Crop plants

A-level Crop plants.
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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 C alvin 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. Thank you for your attention