Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol84.Issue3-4.June2000:
Scientia Horticulturae 84 (2000) 265±273
Effects of nickel concentration in the nutrient solution
on the nitrogen assimilation and growth of tomato
seedlings in hydroponic culture supplied with urea
or nitrate as the sole nitrogen source
Xue Wen Tan, Hideo Ikeda*, Masayuki Oda
Laboratory of Vegetable Crops, College of Agriculture, Osaka Prefecture University,
Sakai, Osaka 599-8531, Japan
Accepted 13 August 1999
Abstract
Both the bene®cial and the adverse effects of nickel supplement on the N assimilation and growth
of tomato plants were evaluated while either urea or nitrate was applied as the sole N source in the
nutrient solution.
The nickel concentration in the plant was related to that in the solution. Urea toxicity to the plants
was reduced by the nickel supplement at 0.01 mg lÿ1, and no symptom of urea toxicity was
observed in the plants when supplemented nickel was at 0.1 or 1 mg lÿ1. Both the plant growth and
chlorophyll concentration in leaves of the urea-fed plants increased when nickel concentration in
the solution was up to 0.1 mg lÿ1, but decreased by nickel at 1 mg lÿ1. The maximum growth of the
urea-fed plants promoted by nickel supplement was still less than 80% of the nitrate-fed plants.
Nickel supplement did not affect the growth of the nitrate-fed plants.
The concentrations of leaf total-N in the urea-fed plants was enhanced by the nickel supplement,
but was still lower than those in the nitrate-fed plants. At 0.1 mg lÿ1 nickel supplement, the
concentrations of leaf urea-N and NH4-N in the urea-fed plants were about one-sixth and four times,
respectively, of those without nickel supplement. Urea assimilation increased with the nickel
supplement up to 0.1 mg lÿ1. No effect of nickel supplement on the N assimilation in the nitrate-fed
plants was found.
Nickel supplement up to 0.1 mg lÿ1 reduced urea toxicity and enhanced chlorophyll
concentration, plant growth, and urea hydrolysis. The symptom of nickel toxicity and the
*
Corresponding author. Tel: 81-722-54-9421; fax: 81-722-54-9918.
E-mail address: ikeda@plant.osakafu-u.ac.jp (H. Ikeda).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 0 7 - 7
266
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
depression of plant growth were only observed with the nickel supplement up to 1 mg lÿ1. # 2000
Elsevier Science B.V. All rights reserved.
Keywords: Tomato; Nickel; Urea; Growth; Urea toxicity; N assimilation
1. Introduction
Urea is an important nitrogen (N) fertilizer in today's agriculture and its
suitability for ®eld crops has been well evaluated (Vavrina and Obreza, 1993).
However, urea in comparison to nitrate is not a desirable hydroponic N source for
tomatoes (Kirkby and Mengel, 1967; Ikeda and Tan, 1998), lettuces (Luo et al.,
1993), zucchinis (Gerendas and Sattelmacher, 1997), and rices (Gerendas et al.,
1998) due to its toxicity. The possible cause of urea toxicity is either by the NH
4
released during urea assimilation (Luo et al., 1993) or by urea itself (Krogmeier
et al., 1989). This latter conclusion was supported by a subsequent experiment in
which nickel de®cient plants showed lower urease activity and more leaf-tip
necrosis than non-de®cient plants (Krogmeier et al., 1991). Because urease is the
®rst enzyme involved in urea assimilation by plant tissues for the hydrolysis of
urea (Hogan et al., 1983), one possible way to avoid urea toxicity is to increase
the activity of urease.
Nickel is an essential micronutrient for some higher plants (Brown et al., 1987),
and the enzyme urease from jack beans (Dixon et al., 1975) and soybean seed
(Polacco and Havir, 1979) is known to be a nickel metalloenzyme. Therefore,
nickel is considered to be an important element for plants applied with urea as a
N source due to its role in the metalloenzyme urease (Gerendas et al., 1998). The
possible way to avoid urea toxicity is to use nickel supplement to enhance the
urease activity (Bekkari and Pizelle, 1992; Marschner, 1995).
