Nitrate-Stimulated Uptake and Transport of Strontium and Other Cations1

W. A. JACKSON AND DORIS CRAIG WILLIAMS 2 3

ABSTRACT

Supplying nitrate to N-depIeted wheat seedlings (Triticumvulgare, var. Atlas 66) stimulated the uptake of both divalent(Sr, Mn, and Mg) and monovalent (Cs, Na, K) cations. Rapidnitrate uptake, and the lower ambient acidity resulting from it,appeared to be partially responsible for the increase in cationuptake. Nevertheless, pretreatments with nitrate exerted somestimulation in Sr and Cs uptake, indicating an additional actionof nitrate after it had entered the tissues. Pretreatment withnitrate, as well as its presence during the Sr uptake period,increased transport of Sr to the shoots more than total uptake.Transport to shoots of previously absorbed Sr was enhanced bysubsequent nitrate treatments, the amount transported beingsignificantly greater with nitrate salts of Ca, Mg, and Na thanwith K or NH4.

Additional Key Words for Indexing: wheat, solution culture.

A SUBSTANTIAL net influx of nitrate occurs when N-de-pleted seedlings are exposed to nitrate salts. In wheat

seedlings the time course of nitrate uptake is characterizedby a slow initial phase4 (13) but after a few hours therate for an individual 14-day-old wheat seedling approaches1.0 to 1.5 ^eq per hour. Associated with this nitrate influxis the onset of a number of metabolic events. Both CO2

evolution and O2 uptake of N-deficient barley root tissuewere increased upon exposure to nitrate (3, 19). The in-crease in CO2 release was larger than the increase in O2

uptake, indicating that both the magnitude and the pat-tern of respiratory activity were altered. Substantial syn-thesis of many amino acids, and incorporation of these

amino acids into protein, takes place (1, 20). Depositionof the incoming nitrate-N into a lipid fraction of wheat-root tissue also occurs within the first three hours suggest-ing synthesis of membrane components.4

It seems reasonable to expect that the rapid nitrateinflux, and/or the accelerated metabolic effects induced,would have a significant influence on the capacity of theroot tissue to take up other inorganic ions from the ambientmedium and perhaps on the distribution of these ionswithin the plant body. Growth of wheat seedlings for twoweeks with ammonium or nitrate N resulted in substantialdifferences, compared to nitrogen-depleted seedlings, incapacity to absorb Cs (12) and Sr.5 Moreover, additionsof nitrate to solutions of SrCl2 and CsCl were found toresult in a considerable enhancement of Sr and Cs accumu-lation in the plant tissue during a 24-hour period.5 Uptakeby the root tissue and transport to shoots were both in-creased, although not to the same extent for each cation.This manuscript presents results of experiments designedto characterize the time course of nitrate-stimulated Sruptake, and to determine whether the phenomenon wasevident with cations other than Sr and Cs.

MATERIALS AND METHODS

The experimental procedures were similar to those describedby Minotti et al. (12) for producing low-N seedlings. Wheat(Triticum vulgare, var. Atlas 66) was germinated for 3 days indarkness. Roots of six seedlings were then inserted throughstainless steel screens held in a polyethylene cup. Forty suchcups were placed in 13-liter tanks, each liter of which con-tained 0.1 meq KC1, 2.0 meq MgSO4, 0.5 mmoles Ca(H2PO4)2,and Vs the standard Hoagland's A-Z trace element solution (4).Each cup of six seedlings thereafter was handled as a unit andhereafter is called a culture. The tanks containing the cultureswere maintained in a growth chamber at 25 ± 2C for 16 hoursat 1,800 ft-c/day and at 20 ± 2C for the remaining 8 hours ofdarkness. The tanks were aerated vigorously. Acidity was ad-justed daily with NaOH or H2SO4 to pH 5.5 and solutionswere changed every other day.

