The distribution of calcium in the grass pollen tube

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Abstract
The distribution of calcium in the terminal regions of actively extending pollen tubes of two Gramineae, Zea mays and Pennisetum americanum, has been investigated by chlorotetracycline (CTC) fluorescence and by energy-dispersive X-ray analysis. Neither method reveals a concentration gradient declining from the tip towards the older parts of the tube comparable with that reported from the pollen tubes of Lilium longiflorum. The difference evidently arises from dissimilarites in pollen-tube growth physiology and the apical organization of the tube. Growth is achieved by the insertion of dictyosome-derived vesicles carrying wall-precursor materials at the tube tip. In L. longiflorum these are produced in a subapical zone of the tube, and a characteristic zonation of cytoplasmic organelles in the distal region develops during growth. In the grasses, large numbers of the wall precursor bodies (''P-particles'') are produced before pollen dispersal, and are stored in the grain; they are distributed throughout the tube during early growth, and the organelle zonation in the apex is less pronounced. CTC-induced fluorescence is strongly associated with mitochondria, membranes and P-particles, suggesting that the observed distribution of calcium may reflect mainly the distribution of the element held in organelles and membranes rather than mobile Ca2+ in the cytosol.