Journal of Experimental Botany, Vol. 49, No. 328, pp. 1863–1868, November 1998
Sodium fluxes in sweet pepper exposed to varying
sodium concentrations
Margaretha Blom-Zandstra1, Sake A. Vogelzang and Bob W. Veen
DLO-Research Institute for Agrobiology and Soil Fertility (AB-DLO), PO Box 14, 6700 AA Wageningen,
The Netherlands
Received 30 April 1998; Accepted 16 July 1998
Abstract
Introduction
The sodium transport and distribution of sweet pepper
(Capsicum annuum L.) under saline conditions were
studied after transferring the plants to a sodium-free
nutrient solution. Sodium stress up to 60 mM did not
affect the growth of sweet pepper, as it appears able
to counteract the unfavourable physiological effects
of sodium efficiently. Sodium was particularly accumulated in the basal pith cells of the stem and in the root
cells, while almost no sodium was directed to the
leaves or the fruits. The sodium concentration in the
pith cells and xylem sap gradually decreased towards
the shoot tip. Removal of sodium from the medium
resulted in a 50% release of sodium from the plant
after 1 week without affecting the gradient in the pith
cells. In contrast, the concentration profile in the xylem
sap was completely changed: the sodium concentration in the xylem sap at the stem base was similar to
that at the top.
Phloem transport was studied in a split root experiment, in which both portions of the roots were exposed
to 15 mM NaCl and one part was fed with additional
22NaCl. During continuous exposure to 15 mM NaCl no
label was detected in unlabelled root parts. However,
after transferring the plants to a sodium-free solution,
22Na was rapidly released from the unlabelled roots,
indicating a downward phloem transport.
It was concluded that pith cells, the intermediates
between the xylem and phloem, play a decisive role in
the recirculation of sodium throughout the plant.
Release of sodium from the plants following transfer
to a sodium-free solution may be explained by changes
in the diffusion resistance for passive sodium efflux
from the cells.
The effects of salinity on plant growth has been the
subject of intensive investigation for many years. Most
studies (recently reviewed by Adams et al., 1995; Niu
et al., 1995; Sanderson et al., 1997) focus on high salt
stress as it poses serious limitations to agriculture in many
areas through yield losses and crop damage. Much less is
known about the effect of mild salt stress. Since the
introduction of hydroponic systems in greenhouse agronomy, problems of mild salt stress have become apparent.
During crop growth, saline rainwater or tapwater is often
used for the preparation of nutrient solutions. Part of the
sodium chloride in the solution is excluded by the plants,
resulting in a steady increase of the salt concentration
within the root nutrient. In order to avoid excessively
high salt concentrations, solutions are regularly replenished, thereby exposing plants to gradually increasing
sodium concentrations and abrupt declines during
cultivation.
Sweet pepper (Capsicum annuum L.) is an important,
widespread agricultural crop, which is often grown on
hydroponic systems under changing mild saline conditions. Yield is slightly diminished by salt stress, but as
also described for tomato (Adams et al., 1995) the crop
is more sensitive to the occurrence of blossom-end-rot in
fruits: When 11 mM sodium is present in the nutrient
medium, fruit yield diminishes by c. 5% (Post and KleinBuitendijk, 1996a). Moreover, a simultaneous increase of
the potassium concentration makes the fruits more sensitive to the occurrence of blossom-end-rot (Post and KleinBuitendijk, 1996b). The question arises as to how sweet
pepper is able to control its internal sodium levels in the
presence of a varying external environment.
From studies on high salt stress it is known that several
plant species show a special mechanism to correct for
saline environments ( Fernandez et al., 1996; Nakamura
Key words: Xylem, phloem, sodium, fluxes, sweet pepper.
1 To whom correspondence should be addressed. Fax: +31 317 423110. E-mail [email protected]
© Oxford University Press 1998
1864
Blom-Zandstra et al.
et al., 1996; Zhu et al., 1997). Some plants avoid serious
growth reduction, or damage to the photosynthetic
system, by compartmentalization of sodium in special
organs, while recirculation of sodium through xylem and
phloem has also been reported for bean (Jacoby, 1979)
and Lupinus albus (Munns, 1988; Munns et al., 1988).
To remove sodium from the cytoplasm, special transporters, i.e. proton pumps and energized Na+/H+-antiporters on the plasma membrane (reviewed by Rausch
et al., 1996), efficiently transfer sodium into glands or
extracellular compartments (Barkla and Pantoja, 1996).
