Nastic responses: non-directional plant movements
What a nastic response is
- A nastic response is a plant movement where the direction of the response is independent of the direction of the stimulus.
- The plant responds the same way no matter which side the stimulus comes from.
- Compare this directly with a tropism, because the exam tests the distinction constantly:
| Tropism | Nastic response | |
|---|---|---|
| Direction | Set by the direction of the stimulus | Independent of stimulus direction |
| Mechanism | Growth — unequal cell elongation | Usually turgor change in existing cells |
| Speed | Slow (hours to days) | Often rapid (seconds to minutes) |
| Reversible? | Usually permanent | Usually reversible |
- Nastic responses are named after the stimulus, with the ending -nasty:
- Thigmonasty — response to touch or vibration.
- Nyctinasty — response to the day/night cycle ("sleep movements").
- Photonasty — response to light intensity.
- Thermonasty — response to temperature.
- Hydronasty — response to humidity or water availability.
How most nastic responses work: turgor and the pulvinus
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Because these responses are fast and reversible, they cannot depend on growth. Growth is neither.
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The mechanism is instead a change in turgor pressure — the pressure of water pushing a cell's contents against its wall.
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Many nastic movements happen at a pulvinus, a swollen joint of thin-walled cells at the base of a leaf or leaflet.
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The sequence is:
- The stimulus triggers active transport of potassium ions (K+) out of the cells on one side of the pulvinus.
- Water follows the ions by osmosis, moving down the resulting water potential gradient.
- Those cells lose turgor and become flaccid.
- The cells on the opposite side stay turgid, so the joint bends and the leaf folds or droops.
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Reversing the ion movement pumps K+ back in, water follows, turgor is restored, and the leaf reopens.
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Some rapid responses use a related mechanism without a pulvinus.
- In the sundew (Drosera), a native NZ carnivorous plant of pākihi wetlands, contact with an insect triggers tentacles to bend inwards over the prey. Here differential growth is involved as well as turgor, so the response is slower — minutes to hours.
Selective advantage of nastic responses
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As always, link the response to survival and reproduction.
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Thigmonasty in the sundew.
- Tentacles fold inwards, holding the insect against digestive glands so enzymes can break it down.
- The plant absorbs nitrogen and phosphorus from the prey.
- This is decisive in its niche: sundews grow in waterlogged, acidic, nutrient-poor wetland soils where nitrogen is the limiting factor for growth. Nitrogen is needed for amino acids, proteins, chlorophyll and nucleic acids, so obtaining it from prey allows the plant to make enzymes and chlorophyll — and therefore to flower and set seed — where competitors cannot.
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Nyctinasty (sleep movements).
- Leaves or petals fold at night and reopen by day.
- Folding reduces the surface area exposed to the sky, cutting radiative heat loss and reducing the risk of frost damage to delicate tissue.
- Folded leaves also present less area to night-feeding herbivores, and closed flowers protect pollen from dew and rain, keeping it viable for the next day's pollinators.
- Less tissue damage and viable pollen mean more successful pollination and more seed set.
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Photonasty in flowers.
- Flowers open in light and close in darkness or cloud.
- Opening only when pollinators are active means nectar and pollen are not wasted when no pollinator will visit, and pollen stays dry.
- Energy saved and pollen preserved increase the number of successfully fertilised ovules.
Worked Example
Worked Example
A student investigates leaf movement in a native plant with compound leaves. They record:
- Leaflets fold upward each evening and reopen each morning.
- Folding still occurs on the first two nights when the plant is kept in constant darkness, at the same time each evening.
- Leaflets fold within 20 seconds if the leaf is touched sharply, from any direction.
- Leaflets folded by touch reopen after about 15 minutes.
Identify the responses shown, and explain how the touch response occurs and why it provides a selective advantage.
Answer:
Identifying the responses. Two separate responses are present.
The evening folding is nyctinasty, a response to the day/night cycle. That it continues in constant darkness shows it is driven by an internal (endogenous) biological clock rather than directly by the light itself.
The rapid folding on contact is thigmonasty. It is nastic rather than tropic because the leaflets fold the same way from any direction of touch — the direction of the response is independent of the direction of the stimulus.
How the touch response occurs.
The response takes 20 seconds and is fully reversible, which rules out growth — new cells cannot be made and unmade on that timescale. The mechanism is therefore a turgor change.
- Touch is detected by mechanoreceptors in the leaflet.
- An electrical signal propagates rapidly through the tissue to the pulvinus at the base of each leaflet.
- This triggers active transport of K+ out of the cells on the lower side of the pulvinus.
- Water follows by osmosis, down the water potential gradient, so those cells lose turgor and become flaccid.
- The turgid cells on the opposite side are now unopposed, so the joint bends and the leaflet folds upward.
- Over the following 15 minutes, K+ is pumped back in, water re-enters by osmosis, turgor is restored and the leaflet reopens.
Why it provides a selective advantage.
The response is triggered by contact — which, for a leaf, most often means a herbivore landing on or beginning to chew it.
- Sudden folding makes the leaf smaller and less conspicuous, and may dislodge a small insect outright.
- A folded leaf presents a thickened edge rather than a broad blade, which is harder for a chewing insect to bite into.
- Less tissue is lost, so the plant retains photosynthetic area and the resources already invested in building that tissue.
Retaining leaf area means a higher total rate of photosynthesis, more glucose available for flower and seed production, and therefore more offspring than an individual that lost the same tissue to herbivory. That the response reverses after 15 minutes matters too: a permanently folded leaf would photosynthesise poorly, so reopening once the threat has passed avoids paying the cost of the response any longer than necessary.