Plant Hormones: Auxins and Tropisms

Plants cannot move from place to place, but they can respond to their environment by growing towards or away from stimuli. These growth responses are called tropisms and are controlled by plant hormones, particularly auxins. This lesson covers auxin action, phototropism and gravitropism, as required by Edexcel GCSE Biology (1BI0) Topic 6.

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What are Plant Hormones?

Plant hormones (also called plant growth regulators) are chemical substances produced in small quantities in one part of the plant that have an effect on another part of the plant. They coordinate and control growth, development and responses to the environment.

Unlike animal hormones, plant hormones are not produced in specific glands — they are made in various tissues (e.g., shoot tips, root tips, developing seeds).

Key Plant Hormones (for Edexcel GCSE)

Hormone	Where produced	Key effects
Auxin (IAA)	Shoot tips, root tips	Promotes cell elongation in shoots; controls tropisms
Gibberellins	Seeds, young leaves	Promote seed germination, stem elongation, flowering
Ethene	Ripening fruits, ageing tissues	Promotes fruit ripening