Na+ on the Growth and Ion Relations of Atriplex amnicola

J. PlantPhysiol. Vol. 133. pp. 228-234 (1988)

Effects of External Concentration of (K + + N a +) and K + IN a + on the Growth and Ion Relations of A triplex amnicola Z. ASLAM i , E.

G. BARRETT-LENNARD2,

and H.

GREENWAY

School of Agriculture, University of Western Australia, Nedlands, W.A., 6009, Australia Received November 26, 1987 . Accepted April 21, 1988

Summary This paper reports on the growth and ion relations of A triplex amnicola in response to different K +I Na+ in nutrient solutions containing either 40 or 400mol m- 3 (K+ + Na+)CI-. Growth was substantially lower at 400 than at 40mol m- 3 external (K+ + Na+)CI-. At 40 mol m- 3 (K+ + Na+)CI-, growth of A. amnicola did not differ at K+/Na+ of 0.025,0.333, and 1.0 in the nutrient solution. At 400 mol m- 3 (K+ + Na+)CI-, growth was substantially decreased at a K+/Na+ of 1.0 in the nutrient solution compared with lower K +IN a +. Although the [K +] and [Na +] in different plant parts were related to the K +INa + in the external solution, the [K+ + Na+] was generally not affected by K+ INa+ in the external medium. The single exception to this observation was at 400mol m- 3 (K+ + Na+)CI-, when the [K+ +Na+] in the shoots was substantially higher at a K +IN a + of 1.0 than at lower K +IN a +. In general, K +IN a + was highest in the apical root segments (10 -12 mm long), lower in the bulk roots and lowest in the leaves.

Key words: A triplex amnicola; Chenopodiaceae; saltbush; sodium and potassium concentrations; selectivity. Abbreviations: 'IT = Water potential,

7r

= Osmotic pressure.

Introduction An inadequate rate of accumulation of osmotic solutes in growing zones of roots andlor shoots might limit the growth of the halophyte Atriplex amnicola at high concentration of NaCI Geschke et aI., 1986). Jeschke (1984) has suggested that in some genera (including A triplex), the turgor-volume maintenance of cells of the shoot and root meristems may depend on the transport of K + in the phloem. Thus at high external NaCl, rates of K + transport in the phloem could limit growth. Assuming this to be true, it appeared possible that an increased concentration of K + in 1 Present address: Nuclear Institute for Agriculture and Biology, Jhang Road, P.O. Box 128, Faisalabad, Pakistan. 2 Address for correspondence: Dr. E. G. Barrett-Lennard, Division of Resource Management, Department of Agriculture, BaronHay Court, South Perth, W.A., 6151, Australia.

© 1988 by Gustav Fischer Verlag, Stuttgart

the external solution would maintain growth at high NaCI particularly of the roots. This speculation was also supported by the report that at a range of isosmotic concentrations, Atriplex halimus produced much higher dry weights with a 1: 1 mixture of KCI and NaCI than with either NaCI or KCI (Mozafar et aI., 1970). The present study aimed to determine whether an increase in the K +INa + in the external solution would enhance the growth of A. amnicola; in this work, the plants were grown at either 40 or 400 mol m - 3 (K + + Na +)CI-. Emphasis was placed on determining the concentrations of K+ and Na+ in different plant organs. The data show that there was no beneficial effect of high K +IN a + on growth. Indeed, at 400 mol m- 3 external(K+ + Na+)CI-,aK+/Na+ of 1.0 was detrimental. The data also show that on a whole plant basis, A. amnicola usually had a high selectivity for K + uptake over Na + uptake. The only exception was at an external K +INa + of 1.0 when there was no selectivity for K + over N a +.

Growth and ion relations of A triplex amnicola 5. 6. 7. 8.

