Tissue zinc distribution in maize seedling roots and its action on growth

Zinc (Zn) distribution over tissues and organs of maize (Zea mays L.) seedlings and its action on root growth, cell division, and cell elongation were studied. Two-day-old seedlings were incubated in the 0.25-strength Hoagland solution containing 2 or 475 μM Zn(NO3)2. Zn toxicity was assessed after the inhibition of primary root increment during the first and second days of incubation. The content of Zn was determined by atomic absorption spectrometry in the apical (the first centimeter from the root tip) and basal (the third centimeter from the kernel) root parts. Zn distribution in various tissues was studied by histochemical methods, using a metallochromic indicator zincon and fluorescent indicator Zinpyr-1 and light and confocal scanning fluorescent light microscopy, respectively. To evaluate Zn effects on growth processes, the average length of the meristem; the length of fully elongated cells; the number of meristematic cells in the cortex row; and duration of the cell cycle were measured. When the Zn concentration in the solution was high, the Zn content per weight unit was higher in the basal root part due to its accumulation in lateral root primordial. Zn was also accumulated in both the meristem apoplast and cell protoplasts. In the basal and middle root parts, Zn was detected essentially in all tissues predominantly in the apoplast. Zn inhibited both cell division and elongation. Under Zn influence, the size of the meristem and the number of meristematic cells decreased, which was determined by an increase in the cell cycle duration. The length of the fully elongated cells was also reduced. A comparison of Zn distribution and growth-suppressing activity with other heavy metals studied earlier allows a conclusion that toxic action of heavy metals is mainly determined by physical and chemical properties of their ions and specific patterns of their transport and distribution. As a result, two basic processes determining root growth, e.g., cell division and elongation, could be affected differently.