Thus far we do not have information concerning the oscillations, if any, in the intracellular activities of these ions, but the extracellular fluxes of both ions oscillate in relation to the changes in growth rate. The extracellular influx of K+ presents a very similar phase relationship to growth as do H+, and is also not significantly different from that of Ca2+; thus, the influx of K+ lags growth by about 100° (Messerli et al. 1999). Cl-, in contrast to these other ions, exhibits a marked efflux from the apex, and an influx along the sides of the clear zone (Zonia et al. 2002). Similarly to H+, only the activity at the apex has been observed to oscillate. In tobacco pollen tubes, Cl- efflux occurs in exact phase with the growth rate (Zonia et al. 2002). However, as noted earlier, these results have been challenged by Messerli et al. (2004) raising the hypothesis that it may be another anion or H+ that are being measured by Zonia et al. (2002).
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