already there is able to stretch it, water enters by osmosis, and the cell lengthens along its axis. Cells on the shaded side of a shoot, receiving more auxin, elongate more than those on the lit side, and the shoot bends towards the light.
Concentration is the part that catches people out. Auxin promotes elongation over a range of concentrations and inhibits it above that range, and roots are inhibited at concentrations perhaps a thousand times lower than shoots. A single concentration can therefore promote growth in a shoot and inhibit it in a root — which is what the last line of the figure means, and what the next section needs.
Why a root and a shoot bend opposite ways
Lay a germinating seedling on its side in the dark. The shoot turns upwards and the root turns downwards, and the same explanation handles both.
Gravity is detected by statoliths, dense starch-filled organelles that sediment to the lower side of particular cells — in the root, the columella cells of the root cap. Their settling is followed by auxin being redistributed to the lower side of the organ, and while exactly how the one leads to the other is still being worked out, the redistribution itself is well established.
In the shoot, more auxin on the lower side means the lower cells elongate more than the upper ones, so the shoot curves upwards: negative gravitropism.
In the root, more auxin on the lower side means the lower cells elongate less, because root cells are inhibited at concentrations that would stimulate a shoot. The upper side therefore elongates more and the root curves downwards: positive gravitropism.
The same stimulus, the same redistribution, opposite bends — because the two tissues respond to the same concentration differently. Questions that give you a graph of growth rate against auxin concentration with two curves on it, one for roots and one for shoots, are testing exactly this and nothing else.
Gibberellins, abscisic acid, and what they are used for
Auxin is the substance AQA examines; OCR A and CAIE both go further, and two more are worth knowing properly.
Gibberellins promote stem elongation by stimulating cell division and cell elongation between the nodes, and dwarf varieties of plants such as maize and peas are often dwarf because they cannot make them. Their other examined role is in germination, and it is a clean example of gene expression being controlled by a chemical signal. In a barley grain taking up water, the embryo secretes gibberellin, which diffuses to the aleurone layer surrounding the starchy endosperm. There it causes the transcription of the gene for amylase, the amylase hydrolyses the stored starch to maltose, and the maltose is absorbed by the embryo and respired. A grain whose embryo has been removed makes no amylase; add gibberellin to it and it does.
Abscisic acid works largely the other way. It maintains dormancy in seeds and buds, and — the effect boards return to — it closes stomata under water stress. Released when a plant begins to lose more water than it can take up, abscisic acid causes solutes to leave the guard cells, so their water potential rises, water follows out by osmosis, the guard cells become flaccid and the stoma closes. Transpiration falls at the cost of photosynthesis, which is the trade a plant in drought is making.
Ethene, a gas, ripens fruit and promotes leaf fall, and cytokinins promote cell division and delay leaf senescence. Both appear in OCR A questions and neither is needed for AQA.
| Use | Substance | How it works |
|---|---|---|
| Selective weedkiller | Synthetic auxin, such as 2,4-D | Broad-leaved weeds absorb more and grow uncontrollably; narrow cereal leaves absorb less |
| Rooting powder for cuttings | Auxin | Stimulates root growth at the cut end, so cuttings establish |
| Seedless fruit | Auxin or gibberellin | Fruit develops without fertilisation, so no seeds form |
| Larger, looser grape bunches | Gibberellin | Elongates the stalks and the fruits themselves |
| Malting barley | Gibberellin | Speeds and evens out amylase production, so starch is converted to sugars sooner |
| Ripening fruit after transport | Ethene | Fruit picked unripe travels undamaged, then is ripened on arrival |
| Tissue culture of plants | Cytokinin with auxin | The ratio of the two decides whether the tissue forms shoots or roots |
TRY IT — Designing the control that decides it
A student claims that gibberellin causes amylase production in germinating barley grains. Describe an experiment using barley grains, some with the embryo removed, that would test the claim, and say what result would support it.
Check your answer
Set up three groups of grains. Group A is intact grains. Group B has the embryo cut away. Group C has the embryo cut away and is supplied with a gibberellin solution. All three are cut in half, sterilised to prevent microbial amylase, and placed on agar plates containing starch, then left at the same temperature for the same time.
Amylase activity is detected by flooding each plate with iodine solution afterwards. Starch that has been hydrolysed gives a clear zone around the grain; starch still present stains blue-black, and the diameter of the clear zone is a measure of how much amylase was produced.
The claim is supported if group A gives clear zones, group B gives none or almost none, and group C gives zones comparable with group A. That pattern shows the embryo is needed for amylase production and that gibberellin alone can replace it.
Group C is the group that does the work, and a plan without it proves nothing: A against B shows only that something in the embryo is required. Two more controls are worth stating — grains with the embryo removed and supplied with water alone, to show the solvent does nothing, and sterilisation of every grain, so that a clear zone cannot be the work of a fungus.
In the exam
- A tropism is a growth response whose direction is set by the direction of the stimulus. Answers that say the plant 'moves towards' the stimulus lose the mark, because nothing moves; growth is unequal.
- Attribute each experiment to what it showed. Darwin's shows the tip detects light; Boysen-Jensen's shows a diffusible chemical is involved; Went's shows the chemical alone causes bending in the dark.
- Say that auxin is redistributed to the shaded side, not that light destroys it. The evidence is that the total quantity is unchanged.
