Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol84.Issue3-4.June2000:
Scientia Horticulturae 84 (2000) 275±283
The absorption, translocation, and assimilation of urea,
nitrate or ammonium in tomato plants at different
plant growth stages in hydroponic culture
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
The absorption, translocation, and assimilation of urea, nitrate, and ammonium in tomato plants
within 24 h after 15N labeled compounds were applied at four different growth stages: seedling,
¯owering, fruiting, and harvesting.
The absorption of urea-N was only 25% of NO3-N at seedling stage, but it was up to about 80%
of NO3-N at the subsequent growth stages. The translocation of urea-N was limited at seedling
stage, but it was as fast as that of NO3-N at the subsequent growth stages. 15N was found higher in
the lamina of urea- or nitrate-fed plant, but higher in the stems and fruits of ammonium-fed plant.
The assimilation of urea-N at seedling stage was less than half of that at the subsequent growth
stages. The poor absorption, limited translocation, and slow assimilation of hydroponically applied
urea may be the cause of growth reduction at seedling stage.
Because as much as 94% of total-15N in the leaves of urea-fed plant at seedling stage and about
84% of that at the subsequent growth stages were found in the form of urea-15N, urea is not only
absorbed but also translocated by the plant in the form of urea itself.
At reproductive growth stage, the absorption, translocation, and assimilation of hydroponically
applied urea were greatly improved, and urea should be a suitable hydroponic N source for tomato
plants. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Tomato; Urea; Growth stage; Absorption; Translocation; Assimilation
*
Corresponding author. Tel: 81-722-54-9421; fax: 81-722-54-9918.
E-mail address: [email protected] (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 8 - 9
276
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
1. Introduction
The absorption or translocation of nitrate or ammonium has been investigated
extensively in hydroponic culture at either seedling stage or reproductive stage of
vegetable crops (Shelp, 1987; Liu and Shelp, 1993; Kosola and Bloom, 1996).
The utilization of nitrate or ammonium was in¯uenced by plant genotype
(Gabelman et al., 1986), solution pH (Yokota and Ojima, 1995), and solution
temperature (Ikeda and Osawa, 1984). Moreover, the utilization of nitrate or
ammonium was also affected by the growth stage in lima beans (McElhannon and
Mills, 1978) and sweet corn (Mills and McElhannon, 1982).
Urea is one of the most important nitrogen (N) fertilizers used for vegetable
production in the ®eld (Vavrina and Obreza, 1993). Urea as an organic N source
is, however, seldom used in hydroponic culture for vegetable production,
although a few successes have been reported in reducing nitrate accumulation in
leafy vegetables by partial replacement of nitrate with urea in the feed (Gunes
et al., 1994).
In recent years much attention has been focused on whether urea should be
used as the sole hydroponic N source for vegetables, especially for the leafy
vegetables (Luo et al., 1993; Khan et al., 1997; Zhu et al., 1997). Up to date
studies of the utilization of hydroponically applied urea by fruit vegetables have
been limited at seedling stage (Kirkby and Mengel, 1967; Gerendas and
Sattelmacher, 1997). According to their ®ndings, urea was not a suitable
hydroponic N source when it was compared with nitrate. Similar results were also
obtained in our previous experiment with tomatoes at seedling stage (Ikeda and
Tan, 1998). The response of fruit vegetables at different growth stages to the
utilization of urea in hydroponic culture has received much less attention. In this
study, we want to know whether urea is always not a suitable hydroponic N
source at all the growth stages of tomato plants.
To assess the possibility of using urea in hydroponic solution for tomato plants
at reproductive growth stage, the absorption, translocation, and assimilation of
urea, nitrate, and ammonium were compared at four different growth stages.
2. Materials and methods
2.1. Plant materials, growth conditions, and treatments
Seeds of tomato (Lycopersicon esculentum Mill., cv. Momotaro) were sown in
well-sterilized sand. Seedlings at the two leaf stage were cultured hydroponically
in a 15 l container in a greenhouse during autumn (328C/208C day/night).
