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Horticulture 1000
HT1091 - Plant propogation 1 - Plant breeding
Thinning seedlings Plant Propagation 2
7

Plant breeding

The fact that sexual reproduction in plants can result in variation in the offspring is useful for plant breeders. They can select plants with the characteristics they want. They can then deliberately cross-pollinate two selected parent plants to try to produce offspring with the most desirable characteristics of both parents.

The objective of a recent breeding programme in pears was to breed juicy, crisp, flavourful fruit. To achieve this, crisp Asian pears called Nashi were crossed with highly flavoured European pears.

The plant breeders found all the characteristics they wanted in the different species and were able to select the ones they wanted to use. The resulting new pears are high yielding and have low susceptibility to major fungal diseases. The new pear is crisp and juicy, with high acid, high sugar, high aroma, and ‘a flavour that really knocks your socks off’.

European pears
Asian Nashi pears European pears Asian Nashi pears New pear cultivars Maxie Crispie crossbreed pears Maxie pear cultivars Crispie crossbreed pears

How a breeding programme works

To produce plants with desired qualities or attributes, two closely related plants from the same genus are cross pollinated. The parent plants must be disease free. The resulting offspring is a hybrid. The seeds from the cross are planted and if the resulting plants have the desirable characteristics then they will be propagated to increase their numbers.

Here are the steps a rose grower would follow to breed an orange rose with no thorns, a strong scent and a long stem.

Creating a new hybrid rose

                                                                               
Step 1
    Select parent flower buds both at the same stage of opening.
Step 2
    Take off all the petals.
Step 3
    Use a small pair of scissors to remove all anthers.
Step 4
    Cover with a paper bag.
Step 5
    Pollen from male parent (yellow) is placed on the female parent (red).
Step 6
    Cover with a bag. Secure and label.
Step 7
    New hybrid.
Parent 1
      The seed parent that is to grow and ripen the seed.
      Flower with dark red petals, smooth stem, little scent
      Dark red petals, smooth stem, little scent.
      Leave the flowers on the plant.
This makes the flower easier to work with.
      Take off all the petals
Use a small pair of scissors to remove all anthers This will prevent the flower from being pollinated with undesirable pollen.
Cover with a bag
Pollen from male parent (yellow) is placed on the female parent (red) New hybrid.
New hybrid
New hybrid    

The outcome in some offspring – bright orange:

     
           
  • no thorns
  •        
  • long stem
  •        
  • strong scent.
  •      
Parent 2
      The pollen parent that will pollinate the flower on Parent 1.
      Flower with yellow petals, prickles on a stem, thorns on the stem, strong scent
      Yellow petals, prickles on a stem, thorns on the stem, strong scent.
      Cut the flower and place it in water for a few days.
This makes the flower easier to work with.
      Take off all the petals
    Brush pollen from male parent onto female parent  

Producing hybrid seed is difficult and time-consuming therefore the seed is expensive. Hybrid seed will germinate faster, more evenly and the plants will be more vigorous and higher yielding.

The resulting hybrid plants in a crop will all be ready for harvest at the same time, which is useful if it is a crop that needs to be harvested all at once. Peas to be processed and frozen or apples for an export market are both examples of crops that need to be harvested all at once.

Complete Activity 7A in your workbook.

Genetics

The characteristics of the parent plants used previously to create the hybrid rose could result in a hybrid that could have:

  • dark red petals or yellow petals
  • smooth stem or prickles on the stem
  • little scent or strong scent. In fact the hybrid has:
  • no thorns
  • long smooth stem
  • strong scent.

However, the colour of the petals are neither red nor yellow. Instead they are orange.

Genes control all the features or characteristics of a hybrid plant. They control how a plant grows and what it looks like. Genes are the units of inheritance found inside every living cell. Genes are located on the chromosomes which are found in the nucleus of a cell.

Chromosomes within a plant

If you could look into the nucleus of a cell not only would you see the genes located on the chromosome but you would also see that the chromosomes come in pairs. One chromosome comes from one parent plant and the other chromosome from the other parent plant.

The genes in the same place in a chromosome pair control the same characteristics but these genes may not be identical. The gene therefore comes in two forms. The term allele (a-leel) is used to describe a different form of a gene that describes the same feature for example flower scent – weak or strong.

Let's look at the hybrid rose again. Scientists use letters to represent the alleles for a gene for example
  S = the strong scented gene
  s = the weak scented gene.