Although the requirement of nickel for the urease activity was discovered more
than 20 years ago (Dixon et al., 1975), effects of nickel concentration in the urea
solution on the growth have received little attention. In this study, we tested
whether the nickel supplement can reduce urea toxicity and investigated effects of
nickel concentration in the solution on the N assimilation and growth of tomato
plants in hydrophonic culture supplied with urea or nitrate as the sole nitrogen
source.
2. Materials and methods
2.1. Plant materials, growth conditions, and treatments
Seeds of tomato (Lycopersicon esculentum Mill., cv. Momotaro) were
germinated on well washed and sterilized sand. Seedlings, 5 days after
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
267
emergence, were transferred to a hydrophonic culture in a greenhouse under
natural sunlight during autumn (average air temperature 32/208C day/night). A
half strength Hoagland's nutrient solution was used before the onset of
treatments.
When the seedlings had grown to the 6±7 leaf stage, eight plants were
transplanted into a 15 l polypropylene container. The basic nutrient solution was
prepared as follows: (in mM) K2SO4: 2, CaCl22H2O: 1.5, MgSO47H2O: 1,
NaH2PO42H2O: 2/3, and micronutrients. Plants were treated with two N sources:
urea and nitrate as NaNO3 at 168 mg N lÿ1, and four concentrations of nickel as
NiSO46H2O: 0, 0.01, 0.1, and 1 mg lÿ1. Treatments were arranged in a
randomized block design with three replicates. All solutions were prepared with
deionized water, aerated using an aquarium air stone, and were renewed every 7
days. Solution pH was daily checked and adjusted to 6.0 with 1 N NaOH or HCl.
2.2. Investigations, samplings and chemical analyses
Before harvest, the concentrations of leaf chlorophyll were measured with a
hand-held chlorophyll meter (SPAD-502, Minolta), and visual leaf symptoms of
urea toxicity were assessed with index from 0 (none) to 3 (severe). Plants were
harvested 4 weeks after treatment and were divided into leaves, stems, and roots.
These plant materials were then dried immediately in a forced-air oven at 608C to
a constant weight, weighed, and were ground to a ®ne powder in a Wiley mill to
pass through a 20-mesh sieve. The concentrations of total-N and nickel in the leaf
samples were determined by a modi®ed Kjeldahl method and ICP, respectively.
To determine the concentrations of urea-N, NH4-N, and NO3-N, the samples were
extracted with hot water. The concentrations of urea-N were determined using the
method of Cline and Fink (1956), and the concentrations of NH4-N and NO3-N
were determined using ion exchange chromatography (Dionex DX-AQ).
A statistical analysis was made using analysis of variance, and the means were
separated by Duncan's multiple range test (DMRT) at the 5% level.
3. Results
Leaf nickel concentrations in the urea-fed plants increased signi®cantly with
the increase of the nickel concentration in the solution from 0.01 to 1 mg lÿ1: 7.5
times from 0.01 to 0.1 mg lÿ1 and 13.9 times from 0.1 to 1 mg lÿ1 (Fig. 1). Leaf
nickel concentrations in the urea- or nitrate-fed plants without nickel supplement
were relatively low. N source did not signi®cantly affect leaf nickel concentration.
The symptoms of urea toxicity were severe in the urea-fed plants without
nickel supplement, and the symptoms were reduced in the plants with nickel
supplement at 0.01 mg lÿ1 (Fig. 2). No symptom of urea toxicity was observed in
the plants supplemented with nickel at 0.1 or 1 mg lÿ1.
268
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
Fig. 1. Effects of nickel concentration in the nutrient solution on the leaf nickel concentration in the
tomato plants applied with urea or nitrate. Leaf samples were obtained 4 weeks after treatment.
Bars having different letters are signi®cantly different at the 5% level by DMRT.
Fig. 2. Effects of nickel concentration in the nutrient solution on the symptom index of urea
toxicity on the leaves of the tomato plants applied with urea. Leaf samples were obtained 4 weeks
after treatment. The indexes of visual symptoms were from 0 (none) to 3 (severe). Bars having
different letters are signi®cantly different at the 5% level by DMRT.
