Planta (Berl.) 93, 227--232 (1970)
9 by Springer-Verlag 1970
A Comparison of Potassium Translocation in Excised Roots and Intact Maize Seedlings W. P. A ~ D ~ s o ~ and E. AL~.~N University of Liverpool, Department of Botany, Liverpool 3, England Received May 15, June 6, 1970 Summary. The water and potassium fluxes exuded by excised primary roots of maize seedlings are compared with the rate of wet weight increase and of potassium uptake by the growing tissue of the shoots of intact seedlings. The exuded potassium flux from 1 mM KC1 solution as the bathing medium is found to correspond very closely to the potassium uptake by the shoots of seedlings of identical age and history as the excised roots.
Introduction
Physiological studies of exudation from excised root preparations have long been a popular and useful technique in efforts at elucidating the mechanism of root pressure exudation; e.g. Ariszetal. (1951), House and Findlay (1966), Baker and Weatherley (1966), Anderson et al. (1970). Although there are still various points of contention, the model generally agreed from these studies is that root pressure exudation proceeds principally b y an osmotic mechanism. Salts are accumulated b y the root into the xylem vessels, thus creating an osmotic pressure gradient between the xylem sap and the external solution which drives a flow of water into the xylem. This water flow eventually appears at the excised stump as the exudation flux. The rate at which an excised root exudes salt is measured b y the salt exudation flux, obtained (House and Findlay, 1966) as the product of the volume (water) exudation flux multiplied b y the salt concentration of the exudate fluid. I t is of interest to enquire how closely an ion flux exuded b y an excised root preparation resembles the ion flux translocated b y an intact plant. Morrison (1965) reports an experiment of this type with rooted cuttings of Salix vitellina and shows evidence for a sudden increase in the p82 content of xylem sap exuded immediately after excision, compared with t h a t of xylem sap extracted from a freshly detached leaf of an intact plant. Thus it is concluded t h a t the exuded phosphate flux is a poor, and overestimate of the phosphate flux translocated in the intact plant. These results contrast with those reported here for potassium flux. Several explanations of the discrepancy are available;
228
W. P. Anderson and E. Allen:
the physiological age of the maize shoots is different from t h a t of the leaves used b y Morrison. Secondly, phosphate has a n e x t r e m e l y complex biochemical r6]e a n d indeed Morrison ascribes the increased P 32 a c t i v i t y i n x y l e m sap to a secretion b y the root of organic phosphates, a n d thirdly, it m a y well be incorrect to assume t h a t the p32 labelled phosphate is everywhere i n tracer equilibrium w i t h i n the p l a n t , the a s s u m p t i o n implicit i n Morrison's assay technique. The work reported here is a n a t t e m p t to compare the K+ flux exuded b y excised roots with the rate of n e t K+ t r a n s l o c a t i o n from the root to the shoot of i n t a c t seedlings of identical age a n d history as the excised roots. F r o m such m e a s u r e m e n t s various inferences m a y be d r a w n regarding possible i n t e g r a t i o n b y the p l a n t of the rate of salt t r a n s l o e a t i o n (as measured on excised root preparations) a n d the growth rate. There are difficulties i n m a k i n g a comparison of this n a t u r e , the principal one being t h a t the a d d i t i o n a l driving force of t r a n s p i r a t i o n a l pull m a y operate on the x y l e m columns of the i n t a c t seedlings. This effects was minimised i n these experiments b y growing the seedlings i n the d a r k a n d i n a n enclosed v o l u m e of high h u m i d i t y .
