| Abstract: | The distribution of calcium (Ca) in caps of vertically- andhorizontally-oriented roots of Zea mays was monitored to determineits possible role in root graviresponsiveness. A modificationof the antimonate precipitation procedure was used to localizeCa in situ. In vertically-oriented roots, the presumed graviperceptive(i.e., columella) cells were characterized by minimal and symmetricstaining of the plasmalemma and mitochondria. No precipitatewas present in plasmodesmata or cell walls. Within 5 min afterhorizontal reorientation, staining was associated with the portionof the cell wall adjacent to the distal end of the cell. Thisasymmetric staining persisted throughout the onset of gravicurvature.No staining of lateral cell walls of columella cells was observedat any stage of gravicurvature, suggesting that a lateral flowof Ca through the columella tissue of horizontally-orientedroots does not occur. The outermost peripheral cells of rootsoriented horizontally and vertically secrete Ca through plasmodesmata-likestructures in their cell walls. These results are discussedrelative to proposed roles of root-cap Ca in root gravicurvature. Key words: Antimonate, calcium, columella cell, peripheral cell, root gravitropism, Zea mays L. |
