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The Influence of Pretreatment of Barley Plants with K+ and Cl− Ions on Subsequent Upward Transport of Absorbed Na+ Ions
The Influence of Pretreatment of Barley Plants with K+ and CI- Ions on Subsequent Upward Transport of Absorbed Na+ Ions
J. J. M. HOOYMANS Botanical Laboratory, State University of Leiden, Nonnensteg 3, Leiden, Netherlands Received August 29, 1980 . Accepted December 28, 1980
Summary The effect of pretreating barley plants with K or Rb salts on the time course of upward translocation of absorbed Na+ ions was studied. It appeared that the moment at which Na+ ions entered the shoots, was advanced when the plants had previously absorbed K+ together with Cl-. No forward shift of the start of upward Na+ transport was observed when the plants were pretreated with a solution of K 2 S0 4 or RbCI. The effect appeared to depend on the lenght of the period during which KCl had been absorbed rather than on the total amount of KCl taken up within this period. The results are discussed in relation to the transport process in the symplast. Key words: Hordeum vulgare upward ion translocation, Na+.
Introduction In low salt barley plants translocation to the shoots of ions absorbed by the roots, begins or at least is considerable accelerated, a few hours after the absorption process has started. The lag phase in upward transport is remarkably independent of the uptake rate at the plasmalemma, whether the 13!tter is manipulated by changes of substmte concentration or addition of Ca ions (Hooymans, 1968, 1971). This study deals with the question whether or not the duration of the lag phase in upward transport of Na+ can be changed by pretreatment of the roots with K+ or Rb+ ions. It will be shown that under certain conditions such a change can be effected. Material and Methods Intact plants of barley (Hordeum vulgare L. c. v. Effendi) were used for the experiments. The plants were grown as follows. After desinfection in 1 Ofo HgCI 2 , the seeds were rinsed in running tap water for half an hour and then soaked for 24 hours in demineralized water with continuous aeration at 25 DC. Thereafter germinanion of the seeds was continued on moist aseptic gauze supported by a stainless steel grid over a solution of 2.10- 4 M CaS04. Two days later the seedlings were mounted in groups of 10 on small PVC grids and transferred to a nutrient solution containing 0.171 mM Ca(NO a)2.4 H 20, 0.075 mM Ca(H 2P0 4)2, 0.0975
z. PJlanzenphysiol. Bd.102. 5.157-165. 1981.
158
J. J. M. HOOYMANS
mM MgS0 4 · 7 H 20, 0.05 mM KNO a, 0.01 mM EDTA-Fe-Na and 1 m! Hoagland A-Z solution per l,itre, and a light/dark regime of 16/8. After three days, this solution was replaced by one of 2· 10-4 M CaS04' On the following day the seedlings were used for the experiment. In the experiment a set of 10 seedlings being treated as a single plant. Each set of seedlings was placed on a polyethylene bottle containing 10 litres of an experimental solution under aeration with compressed air and a temperature of 25°C. Experimental solutions were buffered at pH 7.2 by addition of 0.1 mM Ca (HCO a)2' After the uptake period the roots were rinsed with demineralized water during one minute. Roots and shoots were analysed separately. K, Na and Rb contents were determined with an atomic spectrophotometer. Since considerable variation in the moment of upward translocation occurred between the different plant cultures each set of results presented in this paper was obtained in a single experiment. However, all data have been checked on reproducibility; replicate experiments confirmed the results presented here.
Results A biphasic time curve showing a more or less abrupt decline of the rate of absorption to a new steady value after about 2 to 3 hours, is characteristic for the uptake of monovalent ions by roots of low salt barley plants. This time curve has been interpreted as follows. During the first few hours a relative small root compartment represented by the root symplast is filled. Transport to the shoots is absent or only small during this period and vacuolar accumulation mayor may not occur (Hooymans, 1964, 1968; Bange, 1977). After the first few hours, when transport to the shoots starts or considerable accelerates, the increase in root ion content represents solely vacuolar accumulation by the root cells. In a first experiment plants were pretreated during 3 hours in a solution containing 0.01 mM K 2 50 4 and 5 mM CaCI 2 • Then their Na+ absorption from a solution of 5 mM Na 2504 + 5 mM Ca50 4 and the distribution of absorbed Na+ over roots and shoots was followed over a period of 41/ 2 hours and compared to the Na+ uptake and distribution by unpretreated plants (Fig. 1 A). It appeared that in both sets of plants the overall uptake rMe of Na+ as well as the rate of vacuolar accumulation were almost the same. However the transfer of Na+ ions to the shoots started about 11/2 hours earlier in plants pretreated in the KCI solution. In a separate set of plams the K+ uptake from the pretreatment solution was determined over a period of 8 hours (Fig. 1 B). The K+ content of the roots did not show a further increase after an uptake period of 3 hours. The K+ ions absorbed after this period being transported exclusively to the shoots. This indicates that vacuolar K+ accumulation was lacking under these circumstances. 50 most likely the forward shift of the start of upward Na+ translomtion is the resuh of the presence of K+ in the cytoplasm. In the next experiment the plants were pretreated in a solution containing 0.01 mM K 2 504 + 5 mM Ca50 4 (Fig. 2 A). It appeared that after substitution of 504 2- for CIin the pretreatment solution the moment at which Na+ started to be transported to the shoots was not advanced. With 50 4 2 - as the only anion present there was Z. Pjlanzenphysiol. Bd. 102. S. 157-165. 1981.
