APPLICATION T – CROP PLANTS
(a) Describe and explain the structural features of a named, wind pollinated plant.
Pollination: transfer of pollen from an anther to a stigma.
Structural features of wind-pollinated plant (different from insect-pollinated plant):
-
Flowers borne at end of long stalks, held well above foliage. In some cases,
flowers appear before leaves.
Small inconspicuous petals – often green in colour. Petals may be absent
altogether. Why?
Stigmas are large, branched and feathery – and held outside flower.
Stamens are pendulous and also hang outside flower.
Anthers are versatile, i.e. they are attached at midpoint, so they will swing freely
in the wind.
Pollen grains are relatively light and small. Why?
Pollen grains are produced in very large quantities. Why?
Absence of nectarines. Why?
Absent of scent. Why?
Example: Maize
Maize (called corn in some parts of the world) is pollinated by wind. Male anthers
release their pollen, which is carried by the wind to a nearby female flower on another
corn plant (cross-pollination is favoured, as you will see that inbreeding depression is
not tolerable by maize plants). [Male and female parts are borne on one maize plant
(monoecious)].
Maize flowers are adapted to wind pollination. They do not need pretty petals. The
pollen is light and non-sticky so it can be carried by the wind, and the ends of the
female parts (stigma) are fluffy to catch all the tiny pollen grains.
(b) Compare the outcomes of self-pollination and cross-pollination in terms of genetic
variations.
Self-pollination: when transfer of pollen to stigma occurs in the same flower or from
one flower to another on the same plant.
Cross-pollination: when transfer is from one plant to another.
Assume pollination results in successful fertilization, the main genetic outcomes are:
Self-pollination
Decreased genetic variation/increased
genetic uniformity
Increased homozygosity
Harmful recessive characteristics more
likely to be expressed
Reduction in gene pool
Inbreeding depression reduces fitness
Cross-pollination
Increased genetic variation
Increased heterozygosity
Harmful recessive characteristics less
likely to be expressed
Maintenance of gene pool
Gives hybrid vigour ( aka outbreeding
enhancement), so fitness is maintained
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APPLICATION T – CROP PLANTS
Inbreeding depression: Reduced fitness (survival and fertility) of offspring in a given
population as a result of breeding between related individuals.
The most likely cause for reduction of fitness upon inbreeding involves expression of
deleterious recessive alleles. Deleterious alleles originate when underlying DNA
sequence of a functional allele is altered by mutation to code for a gene product which
is either harmful or simply does not work. With inbreeding, the odds of producing an
offspring homozygous for a deleterious allele are much higher. Because rare
deleterious mutations are transmitted along family lines, brothers, sisters and close
family members are much more likely to be carrying the same deleterious alleles than
unrelated individuals.
In plants, inbreeding happens frequently as 70% of flowering plant species are
hermaphrodites (possess both male and female parts on the same plant). However,
even within these plants, outbreeding is promoted since there is an obvious
evolutionary advantage that is increased vigour. Features of hermaphrodites that
promote outbreeding: self-incompatibility (pollen incapable of functioning on stigmas
of the individual from which it came, seeds produced not viable), male and female
parts are separated within a flower (more effective arrangement would be separate
male and female flowers) or they function at different times during the life of a
flower.
Human example: Charles Darwin – married first cousin, three children died before
age 10 and three who were in long term marriages did not have children, Vadoma
tribe in Zimbabwe – many carry ttrait of having only two toes due to a small gene
pool.
Conclusion: Populations which result largely from cross pollination are
phenotypically more variable, which gives them more evolutionary potential and
means that they are able to adapt to changes in the habitat or environment.
(c) Describe the structure of the fruit in maize and explain the function of the
endosperm.
Caryopsis: individual fruit of maize is a dry fruit and contains a single seed.
Seed has two structures:
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APPLICATION T – CROP PLANTS
1. germ: structure from which a new plant will develop
 Consists of a miniature plant axis to which are attached around 5 embryonic
leaves and a radicle, from which the root ill develop.
 Source of ‘maize oil’.