Seedlings at the 6±7 leaf stage were transplanted into a deep water culture system
with Hoagland's solution. Plants were pruned to one basal stem and were pinched
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
277
at two leaves above the third cluster. Plants were treated with 15N tracers at four
growth stages for 24 h: when the fourth or ®fth leaf was fully expanded (seedling
stage), when 2±3 ¯owers on the ®rst cluster were ¯owering (¯owering stage),
when 2±3 fruits on the ®rst cluster were about 3 cm in diameter (fruiting stage),
and when 2±3 fruits on the ®rst cluster were red (harvesting stage). The basic
solution was formulated as (in mM) K2SO4: 2, CaCl22H2O: 1.5, MgSO47H2O:
1, NaH2PO42H2O: 2/3, and micronutrients. Plants were treated with three N
sources: urea-15N (30.21% 15N atom excess), NO3-15N as NaNO3 (30.12% 15N
atom excess), and NH4-15N as (NH4)2SO4 (30.02% 15N atom excess) at 200 mg
N lÿ1. Treatments were arranged in a randomized block design with three
replicates, made in a growth chamber (258C-12 h/158C-12 h day/night) at
seedling stage, and were made in the greenhouse as described above at other
growth stages.
2.2. Sampling procedures and chemical analyses
Plants were harvested 24 h after treatment and were divided into shoots and
roots. The roots were washed with deionized water. Apart from the basal zero
section, each section of the shoot comprised with two leaves above cluster and
one leaf below that cluster as depicted in Fig. 1. From the base to top, the shoot
was divided into zero, ®rst, second, and third sections. Each section was divided
Fig. 1. The arrangement of shoot section of a tomato plant for sampling.
278
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
into lamina, petiole, stem, and fruit. Samples were dried immediately in a forcedair oven at 1008C for the ®rst 30 min and at 608C thereafter, and were ground to a
®ne powder in a Wiley mill.
The concentrations of total-N and total-15N were determined using a modi®ed
Kjeldahl method and an emission spectrometer (Yoneyama and Kumazawa,
1974), respectively. To determine the concentrations of leaf urea-N and NH4-N in
urea-fed plant, the samples were extracted with hot water. The concentrations of
urea-N and NH4-N were determined using the method of Cline and Fink (1956)
and ion exchange chromatography (Dionex DX-AQ), respectively. NH4-15N in
the extract was collected by microdiffusion and was determined by the method of
Yoneyama and Kumazawa (1974).
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
Compared with the absorption of NO3-N or NH4-N within 24 h, the absorption
of urea-N was most affected by growth stages (Table 1). From seedling to
harvesting stage, absorption of N increased: 64 times for urea-N and only 20 30
times for NO3-N or NH4-N. The absorption of urea-N was only 25% of NO3-N at
seedling stage, whereas it was up to 66%, 82%, and 80% of NO3-N at ¯owering,
fruiting, and harvesting stage, respectively.
The % distribution of 15N in the shoot increased in the third section but
decreased in the zero section from seedling to harvesting stage for all the N
sources (Table 2). The % distribution of 15N in the root of urea-fed plant was
higher than that of nitrate-fed plant at seedling stage, but urea- and nitrate-fed
plant showed a similar % distribution of 15N in each plant section at the
subsequent growth stages. At all growth stages, ammonium-fed plant had a higher
Table 1
The absorption of urea, nitrate, and ammonium by tomato plants at seedling, ¯owering, fruiting, and
harvesting stage within 24 h after 15N application
Nitrogen source
Absorption of
15
N (mg plantÿ1)a
Plant growth stage
Urea
Nitrate
Ammonium
Seedling
Flowering
Fruiting
Harvesting
0.84 (1) c
3.32 (1) a
1.79 (1) b
9.84 (12) c
14.90 (4) a
13.23 (7) b
36.27 (43) b
44.35 (13) a
35.55 (20) b
53.77 (64) b
66.93 (20) a
55.35 (31) b
a
Relative value when absorption of 15N at seedling stage is represented as 1. Means in each
column followed by different letters are signi®cantly different at the 5% level by DMRT.
