Plant growth responses: tropisms and the hormones behind them
Plants change their direction of growth in response to light and gravity. Unequal auxin distribution changes cell elongation across a shoot or root. Experiments distinguish light detection from signal transmission, and growers use hormone responses in rooting, ripening and weed control.
What a learner can do afterwards
- Explain phototropism and gravitropism through unequal cell elongation, distinguishing growth bending from locomotion.
- Interpret shoot-tip experiments and explain the roles of opaque caps, transparent caps, gelatin and mica.
- Name a commercial use of a plant hormone and explain the response it exploits.
1 · Read
A tropism is a growth response whose direction depends on a stimulus. Phototropism responds to light. Positive phototropism is growth towards light; negative phototropism is growth away from it. Young shoots usually grow towards light. One side elongates more than the other, producing a bend. Tissue changes position, but the plant does not travel to a new location.
Auxin is a hormone produced in growing tissues, including shoot tips. In a young shoot receiving one-sided light, auxin becomes more concentrated on the shaded side of the elongating region. It promotes greater cell elongation there, so the shoot bends towards light. This redistribution involves local cell-to-cell transport; phloem mass flow alone does not explain it.
Gravitropism is directional growth in response to gravity. In a horizontal seedling, auxin redistributes towards the lower side. In shoots, this promotes lower-side elongation, bending the shoot up: negative gravitropism. Roots respond differently. Extra auxin inhibits lower-side elongation, so the upper side elongates more and the root bends down: positive gravitropism. Gravity can guide these responses in darkness.
Classic experiments used young grass shoots under one-sided light. Intact shoots bent; removing the tip prevented bending. Removal alone did not identify the tip's role. An opaque tip cap prevented bending, while a transparent cap allowed it. Shielding the lower shoot also allowed bending. These comparisons support light detection at the tip in this experimental system, with elongation occurring below.
To test transmission, researchers replaced a removed tip over permeable gelatin. Bending still occurred. An impermeable mica sheet between tip and shoot prevented bending. The tip was present in both treatments. This supports a diffusible chemical signal passing from tip to elongating tissue. The comparison alone does not identify that chemical as auxin.
Growers apply auxin treatments to promote roots on cuttings. Ethylene gas promotes ripening in fruits such as bananas. Some synthetic auxins are selective herbicides. At suitable doses, they cause uncontrolled growth and other damage that can kill susceptible broad-leaved weeds. A growth-promoting hormone can therefore contribute to weed control. Its effects depend on concentration, tissue and species.
Unequal elongation bends shoots and roots. Auxin affects them differently. Controls distinguish light detection from signal transmission.
Teaching text, questions and flashcards written for this dataset. OpenStax Biology 2e, section 30.6, was consulted for plant-response facts and experimental context.
Reading and flashcards written for this dataset. OpenStax Biology 2e, section 30.6, supplied factual background; its prose is not reproduced.
More to read
What a shoot experiment can tell you
A curved shoot presents several questions. Which stimulus guides it? Where is that stimulus detected? How does the information reach the tissue that elongates?
Removing a young grass shoot's tip prevents its usual bending towards one-sided light. That result shows that the tip matters, but removal affects several functions at once. Further comparisons give a narrower conclusion. A transparent cap allows bending, while an opaque cap prevents it. Covering the lower shoot instead still allows bending. These results support light detection at the tip in this experimental system.
Another comparison tests transmission. A replaced tip can still produce bending when gelatin lies between it and the shoot. An impermeable mica barrier prevents bending. A signal that diffuses through gelatin fits these results. The experiment does not reveal its chemical name by itself.
Auxin explains the unequal elongation in the young shoot. More auxin reaches its shaded side, which elongates more. Roots have a different response to extra auxin: inhibited elongation on their lower side allows the upper side to elongate more, bending the root downwards.
The same hormone can also have different effects at different concentrations. Auxin treatments promote roots on cuttings, while some synthetic auxins at suitable doses cause damaging growth responses in susceptible weeds. Ethylene, a different hormone, is used to promote banana ripening.
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