20 Nastic Responses

Key Idea: Nastic responses are plant responses in which the direction of the plant response is independent of the stimulus direction. They are reversible and often rapid movements. Nastic responses in plants are independent of the stimulus direction and may involve quite rapid, reversible movements, often resulting from localised changes in turgor. Nastic responses can occur in response to temperature (thermonasty), light (photonasty), or touch (thigmonasty). Plant 'sleep movements', in which flowers close or leaves droop at night, are specialised diurnal photonasties. The mechanisms involved in Mimosa's thigmonasty (below) are also responsible for the leaf movements of the Venus flytrap.

Movements of the sensitive Mimosa plant

` The sensitive plant (Mimosa pudica) has long leaves composed of small leaflets. When a leaf is touched, it collapses and its leaflets fold together. Strong disturbances cause the entire leaf to droop from its base. This response takes only a few seconds and is caused by a rapid loss of turgor pressure from the cells at the bases of the leaves and leaflets. ` The message that the plant has been disturbed is passed quickly around the plant by electrical signals (changes in membrane potential) not by plant hormones (as occurs in tropisms). The response can be likened to the nerve impulses of animals, but it is much slower. After the disturbance is removed, turgor is restored to the cells, and the leaflets slowly return to their normal state. ` The adaptive value of these responses is uncertain but may relate to deterring browsers or reducing water loss during high winds.

Leaflet

Leaf

3D

Leaf

Leaflets

RCN

Unstimulated leaf

RCN

Disturbed leaf

Leaflet base

Leaf axis Leaflet Thin walled parenchyma cells specialised as motor cells.

Epidermis

Cells on the upper and lower surfaces are turgid Vascular tissue Pulvinus

The leaves of Mimosa have joint-like thickenings, the pulvini (sing. pulvinus) at the bases of the petioles and at the bases of each leaflet. The pulvini contain specialised motor cells, which are involved in the rapid leaf movements.

K+

Cells on the lower surface lose turgor and the leaf collapses.

K+ H2O

vEpidermis

When disturbed, a change in membrane potential of the leaf cells is transmitted to the cells of the pulvinus. These cells respond by actively pumping potassium ions out of the cytoplasm (see inset above). Water follows osmotically and there is a sudden loss of turgor. This mechanism also operates at the leaflet bases, except that the cells on the upper surface of the pulvinus lose turgor, and the individual leaflets fold up, rather than down (left).

Many plants show movements in relation to light and dark. Oxalis (right) spreads it leaves out during the day to capture sunlight. During the night the leaves are lowered and bend slightly along the midline. This helps to prevent dew accumulating on the leaf and minimises the risk of damage while the leaf is not being used to capture light.

Day

Lazaregagnidze

Night

Thigmonastic responses in carnivorous plants

Some small, specialised plants obtain most of their nitrogen (but not their energy) from trapping and digesting small animals such as insects. This allows them to grow in nutrient-poor (particular low nitrogen), high light environments, such as acidic bogs and rocky outcrops.

Venus flytrap (Dionaea)

When an insect touches the hairs on a leaf of a Venus flytrap (right), the two lobes of the leaf snap shut, trapping the insect. Once the insect has been digested, the empty leaves reopen. The hairs on the leaf must be touched twice in quick succession for the leaf to close. This means false alarms, such as a twig falling onto the leaf, do not set it off. 3D Sundew (Drosera)

Sundews also show a thigmonastic response. An insect landing on a leaf quickly becomes trapped in the sticky hairs. The hairs fold around the insect and in some species the leaf may curl over, completely enclosing the insect. Carnivory has evolved independently nine times in five different orders of plants. 3D

1. Identify the type of nasty involved in each of the following examples: (a) Opening and closing of tulip flowers to changes in air temperature:

(b) Opening of evening-primrose flowers at dusk:

2. How is a nastic response different from a tropism?

3. (a) Describe the basic mechanism behind the sudden leaf movements in Mimosa:

(b) Explain how the movements of the Mimosa pudica help its survival:

4. How could the sleep movements of plants (lowering the leaves at night) benefit a plant?

5. (a) How does the Venus flytrap ensure the closing of the trap is not falsely triggered?

(b) Describe how carnivory benefits this plant in its habitat: