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Influx of potassium (86RB)by roots of intact tomatoplantsLillie Andersen a & Niels Erik Nielsen aa Department of Ornamentals , DanishInstitute of Agricultural Sciences ,Kirstinebjergvej 10, Aarslev, DK‐5792,Denmarkb Plant Nutrition and Soil FertilityLaboratory, Department of AgriculturalSciences , Royal Veterinary and AgriculturalUniversity , Thorvaldsensvej 40,Frederiksberg C, DK‐1871, DenmarkPublished online: 21 Nov 2008.

To cite this article: Lillie Andersen & Niels Erik Nielsen (1999) Influx ofpotassium (86RB) by roots of intact tomato plants, Journal of Plant Nutrition,22:9, 1457-1467, DOI: 10.1080/01904169909365727

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JOURNAL OF PLANT NUTRITION, 22(9), 1457-1467 (1999)

Influx of Potassium (86Rb) by Roots ofIntact Tomato Plants

Lillie Andersena and Niels Erik Nielsenb

aDepartment of Ornamentals, Danish Institute of Agricultural Sciences,Kirstinebjergvej 10, DK-5792 Aarslev, DenmarkbPlant Nutrition and Soil Fertility Laboratory, Department of AgriculturalSciences, Royal Veterinary and Agricultural University, Thorvaldsensvej 40,DK-1871 Frederiksberg C, Denmark

ABSTRACT

Seedlings of tomato (Lycopersicon esculentum Mill. cv. Matador) were grownin complete nutrient solutions at constant concentrations of potassium (K)(0.25,0.75, 1.00, 1.25, 2.20, 2.25, 3.50 mM K+) and three activity ratios, aK/(aca+aMg)> (0-007> 0-02> and 0.10 M½). Influx of K was measured by using 86Rbas a tracer of K. The relative growth rate of dry matter was 0.30 day1. Thecontent of K, calcium (Ca), and magnesium (Mg) in the plant dry matter wasnearly unaffected by the concentration of K in the nutrient solution. Underthese conditions it was observed that the influx of K (86Rb) increased whenthe K+ concentration (activity) and activity ratio, ak/ (aCa + aMg ), were increasedin the nutrient solutions. Hence influx of K in tomato plants seemed to beindependent of the concentration of K in the plant tissue. Possible mechanismsof the regulation of the K net uptake are discussed.

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Copyright © 1999 by Marcel Dekker, Inc. www.dekker.com

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INTRODUCTION

Potassium uptake in tomato plants has been intensively studied (e.g., Alarconetal , 1997; Fellahi and Cornillou, 1994; Satti andAl-Yahyai, 1995). However, thecontrol of K uptake is not very well understood. Processes controlling either influxor efflux may modify the net uptake of K+, which is the difference between influxand efflux of K+. At high K+ concentration (>1 mM) specific inward rectifyingchannels facilitate rapid influx of K (Kourie and Goldsmith, 1992; White, 1993;Wegner and DeBoer, 1997; Dietrich et al., 1998), whereas at low concentrations acarrier-mediated K+ protonsymport has been proposed (Maathuis and Sanders,1993). Furthermore, outward rectifying channels facilitating efflux of K have beenfound in plants (Blatt and Thiel, 1993; White, 1993). Drew and Saker(1984) observedthat influx of K+ was controlled by the flux of K+ from the cytoplasm to the xylemindependently of the root content of K. Others have suggested influx to becontrolled by a feedback from a high internal K+ concentration in the root cells(Glass, 1976; Pettersson and Jensen, 1978).

The apparent contradictions between these results could be related to differencesin the pre-treatment of the experimental plants. Movement of the plants from highto low concentrations of K could affect influx and efflux. Furthermore, net uptakeand then influx of K is affected by growth rate, K+ demand of the plant (Pitman,1972), root temperature (Fellahi and Cornillou, 1994) and amount of K+ recirculatedfrom the shoot through the phloem to the root (Armstrong and Kirkby, 1979;Jeschke and Pate, 1991; Zhong et al., 1998) and the concentration of Ca and Mg,i.e., the activity ratio, (aj (aCa+aMg)), in the soil solution (Hansen, 1972).

The objective of our work was to study the influx and net accumulation of K+ byintact tomato plants grown at constant K concentration and ratio of activity innutrient solution.

