Biology Tutorials > Control of Growth & Development > Plant Auxins – Phototropism & Geotropism

Plant Auxins – Phototropism & Geotropism

Plant Auxins

Auxin as the plant hormone regulating phototropism

As with animals, plants also use a variety of hormones to control their growth and development. A family of hormones called auxins are commonly found in plants, and promote (and sometimes inhibit) growth.

Auxins

Auxins are produced in the meristems of plants (meristems are explained on successive pages).

Auxins are responsible for promoting cell elongation, a process that is required before the differentiation of a cell. It is able to this by promoting the intake of water, increasing the elasticity of the cell to cope with the increase of water taken in by the cell.

One of the most common auxins is indole acetic acid.

Indole Acetic Acid (IAA)

Indole Acetic Acid affects the root and shoot tips of the plant, as described below.

  • Shoot Tip – No matter what the concentration, IAA promotes growth in the shoot area of a plant (though higher concentrations promote growth more).
  • Root Tip – High concentrations of auxin inhibit growth while small amounts are enough to promote growth in the root with indole acetic acid.

Phototropism

Auxins also play a part in phototropism, an occurrence that involves plants bending or moving away from light. The shoot tip is responsible for directional movement by the plant in response to sunlight, as this is the area where auxins can be found.

Sunlight eradicates auxin, meaning that the part of the shoot tip of the plant which is receiving direct sunlight will have the least amount of auxin.

The extra auxin present on the shaded side promotes more cell division and elongation, causing the plant to bend towards the sunlight after this lop-sided growth.

 

Credit: Science Sauce

 

Geotropism

Geotropism is a similar occurrence to phototropism where the plant exhibits directional growth in response to gravity. The shoot tip illustrates negative geotropism (grows against the force of gravity) while the root tip exhibits positive geotropism (grows in the same direction as gravity).

Apical Dominance

The presence of auxins in the lateral areas of the plant (in between the root and shoot tip) prevent lateral growth. If you cut off the shoot tip of a plant, the lack of ‘diffusable’ auxins means that they cannot inhibit growth in these lateral areas. This is known as apical dominance.

Leaf Abscission

The presence of auxins in the lateral areas also prevents leaf abscission. In the colder months, auxin concentrations and the rate of photosynthesis drops.

This lack of auxin in the lateral areas results in the forming of an abscission layer at the stalk of the leaf, which weakens its connection with the plant and soon falls off it.

The next tutorial looks at another family of growth hormones, the gibberellin family, with continuing pages looking at the meristems, the sites of plant growth.

Quiz

Select true if the statement is correct and false if incorrect.

1. Plants produce auxins in their meristematic tissues.
2. Auxins promote cell elongation by promoting water intake and increasing cell elasticity
3. Indole acetic acid is a rare type of auxin.
4. Phototropism refers to the tendency of plants to move towards or away from gravity.
5. The tendency of roots to move away from the source of light is an example of phototropism.

Send Your Results (Optional)

Your Name
To Email
Biology Tutorials > Control of Growth & Development > Plant Auxins – Phototropism & Geotropism

You will also like...

Population Growth and Survivorship
Population Growth and Survivorship

This lesson looks at population attributes, regulation, and growth. It also covers population genetics, particularly gen..

Community Patterns

Learn about community patterns and the ecological factors influencing these patterns. Revisit some of the ecosystems you..

Biosecurity and Biocontrol
Biosecurity and Biocontrol

This lesson explores the impact of biosecurity threats, and why they need to be identified and managed. Examples to incl..

Developmental Biology
Developmental Biology

Developmental biology is a biological science that is primarily concerned with how a living thing grows and attains matu..

The Human Physiology
The Human Physiology

Physiology is the study of how living organisms function. Thus, human physiology deals specifically with the physiologic..

Mātauranga Māori and Science Collaboration
Mātauranga Māori and Science

Mātauranga Māori is the living knowledge system of the indigenous people of New Zealand, including the relationships t..