How to Use This Section
This chapter is not a chapter you can reason your way through in the exam hall. Almost every question on it is "give the exact definition", "which growth regulator would you use", "who discovered it and in what plant", or "which way round does this effect run". The marks sit in exact wording, exact names and exact directions, and they are lost by one swapped word - promotes written where the source says delays, primary written where the source says secondary, canary grass written where the answer is oat.
Two habits will carry you through it.
First, learn every regulator as a grid, not as a list. One event is handled by more than one regulator, and usually they pull in opposite directions. Auxin causes apical dominance; cytokinin overcomes it. Ethylene and abscisic acid promote senescence; cytokinin and gibberellin delay it. Abscisic acid induces dormancy; gibberellin and ethylene break it. If you can say both halves of each pair, the whole "which regulator would you use" family of questions answers itself.
Second, keep the two three-item lists apart. Meristematic, elongation and maturation are zones along a root tip. Lag, log and stationary are phases of a growth curve in time. They are different lists about different things, and a paper that offers one where the other belongs is testing exactly that confusion.
The items below run in three tiers.
- Tier 1 - short recall. The one-line definitions, the numbers, the names. Answer these aloud until none of them needs thinking about.
- Tier 2 - applied reasoning. A situation is described and you have to say what happens and why. This is where NEET lives.
- Tier 3 - longer written answers. Full accounts of a regulator or a phenomenon, written out the way a written paper wants them, with the marking-scheme words in place.
The chapter has ten exercises at the end, which come to twenty-one questions once the four parts of exercise 3, the six parts of exercise 8 and the four parts of exercise 10 are counted separately. Twenty of those twenty-one are already answered in full inside the thirteen teaching sections of this chapter. Only exercise 1, the one that asks for eight definitions in a row, is not answered anywhere else, and it is worked out below. The last block of this section tells you exactly where every one of the twenty-one sits, so you can attempt the whole exercise set yourself and then check each answer against a complete one.
Tier 1 - Short Recall
Question 1
Q. A change in size has taken place in a plant part. What three tests must it pass before you may call it growth?
Answer. Growth is an irreversible permanent increase in size of an organ or its parts or even of an individual cell. Test the change against the three words in that definition.
- Irreversible - the increase must not be able to go back down.
- Permanent - it must stay after the cause is removed.
- Increase in size - of an organ, of its parts, or even of a single cell.
There is a fourth test that the next sentence of the definition supplies. Growth is generally accompanied by metabolic processes - both anabolic and catabolic - that occur at the expense of energy. A change in size with no metabolism behind it is not growth.
Question 2
Q. A block of dry wood is put in water and swells. A wilted leaf is watered and becomes turgid again. Neither is growth. Give the reason that covers both cases.
Answer. Both changes fail on the same two counts.
- Both are reversible. Dry the wood and it shrinks back; withhold water and the leaf wilts again. Growth must be irreversible and permanent.
- Neither has any metabolic activity behind it. The wood takes up water by imbibition and the leaf by simple rehydration; no anabolic or catabolic process is spending energy to bring the change about.
Imbibition is not growth, and rehydration is not growth. Run every example an examiner offers you through the same three words - irreversible, permanent, metabolic.
Question 3
Q. At the cellular level, what actually increases when a plant grows? Since that quantity is not what we measure, name the six parameters that are measured instead.
Answer. At a cellular level, growth is principally a consequence of increase in the amount of protoplasm. That is the true answer to what increases.
The trouble is that increase in protoplasm is difficult to measure directly, so one generally measures some quantity which is more or less proportional to it. The six parameters used are increase in fresh weight, dry weight, length, area, volume and cell number.
Remember that these six are only proxies. A question that asks what growth is principally a consequence of wants protoplasm, not fresh weight.
Question 4
Q. A maize root apical meristem and a growing watermelon are both described as growing, but the growth is expressed differently in each. Give both figures and say what parameter each one illustrates.
Answer. The parameter has to suit the organ, and these two cases are the standard proof of it.
| System | Parameter | The figure |
|---|---|---|
| Maize root apical meristem | increase in cell number | more than 17,500 new cells per hour |
| Watermelon | increase in cell size | cells may increase in size by up to 3,50,000 times |
The meristem grows by making more cells, and its cells stay small. The watermelon grows by enlarging cells it already has, and its cell number hardly changes. The examiner swaps the two organs to see whether you noticed, so fix them in place: maize root - cell number; watermelon - cell size.
Question 5
Q. Name the three phases of growth in the order they lie behind a root apex, and give one giveaway feature of each.
Answer. The period of growth is generally divided into three phases: meristematic, elongation and maturation. Running back from the tip, that is the order.
| Phase | Where | Giveaway feature |
|---|---|---|
| Meristematic | at the root and shoot apex | cells rich in protoplasm, with large conspicuous nuclei and thin primary cellulosic walls |
| Elongation | proximal to the meristematic zone | increased vacuolation, cell enlargement, new cell wall deposition |
| Maturation | further back still | cells attain their maximal size in wall thickening and protoplasmic modifications |
The commonest wrong answer puts vacuolation in the meristematic zone. It is exactly backwards - those cells are packed with protoplasm, not vacuole.
Question 6
Q. In one tissue, only one daughter cell of every mitotic division keeps dividing. In another, both do. Name each type of growth and give the shape of the curve each produces.
Answer. The behaviour of the two daughter cells is the whole difference.
In arithmetic growth, following mitotic cell division, only one daughter cell continues to divide while the other differentiates and matures. The pool of dividing cells stays the same size, so the organ gains cells at a steady rate and plotting length against time gives a linear curve. The standard example is a root elongating at a constant rate.
In geometric growth, both the progeny cells following mitotic cell division retain the ability to divide and continue to do so. The pool of dividing cells doubles each round, so the climb is steep, and plotting the parameter of growth against time gives a typical sigmoid or S-curve.
One daughter divides gives you a straight line. Both daughters divide give you an S.
Question 7
Q. Write the mathematical expression for arithmetic growth and for exponential growth, and define every symbol in each.
Answer. Arithmetic growth is expressed as
- = length at time t
- = length at time zero
- = growth rate, that is elongation per unit time
Read it as a sentence: you start with a length and add the same amount for every unit of time that passes.