Materials and Methods Germination and establishment Seeds of A triplex amnicola Paul G. Wilson (accession number 573 of the collection of Malcolm et al., 1984) were germinated according to the methods described by Aslam et al. (1986). Sixteen days after imbibition, the seedlings were developing their first pair of leaves. At this stage the seedlings were transferred to a temperature controlled glasshouse (30°C during the day, 25°C during the night); the plants were transplanted to aerated nutrient solutions (air flow of 400±50 cc min -1) in three litre pots. These solutions had a pH of 6.0 and contained (mol m -3): NO) and Cl-, 2.0; K +, Ca2+ and Na+, 1.0; Mi+ and sOi-, 0.5; NHt and H 2 PO.j, 0.2. Micronutrients were supplied at the concentrations as in Epstein (1972). On day 22, the concentrations of macro nutrients were increased by a factor of 5, except for Na+ and Ca2 + which were increased by factors of 10 and 2.5 respectively. Nutrient solutions were subsequently renewed every 3 - 4 days.

Salinity treatments Plants were grown at 2 concentrations of (K + + N a +)CI- with various K + INa + ratios (Table 1). For «control treatments», KCI and NaCI were added to the nutrient solutions on day 22 to give a concentration of (K+ + Na+)CI- of 40 mol m- 3 ; these solutions had K + INa + ratios of 0.025, 0.333 or 1.0. For «high salt treatments», KCI and NaCI were added to the nutrient solutions (commencing on day 22) in steps of 40 mol m - 3 (K + + Na +)CI- each morning and evening for five days to a final concentration of 400 mol m - 3 (K + + Na +)CI-; these solutions had K + INa + ratios of 0.0025, 0.025, 0.333 or 1.0. Each treatment had 6 replicates arranged at random in the glasshouse.

Harvesting Plants were harvested 3 and 13 days after the concentrations of (K + + N a +) CI- in the high salt treatments had been increased to 400 mol m - 3; plants were harvested between 12.30 and 4.30 a.m. and were dissected into the following parts. 1. Apical root segments (0-12 mm from the apex). This sample contained meristems, rapidly expanding and recently expanded cells of roots (d. Fig. 1 of Jeschke et al., 1986). 2. Bulk roots. This fraction comprised mostly fully expanded tissue although it also contained a substantial volume of root tips. 3. Mature fully expanded leaves of the main stem. 4. Recently expanded leaves of the main stem.

Table 1: Concentrations of K + and Na + (mol m - 3) required to give the appropriate K +IN a + in the external nutrient solution. K + and Na + were supplied as KCI and NaCl. 0.0025

K+ INa+ 0.025 0.333

Control treatment; [K+ + Na+] ext. [K+] ext. [Na+] ext.

=

1* 39

High salt treatment; [K + + Na+] ext. [K +] ext. [Na+] ext.

1* 399

229

10 390

=

Measurement of leaf area, chlorophyll contents and growth The leaf area of mature fully expanded leaves was measured with a portable leaf area meter. These samples were subsequently frozen at -10°C, to be used later for the estimation of chlorophyll, dry weight and ion concentrations. The oven dry weight of leaves of Atriplex species is not a satisfactory basis for expressing the concentration of chlorophyll because of the very high concentrations of salts in these leaves. Consequently, chlorophyll contents were expressed on a leaf area basis. The frozen leaf material was homogenized using a pestle and mortar in cold (4°C) 80 % vlv acetone in water. The homogenate was centrifuged for 3 min to remove the leaf debris. Chlorophyll was determined from the absorbance of the extract at 663 and 645 nm (Yoshidaetal.,1976). The dry weights of the leaf samples were determined after acetone extraction. The acetone extract was returned to the leaf debris; the acetone was evaporated by heating at 55 - 60°C in a water bath and later in a forced draught oven at 70 0c. Dry weights of other plant samples were determined after drying them in a forced draught oven at 70°C for 3 days.

Measurement of cation concentrations Plant material was either (a) digested in concentrated HN0 3 till the material was translucent, or (b) extracted in 80 % ethanol followed by extractions with 1 N HN0 3 • The latter method gave more than 99 % recovery of ions compared with the former method. Cation concentrations were measured by atomic absorption spectroscopy as described by Jeschke et al. (1986). Concentrations of cations were expressed on a tissue water basis. In the case of dead tissue, concentrations were expressed on a dry weight basis. The selectivity of K + uptake relative to Na + uptake by the root was calculated from the formula of Pitman (1967) as: Sr

Na+ =

.

K+ uptake [K+] ext.

