Page 32 - Volume 1, Issue 1, January 2008

A. Assimakopoulou et al. 30
tion in foliage of lettuce Butterhead and
the concentration of nitrates in foliage of
spinach plants were found lower than the
relevant critical ones (10, 13). At those ni-
trogen supplies the concentration of Mn,
Zn and Cu in the foliage was increased
(Tables 1, 3). The observed accumulation
of these micronutrients was probably due
to the “concentration effect” (increase in
concentration caused by reduced plant
size) and/or due to specific changes in the
absorption, transport and distribution of
the micronutrients in the plants (7). In the
same experiments, the observed reduc-
tion in the concentration of Mn, Zn and Cu
at high concentrations of nitrogen in the
nutrient solution could be explained by
the increased plant size due to increased
nitrogen supply and/or changes in the up-
take rate of micronutrients. The increased
nitrogen supply caused an increased de-
mand for micronutrients, which could
not be satisfied by an enhanced uptake
of micronutrients since their concentra-
tions were stable in the supplied nutrient
solution of the different treatments. Fur-
thermore, it is well known that the form
of nitrogen added (ammonium or nitrate)
causes changes in the pH at root surface
and root apoplast. These changes influ-
ence the uptake, translocation, remobiliza-
tion and utilization of several micronutri-
ents (1, 14, 16). Therefore, it is probable that
the reduction of Mn, Zn and Cu concentra-
tion in the plants could be attributed, as
well, to pH increase at root surface and root
apoplast because of nitrate nitrogen added
in the nutrient solution. Many plants spe-
cies grown on a complete medium with ni-
trate-N usually excrete OH- or HCO3
- ions
into the nutrient solution causing pH in-
crease in the rhizosphere (5, 6, 4, 3, 1).
Regarding lettuce plants grown in the
field (experiment B), an increasewas found
in foliage Zn and Cu concentration with
increasing nitrogen supply up to the level
N5. These results are different from those
in lettuce and spinach plants grown in the
nutrient solution. Comparisons of these
experiments could not be made since into
the soil, greater reserves of the micronu-
trients could be mobilized and satisfy the
increased demand of the plants; besides,
soil analysis of the experimental field (12)
showed that the concentrations of Mn, Zn
and Cu in the soil were found between ad-
equate and high levels.
In all experiments, leaf Fe concentration
of plants grown either in hydroponics or in
the field was not significantly differentiat-
ed among the nitrogen treatments, as well
as, leaf Fe concentration was not significant-
ly correlated with plant growth. This discrep-
ancy could be explained by the assumption
that only a portion of Fe in plants participates
in metabolic reactions or is incorporated into
molecular structures (8) and leaf analysis does
not always reflect plant Fe status (9).
Furthermore, in experiment C, com-
paring the concentration determined in
the root system to that of foliage, a high-
er accumulation of micronutrients was
found in roots (Table 3). Similar results
have been reported by other researchers
(10, 16); according to them, more Fe was
immobilized in the root, due to high pH
(plants fed with 100% NO3-N).
We hope that this preliminary study of
the impact of several nitrogen levels on
plant micronutrient concentrations will
contribute to better understanding of nu-
trient balance and can lead to more effi-
cient crop production.
The authors are grateful to E. Moustaka and
N. Panagopoulou for technical assistance as
well as to A. Rodger for correcting English.
Literature Cited
Fageria, V.D. 2001. 1. Nutrient interactions in
crop plants. J. Pl. Nutr., 24: 1269-1290.
Hewitt, E.J. 1966. 2. Sand andWater CultureMeth-