Crop plants

1. Crop plants • Some of the Staple foods –rice , wheat , barley, Sorghum & millet are Cereal crops. A cereal is a grass , which we grow to harvest and eat the grains (seed) that it produces. Taro is not a cereal ,it is a yam that is grown for its starchy underground stem. Potatoes may also form the staple food. Dates are fruits of palm trees & bananas are also fruits & contain a lot of sugars as well as starch. • The cereal foods are rich in carbohydrate & contains significant amounts of proteins ,but are low in fat. The proteins however only contains a little essential amino acid lysine. So in order to eat a balanced diet ,other food rich in essential amino acid– such as fish, milk should be included.

3. Cereals are rich in vitamin B but tend to have small amounts of vitamins A,C & D. These can be supplied by eating fresh fruits & leafy green vegetables. Cereals also contain plenty of calcium. They are high in fiber ,which helps in peristalsis to take place in the alimentary canal. • Maize, Zeamays ,is also known as corn . It is a sturdy tall grass with broad leaves . Maize grows best in the climates with long ,hot summers which provide plenty of time for its seed heads to ripen

4. Pollination and fertilisation • Like all cereals, maize has wind pollinated flowers. The pollen grains which contain the male gametes ,are produced in the anthers. Wind carries them in the air & some may land on the stigma of the flower of another maize plant. • The plants are also adapted for cross pollination. Cross pollination occurs when the pollen from one plant lands on the stigmas of a different plant of the same species. • After pollination, fertilisation takes place & the zygote that is formed grows into an embryo inside a seed. • Cross pollination results in Outbreeding-sexual reproduction involving two unrelated individuals. • Self pollination results in Inbreeding--- Sexual reproduction involving two closely related individuals or involving only one parent.

5. Cross pollination is advantageous to maize. This happens because the parent plants may contain different alleles for some of their genes . The new individual therefore acquires a mix of alleles from both parents. This may mean it is Heterozygous for some genes. • Inbreeding over several generations causes a decreases in size & vigor and eventually the plant may become so weak & small in size that they do not breed at all & the line dies out. But cross breeding these varieties can produce a hybrid that is very vigorous . This is called Hybrid Vigour . • Cross pollination reduces the chances that the offspring will inherit copies of the same harmful recessive allele from both of their parents. • For example –If there is a recessive allele that causes less chlorophyll to be produced in the leaves.

6. If self –pollination takes place ,the offspring might be homozygous for this recessive allele & would not grow strongly. • Cross-pollination makes it less likely that this will happen , because there is less chance that both parents would contain this recessive allele . So cross pollination can maintain genetic diversity. If the maize is growing wild ,this can provide the genetic variation that underlines the ability to adapt to a changing environment. It also reduces the likelyhood that the whole population could be wiped out by a single disease, as there is a good chance that at least one individual plant will have resistance to it. • Following pollination ,fertilisation takes place. The pollen grains respond to the chemicals on the stigma by growing a pollen tube which grows down through the style & into the ovule in the ovary . The ovule

7. contains the female gametes . The male gamete from the pollen grain moves down the pollen tube & into the ovule, where it fuses with the female gamete. This is fertilisation. Seed and fruit Development After fertilisation, the zygote divides repeatedly by mitosis to form an embryo. This embryo is contained within the fertilised ovule which is now known as seed. The style and stigma shrivel and the ovary becomes a fruit. At the same time the triploid endosperm nucleus divides to produce the Endosperm . This is the food store for the embryo, & it is this that provides us with the starch & protein that makes maize such an important source of food in many parts of the world.

9. Diagram of a Corn (Zeamays) Fruit: PC/Testa = Pericarp/Testa, CN = Cotyledonary Node, SAM = Shoot Apical Meristem, RAM = Root Apical Meristem

10. C4 Plants • In the light-independent stage of photosynthesis CO2 combines with RuBP (RibuloseBisphosphate) It is a 5-carbon molecule which combines with carbon dioxide in the Calvin cycle to produce a six-carbon compound) ,which immediately splits to form two three-carbon molecules. Plants that do this are called C3 Plants (eg wheat , barley ,potato etc) • However, maize plants - and most other tropical grasses –do something different . The first compound that is produced in the light-dependent reaction contains four carbon atoms. They are therefore called C4 plants.

