J. Plant Nutr. Soil Sci. 2020, 183, 455–467 DOI: 10.1002/jpln.201900491 455
Potassium is a potential toxicant for Arabidopsis thaliana under saline
conditions
Wenting Zhao1*, Franziska Faust1, and Sven Schubert1
1 Institute of Plant Nutrition (iFZ), Justus Liebig University, Heinrich-Buff-Ring 26–32, 35392 Giessen, Germany
Abstract
Background and aims: Most physiological and biochemical studies on salt stress are NaCl-
based. However, other ions (e.g., K+, Ca2+, Mg2+, and SO24 ) also contribute to salt stress in spe-
cial circumstances. In this study, salt stress induced by various salts was investigated for a better
understanding of salinity.
Methods: Arabidopsis thaliana plants were stepwise acclimated to five iso-osmotic salts as fol-
lows: NaCl, KCl, Na2SO4, K2SO4, and CaCl2.
Results and Conclusions: Exposure to KCl and K2SO4 led to more severe toxicity symptoms,
smaller biomass, and lower level of chlorophyll than exposure to NaCl and Na2SO4, indicating
that Arabidopsis plants are more sensitive to potassium salts. The strongly reduced growth was
negatively correlated with the accumulation of soluble sugars observed in KCl- and
K2SO4-treated plants, suggesting a blockage in the utilization of sugars for growth. We found
that exposure to KCl and K2SO4 suppressed or even blocked sucrose degradation, thus leading to
strong accumulation of sucrose in shoots, which then probably inhibited photosynthesis via feedback
inhibition. Moreover, K+ was more accumulated in shoots than Na+ after corresponding potassium or
sodium salt treatments, thus resulting in decreased Ca2+ and Mg2+ concentrations in response to
KCl and K2SO4. However, K2SO4 caused more severe toxicity symptoms than iso-osmotic KCl, even
when the K+ level was lower in K2SO4-treated plants. We found that Na2SO4 and K2SO4 induced
strong accumulation of tricarboxylic acid intermediates, especially fumarate and succinate which
might induce oxidative stress. Thus, the severe toxicity symptoms found in K2SO4-treated plants
were also attributed to SO24 in addition to the massive accumulation of K
+.
Key words: fumarate / potassium salts / succinate / sucrose degradation / sucrose transport
Supporting Information
Accepted May 06, 2020 available online
1 Introduction
Salinity is one of the major threats to crop yield worldwide 2008). Thus, it is necessary to conduct experiments with salt
(Munns and Tester, 2008). The physiological and molecular stress induced by other salts (e.g., Na2SO4 and KCl) for a
mechanisms of salt stress in plants have been well docu- more comprehensive understanding of salt stress.
mented (Zhu, 2001; Munns and Tester, 2008). Most of the
results are derived from NaCl–stress experiments, since Na+ Plants suffer salt stress in three phases (Munns, 1993;
and Cl– often dominate in saline soils (Rengasamy, 2006). Munns and Tester, 2008; Schubert, 2011). First, after the
However, apart from Na+ and Cl–, other ions may also contrib- application of salt treatment, transient decreases in turgor
ute to salt stress such as K+ and SO24 (Chang et al., 1983; and growth rate occur in Phase 0 (the first few minutes or
Rengasamy, 2006). The principle sources of K+ in soils are hours) (Schubert, 2011). Second, plants then show stunted
potassium fertilizers and K+-rich minerals such as granite growth which is attributed to salinity-induced osmotic stress
dust, greensand, muscovite, biotite, and feldspars. The un- (Phase 1) (Munns, 2002). The osmotic effect induced by salt
usual K+ accumulation in soils might be attributed to heavy stress in Phase 1 is similar to that caused by drought stress,
application of K+ fertilizers (Parnes, 2013), and extreme thus it is not salt-specific (Chazen et al., 1995). Finally, ion
weathering of K+-rich minerals (Hillel, 2008). Many soils in toxicity symptoms develop resulting from ion accumulation
eastern U.S. show an excess of K+ because of over-applica- over time (Phase 2). Generally, ion toxicity symptoms mainly
tion of organic residues (Parnes, 2013). Minerals such as occur in old leaves as they are transpiring and accumulating
micas and feldspars have significant amounts of potassium ions for a longer time period than young leaves (Munns,
and this element is released by weathering to make it avail- 2002).
able for plants. If the weathering process is prolonged and the
leaching system is incomplete, soils will reach high potassium During Phase 2, the Na+ accumulation interferes with K+
concentrations such as the soils in northwest Missouri (Hillel, homeostasis and the imbalance of K+ is responsible for a
* Correspondence: W. Zhao; e-mail:
wenting.zhao@ernaehrung.uni-giessen.de
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
456 Zhao, Faust, Schubert J. Plant Nutr. Soil Sci. 2020, 183, 455–467
large proportion of Na+ toxicity (Kronzucker et al., 2013). 21C and light (200 mmol m–2 s–1) for 8 h and 18C without
Proper potassium fertilization can alleviate the damages light (0 mmol m–2 s–1) for 16 h. The hydroponic culture con-
caused by NaCl stress (Wakeel et al., 2011). However, K+ sisted of 1 mM KH2PO4, 0.25 mM K2SO4, 1 mM MgSO4,
does not always benefit organisms and it can be harmful 2 mM Ca(NO3)2, 50 mM KCl, 5 mM MnSO4, 1 mM ZnSO4, 1
when taken up in excess. In the human body, excessive K+ in mM CuSO4, 0.1 mM NiSO4, 0.7 mM (NH4)6Mo7O24, 30 mM
the blood leads to tingling, decreased reflexes, and muscle H3BO3, and 100 mM FeNaEDTA. Four-week-old seedlings
weakness (Viera and Wouk, 2015). Previous researchers ob- were stepwise acclimated to five iso-osmotic salt treatments.
served that both NaCl and KCl reduced the biomass of plants For each treatment, 15.00 mM NaCl, 15.34 mM KCl, 11.23
at relatively high concentrations, but plants treated with NaCl mM Na2SO4, 11.70 mM K2SO4, and 10.61 mM CaCl2 were
such as Triticum aestivum (Adiloglu et al., 2007) and Brassi- applied every 24 h until reaching final concentrations of
ca rapa (Reich et al., 2017) showed higher reductions in bio- 105.00, 107.38, 78.61, 81.90, and 74.27 mM, respectively.
mass. For Arabidopsis thaliana, an important model plant in Experiments were conducted in seven biological replicates
plant biology, it is unknown whether potassium salts are toxic. (i.e., seven 3-L pots). For each pot, there were three plants.