279
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
Table 2
The % distribution of 15N in the root and shoot section of tomato plants at seedling, ¯owering,
fruiting, and harvesting stage 24 h after 15N application
Plant section
15
N distribution (%) from three N sourcesa
Urea
Nitrate
Ammonium
Seedling stage
0
Root
77.31 b
22.69 a
82.75 a
17.25 b
78.62 b
21.38 a
Flowering stage
3
2
1
0
Root
3.49
13.83
29.69
22.29
30.70
a
b
a
a
b
2.51
16.38
29.48
23.13
28.50
b
a
a
a
b
1.08
12.75
23.11
23.45
39.60
c
b
b
a
a
Fruiting stage
3
2
1
0
Root
20.51
18.40
23.04
14.49
23.55
a
a
a
b
b
20.67
19.47
21.64
12.35
25.86
a
a
a
b
b
10.90
15.21
15.79
19.46
38.64
b
c
b
a
a
Harvesting stage
3
2
1
0
Root
20.20
14.53
14.17
12.67
38.44
a
a
a
a
b
20.35
14.06
12.61
12.13
40.84
a
a
a
a
b
11.81
12.40
13.39
12.60
49.80
b
b
a
a
a
a
Means among the N sources followed by different letters are signi®cantly different at the 5%
level by DMRT.
% distribution of 15N in the root but a lower distribution of 15N in the third or
second shoot section than the nitrate-fed plant.
The % distribution of 15N in the lamina, petiole, and stem of zero shoot
section decreased from seedling to fruiting stage for all the N sources, and the
% distribution of 15N in each organ of the third section increased generally
from seedling to harvesting stage (Fig. 2). 15N was distributed more in the lamina
of urea- or nitrate-fed plant, while even more in the stems and fruits of
ammonium-fed plant throughout plant growth. The % distribution of 15N in the
lamina of top shoot section was the highest in the urea-fed plant regardless of
growth stages.
The assimilation of urea in the lamina of urea-fed plant at seedling stage was
less than half of that at reproductive growth stages (Table 3). The assimilation of
urea in the lamina of top shoot section was much higher than that of basal section.
Among the total-15N in the lamina, as much as 94% at seedling stage and about
280
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
Fig. 2. The % distribution of 15N in the shoot organs of tomato plants at seedling, ¯owering,
fruiting, and harvesting stages 24 h after 15N application. Each value is the means SE of three
replicates. Values in the bars are % 15N in different shoot organs at different shoot sections. Values
in the bracket are the total-15N distribution in each shoot organ.
84% (average) at the subsequent growth stages remained unassimilated, and 3±
5% was changed from urea-15N to NH4-15N at all the growth stages.
4. Discussion
What form of N is absorbed by the plant when urea is applied as the sole
hydroponic N source? This topic has been investigated in recent years. Our
investigation showed that urea is not only absorbed but also translocated by the
281
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
Table 3
The concentration and assimilation of 15N in the lamina of tomato plants fed with urea at seedling,
¯owering, fruiting, and harvesting stage 24 h after 15N application
Shoot
section
15
Urea- N
NH4- N
Total- N
Seedling
0
346 (94.0)
10.8 (2.9)
368
6.0 c
Flowering
2
0
219 (82.0)
102 (87.2)
13.8 (5.2)
6.2 (5.3)
267
117
18.0 a
12.8 b
Fruiting
3
0
409 (80.7)
200 (87.7)
24.6 (4.9)
11.9 (5.2)
507
228
19.3 a
12.3 b
Harvesting
3
0
254 (81.4)
161 (87.0)
15.1 (4.8)
9.5 (5.1)
312
185
18.6 a
13.0 b
Growth stage
N concentration (mg gÿ1 DW)a
15
15
15
15
N assimilation
(%)b
a
The percentage of the total-15N.
N assimilation: (total-15Nÿurea-15N) 100/total-15N, according to Bowman and Paul
(1992). Means in the column followed by different letters are signi®cantly different at the 5% level
by DMRT.
b 15
plant as urea, not as NH4-N which is the by-product of the hydrolyzed urea. The
evidences are: (1) as much as 94% of total-15N in the leaves of urea-fed plant at
seedling stage and about 84% of that at the subsequent growth stages were
detected in the form of urea-15N and (2) no ammonium was detected in the urea
feed 24 h after treatment.