+

Na+ uptake [Na+] ext.

where Na + and K + uptake refer to the increase in ion content of the whole plant (Ilmol) between days 3 and 13, and [Na+]ext. and [K +]ext. refer to the concentrations of these ions in the nutrient solution.

Results 1.0

40molm- 3 10 30

Rapidly expanding leaves of the main stem. Buds and youngest expanding leaves of the main stem. Stems. Branches of the main stem (consisting of both leaves and stems). Ions were removed from the free space of roots by washing them for 10 min in sorbitol solutions isotonic with the nutrient solutions in which the plants had grown. These solutions also contained 2.5molm- 3 Ca2 +.

20 20

400 mol m- 3

100 200 300 200 " In solutions containing 1 mol m - 3 K +, the [K +] was regularly monitored and maintained between 0.8 and 1.2 mol m- 3 •

Visual symptoms, chlorophyll and growth In general, expanded leaves were paler green on plants grown at 400 than at 40 mol m- 3 external (K+ + Na+)Cl-. Expanded leaves of plants grown at 400 mol m - 3 (K + + Na+)Cl- with a K+/Na+ of 1.0 looked particularly unhealthy and gradually all the leaves on the shoot yellowed. Some of the plants in this treatment drooped and had necrotic spots on their leaves, particularly on the tips. These plants eventually died. Chlorophyll contents (/-tg cm - 2 leaf area) of the expanded leaves were affected both by the salin-

230

Z.

ASLAM,

E.

G. BARRETT-LENNARD,

and H.

GREENWAY

Table2: Effect ofK+/Na+ at40and400molm- 3 (K+ + Na+)Clon the chlorophyll content (/Lg em -2 leaf area) of fully expanded leaves of Atripex amnicola. [K + + Na +]ext. (mol m- 3)

0.025

40 400

ND 34.2± 1.9

1200

A.

B.

1.0 50.6±3.6 ND

47.8±2.3 17.3± 1.3

Plants were harvested on the thirteenth day after the concentration of (K + + Na +)Cl- in the high salt treatments had been increased to 400 mol m -3. Values are the mean ± SEM of 6 replicates. ND = not determined.

+ ., + + ,----..o..-c-~.

+ + + + + + + + T + + + + + + + + + + ~a++ + + + + + + + ++++++++ ++++++++ + + + + + + + +

K+ /Na+ 0.333 0.0025 0.025 Control treatment; [K + + N a +] ext. = 40 mol m - 3 Shoot dry weight (g plant -I) Root dry weight (g plant-I) Root/Shoot Relative growth rate (whole plant) (day-I)

0.025

1.0

0.33

1.0

0.0025

0.025

0.33

1.0

K"NIi in lhe nulrienl soiullon

1.67±0.20

1.56±0.17

1.6S±0.10

ND

0.31±0.OS

0.26±0.03

0.32±0.04

ND ND

0.19 0.19±0.01

0.17 O.l8±0.02

0.19 0.19±0.02

=

+

o

ND

High salt treatment; [K + + N a +1ext.

+ + + + + + + + + + + + + + +

+

Table 3: Effect ofK+/Na+ at40and400molm- 3 (K+ + Na+)Clon the dry weight of A triplex amnicola.

400 mol m - 3

Shoot dry weight 0.74±0.03 0.72±0.09 0.83±0.07 0.5HO.03 (g plant-I) Root dry weight 0.19±0.02 0.21±0.03 0.22±0.OS 0.12±0.03 (g plant-I) Root/shoot 0.26 0.29 0.26 0.23 Relative growth rate 0.13±0.01 0.12±0.02 0.13±0.02 0.08±0.02 (whole plant) (day-I) Dry weight and root/shoot values were determined on the thirteenth day after the concentration of (K + + Na +)CI- in the high salt treatments had been increased to 400 mol m - 3. Relative growth rates were calculated from dry weight increases between 3 and 13 days after the concentration of (K + + N a +)CI - in the high salt treatments had reached 400 mol m - 3. Values are the mean ± SEM of 6 replicates. ND = not determined.