This leads to our first hypothesis that high concentrations of The nutrient solution was changed every 3 d. Plants were har-
sodium salts and potassium salts can reduce the growth of vested after 12 d of treatment with 105 mM NaCl, iso-osmotic
Arabidopsis plants and that plants are more sensitive to salt KCl, Na2SO4, K2SO4, and CaCl2. Four biological replicates
stress induced by sodium salts compared to potassium salts. (i.e., four pots) were harvested to determine the dry weight,
chlorophyll (N-tester), ion concentrations, and sugar concen-
Apart from Na+ toxicity, Cl– toxicity is another important topic trations. In these experiments, the entire rosette leaves were
in salinity research. Many important fruits (e.g., strawberry) harvested. The other three biological replicates (i.e., three
and vegetables (e.g., tomato, bean, and soybean) cannot tol- pots) were harvested for the GC-MS and qRT-PCR analyses.
erate Cl– levels above 40 mg g–1 dry weight (White and Mature rosette leaves from 9th leaf to 14th leaf were harvested
Broadley, 2001; Hütsch et al., 2018). However, sulfate salts because these leaves showed toxicity symptoms under salt
can also exceed the tolerable level of plants in some marine stress especially after exposure to KCl and K2SO4. The most
soils as well as in areas that are affected by secondary salini- severely affected young leaves were not harvested because
zation induced by saline irrigation water (Chang et al., 1983). they were totally bleached under stress in some treatments.
Thus, a comparison of plant responses to chloride salts and
sulfate salts is necessary. Sulfate salts (e.g., Na2SO4) have a
higher inhibitory effect on plant growth than chloride salts 2.2 N-tester values
(e.g., NaCl) in Prosopis strombulifera (Llanes et al., 2013; After 12 d of treatment with the salts described above, all the
Reginato et al., 2014), Thellungiella salsuginea (Leonova rosette leaves were tested by the hand-held Yara N-tester
et al., 2009), T. botschantzevii (Leonova et al., 2009), Triticum (Yara, Norway), except the cotyledons and the first and sec-
aestivum (Hampson and Simpson, 1990), and Brassica rapa ond-produced leaves because they were too small to take a
(Aghajanzadeh et al., 2017; Reich et al., 2017). In those measurement. Thirty random-point measurements on the
plants, Na2SO4 induced higher reductions in germination rosette leaves were pooled in one read and four reads were
rate, shoot height, leaf number, and biomass, even when the
2 obtained for each treatment. Yara N-tester allows estimatingSO4 concentration was much lower than the Cl
– concentra- chlorophyll concentration without damaging the plants (Faust
tion in the NaCl-treated plants. This leads to our second and Schubert, 2016).
hypothesis that sulfate salts have a higher inhibitory effect on
the growth of Arabidopsis plants than chloride salts.
2.3 Quantification of ion concentrations
In order to test our hypotheses, Arabidopsis plants were step-
wise acclimated to five iso-osmotic salts (NaCl, KCl, Na SO , Shoots and roots were harvested separately and dried at2 4
K SO , and CaCl ) to avoid differences caused by osmotic 80C for three days in an oven. Then, the dried samples were2 4 2
stress. Plants were harvested when the toxicity symptoms ground to powder in a coffee grinder. To determine Na
+, K+,
2+ 2+
occurred at least in one treatment to make sure that plants Ca , and Mg concentrations, 200 mg of shoot dry matter
were starting to enter Phase 2. Biomass, chlorophyll concen- (or 100 mg of root dry matter) were weighed into a ceramic
tration, and sugar concentrations were determined to test the alumina crucible, then dry-ashed at 550C for 12 h in a muffle
growth of Arabidopsis plants under various salt treatments. furnace. After cooling down of samples, 2.5 mL 5 M HNO3
Moreover, mineral nutrient composition of plants treated with and 2 mL double-deionized water were added and briefly
different salts were determined to evaluate whether Na+ or K+ heated to dissolve the ash. The mixed solution was filtered
was more toxic in Arabidopsis, as well as to investigate the into a 50-mL volumetric flask with hot double-deionized water,
contribution of Cl– and SO2 to the toxicity symptoms. and diluted to 50 mL with double-deionized water after cool-4
ing down. Samples were automatically diluted by Varian
SPS5 and injected into the atomic absorption spectrophotom-
2 Material and methods eter (SpectrAA220 FS, Varian, Mulgrave, Victoria, Australia)
(Pitann et al., 2013).
2.1 Plant material and growth conditions
To determine Cl– and SO24 concentrations, 50 mg of shoot
Arabidopsis thaliana (L.) (ecotype: Col-0) was used through- dry matter (or 50 mg of root dry matter) were weighed into a
out the study. Plants were cultivated in a growth chamber with test tube, 3 mL double-deionized water were added and
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
J. Plant Nutr. Soil Sci. 2020, 183, 455–467 Potassium is a potential toxicant for Arabidopsis thaliana 457
boiled in a water bath at 100C for 3 h. Samples were filtered for the determination. The soluble sugars were measured
into a 25-mL volumetric flask, cooled, and filled up to 25 mL enzymatically (R-Biopharm, Germany).
with double-deionized water. Samples were automatically in-
jected by a 788 IC filtration sample processor into the ion To measure the starch concentrations, 30 mg of the dried and
chromatograph (761 Compact IC, Metrohm). The determined ground shoot material were mixed with 1.8 mL 18% (w/v) HCl
ion concentrations in shoots and roots were listed in Tab. S1. in a 2-mL reaction tube. Starch was extracted at 5C for
60 min and centrifuged at 5,000 g for 20 min. Then, 300 mL of
Ion (Na+, K+, Ca2+, and Mg2+) uptake was calculated as follows: supernatant were mixed with 300 mL of Lugol’s solution [0.5%
(w/v) KI and 0.25% (w/v) I2 in water] and determined at
Ion uptake ¼ Shoot ion content þ Root ion content; (1) 530 nm and 605 nm. The starch concentration was calculated
based on the method of Appenroth et al. (2010).