At seedling stage, the absorption of urea was only 25% of nitrate, and the
translocation of urea was also less than nitrate. Urea was accumulated more in the
root and was not translocated to the shoot as fast as nitrate at seedling stage.
Furthermore, the assimilation of urea at seedling stage was much slower than that
at the subsequent growth stages. The poor absorption, limited translocation, and
slow assimilation of urea may be the cause of growth reduction when urea is
applied at seedling stage as the sole hydroponic N source for tomato plants. This
®nding is consistent with the earlier ®ndings with tomato in which the relatively
low concentrations of total-N in the tissues of urea-fed plant at seedling stage
were contributed to the insuf®cient absorption of urea in comparison with that of
nitrate or ammonium (Kirkby and Mengel, 1967). On the other hand, the
chlorotic and developed necroses at the leaf edges of urea-fed zucchinis plant at
seedling stage (Gerendas and Sattelmacher, 1997) may be the symptom of urea
excess due to insuf®cient assimilation.
15
N was distributed more in the lamina of nitrate-fed plant, but more in the
stems and fruits of ammonium-fed plant. This ®nding is in agreement with our
previous ®ndings (Ikeda, 1991). There are evidences to support the notion that
urea is transported by transpiration stream: (1) urea is transported as such; (2) a
high distribution of 15N in the lamina of the top shoot section was detected in
282
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
urea-fed plant at all the growth stages; (3) urea- and nitrate-fed plant showed a
similar distribution of 15N in each plant section at reproductive growth stages; and
(4) nitrate is transported from root to top in tomato plant through transpiration
stream (Kirkby and Mengel, 1967).
Although the absorption of urea was only 25% of nitrate at seedling stage, it
was up to about 80% of nitrate at the subsequent growth stages. The translocation
of urea was as fast as that of nitrate at reproductive growth stages, and the
assimilation of urea at these stages was more than twice that at seedling stage.
The absorption, translocation, and assimilation of hydroponically applied urea
were greatly improved after seedling stage. Therefore, it is feasible to use urea as
the sole hydroponic N source for tomato plants at reproductive growth stages.
Acknowledgements
We thank H. Furukawa for his several helpful discussions on the experiment,
and 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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The absorption, translocation, and assimilation of urea,
nitrate or ammonium in tomato plants at different
plant growth stages in hydroponic culture
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
The absorption, translocation, and assimilation of urea, nitrate, and ammonium in tomato plants
within 24 h after 15N labeled compounds were applied at four different growth stages: seedling,
¯owering, fruiting, and harvesting.
The absorption of urea-N was only 25% of NO3-N at seedling stage, but it was up to about 80%
of NO3-N at the subsequent growth stages. The translocation of urea-N was limited at seedling
stage, but it was as fast as that of NO3-N at the subsequent growth stages. 15N was found higher in
the lamina of urea- or nitrate-fed plant, but higher in the stems and fruits of ammonium-fed plant.
The assimilation of urea-N at seedling stage was less than half of that at the subsequent growth
stages. The poor absorption, limited translocation, and slow assimilation of hydroponically applied
urea may be the cause of growth reduction at seedling stage.
Because as much as 94% of total-15N in the leaves of urea-fed plant at seedling stage and about
84% of that at the subsequent growth stages were found in the form of urea-15N, urea is not only
absorbed but also translocated by the plant in the form of urea itself.
At reproductive growth stage, the absorption, translocation, and assimilation of hydroponically
applied urea were greatly improved, and urea should be a suitable hydroponic N source for tomato
plants. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Tomato; Urea; Growth stage; Absorption; Translocation; Assimilation
*
Corresponding author. Tel: 81-722-54-9421; fax: 81-722-54-9918.
E-mail address: [email protected] (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 8 - 9
276
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
1. Introduction
The absorption or translocation of nitrate or ammonium has been investigated
extensively in hydroponic culture at either seedling stage or reproductive stage of
vegetable crops (Shelp, 1987; Liu and Shelp, 1993; Kosola and Bloom, 1996).
The utilization of nitrate or ammonium was in¯uenced by plant genotype
(Gabelman et al., 1986), solution pH (Yokota and Ojima, 1995), and solution
temperature (Ikeda and Osawa, 1984). Moreover, the utilization of nitrate or
ammonium was also affected by the growth stage in lima beans (McElhannon and
Mills, 1978) and sweet corn (Mills and McElhannon, 1982).