2. endosperm: structure which is the store of nutrients which will be made available
to the germinating seedling until it has established sufficient leaf area to
photosynthesize.
 Two-thirds of the volume of seed, 86% of dry weight.
 Principal component is starch, together with 10% protein (gluten).
 Provide nutrition for seedling, and hence basis of nutritional value of maize.
 Whole, ground maize meal has an energetic value of ~1500kJ per 100g.
(d) Explain the significance of the grains of cereal crops in the human diet.
Cereal: It is a grass, which we grow to harvest and eat the grain (seeds) that it
produces.
Major component of human diet in many parts of the world.
Examples: rice, wheat, barley, sorghum, millet
Nutritional value includes:
1. Carbohydrates (mainly starch), 70-80%: main source of energy.
2. Source of protein, 6-14%: do not have a balance of amino acids, some lacking
of essential amino acids.
3. Fat, 2-4% (but for oats 7.5%): high in essential fatty acids, found in germ of
grain.
4. Vitamins: all cereals provide a good source of B group vitamins (niacin,
riboflavin, folic acid) and fat-soluble Vit E. Lacking in Vit A (see Golden rice
example later), D and C.
5. Minerals: potassium, calcium, magnesium, iron, zinc, phosphorus.
6. Fibre: cereals excellent source of dietary fibre. Wholegrain meal and flour will
contain much more fibre than grains that have been processed and refined.
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APPLICATION T – CROP PLANTS
(e) Explain how the anatomy and physiology of the leaves of C4 plants such as
maize or sorghum are adapted for high rates of carbon fixation at high
temperatures in terms of – the high optimum temperatures of the enzymes
involved and – the spatial separation of initial carbon fixation from the light
dependent stage (biochemical details of the C4 pathway are not required).
RECALL:
In the light-independent stage of photosynthesis, carbon dioxide combines with RuBP
to form a 6C compound, which immediately splits to form two 3C molecules. Plants
that do this are C3 plants. (p215, main textbook)
Maize, sorghum and most other tropical grasses are C4 plants. The first compound
that is produced in the light-dependent reaction contains four carbon atoms. WHY do
maize, sorghum and tropical grasses need to do something different?
Reason: problem with RuBisCo (_______________________________________)
enzyme. It catalyses reaction of CO2 with RuBP, but unfortunately, can also catalyse
reaction of O2 with RuBP! When this happens, less photosynthesis takes place, as
some of the RuBP is ‘wasted’ and less is available to combine with CO2. The
unwanted reaction (O2 with RuBP) is known as photorespiration.
Photorespiration occurs easily when?
o high temperatures and high light intensity
These conditions are found at low altitudes in tropical parts of the world. They cause
stomata to partially close, hence building up oxygen concentrations (from respiration)
within the intercellular air spaces of leaves, and promoting photorespiration.
Photorespiration is a wasteful process, as it prevents RuBisCo from fixing as much
CO2 as possible.
How do tropical grasses AVOID photorespiration?
Answer: look into their leaf structure! (C4 leaf structure) This structure is known as
the Kranz anatomy.
Structural features of leaves of C4 plants that provides spatial separation of initial
carbon fixation from the light-dependent stage:
1. Around vascular bundles, there is a group of cells known as bundle sheath
cells. These cells contain RuBP and RuBisCo, but have no direct contact with
the air and, therefore ________ exposed to high [O2]. Bundle sheath cells are
large parenchyma cells with many large chloroplasts.
2. Around bundle sheath cells is another ring of mesophyll cells. These are in
contact with air spaces, but have no air spaces between them, ensuring no O2
reached the bundle sheath cells.
3. Mesophyll cells contain an enzyme called PEP carboxylase, which catalyses
combination of CO2 from the air with a 3C compound called
phosphoenolpyruvate (PEP). This results in the formation of oxaloacetate
(4C).
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APPLICATION T – CROP PLANTS
4. Still inside the mesophyll cells, this oxaloacetate is then converted to malate,
which is passed onto the bundle sheath cells, where CO2 is removed from the
malate and combined with RuBP in the usual way. The light-independent
Calvin cycle (p215, main textbook) then proceeds as normal.