Ion translocation from roots to shoots is described by:
Ion translocation from roots to shoots 2.6 qRT-PCR
¼ Shoot ion content After 12 d of treatments with five iso-osmotic salts described: (2)
Root ion content above, 200–300 mg of fresh mature rosette leaves (from leaf
number 9 to 14) were harvested to extract total RNA using
RNeasy Plant Mini Kit (Qiagen). Then, total RNA was
2.4 Determination of TCA intermediates reversely transcribed by means of RevertAid First Strand
The TCA intermediates were determined by means of GC-MS. cDNA Synthesis Kit (Thermo Scientific). qRT-PCR was per-
For GC-MS analysis, 200–300 mg of fresh mature rosette leaves formed in a StepOnePlus system (Applied Biosystems). The
(from 9th leaf to 14th leaf) were harvested and snap-frozen in relative transcript level of genes was calculated by 2-DDCt. DCt
liquid nitrogen. Approximately 500 mg pre-cooled beads were is the Ct value of tested target gene subtracted by the geo-
added to the samples in an Eppendorf tube and homogenized metric mean of the Ct value of endogenous reference genes,
mechanically in a Retsch MM300 tissuelyzer (3 min, 30 Hz). SAND family protein (At2g28390) and TIP41-like protein
Then, 1 mL of pre-cooled MeOH : chloroform : water (5 : 2 : 2) (At4g34270) (Czechowski et al., 2005; Dekkers et al., 2012 ).
was added, vortexed, and homogenized mechanically in a DDCt is the difference between DCt under experimental condi-
Retsch MM300 tissuelyzer (6 min, 30 Hz). After centrifugation tions and DCt under control condition. Transcriptional Fold-
(30,000 g, 4C, 5 min), 800 mL of the supernatant were trans- Change of a target gene was quantified as the quotient of its
ferred to a new tube and dried under nitrogen flow. Samples 2-DDCt value after salt treatments and that under control condi-
were reconstituted in 100 mL double-deionized water and de- tions. The value of |log2 FoldChange | ‡ 1 was used as cutoff
rivatized with methyl chloroformate (MCF) according to the to evaluate significant differences in gene transcription. Pri-
protocol described by Smart et al. (2010). For MCF derivatiza- mers used for qRT-PCR are listed in Tab. S2. The single tar-
tion, 37 mL NaOH (1 M), 25 mL pyridine, 75 mL methanol, and get bands shown in supplementary Fig. S1 indicate high spe-
10 mL MCF were added to an aliquot of 37 mL reconstituted cificity of primers. The Ct values obtained from real-time PCR
sample and subsequently made a vortex for 30 s. Then, instrument are listed in Tab. S3.
another 10 mL MCF were added and mixed vigorously for
another 30 s. The derivatization reaction was stopped by add-
ing 200 mL chloroform. D3-alanine was added as an internal 2.7 Statistical analysis
standard (0.2 mmol per sample). All samples were analyzed Experiments in this study were conducted in four biological
in a randomized order. Analysis was performed using GC replicates, except for the qRT-PCR analysis and determina-
(7890B, Agilent) coupled with a quadrupole detector (59977B, tion of TCA intermediates (three replicates). Data are pre-
Agilent). The system was controlled by ChemStation (Agilent). sented as means – standard errors (SE). Significant differ-
Chromatograms and mass spectra were processed by means ences among treatments were statistically evaluated by
of Chemstation (Agilent) and Matlab R2014b (Mathworks, Inc.) one-way ANOVA and post-hoc analysis using Tukey’s Hon-
(Johnsen et al., 2017) to obtain analyzable chromatographic estly Significant Difference (HSD) method (SciStatCalc:
peak height and retention time. For the metabolite identifica- http://scistatcalc.blogspot.com/2013/11/home.html) and are
tion, the processed data were then matched with the mass indicated by different letters (P < 5%). Significant differences
spectral library NIST/EPA/NIH Mass spectral library (Version between two treatments are indicated by asterisks (*P < 5%,
2). The peaks of mass fragments were normalized against **P < 1%, ***P < 0.1%, ****P < 0.01%, t-test).
internal standard (d3-alanine) and sample fresh weight.
2.5 Sugar concentrations 3 Results
For the determination of soluble sugars (i.e., glucose, fruc- 3.1 Potassium salts induce more severe toxicity
tose, and sucrose), 200 mg of dried and ground shoot or root than sodium salts in Arabidopsis
material were weighed in a 50-mL volumetric flask, then
30 mL hot double-deionized water were added. Samples Exposure to 105 mM NaCl caused no toxicity symptoms even
were incubated at 60C for 30 min, then filled up to 50 mL after 12 d of treatment, while iso-osmotic KCl-treated plants
with double-deionized water after cooling down, finally filtered showed chlorosis in young leaves after only 2 days of treat-
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
458 Zhao, Faust, Schubert J. Plant Nutr. Soil Sci. 2020, 183, 455–467
ment and the chlorosis progressed to old leaves with the not substantially altered after NaCl, KCl, and Na2SO4 treat-
extending treatment time (Fig. 1). After 2 d of treatment with ments. However, it displayed a dramatic reduction after expo-
78.61 mM Na2SO4, plants showed no obvious toxicity symp- sure to iso-osmotic K2SO4 and even increased in iso-osmotic
toms. In contrast, iso-osmotic K2SO4 induced cell death in CaCl2-treated plants (Fig. 2b). Exposure to NaCl, Na2SO4,
young leaves after 2 d of treatment, which was even more and CaCl2 had no substantial effect on N-tester values, which
toxic than iso-osmotic KCl (Fig. 1a). When the treatment time reflect chlorophyll concentration. In contrast, the N-tester val-
was extended to 12 d, the Na2SO4-treated plants displayed ues decreased dramatically in KCl and K2SO4-treated plants
small-scale necrosis at the leaf margins of old leaves and the (Fig. 2c).
K2SO4-treated plants showed necrosis in almost all the area
of young leaves (Fig. 1b). Hence, plants treated with potas-
sium salts (KCl and K 3.2 Uptake and translocation of Na+, K+, Ca2+, and2SO4) showed more severe toxicity
symptoms compared to the corresponding iso-osmotic so- Mg2+ under various salt stresses
dium salts (NaCl and Na2SO4). Plants treated with iso- The uptake of Na+, translocation of Na+ into shoots, and Na+
osmotic CaCl2 did not show toxicity symptoms even after concentration in shoots were always higher in plants treated
12 d of treatment (Fig. 1). with iso-osmotic Na2SO4 (157.22 mM Na
+ in hydroponic cul-
ture) than in plants treated with 105 mM NaCl (Fig. 3a–c),
Shoot dry weight, root dry weight, and N-tester values were
probably due to the higher external Na+ in the Na SO treat-
determined to examine plant growth under various salt stress- 2 4
ment. However, the uptake of K+ by K SO -treated plants
es. Exposure to 105 mM NaCl for 12 d led to a reduction 2 4
was lower than that by iso-osmotic KCl-treated ones (Fig. 3a).
