1021-4437/04/5104- © 2004
Russian Journal of Plant Physiology, Vol. 51, No. 4, 2004, pp. 525–533. Translated from Fiziologiya Rastenii, Vol. 51, No. 4, 2004, pp. 582–591.
Original Russian Text Copyright © 2004 by Seregin, Shpigun, Ivanov.
Among heavy metals that are not essential for plant
and animal nutrition, Cd and Pb are met most widely.
These metals usually enter plants via the root system
and accumulate in various plant organs and tissues.
Most plant species belong to so-called excluders that
accumulate heavy metals in their underground organs
. The limited translocation of Cd and Pb into the
shoots is believed to result from the barrier function of
the root endodermis: its Casparian strips bar Cd and Pb
entrance into the central cylinder [2–6]. However, it is
not clear whether the Casparian strips are the only lim-
iting factor restricting Cd and Pb transport across the
endodermis into the central cylinder tissues [2–9].
The radial transport of Cd and Pb proceeds mostly
via the apoplast; however, the possibility of symplastic
Pb transport was demonstrated in onion roots  and
garden cress hypocotyls .
When heavy metals enter the cells, even in minute
amounts, they produce the toxic effects upon the numer-
ous processes: they inhibit most enzyme activities, upset
mineral nutrition and water balance, change the hormonal
status, and affect the membrane structure. These disorders
reduce many physiological activities and, at high concen-
trations of heavy metals, lead to cell death [10, 11]. The
toxic effects of heavy metals stem from their binding to the
functional protein groups, in particular to SH-groups,
which are essential for numerous and diverse enzyme
activities. The toxicity level of heavy metals is related to
the strength of their binding to SH-groups . The effects
of various heavy metals on root growth demonstrated that
although the particular heavy metals differed in their tox-
icities, they share some common characteristics, such as
the narrow range of effective concentrations and insigniﬁ-
cant effect of exposure prolongation. The tolerance of root
branching as compared to a high sensitivity of the main
root growth  is a common root response to them.
In the present study, we compared tissue distribution
of Cd and Pb in different root regions and used cell
death criterion to recognize root tissues and organs
most sensitive to these metals.
MATERIALS AND METHODS
Plant growth and treatment with heavy metals.
Maize seeds (
L., cv. Diamant) were sterilized
Distribution and Toxic Effects of Cadmium
and Lead on Maize Roots
I. V. Seregin*, L. K. Shpigun**, and V. B. Ivanov*
*Timiryazev Institute of Plant Physiology, Russian Academy of Sciences, Botanicheskaya ul. 35, Moscow, 127276 Russia;
fax: 7 (095) 977-8018; e-mail: email@example.com
**Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
Received February 28, 2003
—Two-day-old seedlings of maize (
L.) were incubated on Cd and Pb nitrate solutions at the
concentrations that inhibited root growth approximately by 50% after two-day-long incubation (LC
M, respectively) or completely terminated growth of the primary root after one-day-long incubation (LC;
M, respectively). Cd and Pb contents were measured using an anodic inversion voltammetric
technique in a ﬂow injection system and a histochemical method. At LC
, Cd and Pb were discerned, by his-
tochemical techniques, in all root apical tissues, whereas in the root hair zone, the heavy metals were primarily
accumulated in the apoplast of the rhizodermis and cortex and to a lesser extent, in the vascular tissues and
parenchyma cells surrounding the metaxylem vessels. Insigniﬁcant accumulation of Cd and Pb in the pericycle
probably explains why root branching was tolerant to these agents. At LC, Cd and Pb were found in the apoplast
of all root tissues, in accordance with the practically complete inhibition of root growth and branching. Irre-
spectively of Cd and Pb concentrations in the external solution, the metal contents in the root apex exceeded
those in the basal region. Procion dyes were used to assess cell death inﬂicted by Cd and Pb. At LC, the root
cap and meristematic cells perished, together with the rhizodermal cells and the outer cortical cells of the root
apex, whereas only the rhizodermal cells in the root apical region died at LC
. The evidence that Cd and Pb
cross the endodermal barrier at LC presumes that, at lower metal concentrations, the Casparian strip and plas-
malemma of the endodermis regulate the transport of these metals into the central cylinder. The authors con-
clude that the identical barriers control Cd and Pb transport in root tissues.
Key words: Zea mays - cadmium - lead - apoplast - endodermis - toxicity - root
: LC—salt concentration that completely termi-
nated root growth after one day; LC
—salt concentration that
inhibited root growth by 50% after two days of incubation.