Upward ion translocation in barley
Na
mmol/kg. fro wI.
A
K
mmol/kg. fro wi.
159
B
30
012345678012345678 hours
Fig. 1 A: Time curve of absorption of Na+ into the roots (open symbols) and of transport of Na+ to the shoots (full symbols) of intact barley plants. The solution contained 5 mM Na 2 S04 + 5 mM CaCI 2• Plants pretreated during 3 hours in a solution of 0.01 mM K.S04 + 5 mM CaCl. (wiangles) were compared to unpretreated plants (circles). Fig. 1 B: Time curve of absorption of K+ into the roots (open circles) and of transport of K+ to the shoots (full circles) of intact barley plants. The solution contained 0.01 mM K 2S0 4 + 5 mM CaCI 2 • After 3 hours part of the plants was transferred to the Na+ solution mentioned under A and the change in the K content of the roots (open triangles) and shoots (full triangles) followed during Na+ uptake.
considerable vacuolar accumulation of K+ from the pretreatment solution (Fig. 2 B). Consequently, the K+ content of the cytoplasm may have been lower thQ.n in the presence of CI- (Fig. 1 B). Nevertheless, the lack of effect is not the result of a lower K+ content of the cytoplasm. This is demonstrated by the fact that upward Na+ translocation was not advanced when in the absence of Cl- ions the K+ content of the cy,t opl,asm was raised markedly. This condition was realized by pretreating the plants in a solution containing 5 mM K.S04 + 5 mM CaS04 during 3 hours (Fig. 3 A and 3 B).
When the plants are pretreated in a solution of 5 mM K 2S0 4 + 5 mM CaS04 and CI- is supplied together with Na+ in the absorption solution of 5 mM Na 2 S04 + 5 mM CaCl 2, no shift of the moment at which upward Na+ translocation starts, is observed (Fig. 4). So, apart from K+, CI- has to be present in the pretreatment solution to evoke the phenomenon. The effect depends on the duration of the pretreatment. This is demonstrated by an experiment in which plants were pretreated in a solution of 5 mM K 2SO. + 5 mM CaCl 2 during either 1 or 3 hours and then transferred to a solution containing 5 mM Z. Pjlanzenphysiol. Bd. 102. S. 157-165.1981.
160
J. J. M. HOOYMANS Na
mmol/kg. Ir. wI.
A
K
mmol/kg.lr. wt.
B
012345678012345678 hours
Fig. 2 A : Time curve of absorption of Na+ into the roots (open symbols) and transport of Na+ to the Shoots (full symbols) of intact barley plants. The solution contained 5 mM Na 2S0 4 + 5 mm CaS0 4 • Plants pretreated during 3 hours in a solution of 0.01 mM K 2S0 4 + 5 mM CaS0 4 (triangles) were compared to unpretreated plants (circles). Fig. 2 B: Time curve of absorption of K+ into the roots (open circles) and of transport of Kt to the shoots (full circles) of intact barley plants. The solution contained 0.01 mM K 2 S0 4 + 5 mM CaS04' After 3 hours pan of the plants was transferred to the Na+ solution mentioned under A and the change in the K+ content of the roots (open triangles) and shoots (full triangles) followed durin g Nat uptake.