After 14 days, cultures were removed from the tanks and

JACKSON AND WILLIAMS: NITRATE-SIMULATED UPTAKE AND TRANSPORT OF STRONTIUM 699

carried to the laboratory where the roots were rinsed thoroughlywith distilled water. Two cultures were placed in 1-liter beakersof the desired salts. Strontium and cesium were labeled with85Sr and 137Cs, usually at 2-3/iCi/liter. Duplicate, and some-times triplicate, beakers were employed for each treatment com-bination. Beakers were gently aerated with capillaries. Lightintensity was 750 ft-c at the surface of the beakers and tempera-ture was maintained at 25 ± 2C during the experimental period.When solution acidity adjustments were made, dilute H2SO4or NaOH were employed. At the desired times the cultureswere removed, roots rinsed systematically with a jet of distilledwater, and dipped and drained 10 times in 3 successive changesof distilled water. Roots and shoots were excised 5 mm from theseed, blotted uniformly on cheesecloth, weighed rapidly, andassayed for 85Sr and 137Cs activity by gamma scintillation pro-cedures. Each culture was handled as a unit, and the valuesreported are the means ef the four or six cultures from eachtreatment combination. The amounts of Sr and Cs taken upwere calculated from the tissue assays and the initial specificactivity values of the uptake solutions. Percentage transportvalues are calculated as the amount of the ion in the shoots aspercent of total ion taken up per culture. When Mn uptake wasdetermined (Table 5), solutions were labeled with 54Mn andthe experiment was conducted in exactly the same way. Forthe experiment reported in Table 2, unlabeled Sr salts wereused, and Sr was determined by atomic absorption methods.For this analysis and that of other inorganic cations, the tissueswere dried 24 hours at 70C, ashed at 450C, and taken up indilute HC1. Sodium and potassium were determined flame pho-tometrically and Ca and Mg by atomic absorption spectrometry.Nitrate was determined by the method of Johnson and Ulrich(7) and chloride was determined with a Cotlove chloridometer.

RESULTS

Nitrate-Stimulation of Sr and Cs Uptake: Figs. 1 and 2show the enhanced uptake of Sr and Cs over a 24-hourperiod when NaNO3, compared to NaCl, was added to theN-depleted wheat seedlings. Solution analysis revealed thatover the 24-hour period approximately 850 X 10~4 meq

Na Cl Na NO,

O 3 6 9 12 IB 24HOURS

O 3 6 9 12 18 24HOURS

Fig. 1—Time course of Sr uptake by low N-seedlings in pres-ence of NaCl (left) or NaNO3 (right) at 1.0 meq/liter.Amounts in the roots (open circles) and total amounts(open octagons) refer to left ordinate, amounts in shoots(open squares) and percent transported to shoots (closedcircles) refer to the right ordinate. Note that the right ordi-nate is expanded twofold compared to the left ordinate.Solutions contained 0.1 meq CsCl + 1.0 meq SrCl2/liter,pH 5.5.

Table 1—The effect of various concentrations of SrCl2 andSr (NOs)2 on Sr uptake for 2 and 8 hours; solutions

initially were at pH 5.5

TreSalt

atmentmeq/liter

Two hoursRoots Shoots

Eight hours

Roots[meq Sr (X 10*)/culture]

SrClj

Sr(NO,

0.0.0.1.

25507500

)j 0.250.500.751.00

20.1*2.718.6*1.019.4*1.519.9*0.924. tel. 527.4*1.830. 4*2. 031.3*1.8

tr*trtrtrtrtrtrtr

Shoots[meq Sr (

32. 0*1. 530. 1*1. 629. 4*2. 129. 2*2. 749.57,62,64.

3*2.58*3.14*3.65*2.0

12.16.17.16.

TotalPercent

transportx 10<)/culture] %

tr*trtrtr

9*0.86*1.53*1.09*1.9

62.2*3.574.4*3.679. 7*3. 281. 4*0. 5

20.22.22.20.

-

7*1.73*1. 72*1.68*2. 3

* Only trace amounts were detected.

NO3 per culture was absorbed while no net chloride uptakecould be detected. There was little influence of addednitrate on the rapid initial phase of Sr uptake during thefirst hour (Fig. 1). With both NaNO3 and NaCl, uptakeby the root tissue was essentially saturated by about 12hours and the level of Sr in the roots at saturation wassubstantially greater with NaNO3 (—78 X 10~4 meq perculture) than with NaCl (—52 X 10~4 meq per culture).Transport of Sr to the shoots in the NaCl series developedslowly and in ascending fashion after the sixth hour. Therewas a relatively rapid surge of Sr to the shoots of theNaNO3 series after the third hour and, during the entire24-hour period, Sr transport continued to exceed the ratein the NaCl series.

Uptake of Cs by roots in this experiment was not appre-ciably affected by NaNO3 compared to NaCl (Fig. 2) but itcontinued through the 24 hours. In contrast to the roottissue, transport of Cs to the shoots clearly was enhancedby NaNO3, the effect primarily occurring during the first9 hours.

The concentration dependence of nitrate-stimulated Sruptake at 2 and 8 hours is shown in Table 1. The experi-ment was conducted with SrCl2 and Sr(NO3)2 and revealspatterns similar to experiments in which Na salts wereemployed. With SrCl2, the system was saturated at 0.25

80NnCI

20 "S

Is

3 6 9 12HOURS

9 12 18HOURS

Fig. 2—Time course of Cs uptake by low N-Seedlings inpresence of NaCl (left) and NaNOs (right) at 1.0 meq/liter.Amounts in the roots (open circles), and total amounts (openoctagons) refer to the left ordinate, amounts in shoots(open squares) and percent transported to shoots (closedcircles) refer to the right ordinate. Note that the rightordinate is expanded twofold compared to the left ordinate.Solutions contained 0.1 meq CsCl + 1.0 meq SrCl2/liter,pH 5.5.

