ISSN 1021-4437, Russian Journal of Plant Physiology, 2011, Vol. 58, No. 1, pp. 109–117. © Pleiades Publishing, Ltd., 2011. Original Russian Text © I.V. Seregin, A.D. Kozhevnikova, V.V. Gracheva, E.I. Bystrova, V.B. Ivanov, 2011, published in Fiziologiya Rastenii, 2011, Vol. 58, No.1, pp. 85–94. 109 INTRODUCTION Zinc (Zn) is an essential micronutrient because it is a cofactor of many enzymes involved in nitrogen metabolism, photosynthesis, auxin biosynthesis, the synthesis of nucleic acids and proteins, and many other processes [1]. However, like other heavy metals, high Zn concentrations could be toxic for diverse physiological processes, which could result in the dis- turbances of plant growth and morphogenesis [1, 2]. In connection with uneven accumulation of heavy metals in different plant tissues and organs, the prob- lem of their compartmentation deserves an attention at investigation of metal toxicity and the mechanisms of plant tolerance to them. Most of numerous studies of Zn accumulation, concern aboveground organs of excluders and hyperaccumulators, the two contrasting plant groups accumulating metals in underground and aboveground organs, respectively [3–5]. Much less is known about tissue Zn distribution in the root system [3, 6], although just the analysis of heavy metal distribution in the root tissues is required for understanding the mechanisms of their toxicity and morphological and physiological mechanisms of their detoxification. The limited amount of such data is related to insufficient development of histochemical methods for heavy metal detection. Elaboration and modification of such methods of Zn detection and also the analysis of its tissue distribution in different root parts is one of the tasks of this work. To check and con- firm the results of histochemical analysis, the content of Zn in shoots and different root zones of maize seed- lings was determined. Heavy metals penetrate the plant mainly through its root system. Therefore, the primary plant response to their action occurs just in the root, and this is clearly manifested in growth suppression. In this connection, the root is often used as a test-system for investigation of toxic effects of various heavy metals [7–14]. Tissue Zinc Distribution in Maize Seedling Roots and Its Action on Growth I. V. Seregin, A. D. Kozhevnikova, V. V. Gracheva, E. I. Bystrova, and V. B. Ivanov Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, 127276 Russia; fax: 7 (495) 977-8018; e-mail: ecolab-ipp@yandex.ru Received March 23, 2010 Abstract—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(NO 3 ) 2 . Zn toxicity was assessed after the inhi- bition of primary root increment during the first and second days of incubation. The content of Zn was deter- mined 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. Keywords: Zea mays, zinc, root, growth, cell division, cell elongation. DOI: 10.1134/S1021443711010171 RESEARCH PAPERS Abbreviations: T l —time of the cell life in the meristem.