269
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
Table 1
Effects of nickel concentration in the nutrient solution on the dry matter increments of tomato
plants fed with urea or nitrate for 4 weeksa
Nitrogen
source
Nickel
concentration
Shoot
(g/plant)
Root
(g/plant)
Total
(g/plant)
Relative
valueb
Urea
0
0.01
0.10
1.00
2.29d
3.09c
4.34b
2.52d
0.70b
0.91a
1.07a
0.69b
2.99d
4.00c
5.41b
3.21d
43
58
78
46
Nitrate
0
0.10
5.98a
6.05a
0.94a
0.93a
6.92a
6.98a
100
101
a
Means in each column followed by different letters are signi®cantly different at the 5% level by
DMRT. DW increment: DW (4 weeks after treatment)ÿDW (before treatment).
b
The total dry matter increments of the nitrate-fed plants without nickel supplement is
represented as 100.
The dry matter increments of the shoots or roots of the urea-fed plants
increased signi®cantly with the increase of the nickel supplement from 0 to
0.1 mg lÿ1, but decreased sharply with nickel at 1 mg lÿ1 (Table 1). The highest
dry matter increment in plants fed with urea at 0.1 mg lÿ1 nickel supplement was
78% of the nitrate-fed plants without nickel supplement. The dry matter
increments of the shoots or roots of the nitrate-fed plants were not affected by
nickel supplement.
With urea nutrition, leaf chlorophyll concentrations were low in the plants
without nickel supplement and in the plants supplemented with nickel at 1 mg lÿ1
(Fig. 3). Leaf chlorophyll concentrations in the nitrate-fed plants were not
signi®cantly affected by the nickel supplement.
The lowest concentrations of leaf total-N were detected in the urea-fed plants
without nickel supplement (Table 2). The concentrations of leaf urea-N and its
percentages in the total-N decreased greatly with the increase of the nickel
supplement from 0 to 0.1 mg lÿ1, but no further reduction at 1 mg lÿ1. Although
the concentrations of leaf NO3-N in the urea-fed plants were not affected by the
nickel supplement, the concentrations of leaf NH4-N increased sharply with the
increase of the nickel supplement. The concentrations of leaf urea-N and NH4-N
in the urea-fed plants with nickel supplement at 0.1 mg lÿ1 were about one-sixth
and four times of those without nickel supplement, respectively. Urea assimilation
increased as the nickel concentration in the solution increased from 0 to
0.1 mg lÿ1, but no further increase at 1 mg lÿ1. Higher concentrations of total-N
were detected in the nitrate-fed plants than in urea-fed plants. No effects of nickel
supplement on the concentrations of total-N, NH4-N, NO3-N, and the assimilation
of nitrate in the nitrate-fed plants were found.
270
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
Fig. 3. Effects of nickel concentration in the nutrient solution on the leaf chlorophyll
concentrations of the tamto plants applied with urea or nitrate. Values were SPAD readings from
chlorophyll meter (SPAD-502, Minolta) 4 weeks after treatment. Bars having different letters are
signi®cantly different at the 5% level by DMRT.
Table 2
Effects of nickel concentration in the nutrient solution on the concentration and assimilation of N in
the leaves of tomato plants fed with urea or nitrate for 4 weeksa
Nitrogen
source
Nickel
concentration
Total-N
Urea-N
NH
NOÿ
Urea-N/
Assimilated
3 -N
4 -N
ÿ1
ÿ1
ÿ1
(mg g ) (mg g ) (mg g ) (mg gÿ1) total-N (%) (%)b
Urea
0
0.01
0.10
1.00
35.81c
39.74b
41.14b
40.86b
Nitrate
0
0.10
55.04a
54.90a
13.14a
5.68b
2.22c
2.35c
0.91c
1.83b
3.55a
3.54a
0.50b
0.55b
0.49b
0.49b
1.09c
1.10c
9.64a
9.46a
36.7a
14.3b
5.4c
5.8c
63.3c
85.7b
94.6a
94.2a
82.5b
82.8b
a
Means in each column followed by different letters are signi®cantly different at the 5% level by
DMRT. Data were obtained 4 weeks after treatment.
b
Assimilated N (%): the percentage of the concentration of N changed from treatment divided
by the concentration of total-N, according to Bowman and Paul (1992).