MATERIALS AND METHODS

Experimental Conditions

Seeds of tomato (Lycopersicon esculentum Mill. cv. Matador) were sown invermiculite in 1-1 pots. The vermiculite was moistened with tapwater and kept indarkness in a growth-chamber at 20°C ± 2. After emergence (6-7 days) the seedlingswere continuously irrigated by a pre-experimental nutrient solution (Table 1).

After 14-16 days under the pre-experimental condition the roots were washedand nearly all vermiculite was removed from the roots. The plants were thentransferred to the experimental water culture systems at various levels of K+

concentrations.During pre-experimental and experimental periods the plants were illuminated 16

h per day by Osram HQI-T 400 W lamps. The light intensity was ca. 400 /JE iff2 s"1

at the top of the plants. Relative humidity and temperature were kept constant atca. 70% and 20°C ± 2, respectively. The water culture experiments (Experiments 1,

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INFLUX OF K BY ROOTS OF INTACT TOMATO PLANTS 1459

TABLE 1. Concentrations, activity of potassium (a^ (mM and uM) and activityratio (ag/ aCa+aMg) (M*4) of the basic nutrient solutions in treatment A, B, C, D, E,F, G, and H, the maintenance solution (M) and pre-experimental (P) solution.

M PExp.

Treat.

1A

3

B

2

C4

D

1

E

3

F

2

G

4

H

mM

"NO3 237 145 Z45 4J0 109 2436 2T6 246 15?7 4lRf

H2PO4 0.70 0.70 0.70 0.70 0.70 0.70 0.70 0.70 1.13 0.27

SO4 0.35 0.50 0.50 0.50 1.00 1.00 1.00 1.00 3.12 0.72

K 0.25 0.75 0.75 1.00 1.25 2.20 2.25 3.50 8.95 0.74

Ca 1.40 1.40 1.40 1.45 3.70 10.30 10.30 1.00 4.99 1.84

Mg 0.30 0.30 0.30 1.25 2.20 2.20 2.20 0.50 2.47 1.10_

Fi 251) 2T0 25l) 251) 5O0 5O0 5O0 5O0 67b" 25!b~

Mn 3.0 3.0 3.0 3.0 6.0 6.0 6.0 6.0 26.0 3.0

Zn 0.4 0.4 0.4 0.4 0.8 0.8 0.8 0.8 2.1 0.4

Cu 0.4 0.4 0.4 0.4 0.8 0.8 0.8 0.8 2.1 0.4

B 11.0 11.0 11.0 U.O 22.0 22.0 22.0 22.0 11.0 5.0

Mo 0.35 0.35 0.35 0.35 0.70 0.70 0.70 0.70 0.8 0.35

Na 20.0 20.0 20.0 20.0 40.0 40.0 40.0 40.0 120.0 20.1

Cl 9.0 9.0 9.0 9.0 18.0 18.0 18.0 18.0 60.0 9.0

K 069 (T69 O9l Tbl O l L8l 37l5

SE 0.015 0.013 0,015 0.016 0.021 0.053 0.066 0.099

ratio* 0.0066 0.0199 0.0199 0.0210 0.0184 0.0227 0.0233 0.1012

SE 0.0004 0.0007 0.0008 0.0008 0.0008 0.0008 0.0007 0.005

*mM and My-

2, 3, and 4) were carried out in recirculating nutrient solution according to theprinciple of regeneration (Nielsen, 1984) and included eight treatments with threereplicates (Table 1). Each replicate included 12 plants.

The water culture system consisted of a 7-1 black plastic container with fourseedlings in a basic solution, A, B, C, D, E, F, G, or H (Table 1). The solution was

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1460 ANDERSEN AND NIELSEN

circulated at a rate of 5-1 min'1 by a pump from the bottom to the top of the containerthrough a tube equipped with ten irrigation tubes (five cm long). A disc with holeskept the plants with the roots in the nutrient solution. Deionized water was addeddaily to keep the volume constant. Conductivity and concentration of the nutrientsin the basic solutions was kept constant by daily addition of maintenance solution(Table 1), when the conductivity of the solution was lowered maximum 0.05 mScm1. The pH, 6.0±0.2, was maintained by addition of NH4NO3 to the basic solutions(Nielsen, 1984). In order to evaluate the importance of activity ratio, aK/v/(aQ+a,^ ),on influx and net uptake of K, activities (a.) of K+, Ca2+ and Mg2+ were calculatedfrom a.=y. * c, in which Cj is the actual ion concentration and y. is the activitycoefficient. A computer program based on the work of Adams (1971), made thecalculations of activity coefficients and actual ion concentrations by taking intoconsideration the formation of ion pairs and the extended Debye-Huckel law.