11. Difference between C3 and C4 plants C3 plants C4 plants C4 plants use Phosphoenolpyruvatecarboxylase enzyme. The primary product of photosynthesis is oxalacetate. C4 plants use PEP carboxylase to fix CO2 and their first molecule is oxalacetic acid(4 C). At C4 plants photosynthesis is 6 times faster than C3 plants. Eg -Corn • C3 plants use ribulosodiphosphatcarboxyl-ase enzyme. • The primary product of photosynthesis of C3 plants is phosphoglycerat. • C3 plants use RUDP carboxylase to fix CO2 and their first molecule is phosphoglycerat(3 C compound). • Eg– wheat, barley, potatoes & sugar beet

12. C3 plants C4 plants Bundle sheath contains chloroplast. Carbon is fixed in mesophyll cells ,then transported to bundle sheath cells where calvin cycle reactions occur in the absence of oxygen. C4 plants have a concentric arrangement of bundle sheath and bundle sheath is also thicker. • Bundle sheath does not contain chloroplast. • Carbon fixation and calvin cycle reactions occur in mesophyll cells and in the presence of oxygen. • The bundle sheath is thinner in C3 plants

13. Avoiding Photorespiration Photorespiration ----Oxidation of carbohydrates in plants with the release of carbon dioxide during photosynthesis. Photorespiration occurs when a plant runs out of CO2 and begins adding O2 to RuBP in the Calvin cycle. Why do tropical grasses need to do something different. The reason is a problem withrubisco(ribulosebisphosphatecarboxylase). This enzyme ,thought to be the commonest enzyme to catalyse the combination of CO2 with RuBP. But ,unfortunately ,it can also catalyse the reaction of oxygen with RuBP. When this happens less photosynthesis takes place ,because some of the RuBP is wasted & less is available to combine with CO2 . This unwanted reaction is known as Photorespiration. • It happens most readily at high temperatures & high light intensities. • Tropical grasses such as maize, sorghum & sugar cane have evolved a method of avoiding photorespiration.

14. They do this by keeping RuBP & rubisco well away from high O2 concentrations . The cells that contain these two compounds are arranged around the vascular bundles & are called Bundle sheath cells . They have no direct contact with the air inside the leaf. • CO2 is absorbed by another group of cells ,the mesophyll cells, which are in contact with air. These cells contain an enzyme called PEP caboxylase, which catalyses the combination of CO2 from air with the three-carbon substance called Phosphoenolpyruvate or PEP. The compound formed from this reaction is Oxaloacetate. • Still inside the mesophyll cells the Oxaloacetate is converted to malate, & this is passed on to the bundle sheath cells. Now the CO2 is removed from the maltate molecules & is delivered to RuBP by rubisco in the normal way. The light –independent reaction then proceeds as usual.

15. Enzymes in C4 plants generally have higher optimum temperatures than those in C3 plants. There is an adaptation to growing in hot climates . For example –amaranth which is a C4 plant ,the optimum temperature for the activity of PEP carboxylase is around 45°C. If the temperature drops to 15°C the enzyme looses around 70%of its activity . • By contrast, the same enzyme in peas , which are C3 plants ,was found to have an optimum temperature of around 30°C & could continue to work at much lower temperatures than in amaranth.

17. Adaptation of Sorghum for dry conditions • Sorghum ,sorghum bicolor is grown in the parts of the world that are hot & too dry for growing maize. Different kinds of sorghum , are grown to provide grains for food, to provide grazing for cattle & to make hay ,to make a sweet syrup & to make brooms and brushes. • Enzymes in sorghum have a relatively high optimum temperature for a plant , and sorghum needs daytime temperatures of around 30°C to photosynthesise at its maximum rate. • Sorghum plants look very like maize plants. However they have much more widely spreading roots ,to help to draw water from the soil. This is an adaptation to grow in dry conditions ,and sorghum is much more drought resistant than maize.

18. All plants have the problem that ,if they are to allow CO2 to diffuse into their leaves for photosynthesis , water vapour will diffuse out. Sorghum has thick covering of wax on its leaves, which helps to stop water leaving through the leaf surface. • It is able to roll up its leaves when it is short of water, tucking the stomata away & reducing the surface area from which water can be lost. If drought continues for any length of time ,sorghum plants can go into a kind of dormancy ,in which their metabolism slows right down. When it rains ,they begin growing again . It is sorghum’s ability to produce grains even under very dry conditions that makes it such a valuable crop plant in areas where rainfall in infrequent & unpredictable.