Urea is one of the most important nitrogen (N) fertilizers used for vegetable
production in the ®eld (Vavrina and Obreza, 1993). Urea as an organic N source
is, however, seldom used in hydroponic culture for vegetable production,
although a few successes have been reported in reducing nitrate accumulation in
leafy vegetables by partial replacement of nitrate with urea in the feed (Gunes
et al., 1994).
In recent years much attention has been focused on whether urea should be
used as the sole hydroponic N source for vegetables, especially for the leafy
vegetables (Luo et al., 1993; Khan et al., 1997; Zhu et al., 1997). Up to date
studies of the utilization of hydroponically applied urea by fruit vegetables have
been limited at seedling stage (Kirkby and Mengel, 1967; Gerendas and
Sattelmacher, 1997). According to their ®ndings, urea was not a suitable
hydroponic N source when it was compared with nitrate. Similar results were also
obtained in our previous experiment with tomatoes at seedling stage (Ikeda and
Tan, 1998). The response of fruit vegetables at different growth stages to the
utilization of urea in hydroponic culture has received much less attention. In this
study, we want to know whether urea is always not a suitable hydroponic N
source at all the growth stages of tomato plants.
To assess the possibility of using urea in hydroponic solution for tomato plants
at reproductive growth stage, the absorption, translocation, and assimilation of
urea, nitrate, and ammonium were compared at four different growth stages.
2. Materials and methods
2.1. Plant materials, growth conditions, and treatments
Seeds of tomato (Lycopersicon esculentum Mill., cv. Momotaro) were sown in
well-sterilized sand. Seedlings at the two leaf stage were cultured hydroponically
in a 15 l container in a greenhouse during autumn (328C/208C day/night).
Seedlings at the 6±7 leaf stage were transplanted into a deep water culture system
with Hoagland's solution. Plants were pruned to one basal stem and were pinched
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
277
at two leaves above the third cluster. Plants were treated with 15N tracers at four
growth stages for 24 h: when the fourth or ®fth leaf was fully expanded (seedling
stage), when 2±3 ¯owers on the ®rst cluster were ¯owering (¯owering stage),
when 2±3 fruits on the ®rst cluster were about 3 cm in diameter (fruiting stage),
and when 2±3 fruits on the ®rst cluster were red (harvesting stage). The basic
solution was formulated as (in mM) K2SO4: 2, CaCl22H2O: 1.5, MgSO47H2O:
1, NaH2PO42H2O: 2/3, and micronutrients. Plants were treated with three N
sources: urea-15N (30.21% 15N atom excess), NO3-15N as NaNO3 (30.12% 15N
atom excess), and NH4-15N as (NH4)2SO4 (30.02% 15N atom excess) at 200 mg
N lÿ1. Treatments were arranged in a randomized block design with three
replicates, made in a growth chamber (258C-12 h/158C-12 h day/night) at
seedling stage, and were made in the greenhouse as described above at other
growth stages.
2.2. Sampling procedures and chemical analyses
Plants were harvested 24 h after treatment and were divided into shoots and
roots. The roots were washed with deionized water. Apart from the basal zero
section, each section of the shoot comprised with two leaves above cluster and
one leaf below that cluster as depicted in Fig. 1. From the base to top, the shoot
was divided into zero, ®rst, second, and third sections. Each section was divided
Fig. 1. The arrangement of shoot section of a tomato plant for sampling.
278
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
into lamina, petiole, stem, and fruit. Samples were dried immediately in a forcedair oven at 1008C for the ®rst 30 min and at 608C thereafter, and were ground to a
®ne powder in a Wiley mill.
The concentrations of total-N and total-15N were determined using a modi®ed
Kjeldahl method and an emission spectrometer (Yoneyama and Kumazawa,
1974), respectively. To determine the concentrations of leaf urea-N and NH4-N in
urea-fed plant, the samples were extracted with hot water. The concentrations of
urea-N and NH4-N were determined using the method of Cline and Fink (1956)
and ion exchange chromatography (Dionex DX-AQ), respectively. NH4-15N in
the extract was collected by microdiffusion and was determined by the method of
Yoneyama and Kumazawa (1974).
