J. theor. Biol. (1978) 73, 363-366
LETTERS TO THE EDITOR
Potassium and Turgor Pressure in Plants In a recent article in this journal, Gradmann (1977) questioned the commonly held view that potassium plays a major role in the osmotic relations of most plant cells. Citing the fact that most cell membranes have a relatively high permeability to K’, Gradmann infers that the reflection coefficient for K+ will therefore be low and that K+ will have a low “osmotical efficiency”. From this he concludes further that K+ plays a minor role in osmoregulation and that “investigations of the K+ distribution are not likely to illucidate the basic events of osmoregulation”. Gradmann’s analysis assumes that the “osmotical efficiency” of KC is a function of the K” permeability and is independent of the membrane permeabilities to other solutes. Although this approach conforms to the description of the reflection coefficient as originally defined for nonionic solutes (Staverman, 1951; Kedem & Katchalsky, 1958), it conflicts with the concept of the reflection coefficient as defined for ionic solutes. As pointed out by Kedem & Katchalsky (1961), the reflection coefficient for ionic solutes at zero current flow is given by: a,=l-
p,v,=l RTL,
pzvs
-->I-RTL,
PIK RTL,
(1)
where cr,, is the reflection coefficient for the salt, Va is the partial molar volume of the salt, P, is the salt permeability coefficient, P, is the permeability coefficient for the more permeant ion, Pz is the permeability coefficient for the less permeant ion, L, is the hydraulic conductivity and R and T have their usual meanings. As stated in equation (I), substitution of P, for P, will yield a conservative estimate of a,, whereas the use of P, may greatly underestimate a,. However, even the erroneous substitution of P, for P, will still yield a reflection coefficient close to 1.0, provided that P, rs $ R TL,,, which is usually true for biological membranes (see House, 1974). Equation (1) assumes that salts and water move independently through the membrane. However, if water and salt flows are coupled, e.g. due to frictional interactions in charged pores, then a third term must be subtracted from the right hand site of equation (1), and os will be less than 1 - (P, Ei,/ 363
0022-5193/78/0720-0363
$01.00/O
@ 1978 Academic Press Inc. (London)
Ltd.
364
J.
CUTKNECHT
AND
D.
F.
HASTINGS
R TL,) (Kedem & Katchalsky, 1958, 1961). However, this subtraction will not alter the basic fact that the reflection coefficient for a salt will be governed by the less permeant ion. Gradmann suggests that due to the low osmotic efficiency of K+ the turgor pressure changes in plant cells, particularly the guard cells of the stomata1 apparatus, must be due mainly to anions such as malate and Cl-, to which the cell membrane is only slightly permeable. Although we agree that organic anions and Cl- often have much lower permeability coefficients than K+, we do not agree that K+ has therefore a lower osmotic effectiveness than malate or Cl-, for the reason stated in equation (1) and illustrated by the following example: A hypothetical walled cell containing potassium chloride is placed in distilled water. The cell membrane is highly permeable to K+ but only slightly permeable to Cl-. At equilibrium, the turgor pressure, AP, is equal to the effective osmotic pressure difference, i.e.