Fig. 1: Effect of K +INa + in the nutrient solution on the potassium and sodium relations of the expanded leaves of A. amnicola grown at (A) 40 mol m- 3 (K+ + Na+)Cl-, (B) 400moi m- 3 (K+ + Na +)Cl-. Concentration of K + (e, 0); total concentration of (K + + Na +) (., 0). Plants were harvested on the thirteenth day after the concentration of (K + + N a +)Cl- in the high salt treatments had been increased to 400 mol m - 3. Values are the mean ± SEM of 6 replicates.

A.

B.

/ :' +

80

/+

/+

400 + + + + + + + + + + +

++++++++~

ity and the K + IN a + of the external solution. At a K + IN a + of 1.0, chlorophyll contents were 64 % lower at 400 than at 40 mol m- 3 (K+ + Na+)CI-. Further, at 400 mol m- 3 (K+ + Na +)CI-, chlorophyll contents were 49 % lower with the K +IN a + at 1.0 than at 0.025 (Table 2). Growth was slower at 400 than at 40 mol m -3 external (K+ + Na+)CI- (Table 3). This is consistent with earlier data on the response of growth to NaCI (Aslam et aI., 1986). Adverse effects on shoots were slightly more severe than on roots (Table 3). At 40 mol m -3 external (K + + Na +)CI-, variation in the K +INa + from 0.025 to 1.0 had no measurable effect on the growth of shoots or roots. At 400 mol m - 3 external (K + + N a +) CI- , growth was similar with K +I Na+ ranging from 0.0025 to 0.333; however shoot and root growth were both substantially reduced at the K +IN a + of 1.0 (Table 3).

Effects on the K+ INa + and [K+ + Na+] in different plant parts The major trends in the K + and Na + relations of A. amnicola are summarized in Figs. 1-4. Ion relations of the re-

+ + + + + + + +Na+ + + + + +~ + + + + + + + + + + + + + + + + + + + + ~~

K+

+

H+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Na + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + f + + + + + + + + + + + + + ~ ,..~-

T

+

... -

+--r-.,....--o-.,....-~--+-

~-':---.f-l'

++ ++ ++++ ++++ ++ ++ ++++ ++ ++ +~"/ ++++++++++++: ++++++++++++/ + + + + + + + + + + +4

: + : + : +: + : + : + ::~:;t:~!~~~ :!_.:t.j_.----,j-{)-

.,. "

o 0.025

0.33

1.0

0.0025

0.025

0.33

1.0

K' I Na' in the nutrient solution

Fig. 2: Effect of K +INa + in the nutrient solution on the potassium and sodium relations of the buds. Other details as for Fig. 1.

cendy expanded and rapidly expanding leaves of the main stem, the stems, and the branches of the main stem are reported elsewhere (Aslam, 1985). The internal [K + + N a +] in all plant parts was much higher at 400 than at 40 mol m- 3 (K+ + Na+)CI-. Within (K + + N a +) CI- treatments, the [K + + N a +] was lower in the roots than in the leaves (Figs. 1-4). In general, in all tissues there were relatively similar [K + + N a +] over widely different K +INa + in the external solution, although there was a substitution of K + for N a + as the external K + INa + increased. The principal exception was in the plants grown at 400 mol m- 3 (K+ + Na+)CI- withaK+/Na+ of 1.0. In the

Growth and ion relations of A triplex amnicola

A.

Table4:EffectofK+/Na+ at40and400molm- 3 (K+ + Na+)Clon the K + INa + in the tissues of A. amnicola.

B.

:~~~~~~~~~~:::::~:~::::::::i + + + + + + + + + + + + ~.' +++++++++++,.cr + + + + + + + + + + ~. + + + + + + + + "';./' + + + + + + + + ~. + + + + + + +~/ + + + + + + ~~ + + + + +0' +++++,'

+ + + + ,•.:t . .'·

1.0

0.0025

0.025

0.0025

Control treatment; [K + Expanded leaves Buds Bulk roots Apical root segments

+ N a +] ext. = 40 mol m - 3

Expanded leaves Buds Bulk roots Apical root segments

o 0.33

0.33

K+ INa+ 0.Q25 0.333

Tissue

ND ND ND ND

0.32 0.41 1.9 2.7

High salt treatment; [K + + N a +]

K+

7'---

0.025

231

1.0

K'/Na" in the nutrient sokrtlon

0.05 0.10 0.4 0.5

ext.