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
Compared with the absorption of NO3-N or NH4-N within 24 h, the absorption
of urea-N was most affected by growth stages (Table 1). From seedling to
harvesting stage, absorption of N increased: 64 times for urea-N and only 20 30
times for NO3-N or NH4-N. The absorption of urea-N was only 25% of NO3-N at
seedling stage, whereas it was up to 66%, 82%, and 80% of NO3-N at ¯owering,
fruiting, and harvesting stage, respectively.
The % distribution of 15N in the shoot increased in the third section but
decreased in the zero section from seedling to harvesting stage for all the N
sources (Table 2). The % distribution of 15N in the root of urea-fed plant was
higher than that of nitrate-fed plant at seedling stage, but urea- and nitrate-fed
plant showed a similar % distribution of 15N in each plant section at the
subsequent growth stages. At all growth stages, ammonium-fed plant had a higher
Table 1
The absorption of urea, nitrate, and ammonium by tomato plants at seedling, ¯owering, fruiting, and
harvesting stage within 24 h after 15N application
Nitrogen source
Absorption of
15
N (mg plantÿ1)a
Plant growth stage
Urea
Nitrate
Ammonium
Seedling
Flowering
Fruiting
Harvesting
0.84 (1) c
3.32 (1) a
1.79 (1) b
9.84 (12) c
14.90 (4) a
13.23 (7) b
36.27 (43) b
44.35 (13) a
35.55 (20) b
53.77 (64) b
66.93 (20) a
55.35 (31) b
a
Relative value when absorption of 15N at seedling stage is represented as 1. Means in each
column followed by different letters are signi®cantly different at the 5% level by DMRT.
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X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
Table 2
The % distribution of 15N in the root and shoot section of tomato plants at seedling, ¯owering,
fruiting, and harvesting stage 24 h after 15N application
Plant section
15
N distribution (%) from three N sourcesa
Urea
Nitrate
Ammonium
Seedling stage
0
Root
77.31 b
22.69 a
82.75 a
17.25 b
78.62 b
21.38 a
Flowering stage
3
2
1
0
Root
3.49
13.83
29.69
22.29
30.70
a
b
a
a
b
2.51
16.38
29.48
23.13
28.50
b
a
a
a
b
1.08
12.75
23.11
23.45
39.60
c
b
b
a
a
Fruiting stage
3
2
1
0
Root
20.51
18.40
23.04
14.49
23.55
a
a
a
b
b
20.67
19.47
21.64
12.35
25.86
a
a
a
b
b
10.90
15.21
15.79
19.46
38.64
b
c
b
a
a
Harvesting stage
3
2
1
0
Root
20.20
14.53
14.17
12.67
38.44
a
a
a
a
b
20.35
14.06
12.61
12.13
40.84
a
a
a
a
b
11.81
12.40
13.39
12.60
49.80
b
b
a
a
a
a
Means among the N sources followed by different letters are signi®cantly different at the 5%
level by DMRT.
% distribution of 15N in the root but a lower distribution of 15N in the third or
second shoot section than the nitrate-fed plant.
The % distribution of 15N in the lamina, petiole, and stem of zero shoot
section decreased from seedling to fruiting stage for all the N sources, and the
% distribution of 15N in each organ of the third section increased generally
from seedling to harvesting stage (Fig. 2). 15N was distributed more in the lamina
of urea- or nitrate-fed plant, while even more in the stems and fruits of
ammonium-fed plant throughout plant growth. The % distribution of 15N in the
lamina of top shoot section was the highest in the urea-fed plant regardless of
growth stages.
The assimilation of urea in the lamina of urea-fed plant at seedling stage was
less than half of that at reproductive growth stages (Table 3). The assimilation of
urea in the lamina of top shoot section was much higher than that of basal section.
Among the total-15N in the lamina, as much as 94% at seedling stage and about
280
X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
Fig. 2. The % distribution of 15N in the shoot organs of tomato plants at seedling, ¯owering,
fruiting, and harvesting stages 24 h after 15N application. Each value is the means SE of three
replicates. Values in the bars are % 15N in different shoot organs at different shoot sections. Values
in the bracket are the total-15N distribution in each shoot organ.