4. Discussion
We have shown that nickel in the nutrient solution was absorbed by the plants
as the leaf nickel concentration was signi®cantly increased by the increase of the
nickel concentration supplemented in the nutrient solution.
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
271
The concentrations of leaf urea-N in the urea-fed plants without nickel
supplement were relatively high and were decreased signi®cantly with the nickel
supplement. The concentrations of leaf urea-N in the plants supplemented with
nickel at 0.1 mg lÿ1 were only about one-sixth in the plants without nickel
supplement, while the concentrations of leaf NH4-N in the former plants were
about four times of the latter plants. The changes of the concentrations of leaf
urea-N and NH4-N in the urea-fed plants indicate that a very rapid initial
hydrolysis of urea is stimulated by the nickel absorbed from the nutrient solution.
This result is in agreement with the ®ndings by Nicouland and Bloom (1998) with
tomatoes. The rapid hydrolysis of urea may be caused by higher activity of urease
activated by the addition of nickel (Eskew et al., 1984; Brown et al., 1987).
Nickel supplement to the urea solution reduced the symptom of urea toxicity.
The toxicity is caused by urea rather than NH
4 , the urea assimilation product, for
the following reasons: (1) the concentration of leaf urea-N decreased with the
nickel supplement, and the lower urea-N concentration was accompanied by less
toxicity symptom; (2) the symptom of toxicity reported here was different from
that of NH
4 toxicity. In this study, the toxicity symptom was the brown color
around the leaf margin, while partial leaf yellowing, mottled chlorosis, and curing
the symptoms of NH
4 toxicity in our previous experiment with tomato; and (3)
As the concentration of leaf NH
4 increased greatly with the nickel supplement in
urea-fed plants, while the urea toxicity symptom was reduced; urea toxicity is
unlikely caused by NH
4 in the leaf.
Nickel supplement promoted the growth of urea-fed plants, and the growth
promotion is related to the improvement of urea assimilation. In this study, both
the urea assimilation and the chlorophyll concentration in the urea-fed plants
increased when nickel was supplemented. The increase of the chlorophyll
concentration demonstrates strongly the improvement of urea assimilation by the
nickel supplement because chlorophyll is one of the important N-containing
compounds (Barker, 1989). Minotti et al. (1994) demonstrated that the N status in
the plant can be assessed by the measurement of chlorophyll. The growth
promotion is also related to the absorption of urea. The concentrations of leaf
total-N in the urea-fed plants with nickel supplement were higher than those
without nickel supplement, indicating that the absorption of urea is increased by
the nickel supplement. A rapid growth of the urea-fed plants was obtained with
nickel supplement at 0.1 mg lÿ1, while is about 80% of the nitrate-fed plants. We
think that the growth difference between urea- and nitrate-fed plants is caused by
the difference of N absorption. Although the concentrations of leaf total-N in the
urea-fed plants increased with the nickel supplement, they were still lower than
those of the nitrate-fed plants.
Although nickel supplement to the urea solution promoted plant growth, nickel
at high concentration depressed plant growth. The growth depressed by the high
nickel concentration may be caused by nickel toxicity because many small black
272
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
spots on the stems and brown roots were observed in the plants supplemented
with nickel at 1 mg lÿ1. The symptoms are caused by nickel itself, rather than by
urea or NH
4 : (1) these symptoms differ from those caused by urea or NH4
toxicity and (2) the concentration of leaf urea-N or NH4-N in the plants
supplemented with nickel at 0.1 mg lÿ1 was similar to that supplemented with
nickel at 1 mg lÿ1, but these plants had no nickel toxicity symptoms.
The mechanisms of nickel toxicity to the plant are still unclear. Gabbrielli et al.
(1990) reported that root growth and cell division of Silene italica is sensitive to
the nickel concentration in the solution. On the other hand, Yang et al. (1997)
found that the tolerance of some species to the nickel concentration depends on
the interaction of nickel with organic acids.
Acknowledgements
We thank all the students in our laboratory for their assistance during the
experiment. This study was supported by Grants-in-Aid for Scienti®c Research
(H. Ikeda: No. 08456023) from the Ministry of Education, Science, Sports, and
Culture of Japan.
References
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Biochem. 30(2), 187±192.