- For elongation, name the steps: proton pumps, low pH in the wall, expansins, wall loosened, water in by osmosis, cell elongates. 'Auxin makes cells grow' is not an explanation.
- In gravitropism questions, the mark is for saying that roots and shoots respond differently to the same concentration — inhibited in one, stimulated in the other.
- For gibberellin and germination, transcription of the amylase gene in the aleurone layer is the detail that scores. Do not write that gibberellin digests the starch itself.
Check yourself
A coleoptile is illuminated from the left. A student proposes two hypotheses: that light destroys auxin on the lit side, and that auxin moves across to the shaded side. Design an experiment using agar blocks that would distinguish between them, and state the result each hypothesis predicts.
Answer
Cut the tips from a number of coleoptiles and stand each tip on a pair of agar blocks separated by a thin impermeable barrier running down the middle of the tip, so that auxin diffusing from the lit half and from the shaded half is collected separately. Illuminate from one side, and keep a second set of tips in complete darkness as a control.
Measure the auxin in each block by the standard bioassay: place the block on one side of a decapitated stump in the dark and measure the angle of curvature, which is proportional to the quantity of auxin present.
If light destroys auxin, the total collected from the two blocks of an illuminated tip will be less than the total from a tip kept in darkness, and the shortfall will be on the lit side. If auxin moves across instead, the total will be the same as in darkness, but more will be in the shaded block and less in the lit one.
The experiment as actually done gave the second result, so redistribution is supported and destruction is not. Adding the barrier makes the design decisive in another way: with the two halves separated, no sideways movement is possible, and the asymmetry between the blocks disappears — which is difficult to explain if light were simply destroying auxin where it fell.
Questions
Question 14 marks
Describe the experiment carried out by Boysen-Jensen on coleoptiles, and describe what its result establishes about the signal that passes from the tip.
Mark scheme
- B1 the tip was cut off and replaced, with a thin block of gelatine inserted between the tip and the stump, and the coleoptile was then lit from one side
- B1 bending towards the light still happened when gelatine was used
- B1 when the gelatine was replaced by mica, which nothing can diffuse through, bending stopped
- A1 so whatever passes from the tip to the growing region is a chemical that diffuses, rather than an electrical signal or a mechanical pull; the experiment does not identify the chemical
Question 24 marks
Explain how auxin arriving at a cell in the growing region of a shoot causes that cell to become longer.
Mark scheme
- B1 auxin binds to receptors in the cell and stimulates the proton pumps in the cell surface membrane
- B1 the pumps actively transport hydrogen ions out into the cell wall, so the pH of the wall falls to about 5
- B1 at that pH enzymes called expansins become active and loosen the bonds between the cellulose microfibrils, so the wall becomes more plastic
- A1 the turgor pressure already present can then stretch the loosened wall, water enters by osmosis, and the cell lengthens along its axis
Question 34 marks
A germinating seedling is laid on its side in complete darkness. Explain why its shoot grows upwards and its root grows downwards, even though auxin collects on the lower side of both.
Mark scheme
- B1 gravity is detected by statoliths, dense starch-filled organelles that sediment to the lower side of particular cells, such as the columella cells of the root cap
- B1 auxin is then redistributed to the lower side of both the shoot and the root
- B1 in the shoot that concentration stimulates elongation, so the lower cells elongate more than the upper ones and the shoot curves upwards, which is negative gravitropism
- A1 root cells are inhibited at concentrations that stimulate a shoot, so the lower cells of the root elongate less than the upper ones and the root curves downwards, which is positive gravitropism
Question 43 marks
Explain how a synthetic auxin sprayed over a field can kill broad-leaved weeds without seriously harming the cereal crop growing among them.
Mark scheme
- B1 broad-leaved weeds present a large surface area of leaf to the spray, so they absorb much more of the synthetic auxin than the narrow upright leaves of a cereal do
- B1 the high concentration inside the weed causes rapid and uncontrolled elongation and growth
- A1 the weed cannot supply the water and nutrients that growth demands and it dies, while the cereal absorbs too little for the concentration in its tissues to have that effect
Question 53 marks
Maltsters soak barley grains in water and add gibberellin to the steeping tank. Suggest why this makes the starch in the grains turn into sugars sooner and more evenly across the batch.
Mark scheme
- B1 gibberellin diffuses to the aleurone layer surrounding the starchy endosperm and causes transcription of the gene for amylase
- B1 the amylase produced hydrolyses the stored starch to maltose, so sugars become available sooner than if the maltster waited for each embryo to secrete its own gibberellin
- A1 every grain receives gibberellin at the same time and in the same amount, so grains that would otherwise germinate at different rates produce amylase together and the batch is more uniform
Question 62 marks
Give the name of the tropism shown by a shoot growing towards light from one side, and give the name of the plant growth substance responsible for the bending.
Mark scheme
- B1 it is positive phototropism, because growth is towards the stimulus
- B1 the substance is auxin, in practice indole-3-acetic acid
Worth remembering
- A tropism is unequal growth whose direction is set by the direction of the stimulus.
- Tip detects, chemical diffuses, chemical alone bends in the dark — one experiment each, and no more.
- Auxin is redistributed to the shaded side; the total does not fall.
- Elongation is proton pumps, an acid wall, expansins, and water entering by osmosis.
- The same concentration stimulates a shoot and inhibits a root, which is why they bend opposite ways.