19. Adaptation of rice for wet environment • Two species of rice are Oryza sativa grown in subtropical Asia & Oryzaglaberrima grown in parts of Africa. • Rice is thought to provide 20% of energy needs of human population on earth. It is a staple food of around 50% of people. Although it can grow in dry conditions, it is often grown in paddies. Rice can tolerate growing in water whereas most of the weeds that might compete with it are not able to do so. • Most plants cannot grow in deep water because their roots do not get enough O2. O2 is required for aerobic respiration, which provides ATP as an energy source for active transport & other energy consuming processes such as cell division. • Nor , if the leaves are submerged ,can photosynthesis take place because there is not enough CO2 available . This happens because gases diffuse much more slowly in water than they do in the air. Moreover the concentration of dissolved O2

20. &Co2 in water was much less than they are in the air. This is especially true in the rice paddies, where the rich mud in which the rice roots are planted contains large populations of microorganisms, many of which are aerobic & take O2 from water. • Some varieties of rice respond to flooding by growing taller . As the water rises around them, they keep growing upwards so that the top parts of the leaves & flower spikes are always held above the water . This allows Co2 & O2 to be exchanged through the stomata on the leaves. The extra growth is controlled by the ethylene(ethene), a gaseous plant growth substance produced more rapidly in the submerged parts of the plants. It collects in these parts because it diffuses only very slowly through the water. The ethylene stimulates the production of gibberellin which stimulates cell division & cell elongation. • There are seven major kinds of plant hormones: auxin, • cytokinins, gibberellins, brassinosteroids, • oligosaccharins, ethylene, and abscisic acid.

21. The stems of the rice plant contain loosely packed cells forming a tissue known as aerenchyma. Gases are able to diffuse through the aerenchyma to the other parts of the plant , including those under water . This is supplemented by air that is 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 leave surface. • Neverthless ,the cells in the submerged roots do still have to respire anaerobically at least some of the time. This produces Ethanol which can build up in the tissues. Ethanol is toxic ,but the cells in rice roots can tolerate much higher levels than most plants. They also produce more alcohol dehydrogenase , which break down ethanol. This allows the plant to grow actively even when O2 is scarce , using energy from anaerobic respiration.

23. Crop Improvement • The characteristic of a crop plant can be affected by artificial selection & selective breeding. But although the early farmers knew nothing of genes & inheritance ,they did realize that characteristics were passed on from parents to offspring . They will have picked out the best plants that grew in one year, allowing them to breed & produce the grains for next year This has brought about great changes in the cultivated varieties of crop plants ,compared with their wild ancestors. • Today, selective breeding continues to be the main method by which new varieties of crop plants are produced. In some cases, however gene technology is being used to alter or add genes into a species in order to change its characteristics.

24. The origins of wheat • A wild wheat called einkorn, triticumurartu, grows in fertile crescent . Long before human began to cultivate it ,this cereal plant for a close relative of it hybridised with another grass ,wild goat grass, Aegilopsspeltoides. • Both of these plants were diploid with 14 chromosomes- two sets of seven. So the hybrid plant also had 14 one set from einkorn and one set from wild goat grass. Like most hybrids these plants would have been infertile. This happens because ,when meiosis is attempted ,the two sets of chromosomes , cannot match up with one another sufficiently to form bivalents. And if meiosis cannot take place ,gametes cannot be formed.

26. However ,in just a few cases, a kind of faulty meiosis must have happened .Even though the chromosomes did not pair up with each other , they somehow all stayed at one end of the dividing cell , so that one daughter cells got no chromosomes & another got all 14. This produces a diploid cell that could become a gamete . If this happens in two plants then the result can be male and female gametes that are diploid-they have two sets of chromosomes instead of one. They can fuse to form a tetraploid zygote with four sets of chromosomes. • The resulting tetraploid hybrid is called emmer wheat, triticumturgidum . Emmer like ,most polyploids ( organisms with more than two complete sets of chromosomes) is more vigorous than its diploid parents it grows larger and produces larger ears of grains

27. While emmer wheat & einkorn were growing in farmers fields another hybridisation occurred. This time ,the tertaploid emmer wheat crossed with another kind of diploid wild goat grass Aegilopstauschii. The resulting wheat was hexaploid with six sets of chromosomes – 4 sets from emmer wheat & two sets from wild goat grass . It is called Triticumaestivum. Being hexaploid , they produce even larger ears than the tertraploid emmer wheat. Durum wheat triticum durum is tetraploid .

28. Today selective breeding has produced many different varieties of wheat . Much of it is grown to produce grains rich in gluten, which makes them good for making bread flour. • Wheat plants have now much shorter stems than they did 50 years ago. This makes them easier to harvest & have higher yield (because they put more energy into making seeds rather than growing tall ). It also makes them less susceptible to being knocked flat by heavy rains & produce less straw ,which has little value and costs less money to dispose of.