Na 2S04 + 5 mM CaCl 2 • The distribution of absorbed Na+ over roots and shoots of these plants was compared to that of unpretreated plants (Fig. 5 A). It appears that the effect is present only after a pretreatment period of 3 hours though the amount of K+ in the cytoplasm after 1 hour pretreatment in 5 mM K 2 S0 4 + 5 mM CaCb (Fig. 5 B) exceeds the amount of K+ present in this compartment after pretreatment during 3 hours in 0.01 mM K 2S0 4 + 5 mM CaCI 2 (Fig. 1 B) when the effect is observed. The phenomenon is also dependent of the nature of the cation since substitution of RbCI for KCI during the pretreatment period abolished the advancing effect (Fig. 6 A and 6 B). Discussion The results show that, from the combination of cations and anions (K+, Rb+, CI-,
SOl- ) tested, only the pretreatment of K+ together with CI- caused a forward shift of the moment at which Nat ions absorbed by the roots were transported to the shoots. Furthermore, the effect appeared to bear no direct causal relation to the amount of K+ and CI- absorbed during the pretreatment. Z. Pflanzenphysiol. Bd. 102. S. 157-165. 1981.
Upward ion translocation in barley
Na mmol/kg.lr. wI.
A
K
mmol/kg.lr. wI.
161
B
30
12345678012345678 hours
Fig. 3 A: Time curve of a,bsorption of Na+ into the roots (open symbol) and of transport of Na+ to the shoots (full symbols) of intact barley plants. The solution contained 5 mM Na 2 S0 4 + 5 mM CaS04 • Plants pretreated during 3 hours in a solution of 5 mM K 2S04 + 5 mM CaS04 (triangles) were compared to unpretreated plants (circles). Flig. 3 B: Time curve of absorption of K+ into the roots (open circles) and of transport of K' to the shoots (full circles) of intact barley plants. The solution contained 5 mM K 2S0 4 + 5 mM CaS04' After 3 hou'rs p art of the plants was transferred to the Na+ solution mentioned under 3 A and the change in the K + content of the roots (open triangles) and shoots (full triangles) followed during Na+ uptake.
Questions pertinent to these results concern the way in which Na+ is transported through the cortex cells to the xylem vessels and the mechanism by which K+ together with CI-, when present in the cytoplasm, do influence this Na+ transpOl't in such a way that the rate of Na+ release to the xylem vessels is not influenced but the moment alt which this process starts, is advanced. Diffusion is among the processes by which ions can be translocated in the cytoplasm from the absorbing plasmalemmata to the vacuoles and xylem vessels. The diffusion rate is dependent on the gradient of the electrochemical potential. Higher absorption rates from higher external concentrations may be expected to result in higher gradients. Nevertheless, as mentioned in the introduction, the lenght of the lag phase in upward translocation is independent of the external concentration. Therefore diffusion as a retarding factor between absorption from the medium and release to the vessels is less likely. Another possibility is that the ions are transported in the symplast by mass flow. In this case they would move from the uptake sites at the epidermis (Bange, 1973) to Z. Pflanzenphysiol. Bd. 102. S. 157-165. 1981.
162
]. ]. M.
HOOYMANS
Na mmol/kg.lr. wI.
A
K
mmol/kg. Ir. wI.
B
12345678012345678 hours
4 A: Time curve of absoprtion of Na+ into the roots (open symbols) and of transport of Na+ to the shoots (fuJI symbols) of intact barley plants. The solution contained 5 mM Na2S04 + 5 mM CaCL2• Plants pretreated during 3 hours in a solution of 5 mM K 2S04 + 5 mM CaS0 4 (triangles) were compared to unpretreated plants (circles). F~g.
Fig. 4 B: Time curve of absorption of K+ into the roots (open circles) and of transport of K+ to the shoots (full circles) of intact barley plants. The solution contained 5 mM K 2S0 4 + 5 mM CaS0 4 • After 3 hours part of the plants was transferred to the Na+ solution mentioned under 4 A and the change in the K+ content of the rootS (open triangles) and shoots (fuJI triangles) followed durin g Na+ uprake.
the release sites in the stele as ,an advancing front. The lag phase could then be a reflection of the time this front needs to cover this distance. However, autoradiographic studies (Van Iren et a!., in prep.) do not give any support to such an explanation. On the contrary, they indicate that the ions absorbed are present in the whole root cortex and endodermis long before upward translocation starts. In view of these facts it seems worthwhile to consider cytoplasmic compartmentation as an alternative explanation for the phenomenon discussed. Obviously, if the symplast were a homogeneous ion pool between the uptake and release sites whose ion content increases gradually during the initial phase of absorption, it would be difficult to visu.alize a lag phase whose lenght is quite independent of the rate of uptake. On the other hand, if at the start of uptake ions entering the cytoplasm would be accumulated mainly into cytoplasmic compartments giving no direct acces to the sites of release at the xylem vessels, the moment at which sufficient ions would become available for release, would be delayed. Within this concept the phenomenon reported above might be understood from the assumption that preliminary sa'turation of the cytoplasmic compartments involved Z. P/lanzenphysiol. Bd.102. S. 157-165 . 1981.