Exponential growth is expressed as
- = final size - weight, height, number and so on
- = initial size at the beginning of the period
- = growth rate
- = time of growth
- = base of natural logarithms
Hence the final size depends on the initial size . Two plants with the same growing for the same do not end up the same size unless they started the same size.
Question 8
Q. In the expression , what exactly is , and by what other name is it known?
Answer. Here is the relative growth rate, and it is also the measure of the ability of the plant to produce new plant material, referred to as efficiency index.
Learn that line in those words, because it is asked directly. The distractors offer as the absolute growth rate, as the time, or as itself, and all three are wrong.
Question 9
Q. Define the absolute growth rate and the relative growth rate, and say in one sentence what separates them.
Answer. Quantitative comparisons between the growth of living systems can be made in two ways.
- Measurement and the comparison of total growth per unit time is called the absolute growth rate.
- The growth of the given system per unit time expressed on a common basis, for example per unit initial parameter, is called the relative growth rate.
What separates them is simply whether you divide by what was there to begin with. Absolute growth rate asks how much was added. Relative growth rate asks how much was added for every unit the system already had.
Question 10
Q. What must a plant be supplied with before it can grow at all, and what does each supply do? Add the two further conditions the chapter names.
Answer. Water, oxygen and nutrients are very essential elements for growth.
| Condition | What it does for growth |
|---|---|
| Water | cell enlargement requires it; turgidity of cells helps in extension growth; it also provides the medium for the enzymatic activities needed for growth |
| Oxygen | helps in releasing metabolic energy essential for growth activities |
| Nutrients - macro and micro essential elements | required for the synthesis of protoplasm and act as a source of energy |
Two further conditions complete the answer. Every plant organism has an optimum temperature range best suited for its growth, and any deviation from this range could be detrimental to its survival - note that the word is survival, not merely growth rate. And environmental signals such as light and gravity also affect certain phases or stages of growth. Light and gravity are signals, not nutrients - they tell the plant what to do, they do not feed it.
Question 11
Q. In a woody dicotyledonous plant, sort these into products of dedifferentiation and products of redifferentiation: secondary xylem, interfascicular cambium, cork, cork cambium, secondary phloem, phelloderm.
Answer. The rule is simple - a new meristem made out of an ordinary cell is dedifferentiation; a mature tissue made by that meristem is redifferentiation.
Products of dedifferentiation - new meristems formed from fully differentiated parenchyma cells: interfascicular cambium and cork cambium.
Products of redifferentiation - cells that once again lose the capacity to divide but mature to perform specific functions: secondary xylem, secondary phloem and secondary cortex, that is phelloderm, from the vascular cambium, and cork, that is phellem, from the cork cambium.
The examiner's favourite swap is to offer cork cambium as a product of redifferentiation, or secondary xylem as an example of dedifferentiation. Both are the wrong way round.
Question 12
Q. Define development, name the two end points the definition carries, and give the one-line equation for it.
Answer. Development is a term that includes all changes an organism goes through during its life cycle, from germination of the seed to senescence. Both end points belong to the definition - it starts at germination of the seed and runs to senescence, not from flowering to maturity.
Broadly, development is considered as the sum of growth and differentiation. Keep that in exactly those words: development = growth + differentiation.
The full sequence for a cell of a higher plant is meristematic cell -> cell division -> plasmatic growth -> expansion (elongation) -> differentiation -> maturation -> mature cell -> senescence -> death, and it is also applicable to tissues and organs, not only to single cells.
Question 13
Q. Define plasticity, name the phenomenon that illustrates it, and give both of its causes with the plants that show each.
Answer. Plants follow different pathways in response to environment or phases of life to form different kinds of structures. This ability is called plasticity.
The illustration is heterophylly - different leaves on one plant - and it has two different causes.
| Cause | Example plants | What is seen |
|---|---|---|
| Phase of life | cotton, coriander and larkspur | the leaves of the juvenile plant are different in shape from those in mature plants |
| Environment | buttercup | the leaves produced in air differ in shape from those produced in water |
This phenomenon of heterophylly is an example of plasticity. The examiner does not ask what heterophylly is; the examiner asks which cause goes with which plant, and the standard wrong option swaps them.
Question 14
Q. Match each plant growth regulator to its chemical class, and name the only one that is a gas.
Answer. The plant growth regulators are small, simple molecules of diverse chemical composition, and there are five chemical classes.
| Chemical class | Example | Common name or symbol |
|---|---|---|
| Indole compounds | indole-3-acetic acid | IAA |
| Adenine derivatives | -furfurylamino purine | kinetin |
| Derivatives of carotenoids | abscisic acid | ABA |
| Terpenes | gibberellic acid | |
| Gases | ethylene |
Ethylene is the only gaseous PGR. The favourite trick is to swap abscisic acid and gibberellic acid between carotenoid derivative and terpene, or to call kinetin an indole compound because IAA is one.
Note also that PGRs are variously described as plant growth substances, plant hormones or phytohormones - all four names mean the same set of molecules.
Tier 2 - Applied Reasoning
Question 15
Q. A culture of cells is growing so that both products of every division keep dividing. Sketch in words what the growth curve will look like over a long period, name each of its phases, and say why it does not simply keep climbing.
Answer. This is geometric growth, so the curve is a typical sigmoid or S-curve with three phases, in this order.
| Phase | What the curve does | What the cells are doing |
|---|---|---|
| Lag phase | rises very slowly | the initial growth is slow because there are few dividing cells to start with |
| Log or exponential phase | rises rapidly | both progeny cells retain the ability to divide and continue to do so, so the pool of dividers doubles each round |
| Stationary phase | flattens off | with limited nutrient supply, the growth slows down |
The reason it stops climbing is outside the cells, not inside them. The nutrient supply is limited. Nothing grows without end in a closed world.
A sigmoid curve is a characteristic of living organisms growing in a natural environment, and it is typical for all cells, tissues and organs of a plant - so this is not a special case, it is the normal shape.
Question 16
Q. In one week a seedling goes from 2 grams to 6 grams of dry weight, while a young tree in the same plot goes from 200 grams to 210 grams. Which has the higher absolute growth rate, which the higher relative growth rate, and by how much?
