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Quantitative Assessment of Ultrastructural Changes in Primary Roots of Corn (Zea mays L.) after Geotropic Stimulation

I. Root Cap ¹

Division of Biological and Medical Research, Argonne National Laboratory, Argonne, Illinois 60439

The root cap is the site of gravity perception. In the study of caps of primary roots of corn (Zea mays L.), we compared the ultrastructure of geotropically responding roots that had received a 661 nm (red) irradiation (60 second) with nonresponding dark control roots kept in the dark, at comparable times following geotropic stimulation for a total of 150 minutes. The outstanding differences in the light-exposed root caps at the ultrastructural level were: (a) significantly more Golgi apparatus (dictyosomes) were found in the top than in the bottom of red-exposed cells; a random distribution is seen in the dark control cells; (b) the nucleus preferred the top in a greater number of the red-exposed cells; (c) the pattern of mitochondria localization was identical in both treatments, a greater preference for the top; however, the number of mitochondria was reduced in the bottom of red-treated cap cells as compared to the control cells. A lowering in number in the bottom of the red-treated cells was noted also in the dictyosomes; and (d) in a small percentage of cells that showed a preferential distribution of endoplasmic reticulum (ER), more red-exposed cells than controls, during the period 30 to 135 minutes after stimulation, had less ER in the top; however, a majority of the cells in both treatments showed no preferred position for ER distribution. Commonalities in ultra-structural behavior also existed between the red- and dark-treated root cap cells: (a) sedimentation of amyloplasts, with no difference in total number between treatments; and (b) a close association between amyloplasts and ER in both groups.

Polarization of organelles occurred in both the geotropically responding and nonresponding roots. The differences in dictyosome and nuclear localization, and dictyosome and mitochondrial number could be correlated with the tropic response in the red-exposed roots and no response in the dark roots, which in turn could be related to the reported hormonal events in the geotropism of roots.