An Alternative Explanation for the Apparently Active. Water Exudation in Excised Roots. M. T. TYREE. Department of Botany, University of Toronto, Canada.
Journal oj Experimental Botany, Vol. 24, No. 7S, pp. 33-37, February 1973
An Alternative Explanation for the Apparently Active Water Exudation in Excised Roots M. T. TYREE Department of Botany, University of Toronto, Canada Received 10 July 1972
ABSTRACT A standing-gradient osmotic-flow model has been proposed by others to explain the apparently active water uptake in excised maize roots. An alternative model is proposed which incorporates electrogenic coupling between the fluxes of anions and cations and electro-osmotic coupling between the fluxes of cations and water in the cell walls of roote. This model does not exclude the contribution (positive or negative) of a standing-gradient osmotic flow.
In recent years the ionic relations of excised roots have been somewhat extensively studied because the content of the xylem will exude from the cut end and is accessible to measurements. In an exuding root experiment one can (1) measure the rate of exudation, (2) measure the concentration and composition of the exudate, (3) vary the tonicity and composition of the bathing medium and measure the transient and steady-state effects on exudation, (4) measure the net salt uptake from the bathing medium, and (5) measure the electrical potential difference between the exudate and the bathing medium. A number of workers have measured the exudation rate, Jv (cm3 8"1 per cm2 of root surface), as a function of the difference between the osmolar concentration of the exudate, Cf, and the bathing medium, C° (House and Findlay, 1966a; Anderson and Reilly, 1968a, b; Anderson and Collins, 1969). It has always been found that the exudation process seems to have two basic components; (1) an 'active' flux rate which is independent of water potential differences, °, and (2) a purely osmotic component oLPRT(C*—C°) where a and LP are the reflection coefficient and hydraulic conductivity of the bulk root respectively. The equation governing exudation is then Jv = oLPRT{C*-C%)+%. (1) The values for oLP (cm3 s~] cm"2 bar"1) and ° (cm3 s - 1 cm"2) reported in the literature for Zea mays roots are 1-8 x 10~7 and 1-2 x 10~7 respectively (Anderson and Collins, 1969), 1-4 xlO" 7 and 1-8X10"7 respectively (Anderson and Reilly, 1968a), and 5-7x 10"7 and l-3xl0~ 7 respectively (House and Findlay, 1966a). Anderson, Aikman, and Meiri (1970) have recently argued that there may not be an active component of water transport in roots and they have demonstrated in theory how the entire exudation process could be osmotic. Their argument for setting ° = 0 is, however, ultimately circular. 180.5
D
34
Tyree—An Explanation of Water Exudation in Excised Roots
There is reason to believe that the pathway for the movement of some ions into the exudation stream is via the symplasm (Arisz, 1956, 1959; Tyree, 1970; Jarvis and House, 1970); but in view of the very low water permeability of maize root cortical-cell membranes (Jarvis and House, 1967; House and Jarvis, 1968), it can be shown by an analysis similar to that of Tyree (1969) that the symplasm cannot be the pathway for most of the water uptake into the xylem. Indeed it can be shown that if one assumes that the hydraulic conductivity of maize cell walls is equal to that oiNitdlaflexilis cell walls, then the calculated value of LP is very close to the experimental value of a LP. The value of LPP for N. flexilis is l-4x 10~7 cm3 s - 1 cm-2 bar - 1 cm (Tyree, 1968); LP is related to LPP by the tortuous path length from the root surface to the xylem elements in the stele, I, and the relative area occupied by the cell wall, a^; LP
.