Materials and Methods a) Growth o/ Material. Grains of Zea mays (ear. White Horse Tooth) were soaked for 4 hr in running tap water and were then set out to germinate on damp paper in trays kept at 25~ C in the dark. After 3 days the seedlings were arranged on an open mesh nylon net suspended over a container so that the roots dipped into a solution of 1.0 mM CaC12. The solution was continuously aerated with moist air and the seedlings were allowed to grow for a further 3 days in the dark at 25~ C. The seedlings were regularly examined and developing secondary roots were removed so that each seedling had only a primary root. b) Excised Root Preparation. Seedlings at the stage described above were randomly selected and 10 cm lengths of the roots were cleanly excised with a sharp razor blade on a wet glass plate. The cut ends were inserted and sealed with a wax:rosin (1:1, w/w) mixture i n ~ a 100 ~1 Microcap pipette and were mounted vertically on a perspex plate with a graduated scale. They were allowed to exude from an aerated solution of 1.0 mM KC1/0.1 mM CaCI2. The heights of the exudate columns were recorded at regular intervals and the exudate fluid was collected, weighed and diluted to l0 ml with distilled and deionised water for K+ analysis at 24 h and 36 h after setting up. The K + concentrations were estimated by comparison with standard solutions on a Unicam SP 90 A atomic absorption spectrephotometer. At the end of the exudation period the diameter of each root was measured at three points along its length using a travelling microscope with a Vernier scale, in order to make a good estimate of external surface area. c) Growth Rate o/Intact Seedlings. Those seedlings which had not been randomly selected to provide the excised roots were each removed from the growth medium and were carefully blotted and weighed. The length of the primary root and its diameter at three points along its length were rapidly measured before the seedling was returned again to the growth culture solution. Twenty four hours later the above procedure was repeated. Weighed samples of leaf and root tissue were then taken, digested in a nfinimum of cone. HNO 3 and diluted to 10 ml for K + analysis.
Potassium Translocation in Seedlings
229
These seedlings showed a marked development of lateral roots on the upper part of the primary root. A visual estimate of the proliferation of these laterals was made by classifying each seedling into one of three categories having respectively slight, moderate or heavy lateral root development. Careful measurement and counting on typical examples of each of these categories gave estimates of the lateral root surface area. These values together with the measurements of root length and diameter, allowed an estimate to be made of the total root surface area and root volume of each seedling.
Results I n t h e T a b l e are listed t h e m e a n values =L t h e s t a n d a r d errors of t h e w a t e r a n d K + e x u d a t i o n fluxes for t h e excised roots. F i f t y r e p l i c a t e d m e a s u r e m e n t s were used to find these m e a n values. The w a t e r fluxes were c o n v e r t e d d i r e c t l y from t h e readings of t h e e x u d a t e column heights a n d t h e K+ fluxes were o b t a i n e d f r o m t h e i n d i v i d u a l p r o d u c t s of w a t e r flux m u l t i p l i e d b y e x u d a t e c o n c e n t r a t i o n for each root. Table. The mean values o / t h e results on roots and seedlings
10 em roots
Seedling roots Seedling shoots
Water flux (~l' cm -2. h -i)
K flux (nmoles- em -z. h -i)
14.4=~ 1.6
307.2~= 28.1
Wet weight increase (mg" h -1)
K growth requirement (nmoles. cm -2. h -i)
1.22 • 0.21 4.09 • 0.47
137.4 =L29.2 387.4 =L63.7
The K growth requirements of the seedlings have been normalised over unit area of the seedling root system. The T a b l e also shows t h e r a t e s of wet weight increase for t h e shoots a n d r o o t s of t h e seedlings allowed to continue growing d u r i n g t h e t i m e w h e n t h e r o o t s f r o m c o m p a r a b l e seedlings h a d been excised a n d allowed to exude. T h e s e p a r a t i o n i n t o r o o t increase a n d shoot increase was a c h i e v e d as follows. T h e e x t e r n a l dimensions of t h e r o o t s were r e c o r d e d a t t h e t i m e t h e seedlings were weighed a n d t h e volumes of t h e r o o t s were d e d u c e d b y s u i t a b l e a p p r o x i m a t i o n s to g e o m e t r i c a l solids. T h e v o l u m e difference f r o m one d a y to t h e n e x t gave t h e e s t i m a t e d increase in r o o t wet weight a n d t h e difference in t h e weights of t h e seedling less this r o o t increase was a s s u m e d to give t h e w e t weight increase of t h e shoot. The m o s t serious s h o r t c o m i n g of this m e t h o d of a p p r o a c h is t h e a s s u m p t i o n t h a t t h e weight of t h e seed, t h e largest single compon e n t of t h e weight of t h e seedling, does n o t a l t e r over t h e 24 h period.