Bowman, D.C., Paul, J.L., 1992. Foliar absorption of urea, ammonium, and nitrate by perennial
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Production and Environment. Kluwer Academic Publishers, Tokyo, pp. 385±390.
Effects of nickel concentration in the nutrient solution
on the nitrogen assimilation and growth of tomato
seedlings in hydroponic culture supplied with urea
or nitrate as the sole nitrogen source
Xue Wen Tan, Hideo Ikeda*, Masayuki Oda
Laboratory of Vegetable Crops, College of Agriculture, Osaka Prefecture University,
Sakai, Osaka 599-8531, Japan
Accepted 13 August 1999
Abstract
Both the bene®cial and the adverse effects of nickel supplement on the N assimilation and growth
of tomato plants were evaluated while either urea or nitrate was applied as the sole N source in the
nutrient solution.
The nickel concentration in the plant was related to that in the solution. Urea toxicity to the plants
was reduced by the nickel supplement at 0.01 mg lÿ1, and no symptom of urea toxicity was
observed in the plants when supplemented nickel was at 0.1 or 1 mg lÿ1. Both the plant growth and
chlorophyll concentration in leaves of the urea-fed plants increased when nickel concentration in
the solution was up to 0.1 mg lÿ1, but decreased by nickel at 1 mg lÿ1. The maximum growth of the
urea-fed plants promoted by nickel supplement was still less than 80% of the nitrate-fed plants.
Nickel supplement did not affect the growth of the nitrate-fed plants.
The concentrations of leaf total-N in the urea-fed plants was enhanced by the nickel supplement,
but was still lower than those in the nitrate-fed plants. At 0.1 mg lÿ1 nickel supplement, the
concentrations of leaf urea-N and NH4-N in the urea-fed plants were about one-sixth and four times,
respectively, of those without nickel supplement. Urea assimilation increased with the nickel
supplement up to 0.1 mg lÿ1. No effect of nickel supplement on the N assimilation in the nitrate-fed
plants was found.
Nickel supplement up to 0.1 mg lÿ1 reduced urea toxicity and enhanced chlorophyll
concentration, plant growth, and urea hydrolysis. The symptom of nickel toxicity and the
*
Corresponding author. Tel: 81-722-54-9421; fax: 81-722-54-9918.
E-mail address: ikeda@plant.osakafu-u.ac.jp (H. Ikeda).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 1 0 7 - 7
266
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
depression of plant growth were only observed with the nickel supplement up to 1 mg lÿ1. # 2000
Elsevier Science B.V. All rights reserved.
Keywords: Tomato; Nickel; Urea; Growth; Urea toxicity; N assimilation
1. Introduction
Urea is an important nitrogen (N) fertilizer in today's agriculture and its
suitability for ®eld crops has been well evaluated (Vavrina and Obreza, 1993).
However, urea in comparison to nitrate is not a desirable hydroponic N source for
tomatoes (Kirkby and Mengel, 1967; Ikeda and Tan, 1998), lettuces (Luo et al.,
1993), zucchinis (Gerendas and Sattelmacher, 1997), and rices (Gerendas et al.,
1998) due to its toxicity. The possible cause of urea toxicity is either by the NH
4
released during urea assimilation (Luo et al., 1993) or by urea itself (Krogmeier
et al., 1989). This latter conclusion was supported by a subsequent experiment in
which nickel de®cient plants showed lower urease activity and more leaf-tip
necrosis than non-de®cient plants (Krogmeier et al., 1991). Because urease is the
®rst enzyme involved in urea assimilation by plant tissues for the hydrolysis of
urea (Hogan et al., 1983), one possible way to avoid urea toxicity is to increase
the activity of urease.
Nickel is an essential micronutrient for some higher plants (Brown et al., 1987),
and the enzyme urease from jack beans (Dixon et al., 1975) and soybean seed
(Polacco and Havir, 1979) is known to be a nickel metalloenzyme. Therefore,
nickel is considered to be an important element for plants applied with urea as a
N source due to its role in the metalloenzyme urease (Gerendas et al., 1998). The
possible way to avoid urea toxicity is to use nickel supplement to enhance the
urease activity (Bekkari and Pizelle, 1992; Marschner, 1995).
