E13 Movement

Topic

Unlike tropisms, rapid movements of plant organs are not caused by differential growth between two sides of the organ, but rather by reversible changes in turgor within specialized motor cells known as the *pulvinus*. While a tropism takes hours to develop because it relies on actual cell elongation, a rapid movement of this type can occur in seconds or minutes because the *pulvinus* only needs to change volume, not elongate.

Within the *pulvinus*, the reversible turgor change begins with the active transport of potassium and chloride ions into or out of the specialized motor cells. When these ions enter a motor cell, they draw water in via an osmotic flow that follows the active ion transport; this water influx causes a change in cell volume, swelling the motor cell and increasing its turgor. Conversely, when these same ions exit the motor cell, the osmotic water flow reverses, causing the motor cell to lose volume and its turgor to drop.

When the specialized motor cells of the *pulvinus* on one side of the organ gain turgor while those on the opposite side lose it, the organ bends toward the side that lost turgor; this type of organ movement is known as a *nasty* (or nastic movement). Unlike tropisms, the direction of a nastic movement does not depend on the direction of the triggering stimulus, but rather on the fixed arrangement of the specialized motor cells within the *pulvinus*.

A specific type of nastic movement is the nyctinastic movement—the folding and unfolding of leaves or petals that repeats daily in accordance with the plant's circadian rhythms. The plant's own internal clock—without requiring an external trigger each time—causes the *pulvinus* to alter its turgor at the specific time of day when that nyctinastic movement normally occurs. Another type of rapid plant organ movement is the action of carnivorous traps; in these, the same mechanism involving reversible turgor changes—driven by active ion transport and the resulting osmotic water flow—is triggered abruptly when prey touches the trap's sensory hairs. This causes the organ to snap shut around the prey much faster than a standard nastic movement, as the turgor change occurs almost simultaneously across all the specialized motor cells involved.

Ultimately, whether it is a standard nastic movement, a nyctinastic movement governed by circadian rhythms, or the sudden closure of a carnivorous trap, all rapid plant organ movements rely on the same physical mechanism: the active transport of potassium and chloride ions, followed by the osmotic water flow that this transport triggers. This process produces rapid, reversible changes in cell volume that, within the pulvinus, translate into the turgor changes that ultimately move the organ.

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