700 SOIL SCI. SOC. AMER. PROC., VOL. 32, 1968

Table 2—The influence of maintaining ambient acidity at pH6.5 on the uptake of Sr from Sr(NOs )z and SrCl2

(1.0 meq/liter) for 10 hours

Table 4—Effects of various salts on Cs uptake and transport toshoots. All treatment solutions contained 0.1 meq

CsCl +1.0 meq SrCl2/liter (initially pH 5.5);

Roots Shoots TotalPercent

transport

SrCl,Sr(NOj)j

SrCl;Sr(NOs),

I.35. 9*1. 279. 143. 1

II.

52. 941. 984. 844. 4

* Adjusted at frequent (15-min) iwas added to SrCl2 solutions,solutions.

Added Salts* Roots

Acidity Unadjusted0. 340. 05

17. 041. 5Acidity Adjusted*

0. 540. 119. 542. 5

36:241.296. 144. 0

53.442.0104.346.4

0.840.117.741.2

0.940.218.641.5

ntervals to pH 6. 5 with NaOH or HjSOj. When NaOHan equivalent amount of NaCl was added to the Sr(NO,)z

NoneNaCl (5)Na2SO, (5)KC1 (5)K2S04 (5)NaNOj (5)NaNOj(3. 75) + KUO, (1.25)NaNOj (2. 50) + KNOj (2. 50)NaNOj (1. 25) + KNC, (3. 75)KNO,(5)

49.

48.42.

3.3.

73.5.5.5.5.

5*9.65i4.92*7. 6

5±0. 4340.41*16.75*0.4240. 20*0.4240.4

Shootsneq Cs (X 104)/

9.041.49.442. 06. 9*1. 9

1.1*0.11.040.1

21. 2±9. 63. 0±0. 23. 1±0. 22.940.13. 641.2

Total

58.5410.657. 94 6. 549.lt 9.4

4.64 0.54.34 0.4

94. 3422. 98. 54 0. 38. 34 0. 37. 94 0. 58. 84 1. 5

Percenttransport

15.16.14.23.23.

22.35.37.36.40.

%

442.32*2. 10*1.6

942.6342.1

546.3343. 3441.87*1.8945.5

meq per liter and only trace amounts of Sr could be de-tected in the shoots by 8 hours. Uptake of Sr by the roottissue increased with Sr(NO3)2 concentration at both 2 and8 hours but clearly approached saturation at 0.5 meqSr/liter.

Influence of Induced Changes in Solution Acidity—AsSr was taken up from Sr(NO3)2 the acidity of the ambientmedium at first increased slightly and then, decreased asa consequence of the subsequent more rapid nitrate uptake.The initial increase in acidity with SrCl2 was not counter-acted by a subsequent decline because net chloride uptakeby these seedlings was very low. It therefore was likely thatpart of the nitrate-stimulated Sr uptake resulted from thelower acidity prevailing adjacent to the root cells. Table 2presents results of a 10-hour experiment wherein ambientacidity was adjusted at frequent (-~15 min) intervals topH 6.5 with NaOH or H2SO4. Each time NaOH wasadded to the SrCl2 treatment an equivalent amount ofNaCl was added to the Sr(NO3)2 treatment. With SrCl2,sizeably more Sr was taken up when the solutions weremaintained at pH 6.5. Only a slight increase was foundwith Sr(NO3)2. The advantage of Sr(NO3)2 over SrCl2 intotal uptake was lessened, but was not eliminated, by thisattempt to maintain solution acidity at pH 6.5. The acidityadjustment had relatively little effect on the proportion ofthe Sr which reached the shoots with either salt.

Influence of Na and K—Total Sr uptake over a 24-hourperiod from solutions of 1.0 meq SrCl2 -f- 0.1 meqCsCl/liter was depressed only slightly by chloride or sul-fate salts of Na and K at 5 meq/liter (Table 3). The mag-nitude of the stimulation in Sr uptake induced by nitratewas less when K rather than Na was present. Varying the

Table 3—Effects of various salts on Sr uptake and transport toshoots. All treatment solutions contained 0.1 meq

CsCl + 1.0 meq SrCl2/liter (initially pH 5.5);uptake period was 24 hours

Added Salts*

NoneNaCl (5)NajSO, (5)KC1 (5)KjSO, (5)NaNO, (5)NaNO,(3. 75) + KNOj(1.25)NaNO, (2. 50) + KNO, (2. 50)NaNOj (1. 25) + KNOi(3. 75)KNO, (5)

Roots

58. 6* 4. 256. 74 7. 655. 84 6. 155. 8* 4. 852.2* 4.986. 1*10. 373.9* 5.673.0*10.776.1* 8.670. 0* 5. 0