Gametes

The sex cells of the parent plants are called gametes (gam- eats). Pollen formed in the male part of the flower is the male gamete. The egg(s) (ovum) formed in the female part of the flower is the female gamete.

Punnett squares

Punnett squares help to show how the alleles are passed down from the parents to the offspring. Remember each parent will have two alleles for each feature. Only one of these alleles from each parent is passed on to the offspring.

                       
Parent Parent rose with strong scent (male gamete–pollen) Parent rose with little scent (female gamete–egg)
Alleles S S s s

The Punnett square will show how the alleles come together in the new plant. The gametes go across the top and down the side of the Punnett square.

                                               
  Alleles from pollen of a strongly scented rose
  S S
Alleles from ovum of a weakly scented rose s Ss Ss
s Ss Ss

The shaded boxes show the possible combinations of alleles for the offspring. These allele combinations are called the genotype. In this example they are all the same genotype Ss.

What the actual plant looks like or in this case what the roses smell like, is called the phenotype. The phenotypes for all the offspring are roses with a strong scent. But why is this when each individual has an allele for a strong scent and an allele for little scent? This is because the allele for the strong scent is dominant to the allele for little scent, which is recessive.

This means that when the two alleles (Ss) are together in the same plant the roses will always be strong scented since the strong scent allele is the dominant one. That’s why it is given a capital ‘S’ and the recessive allele is a small ‘s’.

The different combinations of alleles have different names.

SS = homozygous dominant – both alleles are the same, both are dominant.
  Ss = heterozygous , the alleles are different – one dominant and one recessive.
  ss = homozygous recessive – both alleles are the same, both recessive alleles.

What happens if two of the hybrid roses are crossed? This is a Ss x Ss cross.

Now the Punnett square:

Remember the alleles from the gametes go across the top and down the side of the square.

                                   
  S s
S SS Ss
s Ss ss

There are three possible genotype combinations SS, Ss and ss. One is homozygous dominant (SS), two are heterozygous (Ss), and one is homozygous recessive (ss).

The Plant breeder can expect certain phenotype and genotype proportions or ratios in the offspring.

The phenotypes show that there will be a ratio of three strong scented roses to one that is lightly scented.

Phenotype ratio 3:1 scented to non-scented

Genotype ratio is 1:2:1 (SS:Ss:ss)

Remember:

  • SS, Ss, Ss, are all strong scented because they have the dominant allele
  • ss are roses with little scent because they have two recessive alleles.

Let’s look at the cross that produced the orange coloured petals. One parent had red petals (RR) and the other yellow (rr).

This is the Punnett square:

                                                   
  Alleles from the female egg
    R R
Alleles from the pollen r Rr    orange Rr    orange
r Rr    orange Rr    orange

You can see that all the offspring have the genotype Rr. However, the phenotype, that is what they look like, are all roses with an orange colour. This is because neither the yellow nor the red-coloured allele is the dominant allele. So the two mixed together produce roses with an orange petal.

Complete Activity 7B in your workbook.

F1 hybrid seed

Some plants are able to self-fertilise over a number of generations. Seeds from these plants will remain true for many generations. They are homozygous for their various characteristics, and so are called pure breeding or pure strains.

Pure strain of plant

By taking the pollen from one plant of a pure strain and putting it on the stigma of another pure strain the offspring will continue the same new mixture of genes and will all be similar. The new plants are called F1 hybrids.

These plants generally grow more strongly than their parents. This is called hybrid vigour. They have advantages because of hybrid vigour. If F1 hybrids go to seed their characteristics get mixed up again and the offspring will be different.

Key points   Key points

  • Plants that are genetically closely related can be  cross pollinated to produce some offspring with the best characteristic of both  parents.
  • F1 hybrid plants will have uniform characteristics.  They will be more vigorous and produce a higher yield than either parent.
  • A geneotype with two genes the same is homozygous.
  • A geneotype with both a dominant and recessive gene is  called heterozygous.
  • The phenotype describes what a plant will look like as  result of its genotype.

Assessment

Complete the teacher marked Activity in your workbook.

What's next?

  1. Complete the self-assessment form and cover sheet.
  2. Return your workbook to your teacher.
  3. Continue with HT1092 Plant propagation 2 next.
Thinning seedlings Plant Propagation 2