Answer. Do the two calculations separately, because the two rates give opposite answers here and that is the point of the question.
| Seedling | Young tree | |
|---|---|---|
| Initial dry weight | 2 grams | 200 grams |
| Final dry weight | 6 grams | 210 grams |
| Absolute growth rate - total growth per unit time | 4 grams per week | 10 grams per week |
| Relative growth rate - growth per unit initial parameter | 4 divided by 2 = 2, that is 200 per cent | 10 divided by 200 = 0.05, that is 5 per cent |
The young tree has the higher absolute growth rate - it added two and a half times as much material. The seedling has the far higher relative growth rate - 200 per cent against 5 per cent, forty times as much per unit of what it started with.
The seedling has tripled itself. The tree has added a twentieth. This is exactly why the relative growth rate is the fairer comparison between systems of different sizes, and why the in , the relative growth rate, is called the efficiency index - it measures the ability of the plant to produce new plant material, not the raw amount produced.
Question 17
Q. Set arithmetic growth against geometric growth in a table, covering the daughter cells, the pool of dividers, the curve, the phases, the standard example and the expression.
Answer. This one table answers almost every question in this area.
| Arithmetic growth | Geometric growth | |
|---|---|---|
| Daughter cells after mitosis | only one continues to divide; the other differentiates and matures | both progeny cells retain the ability to divide and continue to do so |
| Pool of dividing cells | stays constant | doubles each round |
| Shape of the curve | linear | sigmoid or S-curve |
| Phases | none - one steady slope | lag, log (exponential), stationary |
| Standard example | a root elongating at a constant rate | cells in culture, and organs of a plant in a natural environment |
| Expression |
Do not let the phases of the sigmoid curve get mixed with the phases of growth in a root tip. Lag, log and stationary belong to the curve. Meristematic, elongation and maturation belong to the root tip. They are different lists about different things.
Question 18
Q. A callus in a culture flask and a tumour on a plant both consist of cells that have started dividing again. Name the process both show, and say precisely what separates them.
Answer. Both show dedifferentiation - living differentiated cells that had lost the capacity to divide have regained the capacity of division.
What separates them is control.
- A tumour is uncontrolled, unregulated dedifferentiated growth. The cells divide without maturing into anything useful - there is division with no differentiation behind it, and no organ is built.
- The callus is dedifferentiated tissue produced under controlled laboratory conditions. In plant tissue culture, parenchyma cells are made to divide deliberately, and the callus can later be pushed to redifferentiate into shoots and roots.
Same starting event, opposite outcomes, and the word that does the work is "controlled".
Question 19
Q. Cells produced by one root apical meristem end up as root-cap cells in one place and as epidermis in another. What does this show about differentiation, and what actually decides a cell's fate?
Answer. It shows that differentiation in plants is open, in the same sense that growth in plants is open.
Differentiation is open because cells or tissues arising out of the same meristem have different structures at maturity. One meristem, many outcomes; nothing about the cell decides its fate in advance.
What decides the fate is position. The final structure at maturity of a cell or tissue is also determined by the location of the cell within. Cells positioned away from the root apical meristem differentiate as root-cap cells, while those pushed to the periphery mature as epidermis. Same meristem, same starting cells, and the only difference between them is where they ended up.
The same rule runs through the anatomy you already know. Derivatives of the vascular cambium laid down on the inner face become secondary xylem, while those on the outer face become secondary phloem.
Question 20
Q. Plant growth regulators can be sprayed onto a crop from outside, yet this chapter files them as intrinsic factors. Explain, and give the full classification of factors controlling development.
Answer. They are intrinsic because of where the plant's own regulators are made and how they act, not because of how a farmer may apply them. The regulators the plant makes are produced inside the plant and act between its cells, so they are intercellular intrinsic factors.
Development in plants - that is, both growth and differentiation - is under the control of intrinsic and extrinsic factors.
| Factor | Type | What it covers |
|---|---|---|
| Intrinsic | intracellular | genetic factors |
| Intrinsic | intercellular | chemicals, namely the plant growth regulators |
| Extrinsic | from outside the plant | light, temperature, water, oxygen and nutrition |
The commonest wrong answer files plant growth regulators under extrinsic factors exactly because they can be sprayed on. Light and temperature are the extrinsic ones, and the chapter adds a further point about them worth carrying: many extrinsic factors such as temperature and light control plant growth and development via PGRs - the outside world acts through these chemicals, not directly.
Question 21
Q. A student writes that kinetin is the naturally occurring cytokinin of plants, isolated from corn kernels. Two things are wrong. Correct both.
Answer. The two names have been swapped, and the more serious error is the word "naturally".
First error. Kinetin does not occur naturally in plants. Cytokinins were discovered as kinetin, a modified form of adenine, a purine, obtained from autoclaved herring sperm DNA - a laboratory preparation, not a plant extract. It is a discovery product, and every year a paper offers it as the natural cytokinin of plants.
Second error. Zeatin, not kinetin, is the one from corn kernels. The search for natural substances with cytokinin-like activity led to the isolation of zeatin from corn-kernels and coconut milk. Zeatin is the natural one.
The corrected sentence reads: zeatin is a naturally occurring cytokinin, isolated from corn-kernels and coconut milk; kinetin is a modified adenine from autoclaved herring sperm DNA and does not occur naturally in plants. Note also that kinetin is a modified purine, never a pyrimidine.
Question 22
Q. A tea bush is pruned at the tips and becomes dense and bushy instead of tall. Explain what happened. Then say how a gardener could get the same result on an ornamental plant without cutting off the apex at all.
Answer. Start from apical dominance. In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds. The apical bud is the source of the auxin; auxins are produced by the growing apices of the stems and roots, from where they migrate to the regions of their action, and that descending auxin keeps the lateral buds suppressed.
Pruning is decapitation - removal of shoot tips - and it removes the source. With the auxin supply cut off, the axillary buds are released and grow out into branches, so the plant becomes short and bushy. In a tea plantation the crop is the young leaves and shoots, so more branches means more shoots to pluck, and the bush stays at a convenient height for the pickers. In hedge-making, trimming the tips releases the lateral buds all over the hedge, filling in the gaps.
