Control and coordination in plants — common mistakes

A common mistake is to assume auxin always promotes growth. Remember: shoots have an optimal auxin concentration for growth which is much higher than the optimal concentration for roots. In fact, auxin concentrations that promote shoot growth are inhibitory to root growth. This differential sensitivity is key to understanding gravitropism in particular!

In Cambridge A Level Biology 9700, the main plant hormones you need to understand are auxins, gibberellins, abscisic acid (ABA), and ethene. Each has specific roles in controlling plant growth, development, and responses to environmental stimuli. For a comprehensive review, check out our free revision course and practice with past papers.

Auxins control phototropism and gravitropism by causing differential growth. In phototropism, auxins move to the shaded side of shoots, promoting faster growth towards light. In gravitropism, auxins accumulate on the lower side of horizontal shoots (promoting growth upwards) and roots (inhibiting growth downwards), leading to directional growth. The key is the differential sensitivity of shoots and roots to auxin concentrations.

Roots being positively gravitropic (growing downwards with gravity) is crucial for a plant's survival. This ensures roots grow into the soil, anchoring the plant securely and allowing efficient uptake of water and mineral ions. Without this response, roots might grow upwards, exposing them to desiccation and preventing proper nutrient acquisition.