AP=&=(l-~$c’
(2)
where rri is the osmolality (osmol kg H,O-‘) of the intracellular solution and P, is the membrane permeability to KCl. Although PK B Pcl, the net K+flux is tightly coupled electrically to the net Cl- flux. Thus, in spite of its high permeability, Kf cannot leave the cell and thus P, r 0 and o, E 1.O. Furthermore, the osmotic effectiveness of the two ions is identical because each ion makes an equal contribution to the intracellular osmolality and thus to the turgor pressure. Among the evidence which Gradmann cites for the low osmotic effectiveness of K+ are studies which show recovery of cell volume and/or turgor pressure following plasmolysis in solutions made hypertonic with potassium salts. According to Gradmann, these studies suggest that “the high permeability of Kf prevents its role as an efficient plasmolyticum”. Although we agree that some cells may have low reflection coefficients for KC1 (and even NaCl) (see, e.g. Steudle et al., 1975), many of these observations have an alternative explanation, i.e., that cells respond to hypertonic stress by increasing the rate of active uptake of K+ and/or Cl- (see, e.g. Cram, 1976; Hellebust, 1976; Hsiao et al., 1976; Zimmermann, 1977; Gutknecht 8~ Bisson, 1977). Thus, recovery from hypertonic stress often gives the appearance that the osmotically important salt has a low reflection coefficient. The question remains, will investigations of Kf distribution elucidate the basic events of osmoregulation? The answer to this question depends partly on the definition of a “basic event” and partly on the organism
LETTERS
TO
THE
365
EDITOR
being studied. In some plant cells, K” transport is the dominant process in turgor generation and regulation, e.g. Zimmermann & Steudle (1974), Hasting & Gutknecht (1976). In other plant cells, anion transport is the primary process and K+ and Na+ movements are secondary, e.g. Bisson & Gutknecht (1977). In still other plants, the synthesis and metabolism of organic solutes may be the most important process in osmotic regulation, e.g. Kauss (1977). However, in virtually all cells, including guard cells, Kf is the principal intracellular cation and plays a key role in the osmotic relations of the cell, regardless of whether its transmembrane movements are primarily active or passive, e.g. Leaf (1959), Raschke (1975). In conclusion, we agree with Gradmann that the implications of this question go beyond the problem of stomata1 movements. In fact, the question applies to virtually all living cells, i.e. all cells which are high in K+ and have relatively high K+ permeabilities. If in these cells the high Kf permeability makes K+ osmotically ineffective, as Gradmann suggests, then we must search for a new, impermeant intracellular solute in order to account for the turgor pressure of walled cells and the osmotic responses of animal cells. In fact, the search is unnecessary in most cells because the reflection coefficient for K+ salts is close to unity and thus Kf salts account for most of the observed osmotic properties of the cell. Department of Biochemistry Dake University Medical Center Durham, North Carolina 27110, U.S.A. Department of Physiology Duke University, and Duke University Marine Laboratory Beaufort, North Carolina, 28516, U.S.A. (Received 18 December 1917, and in revised form 20 February
DAVID
JOHN
F. HASTINGS
GUTKNECHT
1978)
Supported in part by USPHS grant HL 12157. For helpful comments on the manuscript we thank Drs M. A. Bisson, J. Dainty, K. Raschke and J. F. Thain and Ms. A. Walter. REFERENCES BISSON, M. A., & GUTKNECHT, J. (1977). J. mem. Biol. 37, 85. CRAM, W. J. (1976). In Encyclopedia of PIant Physiology, New Series, (U. Luttge and M. G. Pitman, eds), Vol. II, Part A, p. 284. Berlin: Springer-Verlag. GRADMANN, D. (1977). J. theor. Biol. 65, 597. GUTKNECHT, J. & BISSON, M. A. (1977). In Water Relations in membrane Transport in Plants and Animals, (A. M. Jungreis, T. Hodges, A. M. Kleinzeller, & S. G. Schultz, eds), p. 3. New York: Academic Press. HASTINGS, D. H., & GUTKNECHT, J. (1976). J. mem. Biol. 28, 263, HELLEBUST, J. (1976). Ann. Rev. Pl. Physiol. 27, 485.
366
J. GUTKNECHT
AND
D. F. HASTINGS
HOUSE, C. R. (1974). Water Transport in Cells and Tissues, Baltimore: Williams & Wilkins. HSIAO, T. C., ACEVEDO, E., FERERES, E. & HENDERSON, D. W. (1976). Phil. Trans. R. Sot. Land. B273,479. KAUSS, H., (1977). In Plant Biochemistry (D. H. Northcote, ed.). MTP International Review of Science, Series 2, p. 119. London: Butterworths. KEDEM, 0. & KATCHALSKY, A. (1958). Biochim. biophys. Acta. 27, 229. KEDEM; 0. & KATCHALSKY, A. (1961). J. gen. Physiol. 45, 143. LEAF. A. (1959). Ann. N. Y. Acad. Sci. 72. 396. RAS~HKE,‘K. (i975). Ann. Rev. PI. Physioi 26, 309. STAVERMAN, A. J. (1951). Rec. trav. Chim. 70, 344. STEUDLE, E., LUTTGE, U., & ZIMMERMANN, U. (1975). Planta 126, 229. ZIMMERMANN, U. (1977). In Sot. Exp. Biol. Symp. XXXZ, (D. H. Jennings, ed.), p. 117. Cambridge: University Press. ZIMMERMANN, U., & STEUDLE, E. (1974). J. mem. Biol. 16, 331.