(35%) of shoot dry weight compared to control, while iso-
The translocation of K+ into shoots was not altered by KCl
osmotic KCl had a more severe inhibitory effect (52% reduc-
treatment, but showed an increase under K SO treatment
tion, not significant) on shoot dry weight (Fig. 2a). Similarly, 2 4
compared to control (Fig. 3b). Surprisingly, exposure to
shoot dry weight had a reduction of 28% or 64% in response
K2SO4 (163.8 mM K
+ in hydroponic culture) led to lower K+
to iso-osmotic Na2SO4 or K2SO4, being more severely sup- concentration in shoots compared to iso-osmotic KCl treat-
pressed by K2SO4 treatment (Fig. 2a). Root dry weight was ment (107.38 mM K+ in hydroponic culture), although the
external K+ concentration was even
higher in the K2SO4 treatment (Fig. 3c).
The uptake and translocation of domi-
nant cations (Na+ and K+) under corre-
sponding sodium and potassium salt
treatments were compared. In the pres-
ence of Cl–, KCl-treated plants showed
higher levels of K+ uptake and translo-
cation compared to the Na+ uptake and
translocation in iso-osmotic NaCl-
treated plants, resulting in the strong
accumulation of K+ after exposure to
KCl (Fig. 3a–c). In contrast, exposure to
K2SO4 led to lower K
+ uptake than Na+
uptake induced by iso-osmotic Na2SO4
(Fig. 3a), but the translocation of K+ or
Na+ under K2SO4 or Na2SO4 treat-
ments was similar (Fig. 3b). Neverthe-
less, the K2SO4-treated plants still dis-
played higher K+ concentration in
shoots than Na+ in response to iso-
osmotic Na2SO4 (Fig. 3c).
The Ca2+ and Mg2+ uptake and transloca-
tion into shoots were significantly re-
duced in all salt treatments except the up-
take of Ca2+ in plants treated with CaCl2
(Fig. 3d–f). Exposure to 107.38 mM KCl
induced a higher inhibitory effect on the
uptake and translocation of Ca2+ and
Figure 1: Phenotypes of Arabidopsis thaliana under 105 mM NaCl, iso-osmotic KCl, iso-osmotic
Mg2+ than exposure to 105 mM NaCl,
Na2SO4, iso-osmotic K2SO4, and iso-osmotic CaCl2 treatments for 2 d (a) and 12 d (b). The 2+ 2+
4-week-old seedlings were exposed to gradually increasing concentrations of NaCl (15.00 mM), thus resulting in lower Ca and Mg
KCl (15.34 mM), Na2SO4 (11.23 mM), K2SO4 (11.70 mM), and CaCl2 (10.61 mM) every 24 h until
concentrations in the shoots of KCl-
reaching a final concentration of 105.00 mM, 107.38 mM, 78.61 mM, 81.90 mM, and 74.27 mM, treated plants (Fig. 3d–f). Similarly, plants
respectively. treated with 81.90 mM K2SO4 showed
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
J. Plant Nutr. Soil Sci. 2020, 183, 455–467 Potassium is a potential toxicant for Arabidopsis thaliana 459
lower levels of Ca2+ uptake and concentration in shoots as
well as Mg2+ than those treated with 78.61 mM Na2SO4
(Fig. 3d, f). Considering the translocation of Ca2+ and Mg2+,
K2SO4-treated plants also displayed lower levels of Ca
2+
translocation than Na2SO4-treated ones, but the translocation
of Mg2+ showed similar reductions after exposure to Na2SO4
and K2SO4 (Fig. 3e).
3.3 Effects of chloride and sulfate salts on the
accumulation of TCA intermediates
The Cl– concentration in shoots showed significant increases
after exposure to NaCl, KCl, and CaCl2 (Fig. 4a, b). The
plants treated with KCl showed the strongest accumulation of
Cl– in shoots followed by the CaCl2- and NaCl-treated plants
(Fig. 4a). The shoot concentration of SO 2-4 presented higher
levels in K2SO4-treated plants than in Na2SO4-treated ones
(Fig. 4b). Moreover, the Cl– concentration under chloride salt
treatments was extremely higher than the concentration of
SO24 under iso-osmotic sulfate salt treatments (Fig. 4).
The TCA cycle is a series of chemical reactions that contrib-
ute to the production of chemical energy in the form of NADH
+ H+ and ATP. The concentration of tested TCA intermediates
(sum of concentrations of fumarate, succinate, a-ketogluta-
rate, isocitrate, cis-aconitate, and citrate) had no substantial
differences after exposure to 105 mM NaCl and iso-osmotic
KCl for 12 d, while exposure to Na2SO4 and K2SO4 increased
the total amount of TCA intermediates, twice as much as that
in control (Tab. 1). These increases were mainly due to fuma-
rate and succinate. The fumarate concentration in Na2SO4-
and K2SO4-treated plants showed 2.4- and 2.7-fold increases
compared to control, respectively (Tab. 1). Considering succi-
nate, exposure to K2SO4 induced the highest accumulation of
succinate in shoots (7.4-fold higher compared to control) fol-
lowed by exposure to Na2SO4 which led to 3.9-fold increases
(Tab. 1).
3.4 Effects of sodium and potassium salts on
sugar concentrations
The total concentration of tested soluble sugars (i.e., glucose,
fructose, and sucrose) increased after salt treatments both in
shoots and roots (Fig. 5a, b). Among these, sucrose was the
major soluble sugar in salt-treated Arabidopsis plants. In
shoots, plants accumulated sucrose under all five iso-osmotic
salt treatments, displaying the strongest accumulation in
plants treated with potassium salts (KCl and K2SO4) (Fig. 5a).
Different from sucrose, the concentration of glucose and fruc-
Figure 2: Growth responses of Arabidopsis thaliana to five iso- tose remained unaffected by NaCl, KCl, and Na2SO4 treat-
osmotic salt treatments. Four-week-old seedlings were treated with ments, but increased after exposure to K2SO4 and CaCl2
105 mM NaCl, 107.38 mM KCl, 78.61 mM Na2SO4, 81.90 mM K2SO4, (Fig. 5a). In roots, the concentration of total tested soluble
and 74.27 mM CaCl2 for 12 d. Shoot dry weight (a), root dry weight (b), sugars also increased after salt treatments, but was signifi-
and N-tester values of rosette leaves (c) were determined. Data show cantly lower than that in shoots (Fig. 5b). The starch concen-
means of four replicates. Error bars represent standard errors (SE). Sig- tration was not substantially altered in shoots after exposure
nificant differences are indicated by different letters (P < 5%; one-way to NaCl, KCl, and Na2SO4. However, KCl and K SO -treatedANOVA and post-hoc analysis using Tukey’s Honestly Significant Differ- 2 4plants showed a reduction (38% or 23%) of starch concentra-
ence method) and by asterisks (*, **, ***, **** significant differences of
fresh weight, dry weight, and N-tester values under five iso-osmotic salt tion compared to control (Fig. 5c).
treatments in comparison to the control with P < 5%, 1%, 0.1%, 0.01%
respectively; t-test).
