J. Plant Pbysiol. Vol. 139. pp. 761-763 {1992}

Short COlllmunication

The Effects of Iso-osmotic Salt and Water Stresses on the Growth of Halophytes and Non-halophytes

KE-FU ZHAO and P. J. C. HARRIS I

Department of Biology, Shandong Teachers' University, Jinan, China 1 Department of Biological Sciences, Coventry Polytechnic, Priory Street, Coventry CV 1 5FB, England

Received October 16, 1991 . Accepted October 28,1991

Summary

Seedlings of the halophytes A triplex lentiformis and Suaeda salsa, and the drought resistant Prosopis chi­lensis were grown in liquid cultures with their roots exposed to salt (NaCl) or water stress (PEG). A.lenti­formis and S. salsa were exposed to 0, 0.496 and 0.992MPa osmotic potentials imposed by each treatment, and P. chilensis to 0, 0.248 and 0.496 MPa. Dry weight and moisture contents of A.lentiformis and S. salsa were reduced by PEG but were unaffected, enhanced, or only slightly reduced by NaCl. In contrast P. chilensis was affected to a greater extent by NaCI than by PEG. A.lentiformis and S. salsa accumulated large amounts of N a + with a higher concentration being found in the shoots than in the roots. P. chilensis accumulated less N a + and this was largely confined to the roots. The results are discussed in terms of the distinctive nature of osmotic and ionic stresses and the mechanisms of salt resistance.

Key words: A triplex lentiformis, Prosopis chilensis, Suaeda salsa, halophyte, salt stress, water stress.

Abbreviations: PEG = polyethylene glycol; RH = relative humidity.

Introduction

The injury suffered by plants exposed to sodium chloride is considered to result from both ionic damage and from osmotic damage due to a lowered water potential (Gale, 1975; Levitt, 1980). These two factors are interrelated and coexist under saline soil conditions. Halophytes must possess the ability to counter both of these stresses simultaneously and this had made the study of salt tolerance in plants more difficult (Munns, 1985; Cheeseman, 1988). This has resulted in the misconceptioD that halophytes must be drought-tol­erant as well as salt tolerant and to some confusion in the dis­tinct physiological and ecological responses of halophytes and xerophytes (Waisel, 1972; Levitt, 1980). In fact, some ha­lophytes can tolerate salt stress but not drought, and some xerophytes can tolerate drought but not salt stress. In this study the effects of iso-osmotic salt and water stresses on growth and sodium content of two halophytic species (Suaeda salsa and Atriplex lentiformis) and a drought-tolerant species (Prosopis chilensis) were measured, to give a compa-

© 1992 by Gustav Fischer Verlag, Stuttgart

rison between the two types of stress and to gain further un­derstanding of the mechanism of salt tolerance.

Materials and Methods

Seeds of Suaeda salsa (L.) Pall. and A triplex lentiformis Torr. were soaked for 4 h and germinated in washed sand. Prosopis chilensis Mol. Stuntz seeds were soaked in 98 % H 2S04 for 20 min, then washed for 12 h in running water before sowing in sand. Seedlings were watered with Hoagland's solution until three to five true leaves had been produced. They were then transferred to liquid cul­ture where they were supported on polystyrene trays floating in plastic tanks of aerated Hoagland's solution. One week after trans­fer to liquid culture plants were exposed to NaCI and 6 kD PEG in the culture solution. In addition to controls with Hoagland's solu­tion only, S. salsa and A. lentiformis were treated with NaCl or PEG to give osmotic potentials of 0.496 and 0.992 MPa. P. chilensis plants were treated with NaCI or PEG to give osmotic potentials of 0.248 and 0.496 MPa. All treatments were carried out in a growth chamber with a 15 h photoperiod, day and night temperatures of 25

762 KE-FU ZHAO and P.]. C. fuRRIs

and 20 0 e respectively, a photon flux of 110Itmolm-2s-l, 85% RH and 95 - 98 % oxygen saturation of the medium.

After fresh weight determination, roots and shoots were dried to a constant weight at 80 °e and ashed in a furnace for 24 h at 500°C. Organic dry weight was calculated as the difference between dry weight and ash weight, and is an important parameter for measuring growth increases in halophytes which may accumulate large salt de­Rosits. Na + was extracted from dried plant material with 5 M Hel at 100 °e until dry, redissolved in deionized water, and quantified by flame photometry.

Four replicate plants were selected at random for each analysis. Data was analysed by analysis of variance and least significant differ­ences, calculated using Tukey's t value (Li, 1964), are shown in the figures where appropriate.