29. Inbreeding & Hybridisation of maize • If maize are inbred (crossed with other plants with genotype like their own),the plants in each generation become progressively smaller & weaker. This is because in maize , homozygous plants are less vigorous than heterozygous ones. Outbreeding –crossing with others , less closely related produce heterozygous plants that are healthier , grow taller & produce higher yields. • However if outbreeding is done at random ,the maize plant will have lot of variation between the individual plants. This would make things impossible for the farmers. To be able to harvest and sell his crops easily , a farmer needs the plants to be uniform. They should all be about the same height & all ripen at the same time. • So the challenge when growing maize is to achieve both heterozygosity & uniformity. Farmers buy maize seeds from

30. companies that specialize in using inbreeding to produce homozygous maize plants ,& then crossing them . This produces F1 plants that all have the same genotype. There are many different homozygous varieties &different crosses between them can produce a large number of different hybrids, suited for different purposes. Every year thousands of new maize hybrids are trialled, searching for varieties with characteristics such as high yield, resistance to more pests & diseases & good growth in nutrient –poor soil or where water is in short supply.

31. Herbicide resistant oil seed Rape • Oil seed rape( Brassicanapus) is grown in many parts of the world as a source of vegetable oil which is used as a biodiesel fuel , as a lubricant & in human and animal food. • Natural rape seed oil contains substances (erucic acid and glucosionolates) that are undesirable in oil that is to be used in human or animal food . • A hybrid, bred in Canada to produce low concentration of these undesirable substances , was called canola (CANadian Oil Low Acid) and this name is now often used to name any variety of oil seed rape.

32. Gene technology has been used to produce herbicide- resistant strains . Growing a herbicide -resistant crop allows field to be sprayed with herbicide after the crop has germinated, killing any weed that would otherwise compete with crop with space, light , water or ions . This increases the yield of the crop. • Oil seed rape that is resistant to the herbicide glyphosate, or to the related glufosinate, is grown in a number of countries.

33. Glyphosate inhibits an enzyme involved in the synthesis of three amino acids ; phenylalanine , tryosin &tryptophan. • Glyphosate is absorbed by a plant’s leaves and is transported to the growing tips. The amino acids are needed for producing essential proteins , so the plant dies. Various microorganisms have versions of the enzymes involved in the synthesis of phenylalanine , tryosin and tryptophan that are not affected by glyphosate. • The gene that transfers into crop plants come from a strain of the bacterium Agrobacterium

34. The most likely detrimental effects on the environment of growing a herbicide -resistant crop are that: • The genetically modified plant may become an agricultural weed. • Pollen will transfer the gene to wild relatives producing hybrid offspring that are invasive weeds. • Herbicide –resistant weeds will evolve because so much ifthe same herbicide is used.

35. The results of investigation to compare invasiveness of normal and genetically modified oil seed rape were done . Three genetic lines were compared: non engineered oilseed rape and two different genetically engineered versions of the same cultivar. The rate of population growth increase were compared in plants grown in a total of 12 different environments. The environments differed in the presence and absence of cultivated and uncultivated background vegetation, the presence and absence of different herbivores and pathogens.

36. There was no evidence that genetic engineering increased the invasiveness of oil seed rape plants. Where differences between normal and genetically engineered plants existed ,the genetically engineered plants were slightly less invasive than the unmodified ones,. • The risk of pollen transfer , by wind or by insect is real .Oil seed rape inbreeds easily with two related species , wild radish & wild turnip. • Its flowers are adapted for insect pollination but are also pollinated by wind. • Although safe planting distance are specified for trials of genetically modified plants. • Safe planting distance should be increased to more than 4000m to allow the organic farming

37. industry to maintain its ‘GM- free’ certification. Experimental crosses between glufosinate resistant oil seed rape & both wild radish and turnip have shown that resistance can be passed to the hybrid offspring & that it persists through several generations of their offspring. However , there is as yet little evidence of this occurring outside.

38. Insect resistant maize and cotton • Maize is protected against corn- borer which eats the leaves of the plants and then burrows into the stalk , eating its way upwards until the plant cannot support the ear. • Cotton is protected against pests such as boll weevil. • In both plants yield is improved . The most likely detrimental effects on the environment of growing an insect – resistant crops are • The evolution of resistance by the insect pests. • A damaging effect on other species of insects. • The transfer of the added gene to other species of plant.

39. However, less pesticide is used, reducing the risk of spray carrying to and affecting non –target species of insects in other areas. Only insects that eat the crops are effected. • A gene for a toxin , Bt toxin , which is lethal to insects that eat it but harmless to other animals, has been taken from a bacterium , Bacillus thuringiensis.Different strains of B. thuringiensisproduce different toxins that can be used against different insect species. • Crop plants that contain the Bt toxin gene B. thuringiensis produce their own insecticides.

40. However, the insect populations can evolve resistance to these toxins. The danger is that large numbers of crop plants containing the genes for toxins may simply accelerate the evolution of resistance to the toxins. • For sometime, it has become necessary for the growers to plant up to 50% of their maize as non- genetically modified maize in so called’ refuges’ • Bt resistance in corn borer happens to be a recessive allele. Adult corn borers in refuges are mostly homozygous dominant or heterozygous. • These insects supply the dominant alleles to counteract resistance when adult corn borers

41. from fields and refuge mates. The pollen of Bt maize expresses the gene and has found to disperse at least 60m by wind. • In the USA , milkweed frequently grows around the edge of maize fields and is fed upon by the caterpillars of the monarch butterfly. • An experiment was set up in which caterpillars were fed with milkweed leaves dusted with pollen from Bt maize , pollen from unmodified maize or leaves with no pollen at all. • Caterpillars survival after 4 days of feeding on the leaves dusted with pollen from Bt maize was 56%, whereas, no caterpillar died after eating leaves dusted with pollen from unmodified maize or

42. leaves with no pollen. • There is thought to be a danger , if Bt maize is grown in Mexico, it can pollinate its wild ‘parent’ species , teosinte and transferring genes to it. However, it has been found that maize pollen, once released from the anthers and exposed to the air ,is no longer viable after 2 hours. This requires a two-hours wind drift distance between a genetically modified crop and any teosinte habitats • There has been some evidence of reduced populations of microorganisms in the soil in which Bt maize has been growing.

43. Disadvantages of using genetically modified seeds • Genetically modified crop seeds are very expensive & that its cost may remove the advantages of growing resistant crops . • Growers need to buy seeds each season , which again keeps cost high when compared with those of traditional varieties • In parts of the world where a great deal of genetically modified crop is grown , there is a danger of losing biodiversity.

44. Golden Rice • Rice is a staple food in many parts of the world . Where people are poor rice forms the major part of their diet. Deficiency of vitamin A is a common & serious problem .It can cause blindness. • Vitamin A is a fat- soluble vitamin found in oily fish and dairy products . It is also made in our bodies as carotene ,the orange pigment found in carrots. Vitamin in present in the aleurone layer of rice grains but not in the endosperm. • The aleurone layer is removed from rice when it is polished to produce white rice. • Brown rice still contains aleurone layer . In tropical countries ,the aleurone layer goes rancid if the rice is stored for any length of time , which is why white rice is produced & eaten instead.

45. Gene for the production of vitamin A were extracted from daffodils & the bacteriumErwiniauredovora. • The genes , together with promoters were inserted into plasmids. • The plasmids were inserted into bacteria called Agrobacteriumtumefaciens. • These bacteria naturally infect plants & so could introduce the genetically modified plasmid into rice cells.They were mixed with rice embryos in petri dishes some of which were infected by bacteria carrying the vitamin A genes. • The rice embryos , now containing the vitamin A genes , were grown into adult plants . They produce seeds containing vitamin A in their endosperm.

46. The genetically modified rice is called Golden rice, because it contains a lot of the orange pigment carotene . • Finally ,the genetically modified rice was bred with other varieties of rice ,to produce varieties that would grow well in the conditions in the different parts of the world. • There has been a lot of controversy over golden rice . Several NGO’s have condemned it as being wrong way to solve the problem . They say that main reason that people eat diets that are short of vitamin A is poverty ,& that the way to solve it is to help them out of poverty so that they have access to more varied diet. • They suggest that, if people can afford to buy & grow golden rice ,they can afford to buy better food anyways. • Therefore ,the new rice will not help the people who

47. really need help- the ones who are so poor that they survive on diet made up largely of white rice. • Others say that ,although it would be better if we could somehow lift these people out of poverty ,this cannot be quickly achieved. The scientists agree that they need to solve the root cause of poor diets –which include numerous political , cultural & economic issues- but argue that ,in the meanwhile ,golden rice could help million people to avoid blindness.