Shootsmeq Sr (x 10*)/c

12.9*2.710.1*1.56.641.46.641.44.741.9

44.042.231.142.232.6*1.331.3*1.438. 7*2. 3

Total

71.5* 2.966.84 8.362.4* 5.362.4* 5.456.9* 4.3

130.1*11.0105.0* 4.7105.6*11.3107.44 8.5108.74 4.5

Percenttransport

%18.043.715.1*2.010.6*2.710.6*1.78. 3*3. 1

33. 8*3. 729.6*2.830. 942. 729.1*2.435.6*2.5

* Values in parentheses indicate salt concentration of the ambient solution in meq/liter.

proportion of Na and K revealed that the depressing effectof K was as high at 1.25 meq K as at 5.0 meq K/liter. In-creasing concentrations of NaNO3 or KNO3 showed thatat each concentration KNO3 proved slightly restrictivecompared to NaNO3 (Fig. 3).

Uptake of Cs from the same solutions was very stronglydepressed by K (Table 4). Increasing the amount of KNO3(Fig. 4), or the proportion of K at 5.0 meq NO3/liter(Table 4), distinctly increased the proportion of Cs trans-ported to the shoots. In spite of the strong inhibitory effectof K on Cs uptake an influence of nitrate still was evidentbecause uptake was about twice as great from KNO3 asfrom KC1 or K2SO4.

Nitrate-Stimulated Mn Uptake—Table 5 shows that Mnuptake was also considerably enhanced by presence ofnitrate. Retention in the root tissue, transport to shoots,and percentage transport all were increased.

Nitrate Pretreatment Effects—The enhancement in Sr,Cs, and Mn uptake described above occurred under con-ditions of rapid nitrate uptake. A series of experimentswas therefore conducted to determine whether eventsresulting from relatively short periods of prior nitrate entrycould modify subsequent cation uptake. For the Sr studies,10 meq NaCl or NaNO3 were added to the standard nutrientsolution in the growth tanks 24 hours prior to exposure of

Table 5—Effect of NaCl and NaNOs on Mn uptake and trans-port; in addition to the indicated Na salts, the solutions

contained 0.1 meq MnCl2/liter and were atpH 5.5 initially

Treatment

(1 meq/liter)NaCl

NaNO,

Time(hours)

2828

Roots

3040

3427*2

40. 2*3110. 544

Shootsmeq Mn8 00 02 01 3

Total

(x loVculture -034. 01 30.114. 02 40.07*1 4

0226

40.113

33*2.88142. 027*3. 2. 6*4. 0

Percent,transport

'/0.1*0.34

0.242.7*

05040430

Table 6—Changes in mineral contents of 2-week-old seedlingsinduced by a 24-hour period in the standard solution with

10 meq NaNOs or NaCl added per liter; dataexpressed per unit dry weight of tissue

Component

NaKCaMg

Roots Shoots—— meq/g ——0. 08 0. 040. 72 0. 520. 04 0. 090. 15 0. 15

Roots Shoots— meq/g ——0. 18 0. 040.99 0.690. 05 0. 110.40 0.21

Roots—— meq0.100.840.040. 13

Shoots

1/g ——0.030.590.090.15

1 Values in parentheses indicate salt concentration of the ambient solution in meq/liter.

JACKSON AND WILLIAMS: NITRATE-SIMULATED UPTAKE AND TRANSPORT OF STRONTIUM 701

1.25 2.50 3.75 5.00meq NaNO, PER LITER

1.25 250 375 500aieq KNO, PER LITER

Fig. 3—Effects of NaNOs and KNOs on Sr uptake and trans-port to shoots in low N-seedlings. All treatment solutionscontained 0.1 meq CsCl + 0.1 meq SrCI2/liter (pH 5.5).Uptake period was 24 hours. The left ordinate refers tovalues for roots and total uptake, the right ordinate refersto values for shoots or to percentage transport. Note thatthe right ordinate is expanded twofold relative to the leftordinate.

S

I

t80

Haag,«o no

KUO,

• TOML \ ,-*"-

/ *---*"

20 40

O O

ZO

O 128 2SO in SOOM4 NoNO, P0I LITER

O 1 2 S 2 J 0 3 . 7 S S . 0 0M< KNO, PER LITER

Fig. 4 — Effects of NaNOs and KNOs on Cs uptake and trans-port to shoots in low N-seedlings. All treatment solutionscontained 0.1 meq CsCl + 0.1 meq SrCl2/liter (pH 5.5).Uptake period was 24 hours. The left ordinate refers tovalues for roots and total uptake, the right ordinate refers tovalues for shoots or to percentage transport. Note that theright ordinate is expanded twofold relative to the leftordinate.

the seedlings to the SrCl2 solutions. This pretreatment withNaNO3 significantly increased the Na, K, and Mg contentsof the tissue compared to the NaCl pretreatment (Table6) indicating that the uptake of these cations was alsoenhanced during rapid nitrate uptake. Following the 24-hour pretreatment period, the seedlings were exposed to1.0 meq SrCl2 plus either 1.0 meq NaNO3 or 1.0 meqNaCl/liter. Stronium uptake for the standard 2-week-oldseedlings is shown in Table 7, and Table 8 reveals resultsof a similar experiment with 1-week-old seedlings.