An active sodium pump system on the tonoplast has also
been reported for the salt-tolerant Plantago maritima L.
(Staal et al., 1991). This may be a typical characteristic
for salt-tolerant plants, since the glycophyte Plantago
media L. does not have this active pump system.
In the case of sodium recirculation, uptake from the
xylem into pith cells of the stem must occur, followed by
sodium efflux directed towards the phloem vessel. The
possibility of a differential release of assimilates has been
described ( Van Bel, 1996; Jeschke et al., 1997). With pith
cells of the stem and phloem being symplastically connected, the plasmodesmata will play an important role in
the transport characteristics. Van Bel (1996) observed
that the time of plasmodesmata closure in transverse
directions may differ from those in the longitudinal direction, thus regulating a concentration profile. In a dynamic
version of the original Münch model ( Eschrich et al.,
1972), photoassimilates and nutrients constantly leak
from the phloem into the xylem (Minchin and Thorpe,
1984, 1987; Minchin and McNaughton, 1987; Van Bel,
1996; Jeschke et al., 1997).
To date, no such model has been described for sodium.
This paper describes the effects of mild sodium stress on
sodium distribution between different plant parts in sweet
pepper grown under hydroponic conditions. Using xylem
sap analyses and split-root experiments, the internal transport of sodium, including the role of the phloem, was
evaluated. The effect of transfer to a sodium-free nutrient
solution on sodium accumulation and discharge was
also studied.
Materials and methods
Plant growth conditions
Seeds of Capsicum annuum L., cv. Mazurka, were sown in
Perlite and kept at 30 °C in the dark. After 9 d the seedlings
were transferred to a hydroponic system containing a nutrient
solution. The composition of the solution was: 3.73 mM
Ca(NO ) , 4.40 mM KNO , 0.97 mM KH PO , 1.92 mM
3 2
3
2 4
MgSO , 0.89 mM K SO , trace elements, and Fe-EDTA
4
2 4
(Steiner, 1968). Further growth occurred in a growth
chamber at 20 °C, 70% RH and a photoperiod of 12 h
(150 mmol m−2 s−1).
Sweet pepper shows dichotomic branching (Rylski, 1985)
and following the procedure used in commercial practice, the
largest of each dichotomic branch was retained, while the other
was pruned just above its first leaf.
Sodium stress period
Nine-week-old plants were transferred to pots containing
nutrient solution supplemented with 15 mM NaCl (or otherwise
when indicated in the text). Plants were harvested at distinct
intervals, as indicated in the text and divided into roots, stem,
leaves, and fruits. The stem was subdivided into six internodal
segments of c. 5 cm and fresh and dry weights were determined.
The plant parts were ground to powder and the sodium and
potassium content was analysed by atomic absorption, using a
Varian Techtron (AA4).
Xylem sap analyses
Xylem sap analysis was performed with plants harvested after
exposure to 15 mM NaCl or to 15 mM NaCl, followed by
exposure to a sodium-free nutrient solution for an additional
week. Plant stems were divided into internodal 5 cm segments.
Each stem part was placed in a pressure bomb and pressurized
(0.8 MPa, 5 min), and expressed sap collected. This was
considered to be xylem sap, and was analysed for sodium. To
validate the method, a few plants were cut just above the root
collar and bleeding sap collected. The sodium concentration in
the samples was comparable (c. 15 mM ) to that expressed out
of the basal stem.
Split-root experiments
To evaluate the vertical sodium transport, a split root experiment
was performed with 7-week-old plants. Each part was placed
in a separate container with nutrient solution and allowed to
adapt for 2 d. While 15 mM NaCl was added to both containers,
22Na+ (with a negligible effect on the NaCl concentration) was
added to one of the containers only. The first series of plants
was harvested after 1 week, while the other plants were
transferred to sodium-free solutions under the same split root
design. Daily samples were taken from the nutrient solutions
and plants were harvested after 1, 3 and 7 d. Roots and leaves
were separated and dried at 70 °C. Analyses for 22Na+ in the
nutrient solutions and plant material was performed with
c-spectrometry using a 45% HPGe detector in low background
shielding.
Sodium concentrations were calculated on the basis of the
specific 22Na activity. In shoot tissues a correction was made
for the supply of sodium from the unlabelled roots (based on
root mass), while in the unlabelled roots only the flux from the
shoot was calculated.
Statistical analyses
Differences between treatments were determined by analyses of
variance (ANOVA) using Genstat 5 (Rothamsted Experimental
Station, Harpenden, UK ).