Although the requirement of nickel for the urease activity was discovered more
than 20 years ago (Dixon et al., 1975), effects of nickel concentration in the urea
solution on the growth have received little attention. In this study, we tested
whether the nickel supplement can reduce urea toxicity and investigated effects of
nickel concentration in the solution on the N assimilation and growth of tomato
plants in hydrophonic culture supplied with urea or nitrate as the sole nitrogen
source.
2. Materials and methods
2.1. Plant materials, growth conditions, and treatments
Seeds of tomato (Lycopersicon esculentum Mill., cv. Momotaro) were
germinated on well washed and sterilized sand. Seedlings, 5 days after
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
267
emergence, were transferred to a hydrophonic culture in a greenhouse under
natural sunlight during autumn (average air temperature 32/208C day/night). A
half strength Hoagland's nutrient solution was used before the onset of
treatments.
When the seedlings had grown to the 6±7 leaf stage, eight plants were
transplanted into a 15 l polypropylene container. The basic nutrient solution was
prepared as follows: (in mM) K2SO4: 2, CaCl22H2O: 1.5, MgSO47H2O: 1,
NaH2PO42H2O: 2/3, and micronutrients. Plants were treated with two N sources:
urea and nitrate as NaNO3 at 168 mg N lÿ1, and four concentrations of nickel as
NiSO46H2O: 0, 0.01, 0.1, and 1 mg lÿ1. Treatments were arranged in a
randomized block design with three replicates. All solutions were prepared with
deionized water, aerated using an aquarium air stone, and were renewed every 7
days. Solution pH was daily checked and adjusted to 6.0 with 1 N NaOH or HCl.
2.2. Investigations, samplings and chemical analyses
Before harvest, the concentrations of leaf chlorophyll were measured with a
hand-held chlorophyll meter (SPAD-502, Minolta), and visual leaf symptoms of
urea toxicity were assessed with index from 0 (none) to 3 (severe). Plants were
harvested 4 weeks after treatment and were divided into leaves, stems, and roots.
These plant materials were then dried immediately in a forced-air oven at 608C to
a constant weight, weighed, and were ground to a ®ne powder in a Wiley mill to
pass through a 20-mesh sieve. The concentrations of total-N and nickel in the leaf
samples were determined by a modi®ed Kjeldahl method and ICP, respectively.
To determine the concentrations of urea-N, NH4-N, and NO3-N, the samples were
extracted with hot water. The concentrations of urea-N were determined using the
method of Cline and Fink (1956), and the concentrations of NH4-N and NO3-N
were determined using ion exchange chromatography (Dionex DX-AQ).
A statistical analysis was made using analysis of variance, and the means were
separated by Duncan's multiple range test (DMRT) at the 5% level.
3. Results
Leaf nickel concentrations in the urea-fed plants increased signi®cantly with
the increase of the nickel concentration in the solution from 0.01 to 1 mg lÿ1: 7.5
times from 0.01 to 0.1 mg lÿ1 and 13.9 times from 0.1 to 1 mg lÿ1 (Fig. 1). Leaf
nickel concentrations in the urea- or nitrate-fed plants without nickel supplement
were relatively low. N source did not signi®cantly affect leaf nickel concentration.
The symptoms of urea toxicity were severe in the urea-fed plants without
nickel supplement, and the symptoms were reduced in the plants with nickel
supplement at 0.01 mg lÿ1 (Fig. 2). No symptom of urea toxicity was observed in
the plants supplemented with nickel at 0.1 or 1 mg lÿ1.
268
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
Fig. 1. Effects of nickel concentration in the nutrient solution on the leaf nickel concentration in the
tomato plants applied with urea or nitrate. Leaf samples were obtained 4 weeks after treatment.
Bars having different letters are signi®cantly different at the 5% level by DMRT.
Fig. 2. Effects of nickel concentration in the nutrient solution on the symptom index of urea
toxicity on the leaves of the tomato plants applied with urea. Leaf samples were obtained 4 weeks
after treatment. The indexes of visual symptoms were from 0 (none) to 3 (severe). Bars having
different letters are signi®cantly different at the 5% level by DMRT.