(Z)
If we take o^, = 0-05 and I = 0-05 cm then the calculated value of LP is 1-4 x 10~7 cm3 s- 1 cm"2 bar- 1 , taking the barrier as a plane sheet rather than as a cylinder because our knowledge of a^, and I does not justify a more precise model. Jarvis and House (1967) have reported on the water permeability of the mature cortical cells of Zea mays roots. If we assume no water-filled pores we can calculate from their values the LP of the cortical cell membranes to be 4 X 10~9 cm3 s"1 cm-2 bar- 1 (Tyree, 1970). The above calculations suggest that the cell wall offers the least resistance to water uptake into the xylem elements; but where is the permeability barrier to the solutes ? A semi-permeable barrier must exist if osmotic water flow is to occur and this has been postulated to occur either at the xylem element or at the casparian band. In view of the fact that the Cfiara cell wall has a reflection coefficient to dilute salts near to unity (Barry and Hope, 1969), it is possible that the maize root cell wall itself could be the semi-permeable barrier. Some attempts have been made at locating the site of the salt permeability barrier which separates the exudation compartment from the bathing solution, through an analysis of transient changes in fluid exudation after rapid changes in the bathing medium (House and Findlay, 19666). No conclusions have been reached through lack of sufficient time resolution and because of ignorance of the solute diffusion coefficients in maize root cell walls. Bowling and Weatherley (1965) and Hylmo (1953, 1955, and 1958) have reported a linkage between the influxes of water and certain ions into the exudation compartment. I feel it is possible to link both the site of 'active' water uptake and the site of coupling between the influx of water and ions into the root to the cell wall. Electrochemical investigations of ion transport into roots (Bowling, Macklon, and Spanswick, 1964; Bowling and Weatherley, 1964; Bowling, 1966 and 1968) suggest that cations (K+, Na + , and Ca++) enter the exudation stream passively whereas anions (NO,f, C1-, SO4" ~, H2PO4~~, and HP04~ ~) must be actively accumulated against an electrochemical potential gradient (Bowling, 1968). Furthermore, there is evidence that the anion pumps in higher plant cells are electrogenic (Higinbotham, 1970).
Tyree—An Explanation of Water Exudation in Excised Boots symplasm (cytoplasm + vacuole)
ce
35
., ., " >va11
K+,C1
J epidermis
t
endodermis
Cortex
exudation compartment
uptake of ions into symplasm (active and passive)
transport of Cl" in the symplasm via plasmodesmata (the plasmodesmata are not shown)
electrogenic efflux of Cl
"*" *
from the symplasm inlo the exudation compartment
electrogenicly coupled flux of K+ from the symplasm and the outside into the exudation compartment via the cell wall where electro-osmosis occurs FIG.
1.
It may therefore be postulated that there is an electrogenic pump actively extruding anions into the exudation compartment from the symplasm (see Fig. 1). The electrical potential difference between the exudate and the bathing medium would then be more negative than the diffusion potential (as given by the Goldman constant-field theory) by an amount, A^\ The electrogenic potential would drive cations out of the symplasm and through the cell walla into the exudate; the potential would also drive cations from the bathing medium into the exudate via the cell wall. The amount of cation going by each pathway would depend on the ionic conductances of the two pathways. This electrogenicly coupled flow of cations through the cell wall would cause an inward electro-osmotic water flow with a high
36
Tyree—An Explanation of Water Exudation in Excised Roots
electro-osmotic efficiency. Since the net salt influx has been found to be roughly independent of the external salt concentration for maize roots (House and Findlay, 1966a) the electro-osmotic water flow through the cell wall would appear to be independent of (Cf—CJ}) (cf. Eq. 1). This would give the appearance of active water transport. In order to put some teeth into the argument I submit the following crude calculation. Assume a root in which the only salt entering the xylem is KC1 (see Fig. 1). If electro-osmosis through the cell wall is to account for all the observed apparent active water transport, (f>°, it must be shown that the ionic conductance of K+, A+, in the cell wall is sufficiently great so that the magnitude of the electrogenic potential difference, Ae, is within the range of the total observed potential difference between the exudate and the outside, AE. Preferably we would like AS much smaller than AE, because it is known that the cations are not too far from electro-chemical equilibrium. It must also be shown that the electro-osmotic efficiency, Jv\\, needed to produce £ and to preserve charge neutrality is not unreasonably large for plant cell walls. The lowest value of the specific conductivity, LEE, observed in Nitella cell walls is l-3xlO~ 3 mho cm"1 (Tyree and Spanner, 1969) when the wall is bathed in lO^-lO" 4 M KC1. Since K+ is the most abundant species in the wall under these conditions, LEE probably reflects the K+ conductance; expressed in terms of a whole root this becomes, A+ = " " ^ = 1-3 X 10-3 mho cm-2,
(3)
where, as before, o^ = 0-05 and I = 0-05 cm. The ionic conductance of K+ in plant cell membranes based on the Goldman constant field assumption is of the order of 4 x 10~8 mho cm"2; thus the wall is probably the most conductive pathway for K+. If all the K + influx is via the cell wall pathway then T
V T
AS = -± = £^± ^ - 0 - 8 mV, (4) A+ A+ since J+ observed in roots is of the order of 10"11 equiv. s - 1 cm~2. Thus AS is small compared with the total potential difference, AE, observed between the exudate and bathing medium; AE is usually of the order of —50 mV. If «/„// is the electroosmotic efficiency of maize root walls, then in order to produce a flow of ° = 10~7 cm3 s"1 cm"2 and still preserve charge neutrality (J+ = J_) r-f) \1 /wall
= -^-^ •'•'+ 3
0-1 cm3 C-1.