230
W. P. Anderson and E. Allen:
I t does not seem feasible to test possible variation in the seed weight experimentally. There is a very large variation from one seed to the next so t h a t the standard error associated with the mean seed weight on any one day is sufficiently large to completely mask what would be, one imagines, a rather small alteration in the mean weight from one day to the next. The rates of K+ uptake to the tissue are obtained by multiplying the wet weight increase b y the K + concentration in the tissue, estimated from digested samples of root and shoot as described in the Methods section. This of course implies t h a t the K + concentrations remain constant throughout the experimental period and t h a t the only net uptake of K + which occurs is t h a t necessary to maintain the newly formed tissue at those K + levels. A series of determinations of K+ concentrations in shoots and roots of seedlings from growth day 3 to growth day 7 showed t h a t this assumption is justifiable and therefore it is thought t h a t the quoted values are good estimates of the net K+ uptake.
Discussion The principal object of the experiments reported here was to discover how closely the salt exudation flux produced b y excised roots agrees with the rate of translocation of salt in intact seedlings of identical age and history as the roots. The measured values reported in the Results section show there to be a close correspondence in the K+ fluxes in the two eases. There is no statistically significant difference between them. Thus it must be supposed t h a t there is some degree of integration between the growth rate of maize seedlings and the rate of K + translocation to the shoots, and t h a t the transport mechanisms which provide this K + flux continue to function at the rate previously required b y growth even after the root has been excised. There is still some controversy over the details of the mechanism b y which a root transports ions from the external medium to the xylem sap although the consensus of opinion seems to be t h a t the initial uptake occurs at the plasmalemma of the outer cortical and epidermal cells; Laties (1969), Welch and Epstein (1969). Recently a quantitative analysis of symplasmie transport (Tyree, 1970) has enabled one to gauge for the first time the resistance of this pathway to ion fluxes. Although all the relevant anatomical data are not at present available for maize, it does seem likely t h a t the K + flux in the ease investigated here flows principally through the symplasm, from the initial uptake into the cortical cytoplasm through to the parenchyma cells of the stele. Whether the stele is leaky to ions as supposed b y Crafts and Broyer (1938) or whether the stele is physiologically active in secreting ions to the xylem as some now think (Epstein and L/~uehli, private communication) has
Potassium Translocation in Seedlings
231
still to be resolved. As Tyree (1970) pointed out, rate control of ion transport across the root and into the xylem can be effected either b y controlling the initial cortical uptake of ions or the final efflux of ions from the symplasm into the xylem. Wherever the control m a y exist our evidence is t h a t the K + flux from the xylem of excised roots is equal to the K+ growth requirement of the shoot of the seedling from which the root has been excised. A comparison of the water flux exuded b y the excised roots and the rate of wet weight increase in the shoot shows t h a t b y root pressure alone these excised roots maintain a flow of water which is 2.1 times the growth requirement for water of the shoot. The excess water must be lost by evaporation or conceivably be re-circulated in the phloem. Although the seedlings were grown in a high humidity environment guttation droplets were seldom seen on the coleoptile sheaths (the leaves had not emerged from the sheath b y the end of the experimental period) and it does seem likely t h a t some transpiration did occur. The excess water flux produced b y root pressure m a y have been lost in this way. I t is worth noting t h a t the rate of evaporation required is very small, a matter of 2 to 3 ~ l . h -1 over the surface area of the eoleoptile sheath, an area of approximately 10 cm ~. The second possibility m u s t also be considered, t h a t there is a net efflux of water from the shoot in the phloem. There is no experimental evidence for such a movement and such evidence as m a y be adduced is against this possibility. Firstly the seedlings are kept in the dark and are presumably not photosynthesising and secondly Anderson and Long (1968) found no evidence for net water movement in the phloem of similar roots, although their evidence is probably equivocal. Phloem re-circulation, if it operated here, would also be an obstacle to direct comparison of the K+ fluxes in the two cases. If K+ were circulated down the phloem in the intact seedling and not in the excised root, then the xylem exudate would not be a good measure of the net K + flux translocated from root to shoot in the seedlings. I t does seem to be a good estimate however and exactly meets the K + uptake of the growing shoot so t h a t this correspondence leads to the simplest conclu. sion t h a t the phloem is not translocating a net K + flux in the seedlings studied here. We should like to acknowledge the financial support of this work by the Science Research Council and to thank Miss Louise Goodwin for her technical assistance.
