Growth Factors in Plants

Growth Factors in Plants

Plants are living things that need to adapt to their surroundings to survive. They need to adjust to both internal and external changes to stay healthy. This process is called homeostasis, which helps them maintain a stable internal environment and withstand tough conditions. Growth factors in plants are also affected these understand more, let's explore how these factors are influenced. Learn about the growth factors in plants and how they are impacted by their environment.

Plants respond to stimuli

Plants react to different changes that affect their survival like light, water, gravity, carbon dioxide concentration, and infections by fungi and bacteria. Since plants can't move, they have to respond to these changes where they are rooted. They don't have a nervous system like animals, but use chemical communication systems and plant hormones like plant growth factors, abscisic acid (ABA), and ethene. Plant hormones direct responses to different stimuli by interacting with receptors and triggering chemical reactions that amplify the signal and evoke a response.

The stress hormone, ABA, is produced throughout the plant body and helps control responses to environmental stresses like water shortage by closing stomata in leaves and reducing water vapour loss. Stomata are tiny pores on the underside of leaves that allow substances like oxygen and water vapour to diffuse.

Plant responses to stimuli can also be related to changes in guard cell turgidity, which open and close stomata regulating carbon dioxide intake for photosynthesis. However, most plant responses to stimuli involve changing the growth of roots or shoots with plant growth factors. 

Plant Hormones - Plant growth factors

Plant growth factors are chemical messengers produced in small amounts by different tissues throughout the plant body, unlike animal hormones produced by specialised glands. These messengers are transported through the plant's circulation system, the phloem or xylem sap, or diffuse between cells to target nearby cells. This causes a growth-related response like cell elongation, which affects the direction of plant growth. There are two types of plant growth factors: auxins, with indoleacetic acid (IAA) being the most important, and gibberellins. The phloem and xylem are vascular plant tissues responsible for transporting assimilates like amino acids and sucrose and water, respectively.


Gibberellins are present in high concentrations in young leaves and seeds and are involved in regulating seed germination and stem elongation. During the early stages of germination, gibberellins produced by the plant embryo stimulate cells to synthesise amylase, which breaks down starch into sugars needed for embryo growth.

Gibberellins are also present in plant stems, where they play a crucial role in determining their growth and height. They stimulate cell division and elongation within the stem, resulting in taller plants.

Germination is the process by which a plant develops from a seed, and amylase is an enzyme that catalyses the breakdown of starch into sugars the germinating seed can for.

Indoleacetic acid

Indoleacetic acid (IAA) is the primary auxin produced by plants, and it is made in the dividing tips or meristems of roots and shoots. IAA controls plant growth by influencing the elongation of plant cells.

Meristem cells are undifferentiated cells in plants that undergo cell division and differentiate into. They are the plant's stem cells.

IAA promotes plant cell elongation by increasing the plasticity of young plant cells' cell walls, enabling them to expand. The acid growth hypothesis suggests that IAA increases plant cell wall plasticity by stimulating targeted cells to increase the active transport of hydrogen ions from the cell cytoplasm into the cell wall. This acidifies the wall, breaking bonds in its structure, making it more flexible and allowing it to stretch and become longer.


Plants exhibit tropisms, which are differential growth responses to stimuli, to optimize their environmental conditions. Different types of tropisms exist, depending on the nature of the stimulus, but all cause directional growth of a part of the plant, such as shoots or roots.

The following are some examples of different types of tropisms:

  1. Phototropism: This is the growth response of a plant to light. The plant grows towards the source of light (positive phototropism) or from photrop on the light direction.
  2. Geotropism/Gravitropism: This is the growth response of a plant to gravity. Roots show positive geotropism, which means they grow towards the direction of gravity, while shoots show negative geotropism and grow opposite to the direction of gravity.
  3. Chemotropism: This is the growth response of a plant to chemicals. The plant grows towards or away from the chemical stimulus depending on the direction of the chemical gradient.
  4. Thigmotropism: This is the growth response of a plant to touch. The plant responds by growing towards or away from the point of contact.