An electro-osmotic efficiency of 0-1 cm C - 1 corresponds to — . < 550 mol of H 2 0 per faraday of charge. I have frequently observed electro-osmotic efficiencies of this magnitude in Chara and Nitella cell walls in a low calcium environment (unpublished). Anderson, Aikman, and Meiri (1970) proposed a standing-gradient osmotic-flow model in order to explain the apparently active water flux, %, into roots. In their model they incorporated two ideas: (a) that the concentration of the solutes in the exudation stream varies over the length of the root because the rate of salt influx
Tyree—An Explanation of Water Exudation in Excised Boots
37
varies over the length of the root, and (b) that the water permeability of the root, oLP, could well vary over the length of the root. Both of these ideas are reasonable and realistic. Later in their paper they calculated how oLP would have to vary over the root length given certain profiles of CJ over the root length assuming that % = 0. They
have done a valuable service by showing how ° could be set to zero, but in order to prove that °v is zero oLP would have to be measured along the root by some independent technique. I do not know how this measurement of oLP could be carried out; it would be rather difficult to prove their hypothesis. The model I have proposed was first presented in Quebec City in June 1970 at a meeting of the Canadian Society of Plant Physiologists. The model involves electrical coupling between the fluxes of anions and cations and electro-osmotic coupling between the fluxes of cations and water in the cell wall of roots. The model does not exclude the possibility that standing-gradient osmotic flow could either enhance or reduce %. My model suffers from the same flaw of untestability as that proposed by Anderson et al. (1970), but I believe it is sufficiently within the realm of plausibility to justify presentation to those deeply involved in the study of exuding root systems. REFERENCES ANDERSON, W. P., ATKMAN, D. P., and METBI, A., 1970. Proc. R. Soc. B . 174, 445-58. and COLLINS, J . C , 1969. J. exp. Bot. 20, 72-80.
and REIIJLY, E. J., 1968a. Ibid. 19, 19-30. 19686. Ibid. 19, 648-57. ABISZ, W. A., 1956. Protoplasma, 46, 5-62.
1969. Ada bot. neerl. 18, 14-38. BABBY, P . H., and H O P E , A. B., 1969. Biophys. J. 9, 729-57.
BOWLING, D. J . F., 1966. Planta, 69, 377-82. 1968. Ibid. 83, 53-9. MACKLON, A. E. S., and SPANSWICK, R. M., 1964. J. exp. Bot. 17, 410-16.
and WEATEEELEY, P . E., 1964. Ibid. 15, 413-21. 1965. Ibid. 16, 732-41. HIGLNBOTHAM, N. 1970. Am. Zool. 10, 393-403. HOUSE, C. R., and FINDLAY, N., 1966a. J. exp. Bot. 17, 344-54.
• 19666. Ibid. 627-40. and JABVIS, P., 1968. Ibid. 19, 31-40. HYLMO, B., 1953. Physiologic. PI. 6, 333-411. 1955. Ibid. 8, 433-9. 1958. Ibid. 11, 382-400. JABVIS, P., and HOUSE, C. R., 1967. J. exp. Bot. 18, 695-706.
1970. Ibid. 2 1 , 83-90. TYKEE, M. T., 1968. Can. J. Bot. 46, 317-27.
1969. J. exp. Bot. 20, 341-9. —— 1970. J. theor. Biol. 26, 181-214. •
and SPANNER, D. C , 1969. Can. J. Bot. 47, 1497-503.