Pretreatment with NaNO3 increased subsequent Sr up-

Table 7—The influence of 24-hour pretreatments with NaCl orNaNOs (10 meq/Iiter on subsequent uptake of Sr from

solutions containing 1.0 meq SrCk and either NaClor NaNO3 (1.0 meq/liter); initial pH = 5.5

ment

NaCl

NaNO,

NaCl

NaNO,

Uptake treatmentSalt Hours

NaCl 28

24NaCl 2

824

NaNO, 28

24

NaNO, 28

24

Roots

31.48.56.32.51.61.35.81.

1ZG.

——— met0± 1.55± 1.29± 4.14* 2.50± 2.89* 1.54± 0.86± 5.51±10. 7

Shoots Total1 Sr (x I0*)/culture —

31.0.

13.

3.21.

19.84.

9±0. 31*1.4-

0*1.41±2. 0-

8±0. 53±7. 5

35. 1± 1. 956.89.

1* 1.2li 2. 9

6.58.

3*1. 34±3. 1

49.70.32.54.83.35.

IOI.210.

35.62.

147.

0±4*0±4±0±0±

4*4*

1.51.03.92.52.53.20.85.8

4±14. 4

1*4±5±

1.91.12.1

Percenttransport

1.18.

5.25.

19.

_%

8±0.7±2.-

642.4*1.-

5*1.40. Oi2.

10.39.

-2*1.5±2.

82

86

07

83

take after the first 2 hours when NaCl was present in theuptake solution (Table 7) and the beneficial effects weresomewhat greater in the younger seedlings (Table 8).Total amount of Sr transported to the shoots, and the pro-portion transported, were also enhanced by the NaNO3

pretreatment. Pretreatment with NaNO3 rather than NaClresulted in decreased Sr uptake when NaNO3 was presentduring the subsequent exposure to Sr.

The influence of pretreatments on subsequent Cs uptakefrom solutions of NaCl or NaNO3 is presented in Table 9.

Table 8—The influence of 24-hour pretreatments with NaCl orNaNOs (10 meq/liter) on subsequent uptake of Sr by

1-week-old low N-seedlings from solutions containing1.0 meq SrCl2 and either NaCl or NaNOs (1-0

meq/liter); initial pH = 5.5

Pretreat-ment

Naci

NaNO,

NaCl

NaNO,

Uptake treatmentSalt Hours

Naci 28

24NaCl 2

824

NaNO, 28

24

NaNO, 28

24

Roots

16.26.36.19.30.47.

16.34.89.

21.39.66.

Shootsn C.. /——— meM m ^

3*0.55±0. 96*2.24±0. 72*1.12i3. 7

6±0. 17*1.43±5. 5

0*1. 05±1.48±4. 0

0.

12.

4.26.

9.64.

13.

X 10*

Total

16.9±0.5*1.

9*1.2±3.-

4*2.6i7.-

3iO.

42

48

00

352. 4±4. 8

27.49.19.35.73.

16.44.

153.

21.52.

119.

3i 0.54± 1. 3U 2.24± 0. 71* 2.44± 5. 4

6± 0. 1li 2.89*11. 30± 1.08* 1.62± 7.6

Percenttransport

3.641.25. 5i2.

13.36.

21.41.

25.43.

-

95

9*3. 36*4.-

3±3.8*2.-

2*0.

3

10

69±2. 4

Table 9—The effect of NaCl and NaNOs pretreatments on subsequent 24-hour uptake of Cs by 2-week-old wheat seedlings fromsolutions containing 0.1 meq CsCl and either NaCl or NaNOs; initial pH = 5.5

Uptake treatmentNaClf

Pretreatment*

NoneNaCl (24)NaCl (12) + NaNO, (12)NaNO, (24)

Roots

23. 4 * 2 .18. 8 i 0.29. 7*2 .17. 1 ± 0.

Shootsmeq Cs (x 10* V culture5 3.1*0.66 2. 6 ± 0. 11 3. 4 * 0. 47 1.7*0.2

Total

26.521.433.118.8

±2 .8± 0 . 7±2.1±0 .9

Percenttransport

11. 7 ±1.912.1 ± 0.410. 3 * 1. 39. 0 t 0. 5

Roots

89. 1 ± 3.78. 5*4.64. 2*4.26. 3*3.