Influx Experiments

After seven days in the experimental set up the nutrient solutions were labeledwith 8sRb, to a specific activity of 0.65 MBq 86Rb (mmol K+)' in all treatments. The86Rb was added to the nutrient solutions five hours after the light was turned on inthe day-night cycle (16-h light-8 h dark).

During the uptake period samples (10 mL) were collected from the nutrientsolutions after 0,5,10,20,30, and 60 minutes. The influx of K+ (86Rb) during thefirst 0-5 minutes was assumed to be due to exchange between the apoplasm andthe nutrient solution and not used in calculations of influx (Walker and Pitman,1976). Influx was calculated as the mean difference between the amount of 86Rb inthe nutrient solution at the time of samplings. No efflux was assumed to take placeduring the short uptake period. At the end of the experiment the roots wereseparated, blotted between filter paper, weighed for fresh weight, and dried at 70°Cfor dry weight determinations. Relative growth rate (RGR) of dry matter wascalculated. A determination of the radioactivity in the liquid samples was made byCerenkov counting using a LKB Wallac 1217 Rackbeta liquid scintillation counter.

Analysis

The plant material was wet ashed in acids and total content of K and sodium(Na) was determined by flame photometer and Ca and Mg by atomic absorptionspectrophotometer (Perkin Elmer 5000).

Statistics

Statistical analysis of variance was performed by GLM procedure in SAS andsignificant differences between means were calculated according to Duncan'sMultiple Range Tests (Anon. 1987).

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INFLUX OF K BY ROOTS OF INTACT TOMATO PLANTS 1461

TABLE 2. Fresh (FW) and dry weight (DW) of the tomato plants and DW ofthe roots (g plant"1) at the termination of experiments 1,2,3, and 4, at which theplants were 19, 21, 17, and 23 days old, respectively. Values are means ±SE(n=12).

Exp. Treatment* FW of whole plant DW of whole plant DW og root

mMK+ gpl-1 gpl-1 gpl"1

1A

I E

2 C

2G

3B

3 F

4 D

4 H

0.25

1.25

0.75

2.25

0.75

2.20

1.00

3.50

7.0±0.99

6.9±0.80

10.0±0.49

9.8±0.39

4.0±0.50

4.1±0.60

15.8±1.60

14.9±0.73

0.48±0.06

0.47±0.06

0.58±0.03

0.55±0.03

0.27±0.04

0.27±0.02

1.05±0.18

1.00±0.07

0.07±0.005

0.07±0.008

0.09±0.005

0.08±O.006

0.06±0.006

0.05±0.005

0.17±0.03

0.17±0.01

•See Table 1.

RESULTS

Fresh and Dry Weight of the Tomato Plants

The treatments had no specific influence on the fresh or dry weight of thetomato plants or dry weights of the roots within the single experiment (Table 2).During the experimental period the mean RGR of whole plant was 0.3G±0.008 day1

in all experiments and treatments.

Influx of Potassium (86Rb)

Influx of K by the roots of the tomato plants was measured by labeling thenutrient solution with 86Rb and hence the tomato plants were maintained in thesame nutrient solution as before. Under these experimental conditions influx of K+

(86Rb) increased in proportion to the activity of K+ in the nutrient solution in alinear relationship (Figure 1). Hence an increase in activity ofK+increased influx ofK+ (86Rb) independently of activity ratio and of Ca and Mg concentration in thenutrient solution.

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35-,

30-

20-

1 5 •

10-

5 -

ANDERSEN AND NIELSEN

0.5 1.5 2.5 3.5

aK(mM)

FIGURE 1. Influx of potassium (86Rb) (nmol h 1 gDW1) in relationship to activity ofpotassium (mM) in the nutrient solution. Regression line y=6.1595x + 8.5801, r2=0.8494(PO.0001).

Content of Potassium, Calcium, and Magnesium

The content of K, Ca, and Mg in the plant dry matter was within the optimalrange for tomato seedlings (Bergmann, 1983). The Ca and Mg content in the drymatter was not significantly affected by the treatments. Hence an increase inconcentration of Ca and Mg in the nutrient solution did not significantly influencethe content of Ca or Mg in the dry matter (Table 3).