Tropisms allow plants to adapt to their environment and survive in challenging conditions. They are important mechanisms that allow plants to grow towards or away from stimuli to optimize their growth and survival.


Phototropism is the directional growth of plant body parts, such as shoots or roots, in response to light stimuli. When exposed to unilateral light, plants exhibit phototropism, causing to grow towards the light source (positive phototropism) and roots to grow away from the light source (negative phototropism).

The uneven distribution of the auxin IAA throughout the plant body part is responsible for this bending response. Under normal illumination, IAA is evenly distributed throughout the plant body part. However, when exposed to uneven light stimuli, IAA accumulates on the shaded side of the plant body part. In shoots, cells on the shaded side with more IAA will elongate more, causing the entire structure to bend towards the light source. In roots, higher concentrations of IAA inhibit cell elongation, causing the light side of roots to elongate faster than the shaded side, resulting in the root bending away from the light source and into the soil.

These responses help ensure plant survival by allowing shoots to receive as much light as possible for photosynthesis and roots to remain in the soil for nutrient absorption and anchorage. Positive and negative phototropism are the results of plant body parts' bending towards or away from light stimuli, respectively.


Gravitropism is the growth response of plants to gravity. It is essential for controlling plant growth and is evident in horizontally-growing plants. Roots display positive gravitropism, growing towards the pull into the soil, while shoots display negative gravitropism, growing away from the gravitational pull into the light.

Positive gravitropism describes growth in the direction of a gravitational pull, while negative gravitropism describes growth in the opposite direction of a gravitational pull. Similar to phototropism, gravity influences IAA distribution in plants. When growing laterally, IAA accumulates on the lower side of the plant body part, which is closer to the ground.

In shoots, IAA accumulation promotes cell elongation on the lower side, causing the structure to bend and grow upwards, as it would if the plant were vertical. In roots, however, IAA accumulation inhibits cell elongation on the lower side, causing cells on the opposite side to elongate more, leading to the structure bending and growing downwards.

These responses help ensure plant survival by allowing roots to grow into the soil for nutrient absorption and anchorage and shoots to grow towards the light for photosynthesis. Gravitropism, along with phototropism, is an essential mechanism that allows plants to adapt to their environment and maximize their growth and survival. 

Growth Factors in Plants - Key takeaways

Plant growth is regulated through the use of plant growth factors, with auxins (IAA) and gibberellins being the two primary types. IAA is the most important auxin that regulates plant growth, allowing cell elongation by increasing the cell wall plasticity of targeted cells.

Tropism is a type of differential plant growth response to stimuli that allows plants to maximize their environmental conditions. Phototropism is a plant growth response to light stimuli, while gravitropism is a plant growth response to gravity stimuli. A negative tropism response means that plant growth happens in the opposite direction of the stimulus, while a positive tropism response entails plant growth towards the stimulus. Roots are positively gravitropic and negatively phototropic, meaning they grow towards the gravitational pull and away from light stimuli., shoots positively phototropic and negatively gravitropic, meaning they grow towards light stimuli and away from the gravitational pull. Gravity and light influence IAA distribution in plant cells, enabling plant growth responses. These responses help ensure plant survival by allowing roots to grow into the soil for nutrient absorption and anchorage and shoots to grow towards the light for photosynthesis. The understanding of plant growth factors and tropism is vital in modern agriculture and plant breeding, allowing for the development of crops that can thrive in specific environmental conditions.

Growth Factors in Plants

What are the external and internal factors affecting plant growth?

Factors like light, water availability, gravity, carbon dioxide concentration, or infection by fungi and bacteria can affect plant growth and survival. Changes in these factors often trigger stimuli and consequent plant growth responses.

What are the 4 primary factors that affect plant growth?

Light, temperature, water and nutrients.

What stimulates plant growth?

Plant growth factors stimulate plant growth.

Which hormone inhibits the growth of plants?

ABA hormones can inhibit plant growth.

Why is tropism important?

Tropism allows plants to maximise the use of their environmental conditions like optimal light exposure.

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