CALCIUM MOVEMENT, GRAVIRESPONSIVENESS, AND THE STRUCTURE OF COLUMELLA CELLS IN PRIMARY ROOTS OF AMYLOMAIZE MUTANTS OF ZEA MAYS

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Primary roots of Zea mays cv. Amylomaize were less graviresponsive than primary roots of the wild‐type Calumet cultivar. There were no significant differences in: 1) the partitioning of volume to organelles in columella cells, 2) the size or density of amyloplasts, or 3) rates and overall patterns of organelle redistribution in horizontally‐oriented roots of the two cultivars. Amyloplasts and nuclei were the only organelles whose movement correlated positively with the onset of root gravicurvature. However, the onset of gravicurvature was not directly proportional to the average sedimentation rate of amyloplasts, since amyloplasts sedimented at equal rates in columella cells of both cultivars despite their differences in root gravicurvature. The more graviresponsive roots of Calumet seedlings were characterized by a more strongly polar movement of 45Ca2+ from the upper to lower sides of their root tips than the less graviresponsive roots of Amylomaize seedlings. These results suggest that the decreased graviresponsiveness of horizontally‐oriented roots of Amylomaize seedlings may be due to a delay in or decreased ability for polar transport of calcium rather than to smaller, more slowly sedimenting amyloplasts as has been suggested for their less graviresponsive coleoptiles.