The chemical route needs no knife. Cytokinins help overcome apical dominance and promote lateral shoot growth. Apply cytokinin to the axillary buds and they grow out even though the apex is still there. Auxin causes apical dominance; cytokinin overcomes it - that pair is the whole answer.
Question 23
Q. A gardener sprays 2,4-D over a lawn. The broad-leaved weeds die and the grass is untouched. Explain, and name the class of regulator 2,4-D belongs to.
Answer. 2,4-D, that is 2,4-dichlorophenoxyacetic acid, is a synthetic auxin, and auxins act as herbicides.
The selectivity is the point of the question. 2,4-D is widely used to kill dicotyledonous weeds. It does not affect mature monocotyledonous plants. A lawn is grass, and grass is a monocot, so the spray wipes out the broad-leaved dicot weeds and leaves the grass standing. It is used by gardeners to prepare weed-free lawns.
Keep the natural and synthetic auxins sorted while you are here: IAA and IBA are natural, isolated from plants; NAA and 2,4-D are made in the laboratory. IBA is the trap - the name looks synthetic, but indole butyric acid has been isolated from plants.
Question 24
Q. In the middle of the night, a short day plant and a long day plant are each given a single brief flash of light. One flowers and one does not. Say which is which and explain why the names of the two groups are misleading.
Answer. The flash stops the short day plant from flowering and makes the long day plant flower. One flash, two opposite results.
The reason is that the names lie. What actually matters is the length of the uninterrupted dark period, not the length of the light period.
- A short day plant is really a long night plant. It flowers because the continuous dark period is longer than a critical length. The flash breaks that dark period into two shorter stretches, so the plant no longer gets the continuous darkness it needs and does not flower. The day length was never touched.
- A long day plant flowers when the dark period is shorter than a critical length, so breaking a long night is exactly what it wants, and the flash makes it flower.
The commonest wrong answer says a night break promotes flowering in a short day plant. It does the opposite. In a written answer, give the reason as the uninterrupted dark period, never as the day length.
Question 25
Q. A farmer sows the winter variety of wheat in spring instead of autumn. Predict the result, name the process the crop has been denied, and say what that process does for a plant.
Answer. The crop will not flower or produce grain within that season. Winter varieties, if planted in spring, will not flower or produce grain within a season.
The process denied is vernalisation - the promotion of flowering by a period of low temperature, more formally the qualitative or quantitative dependence of flowering on exposure to low temperature. The winter variety needs a spell of cold before it will flower, and a spring sowing never gives it one.
Sown correctly, the winter variety is planted in autumn. It germinates, and over winter comes out as small seedlings. It resumes growth in spring and is harvested around mid-summer. The spring variety, by contrast, is planted in spring and comes to flower and produces grain before the end of the growing season.
What vernalisation does for the plant is worth the extra line: it prevents precocious reproductive development late in the growing season, and it enables the plant to have enough time to reach maturity. A plant that flowered the moment the weather looked mild would be caught out; the cold requirement makes it wait.
Question 26
Q. Ethylene is the regulator a grower wants in the orchard, yet ethylene itself is almost never sprayed. What is used instead, why does it work, and name three jobs it does.
Answer. Ethylene is a simple gaseous PGR, and a gas is very hard to apply to a field. The most widely used compound as a source of ethylene is ethephon.
Why it works: ethephon in an aqueous solution is readily absorbed and transported within the plant, and it releases ethylene slowly. So it is sprayed as a liquid, moves inside the plant, and then acts as a gas from within - a slow-release source that puts the ethylene where it is needed.
The three jobs named in this chapter:
| Use of ethephon | Crop |
|---|---|
| Hastens fruit ripening | tomatoes and apples |
| Accelerates abscission in flowers and fruits - thinning | cotton, cherry, walnut |
| Promotes female flowers, thereby increasing the yield | cucumbers |
Thinning is worth a sentence of its own. A tree that sets too many fruits gives many small poor ones. Ethephon makes some of the flowers and young fruits drop, and the ones that remain grow bigger - that deliberate shedding is what thinning means.
Tier 3 - Longer Written Answers
Question 27
Q. Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate. This is one of the chapter-end exercises.
Answer. Eight definitions, and each one is a mark on its own, so write them as a list and keep the exact wording.
- Growth - an irreversible permanent increase in size of an organ or its parts or even of an individual cell. It is generally accompanied by metabolic processes, both anabolic and catabolic, that occur at the expense of energy.
- Differentiation - the cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions; this act leading to maturation is termed differentiation. During it, cells undergo few to major structural changes both in their cell walls and protoplasm.
- Development - a term that includes all changes an organism goes through during its life cycle, from germination of the seed to senescence. Broadly, development is considered as the sum of growth and differentiation.
- Dedifferentiation - living differentiated cells that have by now lost the capacity to divide regain the capacity of division under certain conditions. The examples are the formation of the meristems interfascicular cambium and cork cambium from fully differentiated parenchyma cells.
- Redifferentiation - such meristems or tissues are able to divide and produce cells that once again lose the capacity to divide but mature to perform specific functions. The products in a woody dicot are secondary xylem, secondary phloem, phelloderm and cork.
- Determinate growth - growth that stops at a predetermined size. Leaves, flowers and fruits have limited dimensions; they grow to a set size, stop, and in time fall. It is the opposite of the open or indeterminate growth of the stem and root.
- Meristem - a group of cells at certain locations in the plant body whose cells have the capacity to divide and self-perpetuate. Self-perpetuate matters: one product stays a meristem cell, so the meristem is never used up, while the other product soon loses the capacity to divide and makes up the plant body. It is the presence of meristems that gives plants the capacity for unlimited growth throughout their life.
- Growth rate - the increased growth per unit time. It can be expressed mathematically, and the increase it shows may be arithmetic - - or geometrical - .
Two pairs inside that list are the ones papers test against each other. Growth against development - growth is only the size increase, development is the whole life history and is the sum of growth and differentiation. Dedifferentiation against redifferentiation - dedifferentiation makes a meristem out of a mature cell, redifferentiation makes a mature cell out of that meristem.
Question 28
Q. Set out, as one table, every event in this chapter in which more than one growth regulator is involved, showing which regulator drives the event and which opposes it.