Upward ion translocation in barley
Na
mmollkg. fr. wI.
A
K
mmoll kg. fr. wt.
163
B
40
3
10
10
12345678012345678 hours
Fig. 5 A: Time curve of absorption of Na+ into the roots (open symbols) and of transport of Na+ to the shoots (full symbols) of intact barley plants. The solution contained 5 mM Na2S0, + 5 mM CaCI 2 • Plants pretreated during 1 hour (squares) or 3 hours (triangles) in a solution of 5 mM K 2 S0 4 + 5 mM CaCl 2 were compared to unpretreated plants (circles). Fig. 5 B: Time curve of absorption of K+ into the roOts of intact barley plants (circles). The solution contained 5 mM K 2S0 4 + 5 mM CaCI 2 • After 1 hour (squares) and 3 h(;lUrs (triangles) part of the plants was transferred to the Na+ solution mentioned under 5 A and the change in the K+ content of the rOOts followed during Na+ uptake.
with KCI blocks the entrance of Na+ ions and thus results in a shorter lag phase in upward Na+ transport. The observation that neither K 2 SO, nor RbCI pretreatment has any advancing effect, then implies that from K 2S04 and RbCl no ions are accumulated into these organels. In that case the time course of saturation of the cytoplasm and transport to the shoots of K+ from a K 2SO, solution and Rb from a 'RbCl solution might be expected to differ from the pattern observed with KCI. However, this is not the case (Hooymans, 1968 b; Bange, 1978). A second objection to the interpretation proposed is that K+ present in the cytoplasm is translocated to the shoots at a high rate after transfer of the plants from the pretreatment solution to the Na+ solution (Fig. 1 B) and thus makes room for the entrance of Na+ ions. There is autoradiographic evidence that in some way or other the endoplasmic reticulum is involved in ion transport (for Tl+: van Iren et a!., 1975; for Cl-: Stelzer, Z. Pjlanzenphysiol. 8d. 102. S. 157-165. 1981.
164
J. J.
M. HOOYMANS Na mmol/kg. fro wI.
A
Rb mmol/kg. fro wI.
B
40
40
30
20
10
12345678012345678 hours
Fig. 6 A: The time curve of absorption of Na+ into the roots (open symbols) and of transport of Na+ to the shoots (full symbols) of intact barley plans. The solution contained 5 mM Na 2 S04 + 5 mM CaCI 2 • Plants pretreated during 3 hours in a solution of 0.05 mM Rb 2 S0 4 + 5 mM CaCl2 (triangles) were compared to unpretreated plants (circles). Fig. 6 B: Time curve of absorption of Rb+ into the roots of intact barley plants (circles). The solution contained 0.05 mM Rb 2 S0 4 + 5 mM CaCI 2 • After 3 hours part of the plants was transferred to the Na+ solution mentioned under 6 A and the change in the Rb+ content of the roots (triangles) followed during Na+ uptake. 1975; Uiuchli, 1976; Robards, 1976). However, at the moment too little data are available concerning the function of the ER in the translocation process and the exact mechanism involved to frame an hypothesis for the explanation of the phenomenon observed. Acknowledgements Thanks are due to Miss G.
J. J. Logman for
technical assistance.
References BANGE, G. G. J.: Acta Bot. Neerl. 22, 529-542 (1973). - Acta Bot. Neerl. 26, 53-62 (1977). - Acta Bot. Neerl. 27,183-198 (1978). HOOYMANS, J. J. M.: Acta Bot. Neerl. 13,507-540 (1964).
Z. P/lanzenphysiol. Bd.102. S.157-165. 1981.
Upward ion translocation in barley
165
- Plant and Soil 1, 92-101 (1968 a). - Acta Bot. Neerl. 17, 313-319 (1968 b). - Z. Pflanzenphysiol. 65, 309-314 (1971). IREN, F. VAN and A. VAN DER SPIEGEL: Science 187, 1210-1211 (1975). LAUCHLJ, A.: In: WARDLAW, 1. F., and]. B. PASSIOURA (Eds.): Transport and Transfer Processes in Plants, pp. 101-112. Academic Press, New York, San Franc,isco, London, 1976. ROBARDS, A. W.: In: GUNNING, B. E. S. and A. W. ROBARDS (Eds.): Intercellular Communication in Plants: Studies on Plasmadesmata, 15-57. Springer, Berlin, Heidelberg, New York,1976. STELZER, R., A. LAUCHLI, and D. KRAMER: Cytobiologie 10,449-457 (1975).