LETTERS TO THE EDITOR
Potassium and Turgor Pressure in Plants In a recent article in this journal, Gradmann (1977) questioned the commonly held view that potassium plays a major role in the osmotic relations of most plant cells. Citing the fact that most cell membranes have a relatively high permeability to K’, Gradmann infers that the reflection coefficient for K+ will therefore be low and that K+ will have a low “osmotical efficiency”. From this he concludes further that K+ plays a minor role in osmoregulation and that “investigations of the K+ distribution are not likely to illucidate the basic events of osmoregulation”. Gradmann’s analysis assumes that the “osmotical efficiency” of KC is a function of the K” permeability and is independent of the membrane permeabilities to other solutes. Although this approach conforms to the description of the reflection coefficient as originally defined for nonionic solutes (Staverman, 1951; Kedem & Katchalsky, 1958), it conflicts with the concept of the reflection coefficient as defined for ionic solutes. As pointed out by Kedem & Katchalsky (1961), the reflection coefficient for ionic solutes at zero current flow is given by: a,=l-
p,v,=l RTL,
pzvs
-->I-RTL,
PIK RTL,
(1)
where cr,, is the reflection coefficient for the salt, Va is the partial molar volume of the salt, P, is the salt permeability coefficient, P, is the permeability coefficient for the more permeant ion, Pz is the permeability coefficient for the less permeant ion, L, is the hydraulic conductivity and R and T have their usual meanings. As stated in equation (I), substitution of P, for P, will yield a conservative estimate of a,, whereas the use of P, may greatly underestimate a,. However, even the erroneous substitution of P, for P, will still yield a reflection coefficient close to 1.0, provided that P, rs $ R TL,,, which is usually true for biological membranes (see House, 1974). Equation (1) assumes that salts and water move independently through the membrane. However, if water and salt flows are coupled, e.g. due to frictional interactions in charged pores, then a third term must be subtracted from the right hand site of equation (1), and os will be less than 1 - (P, Ei,/ 363
0022-5193/78/0720-0363
$01.00/O
@ 1978 Academic Press Inc. (London)
Ltd.
364
J.
CUTKNECHT
AND
D.
F.
HASTINGS
R TL,) (Kedem & Katchalsky, 1958, 1961). However, this subtraction will not alter the basic fact that the reflection coefficient for a salt will be governed by the less permeant ion. Gradmann suggests that due to the low osmotic efficiency of K+ the turgor pressure changes in plant cells, particularly the guard cells of the stomata1 apparatus, must be due mainly to anions such as malate and Cl-, to which the cell membrane is only slightly permeable. Although we agree that organic anions and Cl- often have much lower permeability coefficients than K+, we do not agree that K+ has therefore a lower osmotic effectiveness than malate or Cl-, for the reason stated in equation (1) and illustrated by the following example: A hypothetical walled cell containing potassium chloride is placed in distilled water. The cell membrane is highly permeable to K+ but only slightly permeable to Cl-. At equilibrium, the turgor pressure, AP, is equal to the effective osmotic pressure difference, i.e.
