Pollen Tube Growt h and the lntracellular Cytosolic Calcium Gradient Oscillate in Phase while Extracellular Calcium Influx

Ratio images of cytosolic Ca2+ (Ca2+i) in growing, fura-2-dextran-loaded Lilium longiflorum pollen tubes taken at 3to 5-sec intervals showed that the tip-focused [Ca2+Ii gradient oscillates with the same period as growth. Similarly, measurement of the extracellular inward current, using a noninvasive ion-selective vibrating probe, indicated that the tipdirected extracellular Ca2+ (Ca2+o) current also oscillates with the same period as growth. Cross-correlation analysis revealed that whereas the [Ca2+Ii gradient oscillates in phase with growth, the influx of Ca2+o lags by -11 sec. lon influx thus appears to follow growth, with the effect that the rate of growth at a given point determines the magnitude of the ion influx -11 sec later. To explain the phase delay in the extracellular inward current, there must be a storage of Ca2+ for which we consider two possibilities: either the inward current represents the refilling of intracellular stores (capacitative calcium entry), or it represents the binding of the ion within the cell wall domain.