Wheat

Dwarf breeds of plants

Much of the wheat you eat comes from dwarf breeds. Calum Wiggins explains how these are developed, what other foods they provide, and the pros and cons of growing them

Dwarf breeds are varieties of plants that are smaller than normal for their species. They stay small because they produce lower levels of certain hormones – often called growth factors or growth substances because of the role they play in development.

Gibberellins, one of the five major classes of plant hormones, cause a plant’s stem to grow longer. Because of a genetic mutation, dwarf plants produce less gibberellin and their stems do not grow as long as normal.

The benefits

Between the 1930s and late 1960s, the technologies used in farming dramatically improved. This significantly increased worldwide food production and saved a huge number of people from starvation. One of the most important developments was that scientists bred rice and wheat plants that produced more food. Many of these plants were either dwarf breeds or semi-dwarf (smaller than normal but bigger than dwarfs).

Wheat plants were bred to absorb more nitrogen from the soil and grow larger ears of grain. However, these large ears were too heavy for the plants’ long stems, causing them to bend until the grain touched the ground. The wet soil would spoil the wheat. An American biologist called Norman Borlaug discovered that he could selectively breed semi-dwarf plants with shorter stems. This made them more stable and able to support the larger ears of grain.

The shorter stem also requires less energy from photosynthesis to grow, meaning the plant devotes more energy to growing the grain. The same is true of nutrients from the soil, which increases the effect that fertilisers have on the amount of grain produced by each plant.

Borlaug’s discovery doubled the amount of wheat produced by Pakistan and India between 1965 and 1970 and greatly improved food security across the world. He was awarded the Nobel Peace Prize in 1970 in recognition of his contribution to world peace through increasing food supplies.

Apple orchards today contain trees that are almost all dwarf or semi-dwarf. This makes the fruit easier to pick and reduces the distance nutrients have to travel from the roots up to the leaves and buds.

The dwarf trees in orchards are often made using grafting. Grafting is a technique where the tissue from one plant is inserted into another. Often, fruit trees are made by grafting together the parts of three different plants: onto the trunk from a dwarf plant are grafted roots with good stability and resistance to diseases in soil, and branches from a plant which produces the best fruit. This produces a tree with the best characteristics from all three plants.

Rice is one dwarf variety being investigated for use by astronauts to grow food in space. Others include wheat, tomatoes, peppers, soybeans and peas.

One of the many challenges that space agencies have to overcome if they want to send astronauts further than ever before is providing food for these longer missions. Rather than sending ready-made food, it would be more efficient if astronauts could grow their own food in the spacecraft. Plants could also be used to purify water and recycle carbon dioxide to oxygen. Dwarf breeds are well-suited to being grown on spacecraft because they need less room to grow than standard plants.

Developing dwarf breeds

Whether they’re grown on Earth or in space, scientists can use selective breeding and genetic engineering to develop plants with defective gibberellin genes.

Selective breeding is an ancient method for develop plants (or animals) with particular traits. Breeders choose two parent plants with desirable characteristics and make them reproduce. The parent plants’ DNA is passed on to the offspring, along with the characteristics. Dwarf breeds can be developed over many generations by breeding parent plants that produce relatively low amounts of gibberellin.

Since the 1970s, scientists have been able to alter organisms’ DNA directly using genetic engineering. Genetic engineering is a set of techniques which includes: copying a piece of DNA from one organism and inserting it into the genome of another; removing specific genes from an organism’s genome; or causing a particular gene to mutate. Genetic engineering is now used to produce dwarf plant breeds.

Dwarfing can also occur in nature if a plant’s environmental conditions aren’t to its liking. The type of conditions that can limit how large a plant grows are poor soil, low light, drought, cold or infection. Plants that have been dwarfed due to environmental conditions are said to be stunted.

The downsides

Dwarf breeds of crops are typically grown in monocultures, meaning a single breed or type of crop is grown on a specific piece of land. Because dwarf plants have been bred and selected for their specific characteristics, it makes sense to only grow the species and breed best suited to a particular location. Dwarf plants are also less able to compete against other plants for light, because they are shorter.

There are advantages to monocultures. For example, they make planting, maintenance and harvesting simpler and therefore cheaper. They also can produce more food from a specific piece of land over the short term. However, there are also disadvantages.

Different breeds need different nutrients. Growing only one breed in a particular location means some nutrients are used up more than others. This can lead to the soil running low in particular nutrients, forcing farmers to use chemical fertilisers.

The single breed grown in the monoculture may also be particularly susceptible to a certain pest or disease. These can spread more quickly in monocultures because every plant is affected by them. To prevent this, farmers may be more likely to use pesticides on monocultures – which can be expensive and also cause pollution.

If multiple breeds are grown alongside each other on a piece of land, this is called a polyculture. Polycultures are more difficult to plant, maintain and harvest. However, they make the best use of all the nutrients in the soil, and reduce the crops’ susceptibility to disease and pests – which means a more secure supply of food. Polycultures also increase biodiversity in the area, and many experts believe polycultures are more environmentally friendly.

Lead image:

Sleepy Claus/Flickr CC BY

References

Questions for discussion

  • What would happen if a plant produced more than the normal amount of gibberellin and when might this be beneficial?
  • What is another example of selective breeding in plants? What about in animals?
  • What other characteristics would suit plants grown in space?
  • What other technological advancements have made agriculture more productive since the 1930s?
  • Why is genetic engineering controversial?

Further reading

About this resource

This resource was first published in ‘Plants’ in May 2016.

Topics:
Genetics and genomics, Ecology and environment, Biotechnology and engineering
Issue:
Plants
Education levels:
14–16, 16–19, Continuing professional development