NaNO, T

Shoots•meq Cs (X10»)/cult3 48. 0 ± 1. 73 38. 6 ± 4. 37 21. 4 ± 0. 84 5.5*2.7

Total

137. 1 * 3. 2117.1* 5.385. 6 * 5. 131. 8 ± 6. 0

Percenttransport

35. 0 ± 1. 333. 0 ± 2. 925.0* 1.417. 3 ± 6. 1

* Figures In parentheses Indicate hours of pretreatment with the standard growth solution plus the Indicated salts at 10 meq/liter.f Present In the uptake solution at 1. O meq/liter.

702 SOIL SCI. SOC. AMER. PROC., VOL. 32, 1968

Table 10—The influence of various nitrate and chloride salts ontransport to shoots and loss to ambient solution of Sr pre-

viously accumulated in the root tissue of low N-seedlings.Prior to the indicated salt treatments (initially at

pH 5.5), the seedlings had been in 0.2 meqSrCb/liter for 4 hours and contained 36.4 X

10-4 meq Sr/culture in the roots and 0.03X 10 meq/culture in the shoots

Six- hourtreatment

(0. 4 meq/llteH2OKNO,KC1NaNOjNaClNH,NOSNH4C1Ca(N03)2CaCl;Mg(NOj)2MgCl;

Nitrateuptake*

r) ————

323429

314434

195453

394429

368435

Srremainingin roots

24,23.14.30.22.

8.8.8.9.

13.10.

Srtransportedto shoots

- meq (X 10*)/culturOti. 1 0. 214. 08941. 1141. 3

942.7242.5841.0

.141.0

,341.2441.6442.3941. 4

I.0.3.0.

774.184.

414.144.

0.514.0. 084.

3.0.3.0.

80±.444,

3111

43031202

40.15

60i. 72454. 14

Sr Sr transported X100

Srretainedt

24. 2141. 025.6741.214.28*1.434. 3142. 622. 3442. 49.3141.0B. 1841.0

12. 1041.69.8441.5

17.0042.611. 3541.5

lost tosolution

12.10.22.

2.12.27.28.

241. 0

741.2. 141. t

142.0142.4

141.03±1. 0

24.341.626.641.519.25.

442.6041.5

Sr remainingin roots

0.94 .7.441.1. 340.

11.042.0. 640.

5. 841.1. 040.

45. 848.4. 742.

26. 942.4. 1±1.

327123364

50

* No net chloride uptake was detectable by solution analysis.J Remaining in roots plus transported to shoots.

When neither NaCl nor NaNO3 were present during theprevious 24 hours, Cs uptake, transport, and percentagetransport were greatly increased by NaNO3 in the uptakemedium. The magnitude of these effects was progressivelydepressed as time of exposure to NaNO3, relative to NaCl,increased during the pretreatment period. When NaNO3was present only during the second 12 hours of the pre-treatment period, a modest increase in Cs retention by theroot tissue was evident although the proportion transportedto the shoots was slightly depressed.

Transport of Previously Accumulated Sr—The experi-ments described above indicate that one of the nitrate influ-ences was to stimulate transport of Sr to the shoots evenmore than accumulation in root cells. An experiment there-fore was conducted to determine the extent to which trans-port of Sr, taken up in the absence of nitrate, would beaffected by subsequent nitrate treatment. Seedlings wereexposed to 0.2 meq SrCl2 for 4 hours. The roots were ex-posed to the standard washing procedure following whichthe seedlings were placed for 6 hours in nitrate and chlo-ride salts of various cations at 0.4 meq/ liter. For the latterperiod three beakers per salt were employed, each contain-ing two cultures in 275 ml. Transport to the shoots duringthis period was rather small but signfkant effects of theindividual salts were evident (Table 10). For each cation,nitrate induced greater Sr transport than chloride. Innitrate solutions, K and NH4 resulted in less Sr transportthan occurred with the other three cations. Although theamounts transported in chloride solutions were small, Caand Mg promoted slightly more Sr transport than wasnoted in H2O or with the other three cations. Substantialloss of Sr from the roots occurred. All chloride salts in-duced more loss of Sr to solution than occurred in theabsence of salt, but this was not true with all nitrate salts.With NaNO3, for example, relatively little Sr was lost tosolution and with KNO3 about the same amounts were lostas to H2O. Ammonium and Ca were most effective in dis-placing Sr from the roots, and there was not a large differ-ence between the nitrate and chloride salts in so doing.

