In this work we present the results of anatomical and ultrastructural investigations of root apices of Brassica rapa 6-day old seedlings grown in the stationary conditions and under slow horizontal clinorotation. The main attention was paid to the structural and functional organization of cells non-specialized for gravity perception during their growth and differentiation in a root. It was shown a decrease in length of the root growth zone due to reduction of the cell number in cell files under clinorotation. We conclude that the meristem proliferation activity was depressed under clinorotation. Some ultrastructural rearrangements of mitochondria in the distal elongation zone (DEZ) and increasing of their partial volume per cell were also observed. It is assumed that these changes reflect the intensification of mitochondrium functional activity in a DEZ cortex cells during clinorotation. An analysis of the experimental results supposes plants sense disorientation in the gravitational field as a stress.
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