RHS Level 2 Certificate Year 1 Week 8


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RHS Level 2 Certificate Year 1 Week 8

  1. 1. RHS Level 2 Certificate Year 1 Week 8 – pollination, fertilization and plant breeding
  2. 2. Learning outcomes <ul><li>10.1 Define the term: ‘pollination’. </li></ul><ul><li>10.2 Define the terms: ‘cross-pollination’ and ‘self-pollination’. </li></ul><ul><li>10.3 State two characteristics of wind pollinated plants and two characteristics of insect pollinated plants (review) . </li></ul><ul><li>10.4 Define the term: ‘fertilisation’. </li></ul><ul><li>10.5 Define the terms: ‘incompatible’ and ‘compatible’ in relation to fertilisation. </li></ul><ul><li>10.6 Define the term: ‘parthenocarpy’ and state its horticultural significance. </li></ul><ul><li>10.7 State the horticultural significance of triploid cultivars. </li></ul><ul><li>10.8 State the significance of F1 hybrid seeds and explain the term hybrid vigour </li></ul>
  3. 3. Pollination <ul><li>The transfer of pollen from the anther to the stigma.. </li></ul><ul><li>Carried out by insects, wind, water, animals </li></ul><ul><li>Flowers show particular adaptations for their particular mode of pollination. </li></ul><ul><li>Role in sexual reproduction – hybridization. </li></ul><ul><li>Cross and self- pollination. </li></ul><ul><li>Compatibility – mechanisms to ensure that only compatible DNA reaches the ovule. </li></ul>
  4. 4. Fertilization – stage 1 <ul><li>In flowering plants the process is called ‘double fertilization’ – because two cells in the ovule are involved. </li></ul><ul><li>Two cells in the pollen grain (each with one set of chromosomes). </li></ul><ul><li>Following pollination, chemical stimulation of pollen grains by secretions of the stigma commences fertilization. If the pollen is compatible, one cell from the pollen grain divides and forms a pollen tube. The pollen tube grows down the style. </li></ul><ul><li>The second cell divides into two identical cells– one new cell becomes the tube nucleus which moves to the tip of the pollen tube. The other new cell divides again to produce two sperm cells containing the male parent’s DNA </li></ul><ul><li>The pollen tube finds a tiny opening in the ovule and enters it, guided by the tube nucleus. The sperm cells enter the ovule. </li></ul>
  5. 5. Fertilization – stage 2 <ul><li>Within the ovule is an egg cell and another cell .One of the two sperm cells unites with the egg cell to produce a zygote or fertilized egg. This will develop to produce the plant embryo within the seed. </li></ul><ul><li>The other sperm cell unites with the other ovule cell to form the endosperm. The endosperm provides food for the development of the embryo. The endosperm also provides food for the initial growth of the plant following germination; either by remaining within the seed (as in monocotyledons) or by transferring food into the cotyledons (as in dicotyledons). </li></ul>
  6. 6. Compatibility <ul><li>Only pollen from the same species (or possibly species in the same genera) will be able to fertilize the ovule. This pollen is compatible. </li></ul><ul><li>However many plants seek to avoid self fertilization by ensuring (whether through male and female organs - or male and female flowers - that mature at different times, dioecious habit, or chemical incompatibility) that the pollen from the same plant does not fertilize the ovule. This pollen is therefore incompatible. Some varieties of the same species may be incompatible- this is significant in top fruit. </li></ul><ul><li>Self fertilization means no genetic mixing in the DNA of the seeds which limits variation. </li></ul>
  7. 7. A bit of genetics -1 <ul><li>Chromosomes are the genetic information that is held in a cells nucleus. The chromosomes contain information from both parent plants. They occur in pairs and are made up of genes. </li></ul><ul><li>We know that in cell division for growth identical daughter cells are produced. Each of these has it’s own nucleus which contains an exact replica of both sets of chromosomes from the original cell. </li></ul><ul><li>Cell division for growth is called mitosis. </li></ul>
  8. 8. A bit of genetics - 2 <ul><li>When cells divide to make egg and sperm cells something different happens. The egg and sperm cells have only one set of chromosomes containing half the usual number (which varies between species). The division is random so each sperm or egg cell can possess a different assortment of genetic information from the next. This sort of cell division is called meiosis . </li></ul><ul><li>When the egg and sperm cells fuse they put together their single sets of chromosomes to give the correct full number of chromosomes again. However the resulting embryo has a random mix of genetic characteristics from each parent plant. This gives rise to many variations in cross pollinated plants and is how new varieties arise. Each seed from a single plant will be slightly different . </li></ul>