Z. Pjlanzenphysiol. Ed. 102. S. 157-165. 1981.
J. J. M. HOOYMANS Botanical Laboratory, State University of Leiden, Nonnensteg 3, Leiden, Netherlands Received August 29, 1980 . Accepted December 28, 1980
Summary The effect of pretreating barley plants with K or Rb salts on the time course of upward translocation of absorbed Na+ ions was studied. It appeared that the moment at which Na+ ions entered the shoots, was advanced when the plants had previously absorbed K+ together with Cl-. No forward shift of the start of upward Na+ transport was observed when the plants were pretreated with a solution of K 2 S0 4 or RbCI. The effect appeared to depend on the lenght of the period during which KCl had been absorbed rather than on the total amount of KCl taken up within this period. The results are discussed in relation to the transport process in the symplast. Key words: Hordeum vulgare upward ion translocation, Na+.
Introduction In low salt barley plants translocation to the shoots of ions absorbed by the roots, begins or at least is considerable accelerated, a few hours after the absorption process has started. The lag phase in upward transport is remarkably independent of the uptake rate at the plasmalemma, whether the 13!tter is manipulated by changes of substmte concentration or addition of Ca ions (Hooymans, 1968, 1971). This study deals with the question whether or not the duration of the lag phase in upward transport of Na+ can be changed by pretreatment of the roots with K+ or Rb+ ions. It will be shown that under certain conditions such a change can be effected. Material and Methods Intact plants of barley (Hordeum vulgare L. c. v. Effendi) were used for the experiments. The plants were grown as follows. After desinfection in 1 Ofo HgCI 2 , the seeds were rinsed in running tap water for half an hour and then soaked for 24 hours in demineralized water with continuous aeration at 25 DC. Thereafter germinanion of the seeds was continued on moist aseptic gauze supported by a stainless steel grid over a solution of 2.10- 4 M CaS04. Two days later the seedlings were mounted in groups of 10 on small PVC grids and transferred to a nutrient solution containing 0.171 mM Ca(NO a)2.4 H 20, 0.075 mM Ca(H 2P0 4)2, 0.0975
z. PJlanzenphysiol. Bd.102. 5.157-165. 1981.
158
J. J. M. HOOYMANS
mM MgS0 4 · 7 H 20, 0.05 mM KNO a, 0.01 mM EDTA-Fe-Na and 1 m! Hoagland A-Z solution per l,itre, and a light/dark regime of 16/8. After three days, this solution was replaced by one of 2· 10-4 M CaS04' On the following day the seedlings were used for the experiment. In the experiment a set of 10 seedlings being treated as a single plant. Each set of seedlings was placed on a polyethylene bottle containing 10 litres of an experimental solution under aeration with compressed air and a temperature of 25°C. Experimental solutions were buffered at pH 7.2 by addition of 0.1 mM Ca (HCO a)2' After the uptake period the roots were rinsed with demineralized water during one minute. Roots and shoots were analysed separately. K, Na and Rb contents were determined with an atomic spectrophotometer. Since considerable variation in the moment of upward translocation occurred between the different plant cultures each set of results presented in this paper was obtained in a single experiment. However, all data have been checked on reproducibility; replicate experiments confirmed the results presented here.
Results A biphasic time curve showing a more or less abrupt decline of the rate of absorption to a new steady value after about 2 to 3 hours, is characteristic for the uptake of monovalent ions by roots of low salt barley plants. This time curve has been interpreted as follows. During the first few hours a relative small root compartment represented by the root symplast is filled. Transport to the shoots is absent or only small during this period and vacuolar accumulation mayor may not occur (Hooymans, 1964, 1968; Bange, 1977). After the first few hours, when transport to the shoots starts or considerable accelerates, the increase in root ion content represents solely vacuolar accumulation by the root cells. In a first experiment plants were pretreated during 3 hours in a solution containing 0.01 mM K 2 50 4 and 5 mM CaCI 2 • Then their Na+ absorption from a solution of 5 mM Na 2504 + 5 mM Ca50 4 and the distribution of absorbed Na+ over roots and shoots was followed over a period of 41/ 2 hours and compared to the Na+ uptake and distribution by unpretreated plants (Fig. 1 A). It appeared that in both sets of plants the overall uptake rMe of Na+ as well as the rate of vacuolar accumulation were almost the same. However the transfer of Na+ ions to the shoots started about 11/2 hours earlier in plants pretreated in the KCI solution. In a separate set of plams the K+ uptake from the pretreatment solution was determined over a period of 8 hours (Fig. 1 B). The K+ content of the roots did not show a further increase after an uptake period of 3 hours. The K+ ions absorbed after this period being transported exclusively to the shoots. This indicates that vacuolar K+ accumulation was lacking under these circumstances. 50 most likely the forward shift of the start of upward Na+ translomtion is the resuh of the presence of K+ in the cytoplasm. In the next experiment the plants were pretreated in a solution containing 0.01 mM K 2 504 + 5 mM Ca50 4 (Fig. 2 A). It appeared that after substitution of 504 2- for CIin the pretreatment solution the moment at which Na+ started to be transported to the shoots was not advanced. With 50 4 2 - as the only anion present there was Z. Pjlanzenphysiol. Bd. 102. S. 157-165. 1981.
