Plant Cell Physiol. 38(4): 495-498 (1997)
JSPP © 1997
Short Communication
Suppression of Calcium Transport to Shoots by Root Restriction in Tomato
Plants
Jae Hee Choi', Gap Chae Chung1>4, Sang Ryong Sun 2 , Jung Ah Yu', Je Hoon Sung 2 and Kyung Ju
Choi 3
1
2
3
Dept. of Horticulture, Institute of Biotechnology, College of Agriculture, Chonnam National University, Kwangju 500-757, Korea
Dept. of Agricultural Engineering, College of Agriculture, Chonnam National University, Kwangju, 500-757, Korea
Kurye Cucumber Experiment Station, Chonnam Provincial R.D.A. Kurye 542-820, Chonnam, Korea
Restriction of tomato roots by growth in small containers strongly suppressed transport of Ca ions to new
leaves and apices. Water transport, expressed on a leaf area
basis, was marginally reduced by root restriction, an indication that calcium transport was more severely limited than
water transport.
Key words: Calcium distribution — Lycopersicon esculentum — Root restriction — Water transport.
Physical restriction of roots by growth in small containers is a powerful technique for controlling the size of
shoot and the partitioning of plant mass between shoot and
root. In addition, the relative rates of vegetative and reproductive growth are also altered in favor of the latter in
tomato (Al-Sahaf 1984). Moreover, the root systems often
display alterations in morphology when roots are subjected
to mechanical constraint (Chung et al. 1989).
The mechanism responsible for the reduced shoot
growth that is due to restricted root growth is not fully
understood. Hameed et al. (1987) proposed that the principal effect of root restriction by growth in small containers
might be the induction of physiological drought stress in
shoots as a result of the higher hydraulic resistance of roots
and shoots. Rieger and Marra (1994), by contrast, suggested that drought was not a limiting factor for small container-grown peach trees and that nutrient deficiency might
be the cause of limitations to their growth. Since root
growth is physically restricted by growth in containers,
it seems possible that plant hormones, such as the gibberellins and cytokinins produced in roots, might be involved in the induced dwarfism, as suggested by Carmi et
al. (1983). In the present study, we measured transpiration
with a stem flow gauge in order to examine the influence of
root restriction on the growth and development of tomato
plants in small containers. Transport of Ca 2+ ions and its
relationship to measured transpiration were also investigatCorresponding author.
495
ed using
45
Ca 2 + ions.
Materials and methods—Tomato (Lycopersicon esculentum Mill cv. Youngkwang) seeds were sown in plastic
trays in vermiculite in a temperature-controlled greenhouse (18°C at night and 28°C in the daytime) with heat
supplied from below (30°C). After germination, seedlings with two true leaves were transferred to 20-liter containers that contained aerated Cooper's nutrient solution
(Cooper 1975) with six seedlings per container. Seedlings
were grown for 7 d and uniform plants were selected for
root-restriction treatment. A small container (190 ml) with
a base of fine stainless-steel mesh, containing one seedling,
was suspended in one 20-liter container while each control
plant was kept in a 20-liter container. Vigorous aeration
was supplied to ensure adequate circulation of the nutrient
solution in the 20-liter container and the small container.
Ten replicates were arranged in a randomized completeblock design for both treatments. After 40 d, five plants of
each treatment were harvested at random for measurements of leaf area, number of fruit, incidence of blossomend rot and dry weights of shoots and roots. Parts of leaves
and roots were ashed at 500° C for the determination of
levels of Ca, K and Mg with an atomic absorption spectrophotometer. The electric conductivity and pH of the
nutrient solution were monitored throughout the experiment and the nutrient solution was replaced every 3 to 4 d.
Two d before harvest (38th d), fresh nutrient solution
was supplied to tomato plants and 45CaCl2 (24.6 MBq;
Amersham, U.K.) was added at midday to the 20 liters of
nutrient solution in which control and restricted plants
were growing to give a specific activity of 1.23 kBq ml" 1 .
Tomato plants were harvested 48 h after feeding. They
were divided into apex, first leaf (LI), second leaf (L2), the
15-16th leaf (L3) from the growing apex, the 2-cm portions
of stem just below L2 (S2) and L3 (S3) and the root system.
The various parts were dried at 80°C for 3 d and powdered
samples were ashed at 500°C for 24 h. The ash was extracted with 1 M HC1 and dried again at 60°C. One ml of
distilled water was added and 0.2 ml of the 1 ml suspension
was mixed with scintillation cocktail (Ultima Gold; Amersham, U.K.) for determination of radioactivity.
