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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Auxin and Growth
Plants might not have brains, but they still need to coordinate their growth and responses to the environment. Instead of a nervous system, plants use chemical messengers called hormones. These special chemicals help plants respond to light, gravity, touch and other stimuli in their environment.
Key Definitions:
Plants need to coordinate their growth and development to survive in changing environments. Unlike animals, they can't move away from danger or towards resources. Instead, they adjust their growth patterns. Plant hormones are the chemical messengers that make this coordination possible.
Plant hormones are produced in tiny amounts but have powerful effects. They can be transported around the plant to affect cells far from where they were made. Different hormones control different aspects of plant growth and development, including responses to environmental stimuli.
Auxin was the first plant hormone to be discovered. The most common natural auxin is called indole-3-acetic acid (IAA). It's mainly produced in the tips of shoots and roots and plays a crucial role in controlling how plants grow.
Auxin stimulates cell elongation by making cell walls more stretchy. When auxin reaches cells, it triggers a process that loosens the cell walls, allowing the cells to absorb water and expand. This is how plants grow taller or bend towards light.
The effects of auxin depend on:
In shoots, moderate concentrations of auxin promote cell elongation, causing growth. This helps plants grow taller towards light.
In roots, the same concentration of auxin actually inhibits growth. This different sensitivity helps roots grow downward into soil.
Auxin moves through the plant in one direction (polar transport), usually from the tip downwards, helping to coordinate growth responses.
Phototropism is the growth of a plant towards or away from light. Most shoots show positive phototropism (growing towards light), while roots often show negative phototropism (growing away from light).
When light hits a plant from one side, auxin moves to the shaded side of the shoot. This causes the cells on the shaded side to elongate more than those on the lit side, making the shoot bend towards the light.
The process works like this:
In the early 1900s, scientists performed experiments on grass seedlings to understand phototropism. They used the coleoptile (a protective sheath covering emerging shoots) of oat seedlings. Here's what they discovered:
These experiments were crucial in discovering how plants respond to their environment using chemical signals.
Gravitropism (also called geotropism) is how plants respond to gravity. Roots show positive gravitropism (growing towards gravity), while shoots show negative gravitropism (growing away from gravity).
Plants detect gravity using specialized cells containing starch grains called statoliths. These dense particles settle at the bottom of cells, indicating which way is down. This triggers a redistribution of auxin:
If you place a potted plant on its side, you'll notice that within hours the shoot begins to bend upwards, while the roots bend downwards. This is gravitropism in action! The plant is using auxin to coordinate its response to gravity.
Scientists have studied plant growth in space, where there is microgravity. Without gravity, plants grow in random directions. This research helps us understand how important gravitropism is for normal plant development on Earth.
Understanding how auxins work has led to many practical applications in agriculture and horticulture.
Scientists have created synthetic auxins that mimic the effects of natural auxin. These are used in various ways:
Synthetic auxins are used in rooting powders to promote root growth in plant cuttings, making it easier to propagate plants.
Auxins can be sprayed on flowers to stimulate fruit development without pollination, creating seedless fruits.
High concentrations of synthetic auxins (like 2,4-D) are used as selective weedkillers, as they cause abnormal growth that kills broad-leaved plants but not grasses.
Commercial banana production relies heavily on auxins. Most bananas in shops are seedless varieties that have been treated with auxins to develop without pollination. This is why commercial bananas don't have the large, hard seeds found in wild bananas. Auxins are also used to control the ripening of bananas after harvest, ensuring they reach supermarkets at the perfect stage of ripeness.
Scientists use various experiments to study how plants respond to stimuli. Here are some classic experiments you might perform in school:
Growing seedlings in a box with a single light source coming from one side allows you to observe phototropism. You can measure the angle of bending over time to quantify the response.
Germinating seeds on damp paper towels and then turning them sideways allows you to observe how roots and shoots respond to a change in the direction of gravity.
When investigating plant responses, it's important to control variables like temperature, water and light (except for the variable being tested). This ensures that any changes observed are due to the factor being investigated.
Collecting quantitative data (measurements) rather than just qualitative observations makes your experiments more scientific. For example, measure the angle of bending or the length of growth in different conditions.
Auxin is a crucial plant hormone that helps coordinate plant growth and development. It enables plants to respond to their environment through processes like phototropism and gravitropism. By redistributing auxin within plant tissues, plants can bend towards light or respond to gravity. Humans have harnessed the power of auxins for various agricultural applications, from rooting powders to weedkillers.
Understanding how auxins work helps us appreciate the sophisticated ways plants coordinate their growth without a brain or nervous system. It also allows us to develop better agricultural practices and technologies to improve crop production.
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