269
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
Table 1
Effects of nickel concentration in the nutrient solution on the dry matter increments of tomato
plants fed with urea or nitrate for 4 weeksa
Nitrogen
source
Nickel
concentration
Shoot
(g/plant)
Root
(g/plant)
Total
(g/plant)
Relative
valueb
Urea
0
0.01
0.10
1.00
2.29d
3.09c
4.34b
2.52d
0.70b
0.91a
1.07a
0.69b
2.99d
4.00c
5.41b
3.21d
43
58
78
46
Nitrate
0
0.10
5.98a
6.05a
0.94a
0.93a
6.92a
6.98a
100
101
a
Means in each column followed by different letters are signi®cantly different at the 5% level by
DMRT. DW increment: DW (4 weeks after treatment)ÿDW (before treatment).
b
The total dry matter increments of the nitrate-fed plants without nickel supplement is
represented as 100.
The dry matter increments of the shoots or roots of the urea-fed plants
increased signi®cantly with the increase of the nickel supplement from 0 to
0.1 mg lÿ1, but decreased sharply with nickel at 1 mg lÿ1 (Table 1). The highest
dry matter increment in plants fed with urea at 0.1 mg lÿ1 nickel supplement was
78% of the nitrate-fed plants without nickel supplement. The dry matter
increments of the shoots or roots of the nitrate-fed plants were not affected by
nickel supplement.
With urea nutrition, leaf chlorophyll concentrations were low in the plants
without nickel supplement and in the plants supplemented with nickel at 1 mg lÿ1
(Fig. 3). Leaf chlorophyll concentrations in the nitrate-fed plants were not
signi®cantly affected by the nickel supplement.
The lowest concentrations of leaf total-N were detected in the urea-fed plants
without nickel supplement (Table 2). The concentrations of leaf urea-N and its
percentages in the total-N decreased greatly with the increase of the nickel
supplement from 0 to 0.1 mg lÿ1, but no further reduction at 1 mg lÿ1. Although
the concentrations of leaf NO3-N in the urea-fed plants were not affected by the
nickel supplement, the concentrations of leaf NH4-N increased sharply with the
increase of the nickel supplement. The concentrations of leaf urea-N and NH4-N
in the urea-fed plants with nickel supplement at 0.1 mg lÿ1 were about one-sixth
and four times of those without nickel supplement, respectively. Urea assimilation
increased as the nickel concentration in the solution increased from 0 to
0.1 mg lÿ1, but no further increase at 1 mg lÿ1. Higher concentrations of total-N
were detected in the nitrate-fed plants than in urea-fed plants. No effects of nickel
supplement on the concentrations of total-N, NH4-N, NO3-N, and the assimilation
of nitrate in the nitrate-fed plants were found.
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X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
Fig. 3. Effects of nickel concentration in the nutrient solution on the leaf chlorophyll
concentrations of the tamto plants applied with urea or nitrate. Values were SPAD readings from
chlorophyll meter (SPAD-502, Minolta) 4 weeks after treatment. Bars having different letters are
signi®cantly different at the 5% level by DMRT.
Table 2
Effects of nickel concentration in the nutrient solution on the concentration and assimilation of N in
the leaves of tomato plants fed with urea or nitrate for 4 weeksa
Nitrogen
source
Nickel
concentration
Total-N
Urea-N
NH
NOÿ
Urea-N/
Assimilated
3 -N
4 -N
ÿ1
ÿ1
ÿ1
(mg g ) (mg g ) (mg g ) (mg gÿ1) total-N (%) (%)b
Urea
0
0.01
0.10
1.00
35.81c
39.74b
41.14b
40.86b
Nitrate
0
0.10
55.04a
54.90a
13.14a
5.68b
2.22c
2.35c
0.91c
1.83b
3.55a
3.54a
0.50b
0.55b
0.49b
0.49b
1.09c
1.10c
9.64a
9.46a
36.7a
14.3b
5.4c
5.8c
63.3c
85.7b
94.6a
94.2a
82.5b
82.8b
a
Means in each column followed by different letters are signi®cantly different at the 5% level by
DMRT. Data were obtained 4 weeks after treatment.
b
Assimilated N (%): the percentage of the concentration of N changed from treatment divided
by the concentration of total-N, according to Bowman and Paul (1992).
