Effect of nickel on growth, proliferation, and differentiation of root cells in Triticum aestivum seedlings

The effect of 10(-6) and 10(-4) M NiSO4 on cell growth, proliferation, and differentiation was studied over 48 h in seminal and lateral roots of five-day-old Triticum aestivum seedlings. 10(-6) M NiSO4 did not significantly affect the root system, whereas 10(-4) M NiSO4 inhibited its development. However, 10(-6) M NiSO4 disturbed the contacts between the groups of closely related cells of the rhizodermis in the meristem. In the exodermis, an additional layer of cells was formed. At the nickel concentration of 10(-4) M, cell divisions in the outer layers of the root cells and metaxylem ceased earlier than in other root tissues positioned both centripetally and acropetally. Differentiation of protophloem sieve elements was completed in the meristem but at a greater distance from the root tip. Cell elongation started at the same distance from the root tip as in control plants. The rate of elongation decreased, and acropetally it stopped. Therefore, the cells of the xylem and metaphloem started to differentiate, and primordia of lateral roots were initiated and formed closer to the root tip. At a lethal concentration (10(-4) M), nickel induced necroses of elongating cells of the endodermis and pericycle. Nickel is supposed to enter the tissues of the central cylinder predominantly via the protoxylem and rapidly translocate along the xylem. As a result, the incubation of the roots at this concentration for 48 h almost did not affect the development of the phloem and probably sugar unloading, that makes possible to maintain the growth of meristematic cells and the cell division of the most important tissues for longer time.