Answer. For any and every phase of growth, differentiation and development of plants, one or the other PGR has some role to play, and such roles could be complimentary or antagonistic; these could be individualistic or synergistic. The events where more than one PGR interacts are dormancy in seeds and buds, abscission, senescence and apical dominance.
| Event | Caused or promoted by | Opposed, delayed or reversed by |
|---|---|---|
| Apical dominance | auxin causes it | cytokinin overcomes it |
| Senescence | ethylene and abscisic acid promote it | cytokinin and gibberellin delay it |
| Dormancy of seeds and buds | abscisic acid induces it | gibberellin and ethylene break it |
| Abscission | ethylene and abscisic acid promote it; auxin promotes the abscission of older mature leaves and fruits | auxin prevents the early drop of young leaves and fruits |
| Stomatal closure | abscisic acid stimulates it | no other PGR in this chapter opposes it |
| Seed germination | gibberellin and ethylene promote it | abscisic acid inhibits it |
Read the abscission row twice, because auxin sits on both sides of it. Auxin prevents the early drop of young leaves and fruits, but promotes the abscission of older, mature leaves and fruits. The age of the organ decides the direction, which is why an option that says only "auxin prevents abscission" is incomplete and one that says only "auxin promotes abscission" is incomplete too.
And keep the whole set in its place. The role of PGRs is only one kind of intrinsic control; along with genomic control and extrinsic factors, they play an important role in plant growth and development. In most situations abscisic acid acts as an antagonist to the gibberellins - that single line covers half the table.
Question 29
Q. The discovery of each of the five groups of growth regulators was accidental. Give the discovery story of each, with the worker, the organism and the year where the chapter names one.
Answer. Interestingly, the discovery of each of the five major groups of PGRs has been accidental. Nobody set out to find a plant hormone; each one turned up while somebody was looking at something else.
| Regulator | Worker | Organism or material | What was found |
|---|---|---|---|
| Auxin | Charles Darwin and Francis Darwin, then F. W. Went | canary grass coleoptiles, then oat seedlings | the bending response and the source of the influence |
| Gibberellin | E. Kurosawa (1926) | rice and the fungus Gibberella fujikuroi | sterile filtrates reproduced the disease |
| Cytokinin | F. Skoog and co-workers, then Miller et al. (1955) | tobacco stem internodal callus | a factor that let the callus proliferate |
| Abscisic acid | three independent researchers, mid-1960s | three inhibitor preparations | all three were one molecule |
| Ethylene | H. H. Cousins (1910) | oranges and bananas | a volatile substance that hastened ripening |
Auxin. The Darwins observed that the coleoptiles of canary grass responded to unilateral illumination by growing towards the light source - phototropism. After a series of experiments it was concluded that the tip of the coleoptile was the site of a transmittable influence that caused the bending of the entire coleoptile. Note the logic: the tip senses the light but the bending happens lower down, so something must travel. Auxin was isolated by F. W. Went from tips of coleoptiles of oat seedlings.
Gibberellin. The bakanae, or foolish seedling, disease of rice seedlings was caused by the fungal pathogen Gibberella fujikuroi. E. Kurosawa (1926) reported the appearance of the symptoms of the disease in rice seedlings when they were treated with sterile filtrates of the fungus. Sterile filtrate is the key word - no living fungus was needed, so the effect came from a chemical the fungus made. The active substances were later identified as gibberellic acid.
Cytokinin. F. Skoog and his co-workers observed that from the internodal segments of tobacco stems, the callus - a mass of undifferentiated cells - proliferated only if, in addition to auxins, the nutrient medium was supplemented with one of the following: extracts of vascular tissues, yeast extract, coconut milk or DNA. Miller et al. (1955) later identified and crystallised the cytokinesis promoting active substance, which they termed kinetin.
Abscisic acid. During the mid-1960s, three independent researchers reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. Later all the three were proved to be chemically identical, and it was named abscisic acid.
Ethylene. H. H. Cousins (1910) confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later this volatile substance was identified as ethylene, a gaseous PGR.
The commonest swap in the whole table is canary grass against oat. The Darwins watched canary grass; Went isolated auxin from oat.
Question 30
Q. Write a full account of the auxins - what the term covers, where they are made, the natural and synthetic members, and their physiological and agricultural effects.
Answer. Auxins come from the Greek "auxein", meaning to grow, and were first isolated from human urine.
The term auxin is applied to indole-3-acetic acid (IAA), and to other natural and synthetic compounds having certain growth regulating properties. So auxin is a job description, not a single molecule.
They are generally produced by the growing apices of the stems and roots, from where they migrate to the regions of their action. Made at the tip, used lower down - that sentence explains apical dominance and explains why the Darwins' coleoptile bent below the point where the light was sensed.
| Natural auxins - isolated from plants | Synthetic auxins |
|---|---|
| IAA - indole-3-acetic acid | NAA - naphthalene acetic acid |
| IBA - indole butyric acid | 2,4-D - 2,4-dichlorophenoxyacetic acid |
All these auxins have been used extensively in agricultural and horticultural practices. The effects, each as a verb and its example:
- Initiate rooting in stem cuttings - widely used for plant propagation.
- Promote flowering, for example in pineapples.
- Prevent fruit and leaf drop at early stages, but promote the abscission of older, mature leaves and fruits.
- Cause apical dominance - the growing apical bud inhibits the growth of the lateral (axillary) buds, and removal of shoot tips, that is decapitation, usually results in the growth of lateral buds. This is widely applied in tea plantations and hedge-making.
- Induce parthenocarpy, for example in tomatoes - the fruit develops without fertilisation, so it is seedless.
- Act as herbicides - 2,4-D is widely used to kill dicotyledonous weeds and does not affect mature monocotyledonous plants, so it is used to prepare weed-free lawns.
- Control xylem differentiation and help in cell division.
Two items from that list appear almost every year. 2,4-D killing dicot weeds but not mature monocots, and the two-directional abscission effect - read the option to the end and check whether it says early or older mature. Keep parthenocarpy in tomato and flowering in pineapple apart from each other; swapping the two examples is the usual distractor.
Question 31
Q. Write a full account of the gibberellins - their number and sources, their chemical character, and every use of them named in this chapter with the crop it belongs to.