4. Discussion
We have shown that nickel in the nutrient solution was absorbed by the plants
as the leaf nickel concentration was signi®cantly increased by the increase of the
nickel concentration supplemented in the nutrient solution.
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
271
The concentrations of leaf urea-N in the urea-fed plants without nickel
supplement were relatively high and were decreased signi®cantly with the nickel
supplement. The concentrations of leaf urea-N in the plants supplemented with
nickel at 0.1 mg lÿ1 were only about one-sixth in the plants without nickel
supplement, while the concentrations of leaf NH4-N in the former plants were
about four times of the latter plants. The changes of the concentrations of leaf
urea-N and NH4-N in the urea-fed plants indicate that a very rapid initial
hydrolysis of urea is stimulated by the nickel absorbed from the nutrient solution.
This result is in agreement with the ®ndings by Nicouland and Bloom (1998) with
tomatoes. The rapid hydrolysis of urea may be caused by higher activity of urease
activated by the addition of nickel (Eskew et al., 1984; Brown et al., 1987).
Nickel supplement to the urea solution reduced the symptom of urea toxicity.
The toxicity is caused by urea rather than NH
4 , the urea assimilation product, for
the following reasons: (1) the concentration of leaf urea-N decreased with the
nickel supplement, and the lower urea-N concentration was accompanied by less
toxicity symptom; (2) the symptom of toxicity reported here was different from
that of NH
4 toxicity. In this study, the toxicity symptom was the brown color
around the leaf margin, while partial leaf yellowing, mottled chlorosis, and curing
the symptoms of NH
4 toxicity in our previous experiment with tomato; and (3)
As the concentration of leaf NH
4 increased greatly with the nickel supplement in
urea-fed plants, while the urea toxicity symptom was reduced; urea toxicity is
unlikely caused by NH
4 in the leaf.
Nickel supplement promoted the growth of urea-fed plants, and the growth
promotion is related to the improvement of urea assimilation. In this study, both
the urea assimilation and the chlorophyll concentration in the urea-fed plants
increased when nickel was supplemented. The increase of the chlorophyll
concentration demonstrates strongly the improvement of urea assimilation by the
nickel supplement because chlorophyll is one of the important N-containing
compounds (Barker, 1989). Minotti et al. (1994) demonstrated that the N status in
the plant can be assessed by the measurement of chlorophyll. The growth
promotion is also related to the absorption of urea. The concentrations of leaf
total-N in the urea-fed plants with nickel supplement were higher than those
without nickel supplement, indicating that the absorption of urea is increased by
the nickel supplement. A rapid growth of the urea-fed plants was obtained with
nickel supplement at 0.1 mg lÿ1, while is about 80% of the nitrate-fed plants. We
think that the growth difference between urea- and nitrate-fed plants is caused by
the difference of N absorption. Although the concentrations of leaf total-N in the
urea-fed plants increased with the nickel supplement, they were still lower than
those of the nitrate-fed plants.
Although nickel supplement to the urea solution promoted plant growth, nickel
at high concentration depressed plant growth. The growth depressed by the high
nickel concentration may be caused by nickel toxicity because many small black
272
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 265±273
spots on the stems and brown roots were observed in the plants supplemented
with nickel at 1 mg lÿ1. The symptoms are caused by nickel itself, rather than by
urea or NH
4 : (1) these symptoms differ from those caused by urea or NH4
toxicity and (2) the concentration of leaf urea-N or NH4-N in the plants
supplemented with nickel at 0.1 mg lÿ1 was similar to that supplemented with
nickel at 1 mg lÿ1, but these plants had no nickel toxicity symptoms.
The mechanisms of nickel toxicity to the plant are still unclear. Gabbrielli et al.
(1990) reported that root growth and cell division of Silene italica is sensitive to
the nickel concentration in the solution. On the other hand, Yang et al. (1997)
found that the tolerance of some species to the nickel concentration depends on
the interaction of nickel with organic acids.
Acknowledgements
We thank all the students in our laboratory for their assistance during the
experiment. This study was supported by Grants-in-Aid for Scienti®c Research
(H. Ikeda: No. 08456023) from the Ministry of Education, Science, Sports, and
Culture of Japan.
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