Photosynthesis takes place within the two concentric layers: mesophyll cells and
bundle sheath cells.
Turn to p332, main textbook:
STUDY Fig 23.9
DRAW Fig 23.10 in the space below
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APPLICATION T – CROP PLANTS
(f) Explain how sorghum is adapted to survive in dry/arid environments.
Sorghum – 5th most important cereal crop in the world. Important in areas of low
rainfall – able to grow successfully in areas of low water supply and show many
characteristics associated with xerophytes:
1. A very dense root system – widespread and deep, allowing efficient water
uptake from the moister, deeper layers of the ground.
2. Leaves covered with thick, waxy cuticle especially on the lower surface –
reduces evaporation of water from surface of leaves.
3. Specalised motor cells (bulliform cells) on upper side and strengthening tissue
(sclerenchyma) below vascular bundles, which cause leaves to roll inwards
when water is in short supply, hiding away half of the stomata. This allows
build up of water vapour, reducing the difference between water potential
inside the leaf, again reducing the diffusion of water vapour from the leaves.
4. Number of stomata is low and air spaces inside leaf are small. They are only
found well away from the vascular tissues, increasing the distance water has to
diffuse before it is lose from the leaf.
5. It is a C4 plant, so as long as there is sufficient water, sorghum can continue to
photosynthesis even when it is very hot and sunny. Otherwise, it will remain
dormant.
6. Sorghum plants have enzymes that have relatively high optimum
temperatures.
If drought conditions continue, sorghum plants can go into a kind of dormancy, in
which their metabolism slows down. When it rains or when water becomes available,
they begin to grow again.
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APPLICATION T – CROP PLANTS
(g) Explain how rice is adapted to grow with the roots submerged in water in terms of
tolerance to ethanol and presence of aerenchyma.
Rice grows in tropical regions and is a C3 plant! Two species are cultivated: Oryza
sativa native to subtropical Asia, and Oryza glaberrima found in parts of Africa.
Rice is a unique cereal because it is able to grow in flooded conditions (paddy firls –
fields intentionally flooded).
Soil which is waterlogged lacks oxygen (anaerobic).
Hence, rice must be adapted to these conditions of low oxygen availability if they
were to grow successfully:





Stems and leaves possess very large air spaces and loosely packed cells
forming a tissue known as aerenchyma – as roots grow down into the
oxygen-deficient soil, continuous formation of aerenchyma behind the
growing tip allows oxygen to diffuse downwards through air spaces.
Air is also trapped in between the ridges of the underwater leaves these leaves
have a hydrophobic, corrugated surface that holds a thin layer of air in contact
with the leaf surface.
The roots are very shallow – this allows some access to higher levels of
oxygen in the surface water.
When oxygen concentrations fall to very low levels, the roots are able to
respire anaerobically. This results in the production of ethanol, which would
normally be toxic. However, rice root cells shown an unusually high tolerance
to ethanol – they are able to produce high levels of the enzyme alcohol
dehydrogenase. This allows the plants to grow with energy from anaerobic
respiration.
Rice seeds are also able to tolerate flooded soil. The seed embryo and
coleoptile (sheath covering the first leaves to emerge) can respire
anaerobically, producing ethanol, which the cells can tolerate.
(h) Outline the following examples of crop improvement by conventional breeding
techniques: - hybridization leading to polyploidy in wheat, and – inbreeding and
hybridization in producing vigorous, uniform maize.
Plant breeding is carried out by crossing different varieties, and then selecting
progeny with particular features from which to breed further. Qualities looked for in
the progeny include:
- fast germination and/or growth
- good response to applied fertilisers
- dwarfed stem growth and enhanced leaf growth
- efficient capture of solar energy
- efficient inflorescence and flower formation (e.g. ‘long ear’ and many grains in
cereals)
- tolerance of occasional unfavourable conditions such as drought
- effective resistance to diseases and common pests
- superior nutritive value of the grain/fruit/leaf product yielded
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APPLICATION T – CROP PLANTS
Producing polyploids in wheat through hybridization
The combining together of whole sets of chromosomes in the formation of polyploids
is a branch of plant/crop breeding. Polyploids have more than two haploid (n) sets of
chromosomes, e.g. 3n triploid, 4n tetraploid etc. Polyploid plants not only have larger
cells but the plants themselves are often larger.