Answer. Gibberellins are another kind of promotory PGR, sitting with the auxins and cytokinins in the growth promoting group.
There are more than 100 gibberellins reported from widely different organisms such as fungi and higher plants. They are denoted as , , and so on, numbered in the order they were characterised. Gibberellic acid was one of the first gibberellins to be discovered and remains the most intensively studied form. All GAs are acidic, and they produce a wide range of physiological responses in plants.
One central ability drives most of the list - gibberellins cause an increase in the length of the axis.
| Effect | Crop or setting |
|---|---|
| Increase the length of the axis | grape stalks - the bunch is looser and the berries larger |
| Elongate the fruit and improve its shape | apple |
| Delay senescence, so fruits can be left on the tree longer, extending the market period | orchard fruit generally |
| Speed up the malting process | the brewing industry, using |
| Increase stem length and so the yield by as much as 20 tonnes per acre | sugarcane |
| Hasten the maturity period, leading to early seed production | juvenile conifers |
| Promote bolting - internode elongation just prior to flowering | beet, cabbages and other rosette plants |
The sugarcane figure comes with its reasoning, so give both: sugarcane stores carbohydrate as sugar in its stems, therefore a longer stem is simply a bigger store, and spraying with gibberellins raises the yield by as much as 20 tonnes per acre.
Bolting needs its two definitions stated. A rosette habit means the plant grows its leaves in a tight circular cluster close to the ground, because its internodes stay extremely short. Bolting is internode elongation just prior to flowering - the short internodes stretch and the flat rosette shoots up into a tall flowering stalk. Gibberellin is the regulator to apply if you want a rosette plant to bolt, and it works because the one thing gibberellin does best is lengthen the axis.
Note the direction of the senescence effect - gibberellin delays it. Do not let an option persuade you that a promotory PGR hurries ageing along. The most-swapped pair in the table is grape against sugarcane, and 20 tonnes per acre is asked as a number.
Question 32
Q. Write a full account of the cytokinins - discovery, where they are made in a plant, their effects, and how they are balanced against auxin in tissue culture.
Answer. The name tells you the job: cytokinins have specific effects on cytokinesis, on the division of the cell itself.
Discovery. Cytokinins were discovered as kinetin, a modified form of adenine, a purine, obtained from autoclaved herring sperm DNA. Kinetin does not occur naturally in plants. The search for natural substances with cytokinin-like activity led to the isolation of zeatin from corn-kernels and coconut milk, and since the discovery of zeatin, several naturally occurring cytokinins, and some synthetic compounds with cell-division-promoting activity, have been identified.
Where they are made. Natural cytokinins are synthesised in regions where rapid cell division occurs - root apices, developing shoot buds and young fruits. That list is a definition in disguise: give an unfamiliar tissue and say it is dividing fast, and cytokinin is being made there.
What they do.
- Help to produce new leaves.
- Help to produce chloroplasts in leaves - the leaf is not only made, it is made green.
- Promote lateral shoot growth.
- Promote adventitious shoot formation.
- Help overcome apical dominance.
- Promote nutrient mobilisation, which helps in the delay of leaf senescence.
Give the mechanism for the last one, not just the fact. Cytokinin pulls sugars, amino acids and minerals towards the treated leaf, and a leaf that keeps being fed does not age - senescence is delayed because nutrients are mobilised towards the tissue.
Against auxin, in the plant and in the flask. Auxin causes apical dominance; cytokinin overcomes it. Apply cytokinin to an axillary bud and the bud grows out even though the apex is still there. The same opposition sets the outcome of a tissue culture:
| What is in the medium | What the tissue does |
|---|---|
| Auxin alone, no cytokinin | root formation is favoured; shoots do not form |
| Cytokinin alone, or a high cytokinin-to-auxin ratio | shoot formation is favoured |
| Both, in balance | the callus divides normally and a complete plantlet, shoot plus root, is regenerated |
So it is the ratio of cytokinin to auxin, not the amount of either one, that decides whether a callus becomes a shoot or a root. High cytokinin-to-auxin means shoots; high auxin-to-cytokinin means roots, and the distractor always reverses it. On senescence, cytokinin and gibberellin delay it, while ethylene and abscisic acid promote it.
Question 33
Q. Write a full account of ethylene - what kind of molecule it is, where it is made, and its effects from the seedling stage to the harvested fruit.
Answer. Ethylene is a simple gaseous PGR, chemically ethene, - about as simple a molecule as a hormone can be. That one word, gaseous, separates it from every other regulator in this chapter and explains almost everything it does in practice: a gas does not have to be carried in a stream of sap, it diffuses - out of one fruit and into the next.
It is synthesised in large amounts by tissues undergoing senescence and by ripening fruits. Both sources are ageing tissues, which fits the work it does.
On the seedling: horizontal growth of seedlings, swelling of the axis, and apical hook formation in dicot seedlings - so the delicate apex is dragged up through the soil rather than pushed through it. In this sense ethylene is an inhibitor: it shortens and thickens.
On the mature plant: ethylene promotes senescence and abscission of plant organs, especially of leaves and flowers.
On the fruit: ethylene is highly effective in fruit ripening. Ripening is an active, energy-hungry process, and ethylene enhances the respiration rate during ripening of the fruits; this rise in the rate of respiration is called respiratory climactic. Learn that term exactly - it is asked both as a name and as a definition. Because ethylene is a gas, ripening spreads: a ripening fruit pours out ethylene, that ethylene reaches the fruits packed beside it, and they begin to ripen too. That is why one rotten fruit spoils a whole basket and why bananas ripen faster in a closed paper bag.
On resting structures: ethylene breaks seed and bud dormancy, initiates germination in peanut seeds and causes sprouting of potato tubers. Abscisic acid induces dormancy; ethylene and gibberellin break it.
Three more effects and the field compound:
- Promotes rapid internode and petiole elongation in deep water rice plants, helping the leaves and the upper parts of the shoot to remain above water. The logic is neat - the plant is drowning, the submerged tissue makes ethylene, the trapped gas cannot escape into water, and the internodes stretch until the leaves reach the air.
- Promotes root growth and root hair formation, thus helping the plant to increase its absorption surface.
