Which of the following stimuli is likely to elicit a nastic response in an organism?

Analyze the question The question asks which stimulus is likely to cause a nastic response. We need to recall what makes a nastic movement different from other plant responses.

Explain the concept

• Tropism is a growth movement in which the direction of the stimulus determines the direction of the response. Examples are phototropism (light), geotropism (gravity), chemotropism (chemicals) and hydrotropism (water).
• Nastic movement is a response in which the direction of the stimulus does not matter. The direction of movement is fixed by the structure of the organ. The stimuli are usually diffuse factors such as light intensity, temperature, humidity or touch.

Common examples:

• Opening and closing of flowers (photonasty, caused by light intensity, and thermonasty, caused by temperature)
• Folding of Mimosa pudica leaves when touched (seismonasty)
• Opening and closing of stomata

Show the solution

• A. Wound lesion: This is not a typical stimulus for a nastic response. It is more linked to injury or defence responses.
• B. Light intensity: A change in intensity, not direction, causes flowers to open or close (photonasty). This fits nastic movement.
• C. Chemical substances: A one-sided chemical stimulus produces chemotropism, which is directional. This is a tropism, so it does not fit.
• D. Gravity: Gravity acts in a fixed direction and produces geotropism. This is also a tropism.

Answer: B. Light intensity

Tip: Remember "Nastic = Not direction-dependent". If the stimulus comes from one direction (light direction, gravity, a chemical gradient), think tropism. If it is about intensity or general conditions (light level, temperature, touch), think nastic.