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
460 Zhao, Faust, Schubert J. Plant Nutr. Soil Sci. 2020, 183, 455–467
Figure 3: Effect of sodium and potassium salts on the uptake, translocation, and concentration of cations (Na+, K+, Ca2+,
and Mg2+). Four-week-old seedlings were treated with 105 mM NaCl, 107.38 mM KCl, 78.61 mM Na2SO4,
81.90 mM K2SO4, and 74.27 mM CaCl . After 12 d, Na
+
2 and K
+ uptake (a), translocation into shoots (b), and concentra-
tions in shoots (c) were calculated. In parallel, the Ca2+ and Mg2+ uptake (d), translocation (e), and shoot concentrations (f)
were determined. DW indicates dry weight, and N.S. indicates no significant difference. Data represent the means of four
replicates (– SE). Significant differences of cation uptake, translocation, and shoot concentration among treatments are
indicated by different letters (P < 5%; one-way ANOVA and post-hoc analysis using Tukey’s Honestly Significant Difference
method). The lower-case letters indicate statistical differences among treatments in regard to sodium and calcium; the cap-
ital letters indicate statistical differences among treatments in regard to potassium and magnesium. The significant differen-
ces of K+ uptake, translocation, and concentration under KCl or K2SO4 treatments in comparison to Na
+ uptake, transloca-
tion, and concentration under NaCl or Na2SO4 treatments are indicated by asterisks (*P < 5%, **P < 1%, ***P < 0.1%,
****P < 0.01%, t-test).
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
J. Plant Nutr. Soil Sci. 2020, 183, 455–467 Potassium is a potential toxicant for Arabidopsis thaliana 461
Figure 4: Effect of chloride and sulfate salts on Cl– and SO 2-4 accumulation in shoots. Four-week-old seedlings were
exposed to 105 mM NaCl, 107.38 mM KCl, 78.61 mM Na2SO4, 81.90 mM K2SO4, and 74.27 mM CaCl2. After 12 d, the
concentrations of Cl– (a) and SO24 (b) in shoots were calculated. DW indicates dry weight. Data represent the means
of four replicates (– SE). Significant differences of Cl– and SO24 shoot concentrations among treatments are indicated
by different letters (P < 5%; one-way ANOVA and post-hoc analysis using Tukey’s Honestly Significant Difference
method). The significant differences of Cl– concentration under KCl and CaCl2 treatments in comparison to that under
NaCl treatment are indicated by asterisks (*P < 5%, **P < 1%, ***P < 0.1%, ****P < 0.01%, t-test). The significant differ-
ences of SO24 concentration in Na2SO4-treated plants in comparison to those treated with K2SO4 are indicated by
asterisks (*P < 5%, **P < 1%, ***P < 0.1%, ****P < 0.01%, t-test).
Table 1: Effect of five iso-osmotic salt treatments on tested TCA intermediates (mmol mg–1 DW). Four-week-old seedlings were exposed to
105 mM NaCl, 107.38 mM KCl, 78.61 mM Na2SO4, 81.90 mM K2SO4, and 74.27 mM CaCl2. After 12 d of salt treatment, the concentrations of
TCA intermediates in rosette leaves were determined by means of GC-MS analysis. Data represent the means of three replicates (– SE). DW
indicates dry weight.
TCA Treatment
Control NaCl KCl Na2SO4 K2SO4 CaCl2
Fumarate 176.80 – 9.44 263.10 – 20.49 234.18 – 18.06 428.66 – 34.75 479.58 – 3.62 120.13 – 5.04
Succinate 8.60 – 1.69 5.92 – 0.85 10.17 – 0.68 33.34 – 0.95 63.67 – 5.71 3.89 – 0.13
a-Ketoglutarate 0.89 – 0.07 1.66 – 0.32 15.00 – 0.95 18.19 – 2.25 14.37 – 1.25 3.03 – 0.24
Isocitrate 0.07 – 0.01 0.07 – 0.01 0.47 – 0.06 1.36 – 0.19 0.14 – 0.02 0.11 – 0.01
cis-Aconitate 4.28 – 0.24 1.74 – 0.26 3.13 – 0.36 7.93 – 0.95 2.51 – 0.33 0.74 – 0.11
Citrate 184.18 – 7.07 64.99 – 5.85 110.19 – 14.02 286.84 – 11.81 190.26 – 4.97 43.66 – 1.76
Total TCA 374.80 – 3.65 337.49 – 17.33 373.14 – 6.39 776.33 – 33.91 750.52 – 9.11 171.57 – 6.73
intermediates
In order to understand the largely accumulated sucrose after SPS2 was induced and SPS4 was repressed (Fig. 6a). The
potassium salt treatments, the transcription of genes related transcription of SPS2 showed a similar level in plants treated
to sucrose synthesis, sucrose degradation, and sucrose with NaCl or KCl, as well as in plants treated with K2SO4 or
transport was determined via qRT-PCR. The absolute value Na2SO4. The transcription of SPS4 was more strongly sup-
of log2 FoldChange ‡ 1 was used as cutoff to evaluate signifi- pressed by potassium salts (KCl and K2SO4) compared to
cant differences in gene transcription. Sucrose is synthesized corresponding sodium salts (NaCl and Na2SO4) (Fig. 6a).
from UDP-glucose and fructose-6-phosphate via sucrose
phosphate synthase (SPS), a key enzyme for sucrose Two sucrose-cleaving enzymes contribute to sucrose degra-
synthesis. Four SPS genes (i.e., SPS1, SPS2, SPS3, and dation: invertase and sucrose synthase (SUS). Invertase
SPS4) have been characterized in Arabidopsis thaliana hydrolyzes sucrose irreversibly into glucose and fructose,
(Ruan, 2014). After exposure to five iso-osmotic salts, the while SUS cleaves sucrose reversibly into fructose and UDP-
transcription of SPS1 and SPS3 was not modified, while glucose (or ADP-glucose) (Stein and Granot, 2019). SUS not
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
462 Zhao, Faust, Schubert J. Plant Nutr. Soil Sci. 2020, 183, 455–467
only involves sucrose cleavage but also mediates sucrose
re-synthesis (Noël and Pontis, 2000; Stein and Granot,
2019). Fifteen isoforms of invertase have been identified in
Arabidopsis (Ruan, 2014). Among these, two cytosolic inver-
tase genes (CINV1 and CINV2) are characterized for their
important roles in plant growth and development (Vargas and
Salerno, 2010). In this study, both CINV1 and CINV2 showed
decreased transcription after five iso-osmotic salt treatments,
CINV2 being more highly suppressed and displayed a lower
transcript level in plants treated with KCl or K2SO4 compared
to those treated with NaCl or Na2SO4 (Fig. 6b).