Upward ion translocation in barley
Na
mmol/kg. fro wI.
A
K
mmol/kg. fro wi.
159
B
30
012345678012345678 hours
Fig. 1 A: Time curve of absorption of Na+ into the roots (open symbols) and of transport of Na+ to the shoots (full symbols) of intact barley plants. The solution contained 5 mM Na 2 S04 + 5 mM CaCI 2• Plants pretreated during 3 hours in a solution of 0.01 mM K.S04 + 5 mM CaCl. (wiangles) were compared to unpretreated plants (circles). Fig. 1 B: Time curve of absorption of K+ into the roots (open circles) and of transport of K+ to the shoots (full circles) of intact barley plants. The solution contained 0.01 mM K 2S0 4 + 5 mM CaCI 2 • After 3 hours part of the plants was transferred to the Na+ solution mentioned under A and the change in the K content of the roots (open triangles) and shoots (full triangles) followed during Na+ uptake.
considerable vacuolar accumulation of K+ from the pretreatment solution (Fig. 2 B). Consequently, the K+ content of the cytoplasm may have been lower thQ.n in the presence of CI- (Fig. 1 B). Nevertheless, the lack of effect is not the result of a lower K+ content of the cytoplasm. This is demonstrated by the fact that upward Na+ translocation was not advanced when in the absence of Cl- ions the K+ content of the cy,t opl,asm was raised markedly. This condition was realized by pretreating the plants in a solution containing 5 mM K.S04 + 5 mM CaS04 during 3 hours (Fig. 3 A and 3 B).
When the plants are pretreated in a solution of 5 mM K 2S0 4 + 5 mM CaS04 and CI- is supplied together with Na+ in the absorption solution of 5 mM Na 2 S04 + 5 mM CaCl 2, no shift of the moment at which upward Na+ translocation starts, is observed (Fig. 4). So, apart from K+, CI- has to be present in the pretreatment solution to evoke the phenomenon. The effect depends on the duration of the pretreatment. This is demonstrated by an experiment in which plants were pretreated in a solution of 5 mM K 2SO. + 5 mM CaCl 2 during either 1 or 3 hours and then transferred to a solution containing 5 mM Z. Pjlanzenphysiol. Bd. 102. S. 157-165.1981.
160
J. J. M. HOOYMANS Na
mmol/kg. Ir. wI.
A
K
mmol/kg.lr. wt.
B
012345678012345678 hours
Fig. 2 A : Time curve of absorption of Na+ into the roots (open symbols) and transport of Na+ to the Shoots (full symbols) of intact barley plants. The solution contained 5 mM Na 2S0 4 + 5 mm CaS0 4 • Plants pretreated during 3 hours in a solution of 0.01 mM K 2S0 4 + 5 mM CaS0 4 (triangles) were compared to unpretreated plants (circles). Fig. 2 B: Time curve of absorption of K+ into the roots (open circles) and of transport of Kt to the shoots (full circles) of intact barley plants. The solution contained 0.01 mM K 2 S0 4 + 5 mM CaS04' After 3 hours pan of the plants was transferred to the Na+ solution mentioned under A and the change in the K+ content of the roots (open triangles) and shoots (full triangles) followed durin g Nat uptake.