References Anderson, W. P., Aikman, D. P., Meiri, A.: Excised root exudation a standing gradient osmotic flow. Proc. roy. Soc. B1. 174, 445---458 (1970). - - Long, J.: Longitudinal water movement in the primary root of Zea mays. J. exp. Bot. 19, 637--647 (1968).
232
W . P . Anderson and E. Allen: Potassium Translocation in Seedlings
Arisz, W. H., Helder, R. J., Nie, R. van: Analysis of the exudation process in tomato plants. J. exp. Bot. 2, 257--297 (1951). Baker, D. A., Weatherley, P. E.: Water and solute transport by exuding root systems of Riclnus communis. J. exp. Bot. 29, 485--496 (1969). Crafts, A. S., Broyer, T. C." Migration of salts and water into xylem of the roots of higher plants. Amer. J. Bot. 24, 515--531 (1938). House, C. R., Findlay, N. : Water transport in isolated maize roots. J. exp. Bot. 17, 3 ~ 354 (1966). Laties, G. G.: Dual mechanisms of salt uptake in relation to compartmentation and long distance transport. Ann. Rev. Plant Physiol. 2{}, 89--116 (1969). Morrison, T.M.: Xylem sap composition in woody plants. Nature (Lond.) 2{}5, 1027 (1965). Tyree, M. T.: The symplast concept. A general theory of symplastic transport according to the thermodynamics of irreversible processes. J. theor. Biol. 20, 181--214 (1970). Welch, R. M., Epstein, E.: The dual mechanisms of alkali cation absorption by plant cells: their parallel operation across the plasmlemma. Proc. nat. Acad. Sci (Wash.) 61, 447--453 (1969). Dr. W. P. Anderson University of Liverpool Department of Botany PO Box 147 Liverpool 3, England
9 by Springer-Verlag 1970
A Comparison of Potassium Translocation in Excised Roots and Intact Maize Seedlings W. P. A ~ D ~ s o ~ and E. AL~.~N University of Liverpool, Department of Botany, Liverpool 3, England Received May 15, June 6, 1970 Summary. The water and potassium fluxes exuded by excised primary roots of maize seedlings are compared with the rate of wet weight increase and of potassium uptake by the growing tissue of the shoots of intact seedlings. The exuded potassium flux from 1 mM KC1 solution as the bathing medium is found to correspond very closely to the potassium uptake by the shoots of seedlings of identical age and history as the excised roots.
Introduction
Physiological studies of exudation from excised root preparations have long been a popular and useful technique in efforts at elucidating the mechanism of root pressure exudation; e.g. Ariszetal. (1951), House and Findlay (1966), Baker and Weatherley (1966), Anderson et al. (1970). Although there are still various points of contention, the model generally agreed from these studies is that root pressure exudation proceeds principally b y an osmotic mechanism. Salts are accumulated b y the root into the xylem vessels, thus creating an osmotic pressure gradient between the xylem sap and the external solution which drives a flow of water into the xylem. This water flow eventually appears at the excised stump as the exudation flux. The rate at which an excised root exudes salt is measured b y the salt exudation flux, obtained (House and Findlay, 1966) as the product of the volume (water) exudation flux multiplied b y the salt concentration of the exudate fluid. I t is of interest to enquire how closely an ion flux exuded b y an excised root preparation resembles the ion flux translocated b y an intact plant. Morrison (1965) reports an experiment of this type with rooted cuttings of Salix vitellina and shows evidence for a sudden increase in the p82 content of xylem sap exuded immediately after excision, compared with t h a t of xylem sap extracted from a freshly detached leaf of an intact plant. Thus it is concluded t h a t the exuded phosphate flux is a poor, and overestimate of the phosphate flux translocated in the intact plant. These results contrast with those reported here for potassium flux. Several explanations of the discrepancy are available;
228
W. P. Anderson and E. Allen:
