Changes in cell length and mitotic index in vascular pattern-related pericycle cell types along the apical meristem and elongation zone of the onion root

Abstract

Three pericycle cell types (opposite xylem, opposite phloem and intervening) distinguished by their location in relation to different elements of the vascular system were studied in the adventitious root ofAllium cepa L. Changes in cell length and mitotic index were analysed in these cells along the apical meristem and elongation zone of the root. The opposite phloem and intervening pericycle cells are significantly shorter than the opposite xylem pericycle cells in the apical half of the meristem. Between 1,200 and 1,400 μm behind the tip, length became similar in all three pericycle cell types, while in more proximal zones the opposite phloem cells were significantly longer. These results suggest that the number of transverse divisions is different in the three types of pericycle cells. In the apical half of the meristem, mitotic index increased in intervening and opposite xylem cells but remained unchanged in opposite phloem cells, a fact likely to account for the relative lengthening of the latter. In the proximal half of the meristem, mitotic index fell in all three cell types until cell division had ceased. However, mitotic index in opposite xylem cells remained high for longer than in the other two cell types, implying that increase of the mean cell length in the former was slower. These results suggest that differences in mean cell length between the three pericycle cell types are due to different rates of proliferation.