0.51 0.52 3.1 7.0

1.0 0.78 0.72 4.5 11.5

= 400 mol m - 3

0.09 0.16 0.9 1.4

0.44 0.37 2.8 4.6

0.98 0.87 5.5 7.1

Plants were harvested on the thirteenth day after the concentration of (K + + N a +)Cl- in the high salt treatments had been increased to 400 mol m - 3. ND = not determined.

Fig. 3: Effect of K + INa + in the nutrient solution on the potassium and sodium relations of the bulk roots. Other details as for Fig. 1.

A.

Concentrations o/K+ and Na+ in dead shoots at 400 mol m- 3 (K+ + Na+)CI- with a K+/Na+ 0/1.0

B.

:;...... ~ ..... j

400

i"'<+

:t- + + + + "::!---"F-F-T+ +++++++++ +j;" + + + + + + + + + T r + + + + + + + + + 1 ' " + + + + + + + + + + +.,+ + }J~+ + + + + + + + ~/ + + + + + + + + +.." + + + + + + + + + + + + +,-,<,7'

'e

:.t~··

: + : +: + : + + + + .,. + + ~.:l;t . . /

200

Effects on selectivity o/K+ over Na+

o 0.025

0.33

1.0

0.0025

0.025

Some plants grown at 400 mol m - 3 (K + + N a +)CI- with a K +INa + of 1.0 were dead and had dried to a considerable degree at the time of the second harvest, In the dead shoots the [K+ + Na+] was 9.2mmolg- 1 dry weight and the K+/Na+ was 1.1. These values were similar to those found in the live shoots of the same treatment (dry weight basis) (data not presented).

0.33

1.0

K'/Na in the nutrient solution

Fig. 4: Effect of K + INa + in the nutrient solution on the potassium and sodium relations of the apical root segments. Other details as for Fig. 1. Assumptions: (1) The loss of ions from apical root segments due to cut cells is 1 %. (2) Dry weightltissue water weight for apical root segment = 0.1. (3) 20 % of tissue water is in the extracellular space.

leaves of these plants, the [K +] was very high (420 - 590 mol m- 3) and the [K+ + Na+] was 15-25% higher than in plants grown with a K +INa + less than 1.0. (Figs. 1 and 2). Interestingly, in the roots of this treatment, the [K +] was also quite high (340 - 390 mol m - 3) but the [K + + Na +] was similar to that of plants grown with a K +IN a + less than 1.0 (Figs. 3 and 4). At both 40 and 400 mol m - 3 external (K + + N a +)CI- , the K +IN a + was higher in plants grown with high K +IN a + in the external solution (Table 4). However, the K +INa + within the plant parts varied over a smaller range than the ratios of K +IN a + in external solutions. It is noteworthy that with all treatments, the K+/Na+ was always highest in the apical root segments (10-12 mm long) followed by the bulk root samples, the leaves and buds (Table 4).

The selectivity of K + uptake over N a + uptake (SK + ,Na + ) in A. amnicola is presented in Table 5. There was a high SK+ ,Na+ with plants grown at 40 mol m- 3 (K+ + Na+)CI- and K+ I N a + 0.025, and with plants grown at 400 mol m - 3 (K + + Na+)Cl- and K+/Na+ 0.0025. The selectivity for K+ over Na + decreased as the K +INa + in the external solution increased. At an internal K +IN a + of 1.0, A. amnicola had no selectivity for K + over Na + (Table 5),

Table 5: EffectofK+/Na+ at 40 and400molm- 3 (K+ + Na+)Clin the external solution on the selectivity of K + uptake over Na + uptake by A triplex amnicola. [K+ + Na+] (mol m- 3) 40 400

K+/Na+

ext.

0.0025

0.025

0.33

1.0

ND 19,0

16.2 4.1

1.8 1.3

1.0 1.1

The selectivity of ion uptake was determined from the ion contents of the plants on days 3 and 13 after the concentration of (K + + N a + )Cl- in the high salt treatments had reached 400 mol m - 3. ND = not determined.

Discussion

Growth An increase in the K +INa + in the nutrient solution might have a variety of effects on plants, In halophytes at high

232

Z.