In Arabidopsis, six isoforms of SUS genes (i.e., SUS1,
SUS2, SUS3, SUS4, SUS5, and SUS6) have been character-
ized (Stein and Granot, 2019) and they were all determined in
this study. After five iso-osmotic salt treatments, the tran-
scripts of SUS1, SUS2, SUS3, SUS4, and SUS6 presented a
higher level compared to control (Fig. 6b). In contrast, SUS5
had no substantial changes at the transcriptional level
(Fig. 6b). The SUS1 and SUS3 genes were more transcribed
in KCl or K2SO4-treated plants compared to those in NaCl or
Na2SO4-treated ones.
In Arabidopsis plants, sucrose-proton symporters (SUC) and
SWEET sucrose transporters are involved in sucrose trans-
port from source to sink. Nine isoforms of SUC genes and
seven isoforms of SWEET transporters were tested in Arabi-
dopsis plants in the study of Durand et al. (2018). They found
that only SUC1, SUC2, SUC3, SUC4, SWEET11, SWEET12,
SWEET13, and SWEET15 were expressed in Arabidopsis
rosette leaves. Thus, these eight sucrose transporters were
used for the determination of sucrose transport at the tran-
scriptional level in this study. The transcription of SUC1,
SWEET11, SWEET12, and SWEET13 was repressed after five
iso-osmotic salt treatments, while the transcription pattern of
SUC2, SUC3, and SUC4 remained unaffected and SWEET15
was enhanced (Fig. 6c). The transcription of SWEET11 and
SWEET12 was more strongly suppressed by KCl and K2SO4
than by NaCl and Na2SO4.
4 Discussion
4.1 Potassium salts are more toxic than sodium
salts in Arabidopsis thaliana (ecotype: Col–0)
According to the three-phase model (Munns, 1993; Schubert,
2011), plants show wilting symptoms during Phase 0 because
of a transient decrease in turgor. Growth rate is limited during
Phase 1 due to osmotic stress, but without water deficit in
Figure 5: Effect of sodium and potassium salts on sugar concentra- shoots. Plants develop chlorosis or even necrosis during
tions. Four-week-old seedlings were treated with 105 mM NaCl, Phase 2 resulting from ion toxicity. The aim of this study was
107.38 mM KCl, 78.61 mM Na2SO4, 81.90 mM K2SO4, and to investigate which ion is responsible for the toxicity symp-
74.27 mM CaCl2 for 12 d. The concentrations of glucose, fructose, toms in Arabidopsis plants under the experimental conditions
and sucrose in shoots (a) and roots (b), as well as the starch concen- in this study. To avoid the differences caused by Phase 1
tration in shoots (c) were determined. DW indicates dry weight. Data (osmotic stress), five different salts (NaCl, KCl, Na2SO4,represent the means of four replicates (– SE). Significant differences
in sugar concentrations among treatments are indicated by different K2SO4, and CaCl2) were applied iso-osmotically. According to
letters (P < 5%; one-way ANOVA and post-hoc analysis using Figs. 1 and 2, plants treated with NaCl, Na2SO4, and CaCl2
Tukey’s Honestly Significant Difference method). The significant dif- displayed no obvious toxicity symptoms except the Na2SO4-
ferences of sugar concentrations after salt treatments in comparison treated ones (promoting small-scale necrosis at leaf margins)
to those under control conditions are indicated by asterisks (*P < 5%, and showed similar reductions in shoot dry weight after
**P < 1%, ***P < 0.1%, ****P < 0.01%, t-test).
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J. Plant Nutr. Soil Sci. 2020, 183, 455–467 Potassium is a potential toxicant for Arabidopsis thaliana 463
Figure 6: Effect of sodium and potassium salts on the transcription of genes related to sucrose synthesis, degradation,
and transport. Four-week-old seedlings were treated with 105 mM NaCl, 107.38 mM KCl, 78.61 mM Na2SO4,
81.90 mM K2SO4, and 74.27 mM CaCl2 for 12 d. The transcription of genes related to sucrose synthesis (a), sucrose deg-
radation (b), and sucrose transport (c) was measured via qRT-PCR. SPS indicates sucrose phosphate synthase; CINV
indicates cytosolic invertase; SUS indicates sucrose synthase; SUC indicates sucrose transporter. Data represent the
means of three replicates (– SE). |log2 FoldChange | ‡ 1 was used as cutoff to evaluate differentially transcribed genes.
The dotted black lines indicate cutoff points.