Hybridisation: crossing of carefully selected varieties in order to bring together the
desirable qualities from both parents in the offspring. The offspring sometimes (not
always) grow more vigorously, a characteristic known as hybrid vigour. It arises
when the new sets of chromosomes not only produce differences but also are
complementary in their effects.
Modern/Domestic wheat is a hexaploid (6n) plant. The probable ancestral number is
though to be 7 (n = 7 chromosomes). Modern wheat is the result of the hybridization
of several wild species of grasses. These species are closely related, so their
chromosome numbers and structures are similar but not identical. Such inter-specific
hybrids (inter-specific hybridization) contain one set of chromosomes from one
parent species and a second, non-homologous, set of chromosomes from the other,
different parent species.
When they attempt to undergo meiosis, it fails, as the chromosomes cannot line up
with their homologous partner chromosome. Such sterile hybrids can only reproduce
asexually.
However, in such sterile organisms, occasional errors can occur whereby during cell
division during production of gametes, the gametes are diploid (2n) instead of haploid
(n)! The chromosomes number of such gametes is doubled as a result of rare failures
of the chromosomes to separate during mitotic division before or during gamete
formation. This failure to separate is termed non-disjunction of chromosomes.
If such diploid male gamete fuses with a diploid female gamete, for example after
self-pollination, the resulting offspring is tetraploid (4n)! Meiosis can occur normally
as there are now homologous pairs of chromosomes, and so a new fertile, species of
plant has been instantly formed.
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APPLICATION T – CROP PLANTS
Among plant species, polyploids are generally more hardy and higher yielding than
their parent species, making them important food crops. The ancestors of wheat were
small, not very robust, produced small ears of small seeds, in contrast to the modern
6n wheat.
Inbreeding and hybridization in producing vigorous, uniform maize
Maize is one of the most widely grown crop plants. Growing conditions vary in
different parts of the world (soil type, prevailing temperature, rainfall etc) hence
through selection, inbreeding and hybridization, growers have been able to produce
varieties that grow vigorously and uniformly under prevailing conditions. Assuming
conditions remain similar year after year, farmers can grow the same variety and
expect to obtain similar crop.
Desirable characteristics:
- High yielding
- Disease resistant
- Good quality in terms of desirability to market
- Vigorous growth under the prevailing conditions
- Plants all grow to a similar height. Why? For easy harvesting using machinery!
- Crops are all ready to harvest at the same time (this only makes sense!)
Initial maize population had wide genetic and phenotypic variation. Plants that did
show some of the above desirable characteristics would be selected and selfpollinated.
Transfer pollen to stigma of chosen plant. Place muslin bag around flower to ensure
pollen from other plants does not reach the stigma. Repeat this self-pollination for
many generations; maize produced will be homozygous for the desired characteristics.
But, maize is a natural outbreeder! It is NOT very tolerant of outbreeding. So the
above process of self-pollination or inbreeding can lead to a loss of vigour and
fertility, as well as a reduction in size and yield. This is known as i___________
d____________. Inbred maize has very little variation, with every plant having the
same alleles of every gene.
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APPLICATION T – CROP PLANTS
However, if two inbred are crossed, it will produce a hybrid that has a greater yield
and is more vigorous than either of the parental lines! This is known as h________
v________. This hybrid is a heterozygous for most genes, so deleterious recessive
alleles are hidden, but at the same time it inherits the lack of variability from its
parents.
Selection for measurable characteristics such as yield is done by measuring
characteristics and choosing for breeding those plants that express it most strongly,
e.g. having the highest yield.
Selection for disease or pest resistance is done by exposing the plants to the disease or
pest, which kills any that are not resistant.
Seeds which result from such breeding are grown and plants showing the desirable
characteristics are bred again – the process can be repeated for many generations.
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APPLICATION T – CROP PLANTS
(i) Outline the following examples of crop improvement by genetic modification and
include any associated detrimental effects on the environment or economy –
herbicide-resistant oil seed rape, - insect-resistant maize and cotton and – Vitamin A
enhanced rice (Golden Rice).
Herbicide-resistant oil seed rape
Herbicide (aka weedkillers): A chemical substance that is used to kill unwanted
plants, e.g. weed.
Herbicide-resistant oil seed rape?
During photosynthesis, chloroplasts produce
ammonia. This is a toxic product. An enzyme,
glutamine synthetase, produced combines
glutamate and ammonia to give harmless
glutamine.
Herbicide’s active ingredient (phospinothricine
aka glufosinate) resembles glutamate, and
binds to the enzyme glutamine synthetase,
blocking its active site. This results in ammonia
accumulating in the chloroplasts and destroys
them, so plant dies.
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APPLICATION T – CROP PLANTS
Genetically engineered oil seed that contains a gene from a soil bacterium. This gene
codes for another enzyme, phosphinothricine acetyl transferase (PAT), which
inactivates the herbicide. The DNA coding for this enzyme has been isolated and
inserted into the plasmid of Agrobacterium tumifaciens to be incorporated into
chromosome of crop plants.
A. tumifaciens infects wounds in plants and causes production of tumour. The
bacterium allows production of tumour due to the presence of Ti plasmid (tumourinducing), carrying tumour-inducing genes. When bacterium invades plant, a copy of
T genes is added to one of the chromosomes of the host. If the wanted gene is inserted
into the Ti plasmid before the bacterium invades the plant cell, it is also added into the
plant genome. The plant cells, in the form of a callus/gall, are engineered using the Ti
plasmid, and then cultured to produce whole plants.
This has made the oil seed rape resistant to herbicide. By making oil seed rape
resistant, herbicide can now be sprayed at any time, killing all weeds but leaving the
crop intact. The disadvantage of this in term of the environment is that there will be
increased herbicide usage.
Advantage: Increase market for herbicides! Companies have developed herbicide
resistant crops as a way to increase the markets for existing herbicides, rather than
develop newer and safer ones. Development of new herbicide costs a lot…
Phosphinothricine/Glufosinate: Non-selective/Broad-spectrum, contact herbicide.
Kills any plant which it comes into contact with. Used for completely clearing
vegetation, and also as a pre-harvest treatment in oil seed rape, to kill off the foliage,
making harvesting easier.
Concerns about herbicide-resistant oil seed rape?
Aventis*s is a herbicide producing company.
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APPLICATION T – CROP PLANTS
Insect-resistant maize and cotton
Maize and cotton are important crops grown in developing countries. They have been
genetically engineered to be resistant towards feeding insects. This has been achieved
by expression of Bt proteins derived from the bacterium Bacillus thuringiensis within
the crop plants.
Advantage?



Synthetic pesticides kill a broad spectrum of insects, including those beneficial
insects (e.g. pollinators). Bt-crops, primarily, kill those insects attacking the crops.
Farmers planting Bt-crops do not require knowledge and equipment for pesticide
application, and hence reduce the farmers’ exposure to pesticides.
Harvest losses caused by insect pests are decreased.
Maize
Problem: European corn borer eats leaves of the plant then burrows into the stalk,
eating its way upwards until the plant cannot support the ear.
European corn borer
European corn borer: shotholes and tunnel in leaf midrib (a), damage and fungal
infection in non-Bt maize (left) and Bt maize (b), stalk tunneling (c), and adult female
(left) and male (d). Source: Nature
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APPLICATION T – CROP PLANTS
Insect-resistance gene from Bacillus thuringiensis, a naturally occurring soil
bacterium is extracted and introduced into maize plant, together with a promoter
region that causes the gene to be expressed in certain parts of the plants or only during
a particular period of time. A marker gene is also introduced to identify which plants
have been transformed. It produces a protein (toxin with insecticidal action) that kills
the larvae of corn borer. Bt maize is grown as an alternative to spraying pesticides for
control of corn borer.