the physiological age of the maize shoots is different from t h a t of the leaves used b y Morrison. Secondly, phosphate has a n e x t r e m e l y complex biochemical r6]e a n d indeed Morrison ascribes the increased P 32 a c t i v i t y i n x y l e m sap to a secretion b y the root of organic phosphates, a n d thirdly, it m a y well be incorrect to assume t h a t the p32 labelled phosphate is everywhere i n tracer equilibrium w i t h i n the p l a n t , the a s s u m p t i o n implicit i n Morrison's assay technique. The work reported here is a n a t t e m p t to compare the K+ flux exuded b y excised roots with the rate of n e t K+ t r a n s l o c a t i o n from the root to the shoot of i n t a c t seedlings of identical age a n d history as the excised roots. F r o m such m e a s u r e m e n t s various inferences m a y be d r a w n regarding possible i n t e g r a t i o n b y the p l a n t of the rate of salt t r a n s l o e a t i o n (as measured on excised root preparations) a n d the growth rate. There are difficulties i n m a k i n g a comparison of this n a t u r e , the principal one being t h a t the a d d i t i o n a l driving force of t r a n s p i r a t i o n a l pull m a y operate on the x y l e m columns of the i n t a c t seedlings. This effects was minimised i n these experiments b y growing the seedlings i n the d a r k a n d i n a n enclosed v o l u m e of high h u m i d i t y .
Materials and Methods a) Growth o/ Material. Grains of Zea mays (ear. White Horse Tooth) were soaked for 4 hr in running tap water and were then set out to germinate on damp paper in trays kept at 25~ C in the dark. After 3 days the seedlings were arranged on an open mesh nylon net suspended over a container so that the roots dipped into a solution of 1.0 mM CaC12. The solution was continuously aerated with moist air and the seedlings were allowed to grow for a further 3 days in the dark at 25~ C. The seedlings were regularly examined and developing secondary roots were removed so that each seedling had only a primary root. b) Excised Root Preparation. Seedlings at the stage described above were randomly selected and 10 cm lengths of the roots were cleanly excised with a sharp razor blade on a wet glass plate. The cut ends were inserted and sealed with a wax:rosin (1:1, w/w) mixture i n ~ a 100 ~1 Microcap pipette and were mounted vertically on a perspex plate with a graduated scale. They were allowed to exude from an aerated solution of 1.0 mM KC1/0.1 mM CaCI2. The heights of the exudate columns were recorded at regular intervals and the exudate fluid was collected, weighed and diluted to l0 ml with distilled and deionised water for K+ analysis at 24 h and 36 h after setting up. The K + concentrations were estimated by comparison with standard solutions on a Unicam SP 90 A atomic absorption spectrephotometer. At the end of the exudation period the diameter of each root was measured at three points along its length using a travelling microscope with a Vernier scale, in order to make a good estimate of external surface area. c) Growth Rate o/Intact Seedlings. Those seedlings which had not been randomly selected to provide the excised roots were each removed from the growth medium and were carefully blotted and weighed. The length of the primary root and its diameter at three points along its length were rapidly measured before the seedling was returned again to the growth culture solution. Twenty four hours later the above procedure was repeated. Weighed samples of leaf and root tissue were then taken, digested in a nfinimum of cone. HNO 3 and diluted to 10 ml for K + analysis.
Potassium Translocation in Seedlings
229
These seedlings showed a marked development of lateral roots on the upper part of the primary root. A visual estimate of the proliferation of these laterals was made by classifying each seedling into one of three categories having respectively slight, moderate or heavy lateral root development. Careful measurement and counting on typical examples of each of these categories gave estimates of the lateral root surface area. These values together with the measurements of root length and diameter, allowed an estimate to be made of the total root surface area and root volume of each seedling.