DISCUSSION

Nitrate-Stimulated Cation Uptake

The stimulation in cation uptake resulting from rapidnitrate uptake appeared to be a rather general effect. Inaddition to Sr uptake, Cs uptake was also enhanced (Figs.2 and 4; Tables 4 and 9), as was Mn uptake (Table 5).Moreover, adding NaNO3, compared to NaCl, to the stand-ard solution used in growing the nitrogen-depleted seed-lings resulted in an enhancement in Na, K, and Mg contentof the tissue at the end of 24 hours (Table 6). Only a veryslight increase in Ca content was found during the pretreat-ment period, but this was probably a result of the lowamount of Ca (0.25 meq/liter) in the solution comparedto the amount of Mg (2.0 meq/liter).

The magnitude of the nitrate-stimulated uptake of Srand Cs was not identical in the various experiments. Someof the variability was due to tissue weights varying amongexperiments; the average weights ranged from 1.31 to 1.75 gper culture for the roots and 0.94 to 1.33 g per culturefor the shoots. Nevertheless, the nitrate-stimulation wasreal and fairly consistent. The increase in Sr uptake dueto presence of nitrate was relatively slight at 2 hours andbecame progressively larger during the subsequent 10hours (Fig. 1). The advantage in total Sr uptake due tonitrate ranged from about twofold (Fig. 1, Table 3) tothreefold (Tables 7 and 8) at the end of 24 hours. In-creases in Cs uptake were more variable ranging from onlya modest stimulation (Fig. 2) to about a fivefold increasewhen Sr was not present (Table 9). The amount of Cstaken up, and the proportion transported to the shoots, isgreatly influenced by the K content of the tissue prior tothe uptake period (5) as well as by the ambient K concen-trations (Fig. 4, Table 4). The data indicate, however,that uptake of Cs was enhanced by nitrate even when totalCs uptake was severely restricted by high amounts of solu-tion K (Table 4). High solution K levels substantially in-creased the proportion of Cs transported to the shoots, aneffect not seen with Sr.

The chloride contents of the tissue were high, and duringthe uptake experiments little net uptake of chloride couldbe detected by solution analysis. During the same 24-hourperiods uptake of nitrate was substantial (850 X 10~4 meqper culture for Figs. 1 and 2; 1,250 X 10~4 meq per cul-ture for 1.25 meq NaNO3 per liter in Figs. 3 and 4), andin fact it was much larger than the combined uptake ofSr, Cs, and Na. The consequent lowering of solution acidityattending the relative rapid nitrate uptake must have ex-erted more favorable conditions for uptake of the solu-tion cations. The depressing effect of hydrogen ion on up-take of various cations has been often demonstrated (6, 14).The acidity developing in the largely unstirred layers closelyadjacent to the root cell surfaces no doubt would not betotally compensated by additions of acid or base to theexternal solution. Nevertheless, the fact that relatively largedifferences still remained when such adjustments weremade (Table 2) suggests that a part of the enhancement incation uptake was a direct result of the rapid nitrate uptake.

After the tissue had been exposed to nitrate for 24

JACKSON AND WILLIAMS: NITRATE-SIMULATED UPTAKE AND TRANSPORT OF STRONTIUM 703

hours, during which considerable nitrate was taken up, thestimulation in Sr uptake resulting from presence of nitratewas not nearly as large as when the tissue had not beenexposed previously to nitrate (Tables 7 and 8). Seedlingsgrown on nitrate for the entire 2-week period showed nostimulation due to presence of nitrate in subsequent 24-hour Sr uptake.6 For all that, the effects on cation uptakecannot be totally a consequence of the parallel rapid nitrateuptake. For example, 24-hour pretreatments in NaNO3,compared to NaCl, resulted in an increased total uptakeof Sr during the following period in absence of nitrate. Theadvantage for nitrate pretreatment in the subsequent 24-hour Sr uptake in NaCl was 18% with 2-week-old seed-lings (Table 7) and 49% for 1-week-old seedlings (Table8). That this indirect effect was reasonably long lasting isindicated by the relatively smaller advantage for nitratepretreatment after 8 hours than after 24 hours. The evi-dence clearly suggests that prior nitrate treatment inducedan enhanced ability for subsequent Sr uptake. This ten-dency was more evident in the younger seedlings whichwere not quite so depleted in N. Three effects thereforewere probably operating in nitrate-stimulated cation up-take: (i) an increased cation mobility associated withrapid nitrate uptake, (ii) a lowering of the H ion concen-tration resulting from rapid nitrate uptake, and (iii) anindirect influence exerted after the nitrate was absorbed.