Na 2S04 + 5 mM CaCl 2 • The distribution of absorbed Na+ over roots and shoots of these plants was compared to that of unpretreated plants (Fig. 5 A). It appears that the effect is present only after a pretreatment period of 3 hours though the amount of K+ in the cytoplasm after 1 hour pretreatment in 5 mM K 2 S0 4 + 5 mM CaCb (Fig. 5 B) exceeds the amount of K+ present in this compartment after pretreatment during 3 hours in 0.01 mM K 2S0 4 + 5 mM CaCI 2 (Fig. 1 B) when the effect is observed. The phenomenon is also dependent of the nature of the cation since substitution of RbCI for KCI during the pretreatment period abolished the advancing effect (Fig. 6 A and 6 B). Discussion The results show that, from the combination of cations and anions (K+, Rb+, CI-,
SOl- ) tested, only the pretreatment of K+ together with CI- caused a forward shift of the moment at which Nat ions absorbed by the roots were transported to the shoots. Furthermore, the effect appeared to bear no direct causal relation to the amount of K+ and CI- absorbed during the pretreatment. Z. Pflanzenphysiol. Bd. 102. S. 157-165. 1981.
Upward ion translocation in barley
Na mmol/kg.lr. wI.
A
K
mmol/kg.lr. wI.
161
B
30
12345678012345678 hours
Fig. 3 A: Time curve of a,bsorption of Na+ into the roots (open symbol) and of transport of Na+ to the shoots (full symbols) of intact barley plants. The solution contained 5 mM Na 2 S0 4 + 5 mM CaS04 • Plants pretreated during 3 hours in a solution of 5 mM K 2S04 + 5 mM CaS04 (triangles) were compared to unpretreated plants (circles). Flig. 3 B: Time curve of absorption of K+ into the roots (open circles) and of transport of K' to the shoots (full circles) of intact barley plants. The solution contained 5 mM K 2S0 4 + 5 mM CaS04' After 3 hou'rs p art of the plants was transferred to the Na+ solution mentioned under 3 A and the change in the K + content of the roots (open triangles) and shoots (full triangles) followed during Na+ uptake.
Questions pertinent to these results concern the way in which Na+ is transported through the cortex cells to the xylem vessels and the mechanism by which K+ together with CI-, when present in the cytoplasm, do influence this Na+ transpOl't in such a way that the rate of Na+ release to the xylem vessels is not influenced but the moment alt which this process starts, is advanced. Diffusion is among the processes by which ions can be translocated in the cytoplasm from the absorbing plasmalemmata to the vacuoles and xylem vessels. The diffusion rate is dependent on the gradient of the electrochemical potential. Higher absorption rates from higher external concentrations may be expected to result in higher gradients. Nevertheless, as mentioned in the introduction, the lenght of the lag phase in upward translocation is independent of the external concentration. Therefore diffusion as a retarding factor between absorption from the medium and release to the vessels is less likely. Another possibility is that the ions are transported in the symplast by mass flow. In this case they would move from the uptake sites at the epidermis (Bange, 1973) to Z. Pflanzenphysiol. Bd. 102. S. 157-165. 1981.
162
]. ]. M.
HOOYMANS
Na mmol/kg.lr. wI.
A
K
mmol/kg. Ir. wI.
B
12345678012345678 hours
4 A: Time curve of absoprtion of Na+ into the roots (open symbols) and of transport of Na+ to the shoots (fuJI symbols) of intact barley plants. The solution contained 5 mM Na2S04 + 5 mM CaCL2• Plants pretreated during 3 hours in a solution of 5 mM K 2S04 + 5 mM CaS0 4 (triangles) were compared to unpretreated plants (circles). F~g.
Fig. 4 B: Time curve of absorption of K+ into the roots (open circles) and of transport of K+ to the shoots (full circles) of intact barley plants. The solution contained 5 mM K 2S0 4 + 5 mM CaS0 4 • After 3 hours part of the plants was transferred to the Na+ solution mentioned under 4 A and the change in the K+ content of the rootS (open triangles) and shoots (fuJI triangles) followed durin g Na+ uprake.
the release sites in the stele as ,an advancing front. The lag phase could then be a reflection of the time this front needs to cover this distance. However, autoradiographic studies (Van Iren et a!., in prep.) do not give any support to such an explanation. On the contrary, they indicate that the ions absorbed are present in the whole root cortex and endodermis long before upward translocation starts. In view of these facts it seems worthwhile to consider cytoplasmic compartmentation as an alternative explanation for the phenomenon discussed. Obviously, if the symplast were a homogeneous ion pool between the uptake and release sites whose ion content increases gradually during the initial phase of absorption, it would be difficult to visu.alize a lag phase whose lenght is quite independent of the rate of uptake. On the other hand, if at the start of uptake ions entering the cytoplasm would be accumulated mainly into cytoplasmic compartments giving no direct acces to the sites of release at the xylem vessels, the moment at which sufficient ions would become available for release, would be delayed. Within this concept the phenomenon reported above might be understood from the assumption that preliminary sa'turation of the cytoplasmic compartments involved Z. P/lanzenphysiol. Bd.102. S. 157-165 . 1981.