ASLAM,

E.

G. BARRETT-LENNARD,

and H.

GREENWAY

salinity we have hypothesized that there could be improvements in turgor-volume maintenance (see Introduction). Other workers have noted increases in starch concentration in sugar beet leaves (Hawker et al., 1974), changes in lipid composition (Stuiver et al., 1981), and increases in protein synthesis in cell free extracts (Gibson et al., 1984). It is therefore interesting that in the present work with A. amnicola, the K +IN a + of the nutrient solution had little effect on plant growth. The only exception to this generalization was at 400 mol m- 3 (K + + Na+)CI-, where a K +INa+ of 1.0 decreased plant growth and the concentration of chlorophyll, and caused leaf necrosis. The adverse effects of the high K +IN a + at high total salt concentration are consistent with results with Atriplex in· flata, A. nummularia, Suaeda maritima and Vigna radiata, which show that growth is reduced more with high concentrations of KCI than of NaCI (Ashby and Beadle, 1957; Yeo and Flowers, 1980; Salim and Pitman, 1983). The exact cause for this decline in vigour and health is unknown, although with Atriplex inflata, A. nummularia and Vigna radiata, the impaired growth coincided with substantially higher concentrations of monovalent cations in shoots of plants grown with KCI than NaCI (Ashby and Beadle, 1957; Salim and Pitman, 1983). Yeo (1981) has suggested that growth of Suaeda maritima may be adversely affected if the [K +] in the cytoplasm of leaf cells increases to toxic levels. Interestingly, in the present work, plants grown at 400 mol m - 3 (K + + Na+)CI- with a K +INa + of 1.0 had a [K +] in the expanded leaves of more than 500mol m- 3 (Fig. 1). High concentrations of K + in the cytoplasm are plausible, as compartmental analysis of leaf slices of S. maritima showed that K + diffuses substantially faster out of vacuoles (to.s of exchange = 126 x 10 3 s) than Na + (to.s of exchange = 533 x 103 s) (Yeo, 1981). An alternative explanation for the adverse growth at high external K +IN a + is that high external K + could have had an antagonistic effect on the uptake of some nutrients essential to the synthesis of chlorophyll. No data are available from this study to choose between these two possibilities. The present results contrast with those of Mozafar et al. (1970), who showed that at a range of external salt concentrations, growth of A triplex halimus was much higher with nutrient solutions containing a 1: 1 mixture of NaCI and KCI than with nutrient solutions containing isosmotic concentrations of either KCI or NaCl. Although the discrepancy in the growth responses of A. halimus and A. amnicola to 1: 1 mixtures of KCI and NaCl might be due to differences between species, it is more likely to be due to the depletion of essential nutrients in the external solution in the experiment with A. halimus. In their study, Mozafar et al. (1970) supplied K + and Na + to the nutrient solutions of all treatments at a minimum of 6 mol m - 3 each; however the authors gave no indication that culture solutions were replaced during their 10 week experiment. Thus depletion of K + from solutions high in Na+, and depletion ofNa+ from solutions high in K + could have prevented growth from reaching the levels found where both K + and N a + were high. The magnitude of the possible detrimental effects of N a + deficiency on the growth of halophytes can be gauged from the study of Williams (1960) with Halogeton glomeratus. After 7 weeks, plants grown in nutrient solutions containing 1.0 mol m - 3 N aCI

had 1.8 fold greater dry weights than plants grown in solutions containing 0.05 -0.08 mol m -3 Na +. The present growth data suggest that the nutritional requirements of A. amnicola for K + are fulfilled at external [K +] as low as 1 mol m - 3, provided that there is an adequate level of total monovalent cations. The results at 40 mol m - 3 external (K + + Na +)CI- are not surprising as higher plants are known to absorb nutrients to adequate levels from very low external concentrations. For example, 13 out of 14 different species tested grew maximally at external K + concentrations of 0.095 mol m - 3 (Asher and Ozanne, 1967). However, it is interesting to note that plants grown at 400 mol m -3 external (K+ + Na+)CI- also achieved comparable growth at 1, 10, and 100 mol m - 3 external K +. This similar growth at very low and high external K +IN a + was undoubtedly due to the very high selectivity of K + uptake over Na + uptake in the plants grown at K +IN a + of 0.0025 (Table 5). It would be of interest to establish whether this high selectivity of K + over Na + at high external [K + + N a +] and low K + IN a + is specific for halophytes or also applies to other species. Selectivity ofK + over Na +