Nitrate-Stimulated Transport of Cations to ShootsThe N-depleted seedlings were only poorly able to trans-

port cations to the shoots, and in the presence of nitrate thetransport processes were increased substantially. Increasesin the Sr percentage transport values due to presence ofnitrate (Figs. 1 and 3; Tables 1, 2, and 3) as well as tonitrate pretreatment (Tables 7 and 8) indicate that thetransport process was in fact increased more than accumu-lation by root cells. This was also true for Mn (Table 5)and Cs (Figs. 2 and 4; Tables 4 and 9). In the absence ofnitrate, it took 6 hours (Fig. 1) or longer (Table 1) forappreciable amounts of Sr to appear in the shoots, but Srcould be detected by the third hour when nitrate was pres-ent (Fig. 1). The roots contained approximately the sameamounts of Sr (—35 X 10~4 meq per culture) "at 6 and 3hours in the NaCl and NaNO3 series, respectively (Fig. 1).Even so, the pattern of transport differed in the two casesbecause the rapid surge between 3 and 6 hours in theNaNO3 series was not evident with NaCl even when theroots contained as much Sr as the NaNO3 series did whenthe surge to shoots was taking place. The greater transportrate therefore cannot be entirely due to a higher concen-tration of Sr in the root tissue.

A similar early surge of Sr to shoots was seen in seed-lings exposed to SrCl2 solutions after they had been grownfor the prior 2 weeks in solutions containing nitrate.6 Thedata suggest that nitrate transport or factors associatedwith nitrate metabolism promoted the rapid initial move-ment of Sr to the shoots. Subsequent experiments7 have

0 Ibid.7 Ashley, D. A. Op. cit.

shown that the amount of nitrate in the shoots of N-depleted seedlings similar to those used in the present ex-periments was around 100 X 10~4 meq per culture after 6hours exposure to Ca(NO3)2. This is probably an under-estimate of the amount of nitrate transported because anadditional amount of reduced N which originated from theexternal solution (~50 X 10~4 meq per culture) was alsofound in the shoots, and a portion of this amount couldalso have been transported as nitrate. The transport ofSr and Cs was substantially less (—20-25 X 10~4 meqper culture) than these nitrate transport estimates suggest-ing that transport of other cations in the roots was en-hanced and that they accompanied the ascending nitrate.The similar quantities of nitrate and K observed in thebleeding xylem sap of wheat roots (11) and other species(8, 9, 10) suggests a close association between these twoions in the transport process. In the absence of external Kother cations supplied externally, as well as internal K,must accompany the ascending nitrate. A sizeable andrather specific effect of nitrate in stimulating xylem exuda-tion from root tissue of decapitated plants was clearlydocumented by van Andel (15), and more recently similareffects have been noted (16, 17). Moreover, when suppliedas nitrate salts, K and Ca from the external solution ap-peared more rapidly in the exudate than when supplied aschloride salts (18). The data strongly suggest that nitrateinduces the entering cations to move more directly to thexylem vessels whereas in the absence of nitrate they areless subject to being deposited there.

Subsequent salt treatments revealed differences amongthe supplied cations in inducing Sr transport to the shootsas well as inducing loss of Sr to the ambient solution(Table 10). The amounts of Sr transported were quitesmall compared to the amounts transported when Sr wascontinually present. Table 1, for example indicates 12 to17 X 10~4 meq Sr per culture transported during a 2- to8-hour exposure to Sr(NO3)2 and similar amounts areshown in Fig. 1 and Table 7 for comparable periods. Buta maximum of only 3.8 X 10~4 meq per culture of previ-ously absorbed Sr was transported during the 4- to 10-hourperiod in absence of external Sr (Table 10). A large partof the failure of the subsequent salt treatments to induce asubstantial movement of Sr to the shoots may have beena result of the relatively large amounts of Sr displaced fromthe roots to the external solution. We have noted that atleast 90% of the displacement occurs within 1 hour (cf. 2)and, thus, much of the previously absorbed Sr was notavailable for subsequent transport. Nevertheless, specificcation effects in the transport process, as well as the nitrate-stimulation, are evident. Assume that the loss of Sr to solu-tion occurred rapidly whereas transport to the shoots oc-curred slowly and steadily. Then the ratio of transportedSr to Sr remaining in the roots should for each anion revealthe same values if the cation influence were nil. This calcu-lation is given in the last column of Table 10 and indicatesfor H2O, KC1, NaCl and NH4C1 that about 1% of theSr remaining in the roots was transported to the shoots.Larger proportions were transported for the CaCl2 andMgCl2 treatments indicating an enhancing effect of the two

704 SOIL SCI. SOC. AMER. PROC., VOL. 32, 1968

divalent cations on Sr transport in the absence of nitrate.All cations induced a greater transport of Sr when they

were present as nitrate salts than as chloride salts (Table10). Again specific cation effects were evident. That Ca,Mg, and Na induced similar amounts of Sr to be trans-ported to shoots but had quite different influences on lossof Sr to solution suggests that the two processes of move-ment from roots had little in common. This conclusion isalso supported by the observation that KC1, NaCl, andNH4C1 had similar effects on transport of Sr to shoots butdiffered considerably in their influence on loss of Sr tosolution.