AP=&=(l-~$c’
(2)
where rri is the osmolality (osmol kg H,O-‘) of the intracellular solution and P, is the membrane permeability to KCl. Although PK B Pcl, the net K+flux is tightly coupled electrically to the net Cl- flux. Thus, in spite of its high permeability, Kf cannot leave the cell and thus P, r 0 and o, E 1.O. Furthermore, the osmotic effectiveness of the two ions is identical because each ion makes an equal contribution to the intracellular osmolality and thus to the turgor pressure. Among the evidence which Gradmann cites for the low osmotic effectiveness of K+ are studies which show recovery of cell volume and/or turgor pressure following plasmolysis in solutions made hypertonic with potassium salts. According to Gradmann, these studies suggest that “the high permeability of Kf prevents its role as an efficient plasmolyticum”. Although we agree that some cells may have low reflection coefficients for KC1 (and even NaCl) (see, e.g. Steudle et al., 1975), many of these observations have an alternative explanation, i.e., that cells respond to hypertonic stress by increasing the rate of active uptake of K+ and/or Cl- (see, e.g. Cram, 1976; Hellebust, 1976; Hsiao et al., 1976; Zimmermann, 1977; Gutknecht 8~ Bisson, 1977). Thus, recovery from hypertonic stress often gives the appearance that the osmotically important salt has a low reflection coefficient. The question remains, will investigations of Kf distribution elucidate the basic events of osmoregulation? The answer to this question depends partly on the definition of a “basic event” and partly on the organism
LETTERS
TO
THE
365
EDITOR
being studied. In some plant cells, K” transport is the dominant process in turgor generation and regulation, e.g. Zimmermann & Steudle (1974), Hasting & Gutknecht (1976). In other plant cells, anion transport is the primary process and K+ and Na+ movements are secondary, e.g. Bisson & Gutknecht (1977). In still other plants, the synthesis and metabolism of organic solutes may be the most important process in osmotic regulation, e.g. Kauss (1977). However, in virtually all cells, including guard cells, Kf is the principal intracellular cation and plays a key role in the osmotic relations of the cell, regardless of whether its transmembrane movements are primarily active or passive, e.g. Leaf (1959), Raschke (1975). In conclusion, we agree with Gradmann that the implications of this question go beyond the problem of stomata1 movements. In fact, the question applies to virtually all living cells, i.e. all cells which are high in K+ and have relatively high K+ permeabilities. If in these cells the high Kf permeability makes K+ osmotically ineffective, as Gradmann suggests, then we must search for a new, impermeant intracellular solute in order to account for the turgor pressure of walled cells and the osmotic responses of animal cells. In fact, the search is unnecessary in most cells because the reflection coefficient for K+ salts is close to unity and thus Kf salts account for most of the observed osmotic properties of the cell. Department of Biochemistry Dake University Medical Center Durham, North Carolina 27110, U.S.A. Department of Physiology Duke University, and Duke University Marine Laboratory Beaufort, North Carolina, 28516, U.S.A. (Received 18 December 1917, and in revised form 20 February
DAVID
JOHN
F. HASTINGS
GUTKNECHT
1978)
Supported in part by USPHS grant HL 12157. For helpful comments on the manuscript we thank Drs M. A. Bisson, J. Dainty, K. Raschke and J. F. Thain and Ms. A. Walter. REFERENCES BISSON, M. A., & GUTKNECHT, J. (1977). J. mem. Biol. 37, 85. CRAM, W. J. (1976). In Encyclopedia of PIant Physiology, New Series, (U. Luttge and M. G. Pitman, eds), Vol. II, Part A, p. 284. Berlin: Springer-Verlag. GRADMANN, D. (1977). J. theor. Biol. 65, 597. GUTKNECHT, J. & BISSON, M. A. (1977). In Water Relations in membrane Transport in Plants and Animals, (A. M. Jungreis, T. Hodges, A. M. Kleinzeller, & S. G. Schultz, eds), p. 3. New York: Academic Press. HASTINGS, D. H., & GUTKNECHT, J. (1976). J. mem. Biol. 28, 263, HELLEBUST, J. (1976). Ann. Rev. Pl. Physiol. 27, 485.
366
J. GUTKNECHT
AND
D. F. HASTINGS
HOUSE, C. R. (1974). Water Transport in Cells and Tissues, Baltimore: Williams & Wilkins. HSIAO, T. C., ACEVEDO, E., FERERES, E. & HENDERSON, D. W. (1976). Phil. Trans. R. Sot. Land. B273,479. KAUSS, H., (1977). In Plant Biochemistry (D. H. Northcote, ed.). MTP International Review of Science, Series 2, p. 119. London: Butterworths. KEDEM, 0. & KATCHALSKY, A. (1958). Biochim. biophys. Acta. 27, 229. KEDEM; 0. & KATCHALSKY, A. (1961). J. gen. Physiol. 45, 143. LEAF. A. (1959). Ann. N. Y. Acad. Sci. 72. 396. RAS~HKE,‘K. (i975). Ann. Rev. PI. Physioi 26, 309. STAVERMAN, A. J. (1951). Rec. trav. Chim. 70, 344. STEUDLE, E., LUTTGE, U., & ZIMMERMANN, U. (1975). Planta 126, 229. ZIMMERMANN, U. (1977). In Sot. Exp. Biol. Symp. XXXZ, (D. H. Jennings, ed.), p. 117. Cambridge: University Press. ZIMMERMANN, U., & STEUDLE, E. (1974). J. mem. Biol. 16, 331.
