Turgor Movements in Thigmotropism
Scanning electron microscopy has revealed that cells on the non-contact side of mechanostimulated tendrils are fully turgid, while those on the contact side are flaccid and folded, suggesting a lower turgor. Additional studies have found that a gradient of K+ ions exists between the contact and non-contact side of the tendril. Specifically, K+ was higher in the contact side cells, and lower in the non-contact side cells. We know that K+ efflux out of a cell is accompanied by an efflux of H2O. This efflux of H2O causes a drop in turgidity, which promotes the contraction of contact-side cells. Conversely, as K+ flows into the non-contact side cells, H2O also flows in, causing a rise in turgidity. It is believed that the initial action potential resulting from mechanostimulation induces these changes in tugidity, establishing the K+ gradient observed in many tendril tissues.
In addition to the K+ gradient, particular substances have been isolated which can alter a cell's turgidity. These substances, termed turgorins, have been isolated primarily in the leaves of the Mimosa "sensitive plant". This plant is known to actually fold up its leaves in response to touch. Specific turgorin receptors have also been found on the outer side of the plasma membrane of Mimosa. Although these substances have been isolated in leaf tissues, it is likely that a similar mechanism is employed in tendril tissues.
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