tions [5]. Colloidal lanthanum caused an almost complete inhibition of cell division and root elon-
gation in the root tips of barley plants [6]. La
3 +
had been shown to inhibit root elongation of wheat
[7,8]. More
recent reports,
however, demonstrated some positive effects of REEs on
plant growth. Diatloff et al. [9] reported that corn root growth increased significantly with the
applications of cerium 0.63 mM and lanthanum 0.63
m M.
Applications of
lanthanum and
cerium were also reported to increase spike pro- duction in wheat [10]. In pot trials, applications
of cerium sulphate up to 100 mgkg enhanced root and shoot growth of Phaseolus radiatus and
Brassica pekinensis [11].
Results from field trials were also inconsistent. The increase in crop yield reported by workers
from China ranged between 8 and 50, with the common response being of the order of 8 – 15
[12,13]. However, no response was found by spraying and seed dressing of a summer fodder
crop Brassica sp. with REEs in a field trial carried out in Australia cited in [4]. In view of
this, it is essential to study and elucidate the effects of REE on essential stages of growth and
development of a model plant species such as Arabidopsis thaliana. In this report, we investi-
gated the effects of cerium Ce and lanthanum La on vegetative and reproductive growth of A.
thaliana and correlated some of the responses to increased sensitivity of cell to plant growth regu-
lators.
2. Materials and methods
2
.
1
. Plant materials, culture media and growth conditions
Seeds of A. thaliana L. Heynh cv. Columbia LEHLE SEEDS, USA were surface sterilized
by soaking in 75 alcohol for 30 s and followed by 15 Clorox
®
for 15 min. The seeds were then rinsed five times in sterilized water prior to cul-
ture. The 14 strength Murashige and Skoog medium [14] was used for seed germination and
as basal medium. The pH of the medium was adjusted to 5.8 before agar Difco, 0.8 was
added. All media were autoclaved for 20 min at 121°C. Cerium nitrate hexahydrate CeNO
3 3
·6H
2
O, Sigma and lanthanum nitrate hexahy- drate LaNO
3 3
·6H
2
O, Sigma were dissolved in Mili-Q water and sterilized by membrane filtra-
tion Millipore, 0.45 mm and stored at room temperature in the dark. Stock solutions of
cerium nitrate and lanthanum nitrate were added to the autoclaved basal medium prior to dispens-
ing into Magenta GA7
®
containers Magenta Corp., USA. Seeds were germinated in the dark
and 2-day-old seedlings were placed under 16 h photoperiod 54 mmol
− 1
m
− 2
s
− 1
provided by Cool White fluorescent lamps at 25 9 2°C.
2
.
2
. Growth measurements Lengths of primary roots were scored 10 days
after seed germination. The number of leaves produced per plant was scored 17 days after seed
germination. Plant heights and dry weights were scored 17 and 35 days after seed germination,
respectively. Dry weights were taken by drying 100 plants in an oven 55°C for 1 week. Floral
initiation was recorded when the plant bolted with at least 1 cm long inflorescence stalk.
2
.
3
. Extraction and determination of endogenous cytokinins and carbohydrates
Approximately 1 g fresh weight of tissues was homogenized in 4 ml of 80 ethanol followed by
1 h incubation at 4°C. After centrifugation at 1670 × g for 3 min, the supernatant was trans-
ferred to another centrifuge tube. The tissues were re-extracted with 2 ml of 80 ethanol, and
the supernatant was pooled together after cen- trifugation. The extracts were vacuum evaporated
at 4°C Eppendorf Concentrator 5301 to 14 volume and then stored at − 20°C after filter-
sterilization Milipore, 0.2 mm. The analysis of zeatin
riboside ZR,
dihydrozeatin riboside
DHZR, and isopentenyl adenosine IPA were performed by immunoassay detection kits Sigma
Chemical Company according to the protocols provided by the manufacturer. All hormonal lev-
els were expressed in terms of pmol per gram fresh weight pmolg.f.wt. The analysis of su-
crose, glucose and fructose were performed by assay kits Sigma Chemical Company according
to the protocols provided by the manufacturer. The levels of sucrose were expressed in term of
mg per gram fresh weight mgg.f.wt. The levels of glucose and fructose were expressed in term of
m g per g fresh weight mgg.f.wt.
3. Results