496
Root restriction and calcium transport
Table 1 Effects of root restriction on the growth of
tomato plants in nutrient solution
Leaf area (m2)
Number of flowers
Number of fruit
Weight of fruit (g)
Incidence of
blossom-end rot (%)
Dry weight (g) Shoot
Root
Fig. 1 Tomato root systems grown in small (190 ml, left) and
large (20,000 ml, right) containers for 40 d. Dry weights were 9.9
and 17.8 g, respectively. The length of small container that was occupied by root system was 10 cm.
A heat-balance sap-flow gauge (Dynagage Flow 32;
Dynamax, Houston, U.S.A.) was used to measure water
flow through the main stem of two control and two rootrestricted plants (Steinberg et al. 1990). Fifteen mm stem
gauges were attached to the stem just above cotyledons
and signals, recorded with a "data-logger" (21X; Campbell
Scientific, Logan, U.S.A.), were collected at 60s intervals
and averaged over 30 min. Measurements were made from
d 36 to d 40. The average temperature and fluence rate of
light quanta at midday on the 4 d on which measurements
were made were 30°C and 700/iE m~ 2 s~', respectively.
Accumulated results were plotted with Quattro Program
(Borland, U.S.A.).
Results and discussion—Tomato plants grew well initially in small containers without any appreciable difference in growth from those in large containers. There were
no signs of wilting or nutrient deficiency in leaves of rootrestricted plants throughout the experimental period even
though plant growth was reduced. The restricted tomato
plants had short stems, as well as reduced leaf area and
stem diameter, as compared to controls. Fig. 1 shows the
Shoot/root (g/g)
Control
Restriction
1.06± 0.05
0.49± 0.04
19.8 ± 2.29
6.2 ± 0.86
160
±26
0
126 ± 7
17.8 ± 2.3
7.10
17
± 3.19
7
± 1.29
104
±33
67.9 ±10.7
90.2 ± 3 . 0
9.9 ± 1.1
9.11
Tomato seedlings were grown in small (190 ml) or large (20,000
ml) containers for 40 d.
Values are means±S.E. of results from five plants.
root systems grown for 40 d in small and large containers.
The root system in small containers occupied most of the
spaces available in the container after 40 d of growth but
the colour of root system was white, an indication of the
normal growth. Leaf area was reduced by 50% but numbers of flower and fruits were not significantly affected by
root restriction (Table 1). A clear difference between control and restricted tomato plants was noted in the incidence
of blossom-end rot, which is known to occur exclusively as
a result of calcium deficiency (Bangerth 1979). Almost 70%
of the fruits were rotten on restricted plants while none of
the non-restricted fruits showed any signs of blossom-end
rot (Table 1). Analysis of levels of Ca, K and Mg in the new
leaves (LI) indicated that root restriction resulted in a considerably reduced level of Ca, while levels of K and Mg
were the same as those in control plants (Table 2). By contrast, roots and old leaves (L3) of restricted plants contained higher levels of Ca than control plants, an indication
that the transport of Ca 2+ ions from root to shoot might
have been suppressed.
Table 2 Effects of root restriction on the level (% of dry matter, w/w) of Ca, K and Mg in tomato leaves and roots
grown in nutrient solution
Ca
Control
K
Mg
Ca
Restriction
K
Mg
Leaves LI
0.07 + 0.05
2.41±0.16
O.33±O.O4
0.47±0.04
2.80±0.24
0.31 ±0.02
L3
6.72±0.09
1.00±0.01
0.28±0.00
8.69±0.29
0.97±0.04
0.33±O.O1
0.29±0.09
4.57±0.27
O.38±O.O3
0.76±0.04
3.01 ±0.37
0. 31 ±0.03
Plant part
Root
Tomato seedlings were grown in small (190 ml) or large (20,000 ml) containers for 40 d.
LI and L3 refer to the first and 15-16th leaves from the growing apex, respectively. Values are means±S.E. of results from five plants.
Root restriction and calcium transport
497
Table 3 Amounts of 45Ca2+ ions (cpm x 10s) in various parts of tomato plants grown in small (190 ml) or large (20,000
ml) containers for 40 d
Plant part
Control (%)
Apex (%)
3.8±0.18 (14.2)
0.07 ±0.003
(0.58)
2.8±0.12
2.5±0.04
3.3±O.O3
5.3±0.28
4.9±0.10
(10.4)
(9.3)
(12.3)
0.05 ±0.001
0.05 ±0.002
0.10±0.004
(0.41)
(0.83)
(19.8)
(18.3)
0.62±0.014 (5.12)
3.04±0.039 (25.1)
Leaves (%)
Stem
LI
L2
L3
S2
S3
4.2±0.09 (15.7)
Root
26.8±0.24 (100)
Total uptake
Restriction (%)
(0.41)
8.18±0.089 (67.6)
12.1 ±0.04 (100)
45
CaCl2 (1.23 kBq ml ') was supplied on the 38th d and the plants were harvested after treatment for 48 h.