Results
Plant growth and time-course of sodium uptake
Figure 1 shows that fresh shoot weight increase is not
affected by sodium concentration up to 60 mM in the
nutrient solution. The time-course of sodium accumulation after supply of 10 mM sodium to the nutrient solution
( Fig. 2) shows a higher concentration in roots than in
the shoot. After about 1 week the increase in sodium
Sodium fluxes in sweet pepper
Fig. 1. Shoot fresh weight production of sweet pepper grown at different
sodium concentrations. (2), control; (%), 1 mM; (6), 5 mM; (#),
15 mM; (l ), 60 mM ). Mean standard deviation of fresh weight values
was 12.8%.
Fig. 2. Sodium concentrations in roots and shoot of sweet pepper
grown in10 mM sodium.
content in roots and shoot levels off at 6 mM for roots
and 4 mM for the shoot, respectively. The observed net
uptake of sodium is probably required to compensate for
increase in biomass.
Distribution of sodium
The average sodium concentration in the roots is similar
to the external sodium concentration (Fig. 3). Of the
investigated shoot tissue, sodium preferentially accumulates in the pith cells of the stem, while hardly any sodium
is directed to leaves and fruits.
Figure 4 shows that the sodium content is highest in
the basal part of the stem. Towards the apex, the sodium
content decreases exponentially. The sodium concentration at the base of the stem is twice that in the root
( Fig. 3).
1865
Fig. 3. Sodium concentration in plant parts of sweet pepper grown in
15 mM NaCl for 3 weeks.
Fig. 4. Sodium concentration at different parts of sweet pepper stems
treated with 15 mM NaCl for 2 weeks. Position in the stem is
represented as cumulative weight from stem base. This agrees with
cumulative stem length but weight is preferred due to the conic form
of the stem towards the top.
Xylem sap analyses
The sodium concentration of the pith cells ( Fig. 4) as
well as in the xylem sap ( Fig. 5) decreases exponentially
towards the shoot tip. This exponential decrease is maintained with longer periods of sodium treatment (data
not shown).
Removal of sodium from the nutrient solution
When plants are transferred to a sodium-free nutrient
solution for 1 week, the amount of sodium in the pith
cells of all stem segments decreases by about 50% (compare Fig. 4 with Fig. 6A), declining exponentially towards
the apex. In contrast, the concentration gradient in the
xylem disappears when the plants are transferred to a
sodium-free nutrient solution (Fig. 6B).
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Blom-Zandstra et al.
Table 1. Sodium content (mmol kg−1 FW) after exposure to
sodium for 1 week in a split-root experiment
Both root parts of the same plant are exposed to 15 mM NaCl, one
supplemented with 22Na+. For calculations of sodium concentrations
in different plant parts: see Materials and methods.
Fig. 5. Sodium concentration of the xylem sap from different stem
segments of sweet pepper in relation to their cumulative fresh weight
towards the top.
Split-root experiments
After 1 week of salt treatment using a split-root design,
the distribution of 22Na+ immediately after exposure to
the labelled sodium was determined ( Table 1). It can be
seen, that sodium uptake was similar to that in Figs 2, 3.
However, only a small amount of 22Na+ was detected in
the roots not exposed to labelled sodium chloride.
In a second split-root experiment, the sodiumcontaining nutrient solutions of both compartments were
replenished with sodium-free nutrient solutions. In contrast to the results from the first split-root experiment,
22Na+ transport into the unlabelled roots and a release
into the medium takes place (Fig. 7A, B).
Discussion
The results clearly indicate that sweet pepper possesses
the characteristics for well-controlled recirculation of
Plant part
Sodium content (±sd)
Labelled root part
Unlabelled root part
Leaves
Stem total
8.87
0.15
3.26
19.14
Stem part I
Stem part II
Stem part III
26.44 (4.10)
20.37 (2.80)
7.03 (2.55)
(0.82)
(0.02)
(1.88)
(3.03)
sodium as described for bean (Jacoby, 1979) and Lupinus
albus (Munns, 1988; Munns et al., 1988): (1) sodium
does not disrupt plant growth, (2) it is preferentially
accumulated in pith cells of the stem, (3) the sodium
concentration in xylem sap decreases towards the apex
and is not influenced by solute uptake and transpiration,
(4) the phloem participates in sodium transport, indicated
by 22Na+ entering the unlabelled root part and leaking
into the medium. As hardly any sodium is transferred to
the leaves, sodium appears to be efficiently restrained
from reaching the photosynthetic tissue.
