Plant Hormones: Gibberellins

This lesson covers: 

  1. How plants overcome limitations in responding to their environment
  2. The role of gibberellins in stem growth
  3. The role of gibberellins in seed germination

Overcoming the limitations of plant responses

Plants are limited in how they can respond to their environment.


Limitations of plant responses:

  1. Plants are rooted to the spot so they are not mobile.
  2. Plants do not have a rapidly responding nervous system.


Plants respond to their environment by producing hormones that are transported throughout the plant and affect different parts of the plant. For example, they allow plants to move by growing towards or away from certain stimuli.

The role of gibberellins in stem growth

Gibberellins are plant growth regulators that stimulate seed germination and regulate plant height via stem growth.


For example, gibberellins cause elongation of plant stems during growth by increasing the length of the internodes, which are the regions between the leaves on a stem.


Plants that are unable to synthesise many gibberellins have shorter stems. This can be beneficial, as it makes the plants less vulnerable to damage by weather and harvesting.

The role of gibberellins in seed germination

Seeds in their mature state exhibit a condition known as dormancy, which is a period of inactivity that allows the seed to withstand various adverse environmental conditions.


Gibberellins play a crucial role in ending this dormant state by initiating the germination process.


This involves a few key structures:

  1. The embryo - This is the developing plant.
  2. The endosperm - This is an energy store that contains starch and surrounds the embryo.
Diagram showing the process of seed germination involving gibberellins, water entry, embryo synthesis of gibberellins, amylase synthesis, and starch hydrolysis to maltose and glucose.

The process of germination occurs as follows:

  1. Water enters the seed.
  2. This absorption of water causes the embryo to synthesise gibberellins.
  3. In response, amylase and maltase enzymes are synthesised.
  4. The amylase enzymes hydrolyse the starch stored in the endosperm into maltose.
  5. This can then be broken down into glucose, which provides the embryo with sugars that are essential for energy and growth.


Through this mechanism, gibberellins activate the mobilisation of stored starch reserves, starting the germination process.


Seeds that cannot produce gibberellins do not typically germinate, but if gibberellins are applied to the seeds externally they germinate normally. This provides evidence for the role of gibberellins in germination.