84% (average) at the subsequent growth stages remained unassimilated, and 3±
5% was changed from urea-15N to NH4-15N at all the growth stages.
4. Discussion
What form of N is absorbed by the plant when urea is applied as the sole
hydroponic N source? This topic has been investigated in recent years. Our
investigation showed that urea is not only absorbed but also translocated by the
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X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
Table 3
The concentration and assimilation of 15N in the lamina of tomato plants fed with urea at seedling,
¯owering, fruiting, and harvesting stage 24 h after 15N application
Shoot
section
15
Urea- N
NH4- N
Total- N
Seedling
0
346 (94.0)
10.8 (2.9)
368
6.0 c
Flowering
2
0
219 (82.0)
102 (87.2)
13.8 (5.2)
6.2 (5.3)
267
117
18.0 a
12.8 b
Fruiting
3
0
409 (80.7)
200 (87.7)
24.6 (4.9)
11.9 (5.2)
507
228
19.3 a
12.3 b
Harvesting
3
0
254 (81.4)
161 (87.0)
15.1 (4.8)
9.5 (5.1)
312
185
18.6 a
13.0 b
Growth stage
N concentration (mg gÿ1 DW)a
15
15
15
15
N assimilation
(%)b
a
The percentage of the total-15N.
N assimilation: (total-15Nÿurea-15N) 100/total-15N, according to Bowman and Paul
(1992). Means in the column followed by different letters are signi®cantly different at the 5% level
by DMRT.
b 15
plant as urea, not as NH4-N which is the by-product of the hydrolyzed urea. The
evidences are: (1) as much as 94% of total-15N in the leaves of urea-fed plant at
seedling stage and about 84% of that at the subsequent growth stages were
detected in the form of urea-15N and (2) no ammonium was detected in the urea
feed 24 h after treatment.
At seedling stage, the absorption of urea was only 25% of nitrate, and the
translocation of urea was also less than nitrate. Urea was accumulated more in the
root and was not translocated to the shoot as fast as nitrate at seedling stage.
Furthermore, the assimilation of urea at seedling stage was much slower than that
at the subsequent growth stages. The poor absorption, limited translocation, and
slow assimilation of urea may be the cause of growth reduction when urea is
applied at seedling stage as the sole hydroponic N source for tomato plants. This
®nding is consistent with the earlier ®ndings with tomato in which the relatively
low concentrations of total-N in the tissues of urea-fed plant at seedling stage
were contributed to the insuf®cient absorption of urea in comparison with that of
nitrate or ammonium (Kirkby and Mengel, 1967). On the other hand, the
chlorotic and developed necroses at the leaf edges of urea-fed zucchinis plant at
seedling stage (Gerendas and Sattelmacher, 1997) may be the symptom of urea
excess due to insuf®cient assimilation.
15
N was distributed more in the lamina of nitrate-fed plant, but more in the
stems and fruits of ammonium-fed plant. This ®nding is in agreement with our
previous ®ndings (Ikeda, 1991). There are evidences to support the notion that
urea is transported by transpiration stream: (1) urea is transported as such; (2) a
high distribution of 15N in the lamina of the top shoot section was detected in
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X.W. Tan et al. / Scientia Horticulturae 84 (2000) 275±283
urea-fed plant at all the growth stages; (3) urea- and nitrate-fed plant showed a
similar distribution of 15N in each plant section at reproductive growth stages; and
(4) nitrate is transported from root to top in tomato plant through transpiration
stream (Kirkby and Mengel, 1967).
Although the absorption of urea was only 25% of nitrate at seedling stage, it
was up to about 80% of nitrate at the subsequent growth stages. The translocation
of urea was as fast as that of nitrate at reproductive growth stages, and the
assimilation of urea at these stages was more than twice that at seedling stage.
The absorption, translocation, and assimilation of hydroponically applied urea
were greatly improved after seedling stage. Therefore, it is feasible to use urea as
the sole hydroponic N source for tomato plants at reproductive growth stages.
Acknowledgements
We thank H. Furukawa for his several helpful discussions on the experiment,
and 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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