Results I n t h e T a b l e are listed t h e m e a n values =L t h e s t a n d a r d errors of t h e w a t e r a n d K + e x u d a t i o n fluxes for t h e excised roots. F i f t y r e p l i c a t e d m e a s u r e m e n t s were used to find these m e a n values. The w a t e r fluxes were c o n v e r t e d d i r e c t l y from t h e readings of t h e e x u d a t e column heights a n d t h e K+ fluxes were o b t a i n e d f r o m t h e i n d i v i d u a l p r o d u c t s of w a t e r flux m u l t i p l i e d b y e x u d a t e c o n c e n t r a t i o n for each root. Table. The mean values o / t h e results on roots and seedlings
10 em roots
Seedling roots Seedling shoots
Water flux (~l' cm -2. h -i)
K flux (nmoles- em -z. h -i)
14.4=~ 1.6
307.2~= 28.1
Wet weight increase (mg" h -1)
K growth requirement (nmoles. cm -2. h -i)
1.22 • 0.21 4.09 • 0.47
137.4 =L29.2 387.4 =L63.7
The K growth requirements of the seedlings have been normalised over unit area of the seedling root system. The T a b l e also shows t h e r a t e s of wet weight increase for t h e shoots a n d r o o t s of t h e seedlings allowed to continue growing d u r i n g t h e t i m e w h e n t h e r o o t s f r o m c o m p a r a b l e seedlings h a d been excised a n d allowed to exude. T h e s e p a r a t i o n i n t o r o o t increase a n d shoot increase was a c h i e v e d as follows. T h e e x t e r n a l dimensions of t h e r o o t s were r e c o r d e d a t t h e t i m e t h e seedlings were weighed a n d t h e volumes of t h e r o o t s were d e d u c e d b y s u i t a b l e a p p r o x i m a t i o n s to g e o m e t r i c a l solids. T h e v o l u m e difference f r o m one d a y to t h e n e x t gave t h e e s t i m a t e d increase in r o o t wet weight a n d t h e difference in t h e weights of t h e seedling less this r o o t increase was a s s u m e d to give t h e w e t weight increase of t h e shoot. The m o s t serious s h o r t c o m i n g of this m e t h o d of a p p r o a c h is t h e a s s u m p t i o n t h a t t h e weight of t h e seed, t h e largest single compon e n t of t h e weight of t h e seedling, does n o t a l t e r over t h e 24 h period.
230
W. P. Anderson and E. Allen:
I t does not seem feasible to test possible variation in the seed weight experimentally. There is a very large variation from one seed to the next so t h a t the standard error associated with the mean seed weight on any one day is sufficiently large to completely mask what would be, one imagines, a rather small alteration in the mean weight from one day to the next. The rates of K+ uptake to the tissue are obtained by multiplying the wet weight increase b y the K + concentration in the tissue, estimated from digested samples of root and shoot as described in the Methods section. This of course implies t h a t the K + concentrations remain constant throughout the experimental period and t h a t the only net uptake of K + which occurs is t h a t necessary to maintain the newly formed tissue at those K + levels. A series of determinations of K+ concentrations in shoots and roots of seedlings from growth day 3 to growth day 7 showed t h a t this assumption is justifiable and therefore it is thought t h a t the quoted values are good estimates of the net K+ uptake.