LI, L2 and L3 refer to the first, second and 15-16th leaves from the growing apex, respectively. S2 and S3 refer to the portions of the
stem just below the L2 and L3 leaves, respectively. Values are means±S.E. of results from three plants.
The severe calcium deficiency, suggested by the high incidences of blossom-end rot, was clearly confirmed by the
distribution of 45Ca2+ ions in tomato plants (Table 3). The
apex and new leaves (LI and L2) of restricted tomato
plants contained only about 2% of the 45Ca2+ ions found in
control plants. Lower old leaves (L3) also contained only
about 3% of the control level. We also found that the transport of Ca 2+ ions in the main stem was significantly reduced. In the stem portion just below L2 (S2), there was
only 12% of the 45Ca2+ ions found in control plants, while
in S3, there was 63%. This gradient in the level of 45Ca2+
ions along the stem suggested strongly that calcium transport might be independent of the transpiration stream, pro500
46
Time (h)
Fig. 2 Mass flow rate of sap in control and restricted tomato
plants. Signals were collected every 60 s and averaged over 30 min
to give hourly mean values. Average fluence rates of light quanta
and temperature at noon for the 4 d were about 700//E m~2 s~'
and 30°C, respectively. The results are the averages of measurements from two control and two restricted tomato plants in each
case.
vided that water transport as mass flow is not limited in
stem. Clarkson (1984) suggested that transpiration might
be the main driving force for the transport of Ca 2+ ions to
various organs. Atkinson et al. (1992) concluded, however,
that long-distance transport of Ca 2+ ions in the xylem does
not occur primarily by mass flow, and that it is independent
of water uptake or transpiration. In the present study, root
restriction induced greater accumulation of 43Ca2+ ions in
the root system. In control plants, 16% of all 45Ca2+ ions
taken up remained in the roots. By contrast, in the restricted roots, 68% of all 45Ca2+ ions taken was in roots.
The sum of 45Ca2+ ions in the root and in S3 accounted for
93% of the total uptake in the restricted plants. Again,
there was a strong gradient of 45Ca2+ ions from the growing apex toward the root (Table 3).
Fig. 2 shows the amount of water transported to the
shoot at 30 min intervals as measured over the course of the
4 d with the stem flow gauge. The highest rate, ranging
from 350 to 450 g h~', was observed around midday in control plants and virtually no water was transported to shoots
during the night. A maximum rate of about 100 g h~' was
consistently measured in restricted plants. The daily accumulated amount of water transported on d 1 was 2,580 and
889 g d~' in the control and restricted plants, respectively.
However, when these values were calculated on the basis of
leaf area (2,450 and 1,810 g(m 2 leaf area)" 1 d"1 for control
and restricted roots, respectively), the difference between
control and restricted plants was not quite so dramatic.
There are conflicting results with respect to reductions in
shoot growth in response to root restriction. Hameed et al.
(1987) concluded that water stress was the major reason for
the reduced growth of tomato plants grown in small containers, even though the plants had been grown in nutrient
solution. However, Bar-Tal et al. (1995) argued that the reduced transpiration associated with root restriction might
498
Root restriction and calcium transport
not be the cause but rather the result of retarded shoot
growth. They also demonstrated an actual increase in transpiration per unit leaf area, indicating that root restriction did not interfere with the uptake of water. Direct measurement of sap flow with a stem gauge provides convincing
results, reflecting the actual amounts of water transported
from root to shoot, as found by other researchers (Devitt et
al. 1993, Heilman and Ham 1990, Steinberg et al. 1990).
The restricted plants transpired at about one-third the rate
of control plants on a daily basis, but this difference became marginal when values were expressed on the basis of
leaf area. Therefore, it appears that water transport from
the root to the shoot was not severely limited by root restriction under the present conditions.
The steep concentration gradient along the stem and
the considerable accumulation of 45 Ca 2+ ions in the root
provide an interesting insight into calcium transport within
the plant. If calcium transport were dependent upon transpiration, we would not expect such a large difference in
the concentration of 45 Ca 2+ ions between control and restricted plants since transpiration on the basis of leaf area
was not so different. This statement agrees with the conclusion of Atkinson et al. (1992), as noted above. It seems,
rather, that the movement of Ca 2+ ions involves a process
of exchange with negatively charged moieties in the xylem
vessel walls, as suggested by Shear and Faust (1970). It
remains to be determined whether root restriction creates
such charged moieties in the xylem vessels.
This work was supported in part by the Non-Directed
Research Fund (1995) of the Korea Research Foundation and by a
special grant from the ministry of Agriculture and Fisheries
(1996), Korea. A critical review of the manuscript by Dr. C.J.
Gantzer of the Department of Soil Conservation and Management, University of Missouri-Columbia, U.S.A., is greatly appreciated.
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(Received October 11, 1996; Accepted January 27, 1997)