- Is used to initiate flowering and for synchronising fruit-set in pineapples, and induces flowering in mango.
Since ethylene regulates so many physiological processes, it is one of the most widely used PGRs in agriculture, applied as ethephon, which in aqueous solution is readily absorbed and transported within the plant and releases ethylene slowly.
Two directions must not slip. Ethylene promotes senescence and abscission; cytokinin and gibberellin delay senescence. Ethylene breaks dormancy; abscisic acid induces it. The other standard trap attaches the apical hook to auxin - it belongs to ethylene.
Question 34
Q. Write a full account of abscisic acid, and justify the name "stress hormone" properly.
Answer. Every other regulator in this chapter pushes growth along. Abscisic acid pulls the other way.
ABA was discovered for its role in regulating abscission and dormancy, and that is where the name comes from - abscission is the shedding of a leaf, flower or fruit, and abscisic acid is the acid found to be regulating it. But like other PGRs, it also has other wide-ranging effects on plant growth and development.
ABA acts as a general plant growth inhibitor and an inhibitor of plant metabolism. Both halves of that sentence are examinable - growth is slowed and metabolism is slowed. ABA does not merely stop a plant getting bigger; it turns the plant's chemistry down. It also inhibits seed germination, where gibberellin and ethylene promote it.
Justifying "stress hormone" needs two facts, not one. ABA stimulates the closure of stomata and increases the tolerance of plants to various kinds of stresses. Therefore it is also called the stress hormone.
- Stomatal closure. A stoma is the pore through which a leaf loses water. A plant short of water cannot afford that loss. ABA stimulates the closure of stomata, the pores shut, transpiration falls, and the plant holds on to the water it has. No other PGR in this chapter closes stomata, so for immediate stomatal closure the answer is always abscisic acid.
- Tolerance of stresses. ABA increases the tolerance of plants to various kinds of stresses - drought, cold, salt, whatever the plant is up against.
An answer that only says "it is produced during stress" scores nothing. The two facts above are the marking scheme.
In the seed, ABA plays an important role in seed development, maturation and dormancy, and by inducing dormancy it helps seeds to withstand desiccation and other factors unfavourable for growth. A dormant seed has stopped, dried down and can wait for a better season, and it is ABA that puts it into that state.
Put the whole hormone in one line. When conditions turn hostile, ABA shuts the plant down and makes it wait. And keep the summary relation: in most situations, ABA acts as an antagonist to the gibberellins - gibberellin lengthens, germinates and mobilises; ABA inhibits, makes dormant and shuts down.
Question 35
Q. Write a full account of photoperiodism - the three groups of plants, what really controls the response, where the stimulus is perceived and how it reaches the place where flowering happens.
Answer. Flowering in many plants depends not on how big the plant is but on how long the day is. A plant may be large, healthy and well fed and still not flower, because the length of the day is wrong for it. The response of plants to periods of day and night length is called photoperiodism.
The number that sorts plants into groups is the critical day length - the day length above or below which a plant will flower. It is a different number for every species, so an option claiming one universal value is always wrong.
| Group | Flowers when | Examples |
|---|---|---|
| Short day plants (SDP) | the day length is shorter than the critical day length | chrysanthemum, tobacco, soyabean, rice, Xanthium |
| Long day plants (LDP) | the day length is longer than the critical day length | wheat, barley, spinach, radish, henbane (Hyoscyamus niger) |
| Day-neutral plants (DNP) | irrespective of day length | tomato, cucumber, maize, sunflower |
Now the correction that carries the marks. What actually matters is the length of the uninterrupted dark period, not the length of the light period. A short day plant is really a long night plant - it flowers because the continuous dark period is longer than a critical length. The two go together in nature, so the plant was named for the wrong half of the cycle. The night-break experiment proves it: interrupt the night of a short day plant with even a brief flash of light and it will not flower, while the same flash makes a long day plant flower.
Perception and action happen in two different places. The leaves perceive the photoperiod - the leaf is the sense organ for day length. The shoot apex, that is the apical meristem, is where flowering actually happens - the leaf never turns into a flower. Since the place that senses the day length is not the place that makes the flower, something must travel between them: a hormonal substance must migrate from the leaves to the shoot apex to induce flowering, and this transmissible flowering hormone is called florigen. The photoperiodic stimulus is perceived by a pigment called phytochrome.
Put the chain together in order: light and dark periods -> perceived by phytochrome in the leaf -> florigen produced -> florigen migrates to the shoot apex -> flowering.
Two consequences follow. A plant must have at least one leaf to perceive a photoperiodic stimulus, and a fully defoliated plant cannot respond to a photoperiodic cycle at all - there is nothing left to perceive the day length and nothing to make the stimulus, so however favourable the photoperiod is, no flowering response follows.
The favourite species trap is Xanthium against henbane - Xanthium is a short day plant; henbane, Hyoscyamus niger, is a long day plant - and rice and wheat sitting in opposite groups is asked constantly.
Question 36
Q. Write a full account of vernalisation - its definition, what it does for the plant, and the two classes of plants that show it.
Answer. Light is only half of what controls flowering. Temperature is the other half.
Vernalisation is the promotion of flowering by a period of low temperature. More formally, it is the qualitative or quantitative dependence of flowering on exposure to low temperature. It is always low temperature, never high, and it promotes flowering - a definition that says cold inhibits flowering is the favourite distractor.
Why a plant would want to be held back by cold is the part students skip and it is a marking-scheme line. Vernalisation prevents precocious reproductive development late in the growing season, and it enables the plant to have enough time to reach maturity. A plant that flowered the moment conditions looked mild would be caught out; the cold requirement makes it wait.
Two classes of plants show it.
- Food plants with spring and winter varieties. Wheat, barley and rye each have two varieties, spring and winter.
- Spring varieties are planted in spring and come to flower and produce grain before the end of the growing season.
- Winter varieties, if planted in spring, will not flower or produce grain within a season. They are instead planted in autumn; they germinate, and over winter come out as small seedlings, then resume growth in spring and are harvested around mid-summer.