Results and Discussion

The halophyte S. salsa grew better with NaCI at 0.496 MPa than with Hoagland's solution alone, with a significantly higher organic dry weight being obtained with 0.496 MPa (Fig. 1). The organic dry weight of A.lentiformis grown in NaCI at 0.496 or 0.992MPa did not differ significantly from that of plants in Hoagland's solution alone. In contrast, the dry weights of A.lentiformis and S. salsa decreased with in­creasing water stress and were significantly lower for S. salsa

240 Suaeda salsa

180 NaCI

120 I

60 I

0 -E 0.496 0.992

~240 Atriplex lentiformis cil

NaCI .s 180 E .~ 120 I ;;: ~ 60 "0 ()

·c 0 ro ~ 0.496 0.992 0

240

180 I 120

60

0 0.248 0.496

Osmotic potential (MPa)

Fig. 1: The effect of 96h exposure to salt stress (NaCl) and water stress (PEG) on organic dry weight of Suaeda salsa, Atriplex lenti­formis and Prosopis chilensis. Where significant differences occur, vertical bars represent the LSD at P < 0.05 for comparing any two means values for that parameter.

93

90

87

84

81 0

~93 ~

~ 90 C 887 ~ ::J ti 84 ·0 ~ 81

0

84

81

78

75

72 0

Suaeda salsa

0.496 0.992

Atriplex lentiformis

0.496 0.992

Prosopis chilensis

0.248 0.496 Osmotic potential (MPa)

I

I

I I

Fig. 2: The effect of 96 h exposure to salt stress (NaCl) and water stress (PEG) on moisture content of Suaeda salsa, Atriplex lenti· formis and Prosopis chilensis. Where significant differences occur, vertical bars represent the LSD at P < 0.05 for comparing any two means values for that parameter.

with PEG at 0.496 and 0.992 MPa, and for A.lentiformis at 0.992MPa, than with Hoagland's solution alone. No signifi­cant change in moisture content was observed in either hal­ophyte exposed to salt-stress, but this declined significantly with increasing PEG concentration (Fig. 2). The pattern of Na+ accumulation was similar in A.lentiformis and S. salsa and increased significantly with increasing NaCI concentra­tion (Fig. 3). Na + accumulated to a greater extent in the shoots than in the roots. The results suggests that these plants may counter the lower water potential imposed by NaCI solutions by absorbing ions from the bathing solution. Suaeda spp. have been shown to be salt accumulating (Zhao, 1991) and Atriplex spp. are known to have salt excreting glands (Goodin, 1979), indicating that plants of this genus employ both salt excretion and salt accumulation to survive in highly saline environments. When exposed to water stress imposed by PEG, in the absence of added Na + and Cl- ions, the plants were unable to adapt and demonstrated reduced growth rate and moisture content.

A statistically significant reduction in the dry weight of the drought-tolerant species P. chilensis was obtained only with NaCI at 0.496 MPa, suggesting that salt was the more inhib­itory of the two stresses (Fig. 1). The moisture content of P. chilensis also differed from that of the two halophytes in that it declined significantly with NaCI at 0.248 or 0.496 MPa and

700 Suaeda salsa

525 I

350 I

175

0 0.496 0.992

s: 700 ci I ~525 OJ :::t

~350 I Q)

§ 175 (J

ill 0 z 0.496 0.992

700 Prosopis chilensis

525 I

350

175

0 0.248 0.496

Osmotic potential (MPa)

Fig. 3: The effect of 96h exposure to salt stress (NaCl) (--) and water stress (PEG) (- - - - -) on Na + content of roots (.) and shoots (.) of Suaeda salsa, Atriplex lentiformis and Prosopis chilensis. Where significant differences occur, vertical bars represent the LSD at P < 0.05 for comparing any two means values for that parameter.

also with PEG at 0.496 MPa (Fig. 2). The Na + content of P. chilensis increased significantly with increasing NaCI concen­tration but only the roots accumulated significant amounts of Na + (Fig. 3). It is clear from the results that P. chilensis is

Iso-osmotic salt and water stresses 763

more sensitive to NaCI than to PEG treatment and more sensitive to NaCI than were either A. lentiformis or S. salsa. This suggests that P. chilensis may be affected by Na+ ionic toxicity. There is some evidence, however, that P. chilensis, although considered primarily a drought-tolerant species, may possess a degree of salt-tolerance. The selective accu­mulation of N a + ions in the roots is similar to that observed with other salt-excluding species such as barley (Munns, 1985) which are tolerant of relatively low salinities. The re­lated species Prosopis farcta has previously been reported to behave in this way when exposed to low concentrations of NaCI (Eshel and Waisel, 1965).

Acknowledgement

This research was funded by the Henry Doubleday Research As­sociation.

References

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saline environments, pp. 168-185. Springer-Verlag, Berlin and New York (1975).

GOODIN, J. R.: In: RITCHIE, G. A. (ed.), New agricultural crops, AAAS Symposium, pp. 133 -146. Westview Press Inc., Boulder, Colorado (1979).

LEVITT, J.: Responses of plants to environmental stress. Volume 2. 2nd edn. Academic Press, New York, pp. 365-434 (1980).

LI, C. c.: Introduction to experimental statistics. McGraw-Hill, New York, pp. 418-429 (1964).

MUNNS, R.: J. Exp. Bot., 36, 1032-1042 (1985). W A1SEL, Y.: Biology of halophytes. Academic Press, New York and

London, pp. 194-211 (1972). ZHAO, K. F.: Plant and Soil 135, 303-305 (1991).