The high selectivity for K + uptake over N a + uptake in A. amnicola grown with 1 mol m - 3 K + is interesting particularly when the external concentration of (K+ + Na+)CI- is 400 mol m - 3. This high selectivity was presumably due to the operation of mechanism I for ion uptake which shows a high affinity for K + uptake at low external K + concentrations; this mechanism is not inhibited by high concentrations of external Na+ (Epstein, 1972). The decrease in SK+ .N.+ as the K+ INa + in the external solution increases is consistent with the trends for barley (calculated from the data of Pitman, 1965). At external K+ INa + of 1.0, the SK+.N.+ was extremely low (Table 5). It is therefore interesting to speculate about what might happen with external K +INa + greater than 1.0; would there be selectivity for Na + uptake over K + uptake? Another interesting aspect is the K +IN a + in young and old leaves. Previous workers have noted that the K +IN a + is generally higher in younger than in older shoot organs (reviewed by Jeschke, 1984). For example, in sugar beet grown at a K +I Na+ of 0.05 and a[K + + Na+] of 5 mol m- 3, the K +INa+ for old, middle and young leaves was 0.06, 0.17, and 0.30 respectively (calculated from the data of Marschner et al., 1981). With some treatments in the present work, the K +IN a + of the buds was not greater than that of the expanded leaves (Table 4). However it should be stressed that these are probably not exceptions to the general case noted above. In the present work we did not remove the salt bladders from the surfaces of the leaves; these bladders accumulate 55 - 85 % of the N a + in young leaves, 5 - 10 % of the N a + in older leaves, and virtually no K + (calculated from Fig. 3 in Aslam et al., 1986). Thus the K +IN a + in the «Ieaftissues» (i.e. leaves without bladders) would have been substantially greater for the buds than the expanding leaves (d. Aslam et al., 1986). Concentrations ofK+ and Na+ in roots and shoots

Roots: The maintenance of a similar internal concentration of (K + + N a +), with widely different K +IN a + at con-

Growth and ion relations of A triplex amnicola Table 6: Estimated internal osmotic pressure (MPa) in roots of At· riplex amnicola grown at different external K +INa + and 40 or 400 molm- 3 (K+ + Na+}Cl-. [K+ + Na+]

ext.

=

40 mol m- 3 ('11" = 0.2 MPa) K+/Na+ 0.025 0.333 1.0 Cations balanced by monovalent anions Apical root segments 0.87 0.95 0.97 0.91 Bulk roots 0.86 0.96 Cations balanced by divalent organic anions 0.71 0.73 Apical root segments 0.65 Bulk roots 0.64 0.72 0.69

[K+ + Na+]

ext.

