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Figure 3.61. Guard cells are structured so that high turgor pressure opens the stomatal pore, and low turgor pressure closes the pore. Left (Lower image), Partially open Tradescantia virginiana stoma with 0.2 MPa guard cell turgor, as measured directly with a guard cell pressure probe (shown). Right (upper image) same stoma showing the aperture almost fully open, with 3.6 MPa guard cell turgor. (Images and data courtesy P. Franks).
Water uptake by cells is driven by solute uptake. Osmotically-driven water uptake generates the turgor pressure needed for maintenance of cell volume, and for specific cell functions that depend on controlled changes in turgor. The rapid influxes or effluxes of water that cause rapid changes in cell volume in key cells are brought about by ion influxes or effluxes.
Rapid changes in turgor cause the swelling or shrinking of guard cells in leaves that controls the opening and closing of stomata. The cell walls of guard cells are structured so that high turgor pressure pushes them apart and opens the stomatal pore. Low turgor pressure allows them to collapse and close the pore (Figure 3.61). Influxes or effluxes of K+ along with accompanying anions causes the osmotically-driven water uptake or loss
Whole leaves or parts of leaves can move quickly. Changes in the turgor of a group of cells at the base of leaves, the pulvinus, cause leaves to fold quickly in response to touch, as in the ‘sensitive plant’ Mimosa pudica, (Figure 3.62)
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Figure 3.62. Seismonastic movement of pinnae and pinnules in leaves of the sensitive plant (Mimosa sensitiva) (a) before and after touch stimulation. (Photographs courtesy J.H. Palmer)
Turgor changes also change the curvature of hairs of insectivorous plants. In the case of the Venus fly trap, sensory hairs coupled to an electrical signalling system require stimulation at least twice within a 30 s period (Simons 1992). This appears to allow the plant to discriminate single pieces of debris from an insect crawling within the trap. Most seismonastic movements result from the explosive loss of water from turgid ‘motor’ cells, causing the cells temporarily to collapse and inducing very quick curvature in the organ where they are located.
Similar mechanism causes the slower folding of leaves at night into special positions to reduce heat loss, as in the prayer plant. This also occurs in many legumes. Charles Darwin measured the folding of leaves at nightfall in white clover, and wrote: “The two lateral leaflets will be seen in the evening to twist and approach each other, until their surfaces come into contact, and they bend downwards. This requires a considerable amount of torsion in the pulvinus. The terminal leaflet merely rises up without any twisting and bends over until it forms a roof” (Figure 3.63).
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Figure 3.63 The nictitropic movements of leaves of white clover (Trifolium repens) from daytime (A) to ‘sleeping’ position (B). (Charles Darwin, The Power of Movement in Plants, 1881). (Diagram courtesy R. Purdie)
Movement of some plant organs can be staggeringly fast. The firing of the reproductive structure (“column”) of trigger plants (Stylidium genus) in response to the landing of a pollinating insect may take only 15 msec (Figure 3.64).
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Figure 3.64 Sequence of superimposed images captures the flower column of a trigger plant (Stylidium crassifolium) as it ‘fires’ in response to a physical stimulus (insect landing, seen on upper right). The column rotates through 200° from a ‘cocked position’ to a relaxed position in 20 ms (photographs taken at 2 ms intervals). (Findlay and Findlay 1975)
The kinetic energy manifested in this rapid firing is derived from events controlled at a membrane level. Ions transported into specialised cells cause hydrostatic (turgor) pressure to develop which is suddenly dissipated following mechanical stimulation
Water transport into a cell anywhere in the plant is governed by solute uptake which in turn is governed by the permeability of membranes to water as well as solutes.