- Biennials. Biennials are monocarpic plants that normally flower and die in the second season. The examples are sugarbeet, cabbages and carrots. In these plants, subjecting the growing plant to a cold treatment stimulates a subsequent photoperiodic flowering response - the cold does not make the flower by itself, it makes the plant able to answer the photoperiod.
That last sentence is where the two halves of flowering control join up: cold first, photoperiod after. Keep the biennial examples sugarbeet, cabbages and carrots separate from the cereal examples wheat, barley and rye, and remember that the winter variety is the one that needs the cold.
Question 37
Q. Growth, differentiation and development are treated as three topics but the chapter calls them one story. Tie them together, and say what controls the whole of it.
Answer. Growth, differentiation and development are very closely related events in the life of a plant. They are three views of the same thing.
Growth adds the material. It is an irreversible permanent increase in size, and in a plant it is open - new cells are always being added to the plant body by the activity of the meristem, so plants retain the capacity for unlimited growth throughout their life. Not every part is open, though: leaves, flowers and fruits have limited dimensions and show determinate growth.
Differentiation shapes that material. The cells derived from the meristems mature to perform specific functions, and differentiation in plants is open too, but for a different reason - cells or tissues arising out of the same meristem have different structures at maturity, because the final structure at maturity is determined by the location of the cell within. Give the wrong reason for the wrong half and the mark is gone: growth is open because the meristem keeps supplying cells; differentiation is open because fate is set by position.
Development is the whole life history that results. It includes all changes an organism goes through during its life cycle, from germination of the seed to senescence, and broadly, development is the sum of growth and differentiation. Its sequence - meristematic cell -> cell division -> plasmatic growth -> expansion -> differentiation -> maturation -> mature cell -> senescence -> death - applies to tissues and organs as well as to cells.
Plasticity is what the whole system buys the plant. Plants follow different pathways in response to environment or phases of life to form different kinds of structures, and heterophylly is the example - juvenile against mature leaves in cotton, coriander and larkspur; air against water leaves in buttercup. A plant cannot walk away from a bad spot, so it changes what it builds instead.
What controls all of it is a two-part list. Development is under the control of intrinsic and extrinsic factors - intrinsic being the intracellular genetic factors and the intercellular chemicals called plant growth regulators, extrinsic being light, temperature, water, oxygen and nutrition. And the two lists are not independent: many extrinsic factors such as temperature and light control plant growth and development via PGRs, in events such as vernalisation, flowering, dormancy, seed germination and plant movements. The outside world acts on the plant through the plant's own chemistry.
Where Every Chapter-End Exercise Is Answered
This chapter has ten exercises at the end. Counted properly they come to twenty-one questions, because exercise 3 has four parts, exercise 8 has six and exercise 10 has four. Twenty of those twenty-one are already answered in full inside the thirteen teaching sections of this chapter; the twenty-first, exercise 1, is the eight-definition question and it is answered above as Question 27.
The table gives one row per part. Attempt each one on paper first and then turn to the section named - the answer there is written out in full, with the wording a marking scheme is looking for.
| Exercise | The question, in short | Section | Answered as |
|---|---|---|---|
| 1 | Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate | this section | Question 27 above |
| 2 | Why no one parameter demonstrates growth throughout a plant's life | Section 2 - Measuring Growth and the Three Phases of Growth | Question 4 |
| 3 (a) | Describe briefly: arithmetic growth | Section 3 - Growth Rates - Arithmetic and Geometric Growth | Question 2 |
| 3 (b) | Describe briefly: geometric growth | Section 3 - Growth Rates - Arithmetic and Geometric Growth | Question 3 |
| 3 (c) | Describe briefly: sigmoid growth curve | Section 3 - Growth Rates - Arithmetic and Geometric Growth | Question 4 |
| 3 (d) | Describe briefly: absolute and relative growth rates | Section 4 - Absolute and Relative Growth Rates, and the Conditions for Growth | Question 1 |
| 4 | List the five main groups of natural growth regulators, and write a note on one of them | Section 7 - Plant Growth Regulators - Characteristics and Discovery | Question 7 |
| 5 | Why abscisic acid is known as the stress hormone | Section 12 - Abscisic Acid and How the Growth Regulators Interact | Question 4 |
| 6 | "Both growth and differentiation in higher plants are open." Comment | Section 5 - Differentiation, Dedifferentiation and Redifferentiation | Question 9 |
| 7 | How a short day plant and a long day plant can flower simultaneously in one place | Section 13 - Photoperiodism and Vernalisation | Question 9 |
| 8 (a) | Which regulator to induce rooting in a twig | Section 8 - Auxins | Question 5 |
| 8 (b) | Which regulator to quickly ripen a fruit | Section 11 - Ethylene | Question 6 |
| 8 (c) | Which regulator to delay leaf senescence | Section 10 - Cytokinins | Question 9 |
| 8 (d) | Which regulator to induce growth in axillary buds | Section 10 - Cytokinins | Question 10 |
| 8 (e) | Which regulator to "bolt" a rosette plant | Section 9 - Gibberellins | Question 12 |
| 8 (f) | Which regulator to induce immediate stomatal closure in leaves | Section 12 - Abscisic Acid and How the Growth Regulators Interact | Question 5 |
| 9 | Would a defoliated plant respond to a photoperiodic cycle, and why | Section 13 - Photoperiodism and Vernalisation | Question 10 |
| 10 (a) | What happens if is applied to rice seedlings | Section 9 - Gibberellins | Question 13 |
| 10 (b) | What happens if dividing cells stop differentiating | Section 5 - Differentiation, Dedifferentiation and Redifferentiation | Question 10 |
| 10 (c) | What happens if a rotten fruit gets mixed with unripe fruits | Section 11 - Ethylene | Question 13 |
| 10 (d) | What happens if you forget to add cytokinin to the culture medium | Section 10 - Cytokinins | Question 12 |
Notice how the set is built, because it tells you what to revise. Ten of the twenty-one parts - the six parts of exercise 8 and four of the "what would happen if" items - are answered by naming the right growth regulator and giving its effect. Learn the five regulators as the grid in Question 28 above, with every effect and its opposite number, and half the exercise set answers itself. Four more parts - exercise 1 and the four parts of exercise 3 - are pure definitions, which means the marks are in the exact wording and nothing else. Write those out by hand until the sentences come back word for word.