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 after15N 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).
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
at two leaves above the third cluster. Plants were treated with15N 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
into lamina, petiole, stem, and fruit. Samples were dried immediately in a forced-air 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 2030
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 after15N application
Nitrogen source Absorption of15N (mg plantÿ1
)a
% 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 of15N 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
Table 2
The % distribution of15N in the root and shoot section of tomato plants at seedling, ¯owering, fruiting, and harvesting stage 24 h after15N application
Plant section 15N distribution (%) from three N sourcesa
Urea Nitrate Ammonium
aMeans among the N sources followed by different letters are signi®cantly different at the 5%
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
Fig. 2. The % distribution of 15N in the shoot organs of tomato plants at seedling, ¯owering, fruiting, and harvesting stages 24 h after15N application. Each value is the meansSE of three
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
Table 3
The concentration and assimilation of15N in the lamina of tomato plants fed with urea at seedling, ¯owering, fruiting, and harvesting stage 24 h after15N application
Growth stage Shoot 15N concentration (mg gÿ1
DW)a 15N assimilation section
Urea-15N NH4-15N Total-15N
(%)b
Seedling 0 346 (94.0) 10.8 (2.9) 368 6.0 c Flowering 2 219 (82.0) 13.8 (5.2) 267 18.0 a 0 102 (87.2) 6.2 (5.3) 117 12.8 b Fruiting 3 409 (80.7) 24.6 (4.9) 507 19.3 a 0 200 (87.7) 11.9 (5.2) 228 12.3 b Harvesting 3 254 (81.4) 15.1 (4.8) 312 18.6 a 0 161 (87.0) 9.5 (5.1) 185 13.0 b
a
The percentage of the total-15N.
b 15
urea-fed plant at all the growth stages; (3) urea- and nitrate-fed plant showed a similar distribution of15N 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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