Upward ion translocation in barley
Na
mmollkg. fr. wI.
A
K
mmoll kg. fr. wt.
163
B
40
3
10
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Fig. 5 A: Time curve of absorption of Na+ into the roots (open symbols) and of transport of Na+ to the shoots (full symbols) of intact barley plants. The solution contained 5 mM Na2S0, + 5 mM CaCI 2 • Plants pretreated during 1 hour (squares) or 3 hours (triangles) in a solution of 5 mM K 2 S0 4 + 5 mM CaCl 2 were compared to unpretreated plants (circles). Fig. 5 B: Time curve of absorption of K+ into the roOts of intact barley plants (circles). The solution contained 5 mM K 2S0 4 + 5 mM CaCI 2 • After 1 hour (squares) and 3 h(;lUrs (triangles) part of the plants was transferred to the Na+ solution mentioned under 5 A and the change in the K+ content of the rOOts followed during Na+ uptake.
with KCI blocks the entrance of Na+ ions and thus results in a shorter lag phase in upward Na+ transport. The observation that neither K 2 SO, nor RbCI pretreatment has any advancing effect, then implies that from K 2S04 and RbCl no ions are accumulated into these organels. In that case the time course of saturation of the cytoplasm and transport to the shoots of K+ from a K 2SO, solution and Rb from a 'RbCl solution might be expected to differ from the pattern observed with KCI. However, this is not the case (Hooymans, 1968 b; Bange, 1978). A second objection to the interpretation proposed is that K+ present in the cytoplasm is translocated to the shoots at a high rate after transfer of the plants from the pretreatment solution to the Na+ solution (Fig. 1 B) and thus makes room for the entrance of Na+ ions. There is autoradiographic evidence that in some way or other the endoplasmic reticulum is involved in ion transport (for Tl+: van Iren et a!., 1975; for Cl-: Stelzer, Z. Pjlanzenphysiol. 8d. 102. S. 157-165. 1981.
164
J. J.
M. HOOYMANS Na mmol/kg. fro wI.
A
Rb mmol/kg. fro wI.
B
40
40
30
20
10
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Fig. 6 A: The time curve of absorption of Na+ into the roots (open symbols) and of transport of Na+ to the shoots (full symbols) of intact barley plans. The solution contained 5 mM Na 2 S04 + 5 mM CaCI 2 • Plants pretreated during 3 hours in a solution of 0.05 mM Rb 2 S0 4 + 5 mM CaCl2 (triangles) were compared to unpretreated plants (circles). Fig. 6 B: Time curve of absorption of Rb+ into the roots of intact barley plants (circles). The solution contained 0.05 mM Rb 2 S0 4 + 5 mM CaCI 2 • After 3 hours part of the plants was transferred to the Na+ solution mentioned under 6 A and the change in the Rb+ content of the roots (triangles) followed during Na+ uptake. 1975; Uiuchli, 1976; Robards, 1976). However, at the moment too little data are available concerning the function of the ER in the translocation process and the exact mechanism involved to frame an hypothesis for the explanation of the phenomenon observed. Acknowledgements Thanks are due to Miss G.
J. J. Logman for
technical assistance.
References BANGE, G. G. J.: Acta Bot. Neerl. 22, 529-542 (1973). - Acta Bot. Neerl. 26, 53-62 (1977). - Acta Bot. Neerl. 27,183-198 (1978). HOOYMANS, J. J. M.: Acta Bot. Neerl. 13,507-540 (1964).
Z. P/lanzenphysiol. Bd.102. S.157-165. 1981.
Upward ion translocation in barley
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- Plant and Soil 1, 92-101 (1968 a). - Acta Bot. Neerl. 17, 313-319 (1968 b). - Z. Pflanzenphysiol. 65, 309-314 (1971). IREN, F. VAN and A. VAN DER SPIEGEL: Science 187, 1210-1211 (1975). LAUCHLJ, A.: In: WARDLAW, 1. F., and]. B. PASSIOURA (Eds.): Transport and Transfer Processes in Plants, pp. 101-112. Academic Press, New York, San Franc,isco, London, 1976. ROBARDS, A. W.: In: GUNNING, B. E. S. and A. W. ROBARDS (Eds.): Intercellular Communication in Plants: Studies on Plasmadesmata, 15-57. Springer, Berlin, Heidelberg, New York,1976. STELZER, R., A. LAUCHLI, and D. KRAMER: Cytobiologie 10,449-457 (1975).
Z. Pjlanzenphysiol. Ed. 102. S. 157-165. 1981.