Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved.
How fast can the leaflets close? How does this occur?
And more importantly, what benefit could this unusual response provide to the plant?
The mechanism of leaflet closure is similar to what happens during the shrinking of guard cells and closing of stomata (see Figure 37.22). When in the open position, special cells on the upper surface of Mimosa leaflets are filled with water and are under pressure.
Explain how osmosis and flow of ions into and out of these special cells may be involved in leaflet closure and reopening.

Verified step by step guidance

1. Step 1: Mimosa pudica leaflets close in response to touch due to changes in turgor pressure within specialized cells on the upper surface of the leaflets. This mechanism is similar to the process of stomatal opening and closing in guard cells.
2. Step 2: When the leaflets are open, the specialized cells are filled with water, creating high turgor pressure. This is maintained by the active transport of ions such as K⁺ and Cl⁻ into the cells, which creates an osmotic gradient that drives water into the cells via osmosis.
3. Step 3: In response to touch, ion channels in the specialized cells open, allowing K⁺ and Cl⁻ to exit the cells along their electrochemical gradients. This reduces the osmotic gradient, causing water to flow out of the cells by osmosis.
4. Step 4: As water exits the cells, the turgor pressure decreases, leading to the collapse of the cells and the closure of the leaflets. This rapid movement is facilitated by the plant's ability to quickly regulate ion flow and water movement.
5. Step 5: The benefit of this response is likely to protect the plant from herbivory or physical damage. By closing its leaflets, Mimosa pudica may deter animals or insects from feeding on it, as the sudden movement can startle predators or make the plant less appealing.

Key Concepts

Osmosis

Osmosis is the movement of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. In the context of Mimosa pudica, when the leaflets are stimulated, water moves out of the specialized cells, leading to a decrease in turgor pressure and causing the leaflets to close. This process is crucial for understanding how the plant responds to mechanical stimuli.

Ion Transport

Ion transport refers to the movement of ions across cell membranes, which is essential for various physiological processes. In Mimosa pudica, potassium (K+) and chloride (Cl-) ions play a significant role in leaflet closure. The influx of these ions into the specialized cells increases turgor pressure, while their efflux during closure leads to a decrease in pressure, facilitating the rapid movement of the leaflets.

Turgor Pressure

Turgor pressure is the pressure exerted by the fluid inside the plant cells against the cell wall, which helps maintain the plant's structure and rigidity. In Mimosa pudica, changes in turgor pressure are responsible for the opening and closing of leaflets. When the cells lose water and ions, turgor pressure decreases, causing the leaflets to fold, which is a protective response to potential threats.