Agrobacterium tumefaciens induces the growth
of tumours on woody plants. These tumours are
engineered by A. tumefaciens to produce a
special food for the bacteria (opines) that plants
normally cannot make. These tumours arise
from a unique bacterial transformation
mechanism involving the Ti-plasmid which
coordinates the random insertion of a subset of
its DNA (t-DNA) containing opine synthase
genes into a plant chromosome. By replacing
portions of the t-DNA sequence with genes of
interest (such as Bt Cry gene from B.
thuringiensis that codes for toxins), researchers
have been able to harness this transformational
mechanism and confer new traits to many
flowering plants including grasses such as maize
and cotton. Cry-transformed corn varieties,
called ‘Bt corn’, produce sufficient levels of Cry
proteins to provide an effective measure of
resistance against corn borer and are now widely
grown in North America.
This protein is selective for corn borer, and
generally does not affect other insects. Hence,
genetic engineering with the Bt gene is
compatible with biological programs because
they are not broad-spectrum.
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APPLICATION T – CROP PLANTS
The protein is ingested, and within minutes it binds to the gut wall and the insect stops
feeding. Within hours, gut wall breaks down, and normal gut bacteria invade the body
cavity. The insect dies of septicaemia (blood poisoning) as bacteria multiply in the
blood.
Vitamin A enhanced rice (“Golden rice”)
Rice is a staple food in many parts of the world.
In parts where rice is the staple food and people are poor, they often suffer from Vit A
deficiency. This causes blindness.
Vit A is a fat-soluble vitamin found only in oily fish and dairy products. It is also
made in our body using the precursor, β-carotene. This is the orange pigment found
in carrots. Vit A is found in the aleurone layer of rice grains but not in the endosperm.
The aleurone layer is removed when rice is polished to produce white rice. Brown
rice, however, still contains aleurone layer. So why is white rice produced? This is
because if rice is stored for any period of time, the aleurone layer rots.
In the 1990s, a genetically modified strain of rice was produced, which stores
significant levels of β-carotene in the grains. This strain is known as “Golden rice”
and contains genes which were transferred from the daffodil and a bacterium.
The genes were incorporated into plasmid of A. tumifaciens and the bacteria used to
carry the genes into plant cells.
A new method has been developed whereby the genes are delivered directly into the
plant cells using micrometer-sized tungsten or gold bullets coated with the DNA. The
bullets are fired from a device that works similar to a shotgun. This delivery device is
known as a ‘gene gun’.
Controversy: is this the way to solve this problem? YES: help millions of people
avoid blindness. NO: issue here is poverty – because of poverty, these people have
diets lacking in Vit A. the way to solve it is to help them out of poverty so that they
have access to a more varied diet.
Detrimental environmental effects:
- GM crops leads to increase in the use of herbicides, e.g. glufosinate (refer to table
above – Aventis*s vs Independent research finding table - to see why this is
harmful).
- Pollution arising from use of harmful chemicals although biotechnology
companies claim that GM crops would reduce the use of herbicides and
insecticides.
- New ‘super-weeds’ may evolve which will be difficult or even impossible to
eradicate. This may drive up the use of other more damaging herbicides.
- Pest resistance arises. Instead of producing crops that are resistant to pests, the
opposite effect comes about as pests can adapt and become resistant to the GM
crop.
- Wildlife and diversity is being harmed by new toxins and changes in agricultural
practices.
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APPLICATION T – CROP PLANTS
Detrimental economy effects:
When adopting GM crops, farmers also meet a number of constraints, namely: seedsaving is illegal and the cost of seed or of fees for technology agreement is increased.
Furthermore, if the HR crop readily cross-pollinates with adjacent non-GM crops,
then it raises questions both about property rights of these unintentionally resistant
plants. Furthermore, public concern over GM food may lead to demands for the
segregation of GM crops and non-GM crops. Such measures increase costs. They may
therefore result in different prices for GM crops and non-GM crops, and this
differentiation may lead to crops being grown under contract.
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