4.2.5  Turgor pressure controlling ion flow

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Figure 4.15 (a) Microcapillaries can be used hydrostatically to increase the turgor pressure in large cells of the alga Valonia. Solute flux into cells was estimated from solution flowing out of the cell through one microcapillary while pressure was applied through another. (b) A decrease in K+ influx as a pressure of 1 atmosphere was applied to cell contents, followed by an increase in influx as pressure was released.

In some large-celled algae, such as Valonia ventricosa, turgor pressure (P) appears to control solute uptake to achieve P regulation. As P decreases, solute uptake increases in what appears to be turgor regulation. To demonstrate this, two fine capillaries (Figure 4.15a) were inserted into Valonia cells (Gutknecht 1968). The capillaries allowed P to be controlled and for cells to be perfused with solutions containing radio-active tracers. When P was increased by even modest amounts, there was a dramatic decrease in the net flux of K+ into the cell (Figure 4.15b). This was reversed when the pressure difference was removed. Both influx and efflux were affected although influx was only affected at low P. Efflux was almost directly proportional to P over the range studied. How does the cell ‘measure’ P? It has been suggested that it is done by specialised areas of membrane that are compressed as P increases. Initial experiments to test this involved compressing the mem-brane with brief pulses of voltages that are much higher than plant membranes normally experience. While this makes those of us who are scientifically conservative a little uncomfortable about such results, subsequently experiments demonstrate the presence of pressure-sensitive ion channels. These channels open under the influence of pressure to allow ions to flow across the membrane and may provide an explanation for the effect of P on ion efflux. Pressure-sensitive efflux appears to play a role in assimilate unloading from legume seed coats (Section 5.6.3(c)) and there is definitive evidence for mechano-sensitive channels in higher plants.