A-level Crop plants. I do not own anything. No copyright infringement is intended. For educational purpose only

1. CROP
PLANTS
Bilegdemberel.M

2. WHAT IS CROP PLANT ?
Crop plants are plants that are grown by humans
for food and other resources. In this section, we
will consider four cereal crop is a grass-like plant
that is grown for its seeds.

3. MAIZE
Maize is grown in tropical and temperate
climates. It is able to grow well in high
temperatures, but varieties have also been
bred that can grow in cooler climates

4. SORGHUM AND RICE
Sorghum is able to grow in hotter, drier conditions
than other cereal crops
Rice is mostly grown in tropical and sub-tropical
climates, as it generally requires temperatures of at
least 20*C. Rice is adapted to grow in wet
conditions.

5. WHEAT
Wheat is grown in temperate climates.
It is able to survive temperatures well
below freezing.

6. WIND POLLINATION
All cereal crops have flowers that are
pollinated by the wind.The structure of a
maize plant and its flowers shows features
characteristic of wind-pollinated plants.
Maize plants have separate male flowers and
female flowers , both on the same plant.

7. A MAIZE FLOWER
Flowers are borne at the end of long stalks, held
well above the foliage.
 In some cases, the flowers appear before the
leaves.
Small, inconspicuous petals – often green in colour.
Petals may be absent altogether
Stigmas are large, branched and feathery – and held
outside the flower.
• Stamens are pendulous and also hang outside the
flower.

8. MAIZE FLOWER
Anthers are versatile. For example, they are
attached at the midpoint, so they will swing
freely in the wind.
Pollen grains are relatively light and small.
Pollen grains are produced in very large
quantities.
Absence of nectaries.
Absence of scent.

9. SELF POLLINATION & CROSS-
POLLINATION
Self pollination occurs when pollen from one
flower lands on the stigma of the same flower, or of
another flower on the same plant.
Cross-pollination occurs when pollen from one
flower lands on the stigma of a flower on a
different plant of the same species.

10. COMPARISON
Self pollination Cross pollination
Decreased genetic variation / increased
genetic uniformity
Increased genetic variation / decreased
genetic uniformity
Increased homozygosity / decreased
heterozygosity
Increased heterozygosity / decreased
homozygosity
Harmful recessive characteristics more
likely to be expressed
Harmful recessive characteristics less
likely to be expressed
Reduction in gene pool Maintenance of gene pool
Inbreeding depression reduces fitness Gives hybrid vigour (heterosis
outbreeding enhancement), so fitness
maintained

11. FRUIT FORMATION
After pollination and fertilasation, fertilised ovule
develops into a seed.The zygote develops into an
embryo plant, inside the seed.
Other tissues in the ovary develop into the
endosperm tissue.This contains food stores, mostly
starch, which will be used by the embryo when the
seed germinates.
Around the edge of the endosperm is the aleurone
layer.This contains enzymes that are activated
when the seed germinates.The enzymes break
down the starch in the endosperm

12. FRUIT FORMATION
The ovary develops into fruit, with the seeds inside
it. In maize each ovary contained a single ovule, so
each fruit contains a single seed.The outer layer of
a seed is the testa, and the fruit wall is the pericarp.
In maize grain, the testa and pericarp are fused
together.

13. CEREAL GRAINS IN HUMAN DIET
All cereal grains have a structure similar to
that of maize fruit. Cereal grains are staple
foods in many parts of the world. They
contain:
Large amount of carbohydrate
Protein, mostly in the aleurone layer and
embryo
Only small amounts of lipid
Vitamin B, Calcium and fibre.

14. ADAPTATIONS OF THE LEAVES IN C4
PLANTS
Maize and Sorghum are C4 plants. This means that,
instead of first making a 3-carbon compound
during the Calvin cycle, they produce a 4-carbon
compound. This is an adaptation to growing
environments where the temperature and light
intensity are high.

15. PHOTOSYNTHESIS IN MAIZE AND
SORGHUM

16. PHOTOSYNTHESIS IN MAIZE AND
SORGHUM
At high temperatures and high light
intensities, the enzyme rubisco tends to
catalyse the combination of RuBP with
oxygen rather than with carbon dioxide.
This is wasteful, and reduces the rate of
photosynthesis. It is sometimes called
‘photorespiration’, because it uses oxygen.
The leaves of C4 plants have structural
adaptations that prevent photorespiration
taking place.

17. PHOTOSYNTHESIS IN MAIZE AND
SORGHUM
In a C4 plant, rubisco and RuBP are kept
away from the air spaces inside the leaf, so
they do not come into contact with oxygen.
The rubisco and RuBP are inside the
chloroplasts of the bundle sheath cells.They
are separated from the air spaces by a ring
of mesophyll cells.These also contain
chloroplasts, where the light-dependant
reactions of photosynthesis take place.

18. PHOTOSYNTHESIS IN MAIZE AND
SORGHUM
Carbon fixation happens like this:
In the mesophyll cells, CO2 combines with PEP to
form 4-carbon compound.
The 4-carbon compound moves into the bundle
sheath cells.
The 4-carbon compound breaks down and releases
carbon dioxide.
The rubisco in the bundle sheath cells catalyse the
reaction of the carbon dioxide with RuBP.
The Calvin Cycle then proceeds as normal, inside
the sheath sheath cells.

19. ADAPTATIONS OF SORGHUM FOR ARID
ENVIRONMENTS
Feature How it helps the plant to survive in
dry conditions
Sorghum plants have a relatively small leaf
area
This reduces the area from which water
can evaporate in transpiration, therefore
reducing the rate of water loss
Sorghum leaves and internodes are
covered with a layer of wax
This is impermeable to water and
therefore decreases water loss.
Sorghum leaves have a row of motor
cells along the midrib that allow the
leaves to roll up when the cells are short
of water
This decreases the surface are of the
leaves in contact with air, and therefore
reduces the rate of loss of water vapour
from the leaves to the air. Moist air is
trapped inside the rolled leaf.

20. ADAPTATIONS OF SORGHUM FOR ARID
ENVIRONMENTS
Sorghum leaves have relatively few
stomata, and these are sunken
below the leaf surface
Moist air, with a high water potential,
is trapped around the stomata.This
reduces the water potential gradient
between the air spaces in the leaf and
the outside, reducing the rate of loss
of water vapour from the leaf.
The root system is extensive and finely
branched
The roots are able to absorb water even
when there is very little water in the soil
The plant can close its stomata and
become dormant for long periods.
The plant is able to survive during a
prolonged drought, resuming growth when
conditions improve.

21. ADAPTATIONS OF RICE FOR WET
CONDITIONS
Feature How it helps the plant to survive
when roots are submerged
Cells are tolerant of high
concentrations of alcohol
When roots are submerged in water,
less oxygen is available than when the
soil contains air spaces. Cells therefore
respire anaerobically, producing ethanol.
Stems have tissues called aerenchyma,
containing large air spaces
Aerenchyma allows oxygen from the air
to diffuse down to the roots.
Some types of rice are able to grow
elongated stems to keep their leaves
above the water as its level rises.
The leaves remain exposed to the air,
which facilitates gas exchange for
photosynthesis and respiration

22. POLYPLOIDY IN WHEAT

23. INBREEDING AND HYBRIDASATION IN
MAIZE

24. VITAMIN A ENHANCED RICE

25. THANKYOU FORYOUR ATTENTION