Studies showing that the application of an increased level of osmoticum in the medium inhibits pollen tube growth suggest that osmotic regulation and the generation of turgor pressure are essential for cell elongation (Pierson et al. 1996). Messerli and Robinson (2003) have shown that an abrupt increase in turgor pressure, brought about by decreasing the osmolarity of the medium, can generate a brief increase in the growth rate. However, it does not follow that changes in turgor pressure underlie changes in growth rate, since studies using a pressure probe failed to detect a correlation between turgor pressure and growth rate, even during oscillatory growth (Benkert et al. 1997). Given the importance of maintaining turgor pressure, it is attractive to imagine that both K+ and Cl- play a key role (Fig. 4), since substantial fluxes of both ions have been measured. As noted by Zonia et al. (2002), the large efflux of Cl-observed in pollen tubes could contribute to salt extrusion and turgor regulation, as has been shown in other systems, notably guard cells (Cosgrove and Hedrich 1991).
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