The sodium concentration gradient in the plant reached
a steady-state after 7 d (Fig. 2), indicating that sodium
uptake, recirculation, and release by the roots is well
regulated. The observed exponential gradient of sodium
within the xylem sap can be explained by a continuous
supply of sodium from the roots (or stem base) to the
shoot with loss to the pith cells at a rate dependent upon
the local concentration, followed by recirculation back to
the roots (or stem base) via the phloem. Thus, the pith
cells, adjacent to both the xylem and the phloem vessels,
play an important role in the recirculation of sodium.
Fig. 6. Sodium concentration in various parts of the stem (A) and of the xylem sap in different stem segments (B) of sweet pepper, treated with
15 mM NaCl for 2 weeks and transferred to a sodium-free nutrient solution for1 week. Position of the stem from basis towards the top is calculated
by summation of the fresh weight of the stem segments.
Sodium fluxes in sweet pepper
1867
Fig. 7. (A) Time-course of sodium concentrations of labelled and non-labelled root parts after transfer to sodium-free solution. (B) Sodium release
from the labelled and non-labelled root part after transfer of the plants to a sodium-free nutrient solution.
Sodium transport model
To understand sodium recirculation through the stem it
is necessary to consider the transport mechanisms in the
pith cells. The concentration of sodium in these cells
( Fig. 4) is equal to the concentration in the xylem sap
( Fig. 5). Therefore, the presence of an active pump system
in the plasma membrane seems unlikely, thus sodium
influx probably occurs by passive diffusion. The occurrence of diffusion-controlled influx is supported by the
decrease in sodium content in the pith cells towards the
stem top, which is proportional to the diffusion resistance
for sodium influx through the plasma membrane. The
tonoplast probably contains an active sodium pump
system, as described for Plantago maritima L. (Staal et al.,
1991), by which the concentration in the cytoplasm can
be kept low and thus maintaining the steep gradient in
the xylem sap.
In addition, an efflux system principally directed
towards the phloem must be present. If a sodium efflux
system into the phloem similar to that for assimilates
( Van Bel, 1996; Jeschke et al., 1997) is assumed, the
question remains why only a small amount of label was
found in the unlabelled roots in the split root experiment
( Table 1). This can only be explained by a model in
which sodium leaks from the phloem into the xylem, as
described in the Münch model for photoassimilates and
nutrients ( Eschrich et al., 1972). For sodium, no such
model has been described to date, but the transport
characteristics may be similar as far as a gradual release
into the xylem along the pathway is concerned.
Sodium release
A model in which net transfer of sodium is directed from
the phloem towards the xylem applies when sodium is
constantly supplied in the nutrient solution. When sodium
is withdrawn from the medium, sodium enters the roots
and is released into the nutrient solution as shown in the
second split root experiment ( Fig. 7A, B). Pith cells
release about 50% of the sodium taken up after a week
( Fig. 6A), while maintaining their concentration profile.
However, the profile in the xylem sap changes completely
( Fig. 6B), becoming almost uniform throughout the stem.
Yet, it may be assumed that the transport system described
above has not been principally changed. The sudden
release of sodium in sodium-free solution may be
explained either by an activation of a transport mechanism or a decrease of the diffusion resistance for passive
sodium efflux from the cells. A patch clamp study (to be
published) indicates the latter: when plants are exposed
to sodium, the highly selective potassium channels become
more permeable for sodium. Retrieving sodium from the
external medium will open these channels, enabling
sodium efflux from the plant. As a result, sodium transfer
from the phloem to the xylem stops and the concentration
gradient is reduced.
It is concluded here that sodium recirculation is strictly
regulated: when the sodium concentration in the nutrient
solution is kept constant, it accumulates in xylem and
pith cells and is continuously released into the phloem
where it is transferred downward through the stem and
pumped into the xylem, either in the roots or at the stem
base. As soon as sodium is deprived from the roots, the
diffusion resistance for passive sodium efflux will decrease
and sodium is released into the medium in the roots or
maybe at the stem base. Thus, the external sodium
concentration controls regulatory mechanisms for internal
sodium fluxes.
For growers these results have important consequences.
A replenishment of the nutrient solution results in a rapid
release of sodium into the nutrient solution, increasing
the sodium concentration.
Acknowledgements
The authors wish to thank Mr TDB van der Struijs for his help
with the c-spectrometry measurements and Dr JPFG Helsper
and Dr ThA Dueck for the critical reading of the manuscript.
1868
Blom-Zandstra et al.
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