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
464 Zhao, Faust, Schubert J. Plant Nutr. Soil Sci. 2020, 183, 455–467
12 days of treatment, indicating that these reductions mainly Another explanation is that excessive K+ inhibits the uptake
resulted from osmotic stress rather than ion toxicity. In con- and/or translocation of calcium and magnesium. In this study,
trast, exposure to KCl and K2SO4 led to severe toxicity potassium salts (KCl and K2SO4) had a higher inhibitory effect
(Fig. 1) and induced higher reductions in shoot dry weight on the uptake and translocation of Ca2+ and Mg2+ than iso-
than exposure to the other three salts (Fig. 2a), suggesting osmotic sodium salts (NaCl and Na2SO4) (Fig. 3d, e). Calci-
that these severe damages were not only due to osmotic um is an important macronutrient, functioning in signal trans-
stress but can also be attributed to additional ion toxicity duction, cell-wall structure, membrane permeability, and
which is presumably due to strong accumulation of K+ enzyme activation (Hepler, 2005; Dodd et al., 2010). The salt
(Fig. 3). stress-induced reduction of Ca2+ concentration in Arabidop-
sis (Fig. 3f) is in line with the observations in sugar beet,
maize, and sorghum (Bernstein et al., 1993; Fortmeier and
4.2 A high preference for K+ over Na+ in Schubert, 1995; Wakeel et al., 2011). Magnesium is an
Arabidopsis essential constituent of chlorophyll and is thus crucial for pho-
In this study, potassium salts (KCl and K SO ) induced more tosynthesis (Gerendás and Führs, 2013). The reduced level2 4 2+ 2+
severe toxicity than sodium salts (NaCl and Na SO ) (consid- of Ca and Mg under salt stress can be explained from2 4
ering toxicity symptoms and biomass) in Arabidopsis (Figs. 1 physiological and chemical views. First, the translocation of2+ 2+
and 2). There are two explanations for the differences de- Ca and Mg mainly occurs in xylem which is mainly driven
tected among these five iso-osmotic salt treatments. by transpiration. Salt stress reduces the absolute transpira-
tion rate in maize (Schubert, 2011). In this study, the absolute
One explanation could be that Arabidopsis plants accumulate transpiration rates under potassium salt treatments (KCl and
more K+ than Na+ in shoots under iso-osmotic concentrations K2SO4) were lower than those under sodium salt treatments
of potassium or sodium salt treatments, as shown in Fig. 3c. (NaCl and Na2SO4) (Fig. S2), which may explain the lower2+ 2+
First, K+ is more easily taken up than Na+ by Arabidopsis. As Ca and Mg translocation observed in plants treated with2+ 2+
shown in Fig. 3a, the uptake of K+ under KCl treatment was KCl and K2SO4 (Fig. 3e). Second, Ca and Mg enter roots
higher than the uptake of Na+ under iso-osmotic NaCl treat- through NSCCs which are permeable to various cations such+ + 2+ 2+
ment. This is in line with the observation of Lazof and as Na , K , Ca , and Mg . Davenport and Tester (2000)
Cheeseman (1988) that also revealed that K+ accumulation in demonstrated competition among these cations during the
roots was faster and higher than Na+ in Lactuca sativa. The uptake via NSCCs. In Arabidopsis, NSCCs have a higher af-
higher preference for K+ in plants can be explained from an finity to K
+ than Na+ (Demidchik and Tester, 2002). Hence, K+
evolutionary standpoint. As potassium has important func- has a stronger competition with Ca
2+ and Mg2+ than iso-
+
tions in enzyme activation, charge balance, and osmoregula- osmotic Na , which may explain the higher inhibitory effect on2+ 2+
tion, cytosolic K+ concentration is tightly regulated around the uptake of Ca and Mg observed in KCl
– and K2SO4-
100–200 mM (Leigh and Wyn Jones, 1984; Walker et al., treated plants (Fig. 3d).
1996; Maathuis, 2009). Therefore, plants need to create strat-
egies for efficient K+ uptake to meet the high demand of K+ in The first hypothesis that Arabidopsis plants are more sensi-
cytosol. In contrast, excessive Na+ in cytosol is toxic. Plants tive to sodium salts than potassium salts has to be rejected.
usually cannot tolerate cytosolic Na+ concentrations greater In Arabidopsis, exposure to Na2SO4 and K2SO4 (1) induced
than 20 mM (Walker et al., 1996; Munns and Tester, 2008). much more severe toxicity symptoms, (2) led to dramatic re-
Moreover, previous studies have demonstrated that the non- ductions in shoot biomass and chlorophyll concentration, and
selective cation channels (NSCCs), which are the main path- (d) strongly inhibited the uptake of nutrients Ca
2+ and Mg2+
way for ion uptake at high ion external concentrations, have a relative to the exposure to iso-osmotic NaCl and KCl. Accord-
higher permeability for K+ than Na+ in plants (Zhang et al., ing to the study of Reich et al. (2017), high concentrations of
2010; Kronzucker and Britto, 2011). For example, the volt- NaCl and KCl both reduced plant biomass in Brassica rapa
age-insensitive NSCCs permeability of K+ is 1.49-fold of Na+ plants, but plants treated with NaCl showed more reductions.
in Arabidopsis plants (Demidchik and Tester, 2002). The low- It seems that Brassica is more sensitive to sodium salts,
er permeability of Na+ may be attributed to its larger hydration which is different from the observation in Arabidopsis plants
shell relative to K+, thus, hindering the transport of Na+ in this study. Arabidopsis and Brassica belong to the same
through cation channels in plasma membrane (Schubert, family, Brassicaceae. The difference between Brassica and
2015). Although the uptake of K+ in K SO -treated plants was Arabidopsis might be attributed to two reasons. First, the salt2 4
lower than the Na+ uptake after exposure to iso-osmotic sensitivity differs between Brassica rapa and Arabidopsis
Na2SO4, the K
+ concentration in shoots was still higher than thaliana. According to the salt-reduced shoot dry matter, Ara-
Na+ after corresponding K SO and Na SO treatments. This bidopsis thaliana is much more sensitive to salinity than most2 4 2 4
may have resulted from the severely reduced biomass under crops including rice, barley, and maize (Munns and Tester,+
K2SO4 treatment; thus, the K
+ was concentrated in K SO 2008). After exposure to similar concentrations of KCl, the K2 4-
treated plants. Second, the translocation of K+ from roots to concentrations in Brassica and Arabidopsis plants are simi-
shoots was higher than Na+ at iso-osmotic concentrations as lar. Brassica can endure such high levels of K
+ in shoots. In
shown in Fig. 3b. Previous studies demonstrated that some contrast, Arabidopsis plants cannot. Second, Arabidopsis+
Na+ transporters (e.g., Nax1, Nax2, Kna1, and Skc1), which plants might lack the capacity of K exclusion. In Arabidopsis,+
mediate Na+ translocation, have higher translocation rates of the K concentration was more than three times that of Na
+
K+ (Gorham et al., 1990; Ren et al., 2005; James et al., after exposure to similar concentrations of NaCl and KCl. This
2006). indicates that Arabidopsis plants can exclude excessive Na
+
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
J. Plant Nutr. Soil Sci. 2020, 183, 455–467 Potassium is a potential toxicant for Arabidopsis thaliana 465
to avoid severe damages, but apparently have a problem in reflect suppressed photosynthetic activity, implying a reduc-
K+ exclusion. Similarly, another crop plant, wheat, is also tion in the primary income of sucrose from photosynthesis.