=

400 mol m- 3 ('11" = 1.9 MPa) 0.0025 0.Q25 0.333 1.0 1.99 2.09

1.89 2.12

1.89 1.99

2.13 1.92

1.49 1.57

1.41 1.59

1.42 1.49

1.60 1.44

The estimates in this table were prepared using the data in Figs. 3 and 4. We have assumed that 0.1 MPa is generated by 21 mol m- 3 monovalent salts or 14 mol m -3 of a salt of K + or Na + with a di· valent anion. stant concentrations of (K + + Na +)CI- in the external solution, can be interpreted more easily for roots than for shoots. The easiest interpretation of the present data is the assumption that the [K + + N a +] is maintained by a system of turgor-volume maintenance. It is interesting that within this system of turgor-volume maintenance, the roots have the option of using K + andlor N a +, depending on the composition of the external medium. The K +IN a + in the roots presumably depends on the active and passive fluxes of Na + and K + into and out of the root. In the present work, the contribution of (K + + N a +) to turgor-volume maintenance can be assessed because the plants were harvested between 12.30 a.m. and 4.30 a.m. (see Materials and Methods). Since there was no substantial water flow through the roots for several hours prior to the time of harvest, the'll in the roots would have been about the same as the 'II of the external solution. The contribution of (K + + N a +) to the 7r of the root cells can therefore be estimated. Table 6 presents estimates of internal 7r assuming that cations are balanced by either monovalent or divalent anions. Calculations of turgor using the data in Table 6 show that if the balancing anions in the roots had been monovalent, then the turgor pressures would have been at least 0.66-0.77MPa and 0-0.23MPa in the roots grown at 40 and 400 mol m - 3 (K + + N a +)CI- respectively. Actual turgor pressures would be higher than these estimates if there were other solutes in the roots, and lower than these estimates, if there were substantial concentrations of divalent instead of monovalent anions. The difference between our estimates of turgor in roots at 40 and 400mol m- 3 (K+ + Na+)CI- is substantial (0.5-0.7MPa equivalent to 200 - 300 os moles m - 3 of ions) and suggests that there may be a lower «reference value» (c.f. Cram, 1976) for turgor maintenance in plants at high than at low salinities. Confirmation of this however, will require direct measurements of turgor with the micropressure probe in roots grown at various external salt concentrations. The principal difference between root tips and the bulk root sample was a higher K+ INa+ in root tips than in mature roots. This observation reinforces the earlier conclusion that in A. amnicola, young tissues had a preference for K + over Na + Geschke et aI., 1986). Shoots: At a given external salt concentration, the [K + + Na +] inside the different parts of the shoot remained at rela-

233

tively steady levels despite large differences in K +IN a + in the external solution. The only exception to this was at a K+/Na+ of 1.0 with 400 mol m- 3 external(K+ + Na+)CI-. It is not clear whether the observed constancy of uptake of (K + + N a +) was due to regulatory processes or was merely circumstantial. In the present study it is not possible to relate [K + + N a +] in shoot tissues to turgor-volume maintenance because these ions would be localized in salt bladders and cell walls as well as within the cells of the stems and laminae (Aslam et aI., 1986). The higher [K + + N a +] in shoots of plants grown with K+/Na+ of 1.0 than in shoots grown with K+/Na+ lower than 1.0 could have been due to the slower growth of these plants (for the general case on the relation between growth rate and ion concentration see Greenway and Thomas, 1965). Alternatively, the rate of transport of (K + + Na +) from root to shoot might have been higher in plants grown with K +IN a + of 1.0 than in plants grown with K +IN a + lower than 1.0.

Acknowledgements Thanks are extended to Dieter Jeschke for criticism of the manu· script, and to Clive Malcolm for the supply of seed. The senior author is grateful to the University of Western Australia for the grant of a University Research Studentship.

Definitions Leaves: Leaf blades including salt bladders. Salt bladders: Vesicular hairs that develop from epidermal cells on the leaf surfaces of a large range of species of A triplex and other Chenopodiaceae.

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ASHER, C. J. and P. G. OZANNE: Growth and potassium content of plants in solution cultures maintained at constant potassium concentrations. Soil Science 103, 155-161 (1967). ASLAM, Z.: Growth, ion relations and carbohydrate levels of At· riplex amnicola at high salinities. PhD Thesis. University of Western Australia (1985). ASLAM, Z., W. D. JESCHKE, E. G. BARRETT-LENNARD, T. L. SETTER, E. WATKIN, and H. GREENWAY: Effects of external NaCI on the growth of A triplex amnicola and the ion relations and carbohydrate status of the leaves. Plant, Cell and Environment 9, 571-580 (1986).

CRAM, W. J.: Negative feedback regulation of transport in cells. The maintenance of turgor, volume and nutrient supply. In: LUTTGE, U. and M. G. PITMAN (ed.): Encyclopedia of Plant Physiology Vol. 2A, pp. 284-316. Springer-Verlag, Berlin (1976). EpSTEIN, E.: Mineral Nutrition of Plants: Principles and Perspectives. pp. 38, 134. John Wiley and Sons, New York (1972). GIBSON, T. S., J. SPEIRS, and C. J. BRADY: Salt tolerance in plants. II. In vitro translation of m-RNAs from salt-tolerant and salt-sensi· tive plants on wheat germ ribosomes. Responses to ions and compatible organic solutes. Plant, Cell and Environment 7, 579 - 587 (1984).

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