As for Ca and Mg, K content was high in all treatments and within optimal range.However, content of K was affected by the different treatment. Hence concentrationof K in the nutrient solution had a significant effect on the K content in the plants,when K concentration was increased from 0.25 to 0.75 mM or more. Further increasein concentration did not have a specific effect on the K-content, except for thehighest concentration (3.50) of K, where activity ratio was higher than in the otherexperiments.

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INFLUX OF K BY ROOTS OF INTACT TOMATO PLANTS 1463

TABLE 3. Potassium, calcium and magnesium concentration, umol(g DW)'1, of tomato plants grown at different basic nutrient solutionafter 19,21,17, and 23 days respectively.

Exp. Treatment aK/V(aca+aMg) ^ ^* ^

mMK+ M'/i ilmol(gDW)-l

T A 025 00066 1048a 374 a 210 a

IE 1.25 0.0184 1481 be 424 a 240 a

2C 0.75 0.0199 1507 be 415 a 243 a

2G 2.25 0.0233 1510 be 399a 235 a

3B 0.75 0.0199 1394 b 402 a 251a

3F 2.20 0.0227 1405 b 399 a 240 a

4D 1.00 0.0210 1387 b 372 a 248 a

4H 3.50 0.1012 1522 c 377 a 243 a

Figures followed by the same letter are not significantly differentwithin the column according to Duncan's Multiple Range Test (p<0.05).

DISCUSSION

Tomato plants were grown at constant concentration of nutrients and at constantrelative growth rate. Under these conditions influx of K increased, when the K+

activity was increased in the nutrient solutions (Figure 1). Kochian and Lucas(1988) found similar results with excised roots and Erdei et al. (1984) observedincreasing influx of K+ by intact wheat plants, when the K+ concentration wasincreased above 1 mM.

Content of K was high in all treatments. Hence, no negative feedback from theinternal K content seems to operate on the influx of K. These results imply that theinflux of K+ by tomato plant roots could not be subject to allosteric regulation ofthe influx (Jensen and Pettersson, 1978; Siddiqi and Glass, 1986).

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1464 ANDERSEN AND NIELSEN

A high recirculation of K is found in tomato plants (Kirkby et al., 1981; Ferrarioet al., 1992; Niedziela et al., 1993), and hence xylem loading of K could affect influxofK(Kochian and Lucas, 1988; Drew and Saker, 1984). In the present experimenthigh contents of K in the plant dry matter had no effect on the influx of K+ (8sRb).

Efflux of K+ was not measured in the present experiments, but Erdei et al. (1984)found efflux of K+ in wheat plants at K+ concentration above 1 mM. Hence, theincrease in influx together with the high content of K in the dry matter could reveal,that efflux could be a way to control net uptake of K in tomato roots. The highinflux and content of K had a small and not significant influence on the concentrationof Ca and Mg in the dry matter (Table 3). Similar results have been found by others(Fellahi and Cornillou, 1994; Alarcon et al., 1997) whereas others have found adecrease in Ca and Mg content (Nukaya et al., 1995; Tareda et al., 1997). Anexplanation could be that in the present experiment the concentration of K in thenutrient solution was rather low compared to other experiments concerning tomatoplants (Bar-Tal and Pressman, 1996; Alarcon et al., 1997) and cultivars might reactdifferently.

Content of Ca and Mg seemed more or less unaffected by the increase in Ca andMg concentration in the nutrient solutions (Table 2).

Although the experiments included three activity ratios the content of K in theplants was only significantly lower at the smallest activity ratio. Activity ratio hada certain, but small effect on the content of Ca and Mg, implying that Ca and Mguptake in tomato plants is only to some extent controlled by activity ratio in thenutrient solution as found by Hansen (1972) at the present concentration range.

CONCLUSIONS

Influx of K (8SRb) increased in accordance to activity of K in the nutrient solutionalthough K content in dry matter was high, showing that feedback on uptakemechanism from internal K content was absent and that efflux might be substantialwhen tomato plants are grown in nutrient solutions. Calcium and Mg content indry matter was high and nearly unaffected by the concentration of K, Ca, and Mg,probably due to the constant concentrations in the nutrient solutions.

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