more sensitive to NaCl than KCl (Adiloglu et al., 2007). However, the strongly accumulated sucrose in plants treated
Compared to crop plants, Arabidopsis thaliana, a wild plant, with KCl and K2SO4 shows that plants had sufficient photo-
might lack the ability to regulate the K+ transporters related to synthates and that sucrose might have repressed the photo-
K+ uptake and translocation under high K+ circumstances, synthesis via a feedback inhibition. Hence, the increased
thus resulting in strong accumulation of K+ in shoots. sucrose concentration in response to KCl and K2SO4 was not
due to enhanced photosynthesis but due to decreased
sucrose consumption. The SPS genes, encoding key en-
4.3 Sulfate salts are more toxic than chloride salts zymes for sucrose synthesis, did not show notable differen-
Although the KCl-treated plants showed a higher concentra- ces in transcription pattern between potassium and sodium
tion of K+ in shoots than those treated with K SO (Fig. 3c), salt treatments (Fig. 6a). Second, the suppressed transcrip-2 4
exposure to K SO induced more severe toxicity symptoms in tion of tested invertase genes CINV1 and CINV2, especially2 4
Arabidopsis (Fig. 1). Thus, SO24 may also contribute to the
CINV2 being more severely repressed under potassium salt
toxicity symptoms induced by K2SO4. It has been previously
treatments (Fig. 6b), indicates a reduction of irreversible
demonstrated that sulfate salts inhibit plant growth more sucrose degradation mediated by CINV1 and CINV2. In con-
strongly than chloride salts, even when SO2 level is much trast, the transcription of SUS genes (i.e., SUS1, SUS2,4
lower than Cl– (Leonova et al., 2009; Llanes et al., 2013; SUS3, and SUS4) was enhanced. SUS catalyzes sucrose
Reginato et al., 2014; Reich et al., 2017). Similar results were degradation reversibly, facilitating sucrose degradation as
observed in this study: plants treated with sulfate salts well as sucrose re-synthesis (Noël and Pontis, 2000; Stein
(Na2SO4 and K2SO4) showed more severe damages than
and Granot, 2019). The sucrose cleavage facilitated by SUS
plants treated with chloride salts (NaCl and KCl) (Figs. 1 and may be suppressed or even blocked in plants treated with
2), and Arabidopsis plants preferred to accumulate Cl– over potassium salts (KCl and K2SO4). This hypothesis can be
SO2 in shoots (Fig. 4). Exposure to sulfate salts (Na SO supported by the lower concentration of starch observed in4 2 4
and K2SO4) induced stronger accumulation of TCA intermedi-
KCl and K2SO4-treated plants (Fig. 5c), because the produc-
ates (especially fumarate and succinate) than exposure to tion of SUS-mediated sucrose degradation, UDP-glucose or
chloride salts (NaCl and KCl) (Tab. 1). Succinate and fuma- ADP-glucose, determines the rate of sucrose-starch conver-
rate are intermediates of the TCA cycle, and they are inter- sion (Baroja-Fernández et al., 2001; Stein and Granot, 2019).
changeable via SDH (succinate dehydrogenase) (Dröse, Third, sucrose is generated in source tissues such as mature
2013; Tretter et al., 2016). SDH is a component of the respira- leaves, and then transported via phloem to different sink
tory Complex II in the mitochondrial electron transport chain, organs (Ruan, 2014). In this study, root is a major sink in the
passing electrons via ubiquinone to Complex III (Cecchini, developing stage of Arabidopsis plants. The genes related to
2003; Dröse, 2013). It has been demonstrated that the accu- sucrose transport, especially SWEET sucrose transporters
mulation of succinate can stimulate reverse electron transport (i.e., SWEET11, SWEET12, and SWEET13), were mostly sup-
and generate reactive oxygen species (ROS) to induce sec- pressed after various salt treatments (Fig. 6c), suggesting
ondary oxidative stress in plants (Muller et al., 2008; Starkov, inhibited sucrose translocation, which may explain the lower
2008; Dröse, 2013). level of sucrose presented in roots compared to that in shoots
(Fig. 5b).
The results presented above support the second hypothesis
that sulfate salts have a higher inhibitory effect on the growth In conclusion, when Arabidopsis plants were treated with KCl
of Arabidopsis plants than those treated with chloride salts, or K2SO4, the SUS mediated-sucrose degradation was sup-
because exposure to sulfate salts induced the overproduction pressed or even blocked, subsequently resulting in an accu-
of organic acids especially fumarate and succinate, which mulation in shoots in contrast to starch. In addition, the irre-
might induce oxidative stress. versible sucrose cleavage promoted by invertase CINV1 and
CINV2 and the sucrose transport mediated by SUC1,
SWEET11, SWEET12, and SWEET13 were also suppressed
4.4 Potassium salts suppress or even block according to the transcription pattern.
sucrose degradation
As growth was inhibited after salt treatments and sugar is an 5 Conclusions
important source for plant growth and development, the con-
centrations of glucose, fructose, sucrose, and starch were The results demonstrate, for the first time, that potassium
determined. Plants treated with KCl and K2SO4 presented the
salts can also induce toxicity symptoms and that they are
strongest accumulation of sucrose in shoots, twelvefold as even more toxic than iso-osmotic sodium salts in Arabidopsis
much as in control ones (Fig. 5a), the contrary was found for thaliana. We propose three possible explanations: (1) the
the starch level, which showed the lowest concentration uptake and translocation of K
+ were much higher than of Na+
(Fig. 5c). The massive accumulation of sucrose observed under iso-osmotic concentrations of potassium or sodium salt+
above can be explained in three aspects: (1) sucrose synthe- treatments, thus resulting in higher accumulation of K in
sis, (2) sucrose degradation, and (3) sucrose partitioning from plants treated with potassium salts; (2) potassium salts had a
source to sink. First, the decreased concentration of chloro- higher inhibitory effect on the uptake and translocation of2+ 2+
phyll observed in KCl and K SO -treated plants (Fig. 2c) may Ca and Mg than iso-osmotic sodium salts; (3) massive2 4 accumulation of K+ suppressed or even blocked the SUS
ª 2020 The Authors. Journal of Plant Nutrition and Soil Science published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com
466 Zhao, Faust, Schubert J. Plant Nutr. Soil Sci. 2020, 183, 455–467
mediated-sucrose degradation, thus leading to strong accu- cation of reference genes for RT-qPCR expression analysis in
mulation of sucrose, subsequently inhibiting photosynthesis Arabidopsis and tomato seeds. Plant Cell Physiol. 53, 28–37.
via feedback inhibition. Hence, an excessive supply of K+ Demidchik, V., Tester, M. (2002): Sodium fluxes through nonselective
needs to be avoided in physiological experiments with Arabi- cation channels in the plasma membrane of protoplasts from
dopsis in the future. Most salinity experiments are based on Arabidopsis roots. Plant Physiol. 128, 379–387.
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