Improvement of Horticultural Crops

1
Improvement of Horticultural Crops

• Objectives of the next two lectures
• Learn how selection of agricultural plants
  differs from natural selection
• Distinguish between variation caused by
  environmental and genetic factors
• Learn the difference between simply inherited and
  quantitative traits
• Examine methods used to improve horticultural
  crops

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Plant improvementan ancient art

• Development of agriculture requires
  identification of plants that can be cultivated,
  harvested and stored reliably

Cave painting from North Africa showing women
harvesting grain, approximately 6,000 years ago
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Plant improvementan ancient art

• Productivity and quality of crops has been
  improved continuously since the start of
  organized agriculture about 10,000 years ago

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Plant improvementan ancient art

• What traits have been improved?
• Yield, especially for agronomic crops (corn,
  wheat, soybeans, etc.)

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Plant improvementan ancient art

• What traits have been targets for improvement?
• Yield is also important for many horticultural
  crops

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Plant improvementan ancient art

• What traits have been improved?
• Resistance to pathogens, other pests
• Tolerance of environmental conditions
  (temperature, drought)
• Efficient utilization of nutrients (N, P)

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Important horticultural traits

• For horticultural crops, an even wider range of
  traits have been altered by selection and
  breeding
• Quality traits
• Taste
• Color
• Storage quality
• Size and shape

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Important horticultural traits

• For some crops, e.g. tomato, traits that are
  important for mechanical harvesting have been
  modified
• uniform ripening
• thicker skinned
• resistant to bruising

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Important horticultural traits

• Additional traits are important for ornamental
  crops
• flower shape, size, color, longevity
• plant form
• size (dwarf, standard)
• shape (columnar, horizontal)
• foliar characteristics
• variegation, fall color, leaf retention

10
Plant Selection

• Two important points about selection of
  horticultural crops
• Selection is based on the utility of plants to
  the grower
• Selection is performed on a wider range of traits
  in horticultural crops
• This is quite different from "natural selection,
  which is the basis for evolution

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Plant Selection

• Charles Darwin proposed natural selection as the
  driving force for evolution - survival of the
  fittest
• Plants that are used in agriculture are selected
  for different characteristics than those that
  allow plants to survive in a natural environment

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Plant Selection

• In most cases, plants selected for agricultural
  purposes would not be successful under conditions
  of natural selection

Seeds that are not released from the plant are
not dispersed
Showy double flowers are normally sterile
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Variation

• Variation between individuals is the "raw
  ingredient" for plant improvement
• There is no basis for selection if all
  individuals are the same

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Variation

• Variation between individuals is caused by two
  factors
• Environmental variation
• if the environment in which genetically identical
  plants are grown is varied, these plants may grow
  differently
• this is the basis for most of the lab experiments
• accounts for variation (in yield, quality, etc.)
  between different fields, production areas

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Variation

• Variation between individuals is caused by two
  factors
• Genetic variation
• if plants differ when they are grown under
  uniform conditions, they are likely to be
  genetically different
• look at the different responses observed among
  the chrysanthemum varieties in your experiments

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Genetic variation

• Review of genetics
• The gene is the unit of inheritance
• Genes are encoded by the sequence of bases
  (A,C,G,T) in DNA
• DNA is packaged in chromosomes
• Chromosomes are contained in the nucleus

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Genetic variation

• Review of genetics
• Most genes encode proteins
• Information in genes is converted into protein
• DNA ? RNA ? protein
• Proteins are responsible for carrying out most of
  the essential functions in all organisms

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Genetic variation

• Organisms have lots of genes
• 20 to 30,000 genes in plants
• 30 to 40,000 genes estimated in humans
• Differences between individuals in the sequence
  of bases (A,C,G,T) in genes is the source of
  genetic variation
• different forms of the same gene are called
  alleles

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Genetic variation

• Some alleles are deleterious and have negative
  effects
• In humans, a number of diseases, such as cystic
  fibrosis and sickle cell anemia, are caused by
  genetic defects
• Most alleles (variants) of genes do not have such
  drastic effects

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Basics of genetics

• The genetic information carried by an individual
  is called its genotype
• The appearance and performance of a plant are
  referred to as its phenotype
• Phenotype is determined both by the plants
  genotype (its collection of genes) and by the
  environment in which that plant is grown (where
  those genes are expressed)

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Basics of genetics

• Some traits are controlled by only one or two
  genes and plants can be placed in categories
• Referred to as qualitative, or simply inherited,
  traits
• Flower color was one of the traits used by Gregor
  Mendel

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Basics of genetics

• Most traits are affected by the actions of many
  genes
• Individuals cannot be placed into a small number
  of categories
• Referred to as quantitative traits

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Basics of genetics

• For these traits there is continuous variation
  within the population
• What are examples of quantitative traits?
• In humans
• In plants

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Basics of genetics

• Many traits that are important for production of
  agronomic and horticultural plants are controlled
  by large numbers of genes
• Yield
• Flavor
• Drought tolerance

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Strategies for plant improvement

• Identification of desirable varieties in natural
  populations
• strategy used by early agriculturists
• still used, especially for woody ornamental
  plants which have long generation times

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Strategies for plant improvement

• Sports
• Sports are the result of rare spontaneous
  mutations in cultivated plants that result in new
  plants with desirable horticultural
  characteristics

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Strategies for plant improvement

• Examples of sports include
• Pink-fleshed grapefruit
• Seedless Navel oranges
• Color variants of many apple varieties

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Strategies for plant improvement

• Another type of sport is known as a chimera,
  where only one layer of tissue is altered
• Thornless blackberries have an epidermal layer
  that doesnt produce thorns
• Stable trait if vegetatively propagated
• However, stems produced by propagation from other
  tissues will be thorny

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Strategies for plant improvement

• White Sim carnation is a sport of Red Sim the
  epidermal layer is not colored
• Red flecks on these flowers indicate the
  underlying mesophyll tissue is able to produce
  the red pigments
• Many cultivars with variegated leaf color are
  chimeras that were first identified as sports

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Strategies for plant improvement

• Polyploidy
• Where an organism has more than the normal
  diploid (2n) number of chromosomes
• Why are polyploids useful?
• Polyploids tend to have larger cells, resulting
  in larger fruit
• Strawberries 8n
• Tart cherries 4n
• Winesap apples 3n

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Strategies for plant improvement

• Polyploids also have larger flowers
• Marigold
• Daylily

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Strategies for plant improvement

• Seedless watermelons are triploid (3n)
• Triploid fruit are larger
• Seeds cannot develop properly, remain immature
  and soft
• Produced from a cross between a diploid (2n) and
  a tetraploid (4n)