Discussion The principal object of the experiments reported here was to discover how closely the salt exudation flux produced b y excised roots agrees with the rate of translocation of salt in intact seedlings of identical age and history as the roots. The measured values reported in the Results section show there to be a close correspondence in the K+ fluxes in the two eases. There is no statistically significant difference between them. Thus it must be supposed t h a t there is some degree of integration between the growth rate of maize seedlings and the rate of K + translocation to the shoots, and t h a t the transport mechanisms which provide this K + flux continue to function at the rate previously required b y growth even after the root has been excised. There is still some controversy over the details of the mechanism b y which a root transports ions from the external medium to the xylem sap although the consensus of opinion seems to be t h a t the initial uptake occurs at the plasmalemma of the outer cortical and epidermal cells; Laties (1969), Welch and Epstein (1969). Recently a quantitative analysis of symplasmie transport (Tyree, 1970) has enabled one to gauge for the first time the resistance of this pathway to ion fluxes. Although all the relevant anatomical data are not at present available for maize, it does seem likely t h a t the K + flux in the ease investigated here flows principally through the symplasm, from the initial uptake into the cortical cytoplasm through to the parenchyma cells of the stele. Whether the stele is leaky to ions as supposed b y Crafts and Broyer (1938) or whether the stele is physiologically active in secreting ions to the xylem as some now think (Epstein and L/~uehli, private communication) has
Potassium Translocation in Seedlings
231
still to be resolved. As Tyree (1970) pointed out, rate control of ion transport across the root and into the xylem can be effected either b y controlling the initial cortical uptake of ions or the final efflux of ions from the symplasm into the xylem. Wherever the control m a y exist our evidence is t h a t the K + flux from the xylem of excised roots is equal to the K+ growth requirement of the shoot of the seedling from which the root has been excised. A comparison of the water flux exuded b y the excised roots and the rate of wet weight increase in the shoot shows t h a t b y root pressure alone these excised roots maintain a flow of water which is 2.1 times the growth requirement for water of the shoot. The excess water must be lost by evaporation or conceivably be re-circulated in the phloem. Although the seedlings were grown in a high humidity environment guttation droplets were seldom seen on the coleoptile sheaths (the leaves had not emerged from the sheath b y the end of the experimental period) and it does seem likely t h a t some transpiration did occur. The excess water flux produced b y root pressure m a y have been lost in this way. I t is worth noting t h a t the rate of evaporation required is very small, a matter of 2 to 3 ~ l . h -1 over the surface area of the eoleoptile sheath, an area of approximately 10 cm ~. The second possibility m u s t also be considered, t h a t there is a net efflux of water from the shoot in the phloem. There is no experimental evidence for such a movement and such evidence as m a y be adduced is against this possibility. Firstly the seedlings are kept in the dark and are presumably not photosynthesising and secondly Anderson and Long (1968) found no evidence for net water movement in the phloem of similar roots, although their evidence is probably equivocal. Phloem re-circulation, if it operated here, would also be an obstacle to direct comparison of the K+ fluxes in the two cases. If K+ were circulated down the phloem in the intact seedling and not in the excised root, then the xylem exudate would not be a good measure of the net K + flux translocated from root to shoot in the seedlings. I t does seem to be a good estimate however and exactly meets the K + uptake of the growing shoot so t h a t this correspondence leads to the simplest conclu. sion t h a t the phloem is not translocating a net K + flux in the seedlings studied here. We should like to acknowledge the financial support of this work by the Science Research Council and to thank Miss Louise Goodwin for her technical assistance.
References Anderson, W. P., Aikman, D. P., Meiri, A.: Excised root exudation a standing gradient osmotic flow. Proc. roy. Soc. B1. 174, 445---458 (1970). - - Long, J.: Longitudinal water movement in the primary root of Zea mays. J. exp. Bot. 19, 637--647 (1968).
232
W . P . Anderson and E. Allen: Potassium Translocation in Seedlings
Arisz, W. H., Helder, R. J., Nie, R. van: Analysis of the exudation process in tomato plants. J. exp. Bot. 2, 257--297 (1951). Baker, D. A., Weatherley, P. E.: Water and solute transport by exuding root systems of Riclnus communis. J. exp. Bot. 29, 485--496 (1969). Crafts, A. S., Broyer, T. C." Migration of salts and water into xylem of the roots of higher plants. Amer. J. Bot. 24, 515--531 (1938). House, C. R., Findlay, N. : Water transport in isolated maize roots. J. exp. Bot. 17, 3 ~ 354 (1966). Laties, G. G.: Dual mechanisms of salt uptake in relation to compartmentation and long distance transport. Ann. Rev. Plant Physiol. 2{}, 89--116 (1969). Morrison, T.M.: Xylem sap composition in woody plants. Nature (Lond.) 2{}5, 1027 (1965). Tyree, M. T.: The symplast concept. A general theory of symplastic transport according to the thermodynamics of irreversible processes. J. theor. Biol. 20, 181--214 (1970). Welch, R. M., Epstein, E.: The dual mechanisms of alkali cation absorption by plant cells: their parallel operation across the plasmlemma. Proc. nat. Acad. Sci (Wash.) 61, 447--453 (1969). Dr. W. P. Anderson University of Liverpool Department of Botany PO Box 147 Liverpool 3, England
