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? Mimosa leaflets also close on their own at dusk and reopen at dawn (this cycle develops as a circadian rhythm). What receptor molecule mentioned in this chapter is likely involved in regulating this process?

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1. The leaflets of Mimosa pudica can close within seconds after being touched or shaken. This rapid movement typically occurs within 1 to 2 seconds.
2. The closing mechanism is due to a rapid change in turgor pressure within the cells at the base of the leaflets. When stimulated, cells release ions causing water to flow out, resulting in the collapse of the cell and the closing of the leaflets.
3. This rapid closing can serve as a defense mechanism to deter herbivores. The sudden movement can startle the herbivore, and the closed leaves might also be less appealing as they appear smaller and less accessible.
4. The leaflets' movement in response to the day-night cycle is likely regulated by a light-sensitive receptor molecule. In plants, phytochromes are a common type of photoreceptor that detect light and are involved in controlling processes like photoperiodism and circadian rhythms.
5. Phytochromes in Mimosa pudica likely play a role in the opening and closing of the leaflets in response to the presence or absence of light, aligning the leaf movement with the circadian rhythm.

Key Concepts

Thigmonasty

Thigmonasty is a rapid plant movement in response to touch or physical stimuli, exemplified by the leaflets of Mimosa pudica. This movement occurs due to changes in turgor pressure within the cells, leading to a quick closure of the leaflets. This response is thought to protect the plant from herbivores and environmental stressors, showcasing an adaptive mechanism in plant behavior.

Circadian Rhythms

Circadian rhythms are biological processes that follow a roughly 24-hour cycle, influencing various physiological functions in organisms, including plants. In Mimosa pudica, these rhythms regulate the opening and closing of leaflets at dawn and dusk, allowing the plant to optimize photosynthesis and conserve water. This internal clock is crucial for synchronizing the plant's activities with environmental changes.

Receptor Molecules

Receptor molecules are proteins located on the cell surface that bind to specific signaling molecules, triggering a response within the cell. In the context of Mimosa pudica, certain receptor molecules are likely involved in detecting mechanical stimuli and regulating the thigmonastic response. Understanding these receptors helps elucidate how plants perceive and react to their environment.