  9. 9. Why does the genetics matter? Part 1 <ul><li>To the plant genetic variation in offspring is a vital insurance for survival of changed circumstances. To the plant breeder trying to produce stable, consistent plants for sale it can be a nightmare (buyers will not be happy if their dwarf, blue lupin turns out to be 5 foot tall and pink!). </li></ul><ul><li>Each characteristic is the result of a pair of genes carrying the genetic information for that characteristic (e.g. flower colour, height etc). If the two genes are the same the characteristic is clear. However each gene of the pair may be different e.g. one carrying information for a red flower and the other for white. </li></ul><ul><li>We need to appreciate that some genes are dominant – that means that if they are present then the physical appearance of the plant is determined by that gene for that characteristic. So if the red colour gene is dominant the flower will be red, even if the other gene of the pair carries the white flower information. </li></ul>
  10. 10. Why does the genetics matter? Part 2. <ul><li>We have seen that some genes are dominant. Those that are not dominant are known as recessive genes. For each gene there is the possibility of four combinations (because there are two pairs of genes for each characteristic – one pair in the mother and one in the father – which are split when the egg or sperm cell is produced.) </li></ul><ul><li>If each parent has a dominant red flower and a recessive white flower gene then three of the resulting combinations will include the dominant gene and one will have a pair of recessive white flower genes. Three of the offspring will have red flowers and one white. This three to one ratio tells the plant breeder that both parents have dominant red flower genes. If the offspring were half white flowered and half red then only one parent has the dominant red gene. </li></ul>
  11. 11. F 1 Hybrids <ul><li>Plant breeders want the plants they grow to be predictably the same but also to create new varieties. They do this by identifying a parent that has two dominant genes for the desired feature (red flowers) and the other parent that has one dominant and one recessive. They then cross (by hand pollination) the parent strains. </li></ul><ul><li>All of the offspring will have one dominant gene for red flowers so the breeder can be certain they will be the same in this respect. The offspring are called F 1 hybrids – F 1 means ‘first filial generation’. </li></ul><ul><li>However if the offspring reproduce by seed then their offspring (F 2 ) will not necessarily all have red flowers. If the other parent plant has one dominant (red) and one recessive gene (white) then the offspring will be three to one red to white. This is why it is said that F 1 hybrids do not ‘come true from seed’. </li></ul>
  12. 12. Hybrid vigour <ul><li>The genetic mixing in hybrids often leads to the offspring being stronger or more disease resistant than the parents. This may be due to the distribution of such dominant genetic characteristics in the offspring or as a result of the particular interactions between the gene pairs. F1 hybrids often have larger flowers or fruit than their parents which is why they are desirable, despite being difficult and expensive to produce. </li></ul>
  13. 13. Triploids <ul><li>We have seen that cells produced for growth have two sets of chromosomes (these are called diploid cells) and cells produced for reproduction have only one (haploid cells). </li></ul><ul><li>It is possible to have more than two sets of chromosomes in a cell. A triploid plant (produced when cell division is incomplete in the ovule and fertilization then takes place) has three sets of chromosomes in each cell. Such plants are often more vigorous and produce larger flowers and fruit than their parents. However they are often difficult to fertilize or may be sterile. The apple M. domestica ‘Bramley’s Seedling’ is a triploid plant and does not produce fertile pollen. It is therefore a poor pollinator (self or cross) which is why another pollinator is required in orchards which have Bramley’s in them. </li></ul>
  14. 14. Parthenocarpy (literal meaning ‘virgin fruit’) <ul><li>In many plants it is possible for fruit to form without fertilization taking place. This is known as parthenocarpy. </li></ul><ul><li>Parthenocarpy is encouraged as a trait in some varieties of fruit and breeding produces varieties that are solely parthenocarpic – for example Musa acuminata (dessert banana) as the seeds are large and hard in fertile varieties. </li></ul><ul><li>However parthenocarpy means that the variety can only be propagated vegetatively. This led to one of the major varieties of banana being wiped out by disease – all the plants were clones so none had resistance. </li></ul><ul><li>In other seedless fruit such as seedless grapes pollination and fertilization proceed as normal but the embryo does not develop and the seeds are tiny or absent. Strictly speaking this is not parthenocarpy. </li></ul>
  15. 15. Learning outcomes <ul><li>10.1 Define the term: ‘pollination’. </li></ul><ul><li>10.2 Define the terms: ‘cross-pollination’ and ‘self-pollination’. </li></ul><ul><li>10.3 State two characteristics of wind pollinated plants and two characteristics of insect pollinated (review) plants. </li></ul><ul><li>10.4 Define the term: ‘fertilisation’. </li></ul><ul><li>10.5 Define the terms: ‘incompatible’ and ‘compatible’ in relation to fertilisation. </li></ul><ul><li>10.6 Define the term: ‘parthenocarpy’ and state its horticultural significance. </li></ul><ul><li>10.7 State the horticultural significance of triploid cultivars. </li></ul><ul><li>10.8 State the significance of F1 hybrid seeds and explain the term hybrid vigour </li></ul>