Planta (Berl.) 86, 301--314 (1969)
The Absorption and Translocation of Sodium by Maize Seedlings M. G. T. StrOPhE, D. T. CLAUKSOX a n d J. SA~D~USO~ Agricultural Research Council Letcombe Laboratory (incorporating Radiobiological Laboratory) Wantage, Berkshire, U.K. Received March 3/April 14, 1969
Summary. The absorption and subsequent distribution of sodium and potassium has been examined in maize seedlings in short-term experiments using sodium-22 and potassium-42. The absorption and translocation of sodium by different segments of intact seedlings was also investigated. Although absorption of potassium exceeded that of sodium by a factor of about 50, there was no evidence that the entry of sodium was confined to a small region of the root. Determinations of the relative quantities of sodium and potassium in the xylem exudate of detached roots showed that the ratio of sodium to potassium decreased with increasing length of the root. These results suggested that upward movement of sodium in the xylem vessels was progressively reduced towards the basal part of the root. This conclusion was supported by microautoradiographs, which showed that although the concentration of sodium within the endodermis was greater than that in the cortex, there was an apparent decrease in the sodium content of the major xylem vessels at the basal end of the root. Introduction Most crop plants, a p a r t f r o m those of h a l o p h y t i c origin t r a n s l o c a t e o n l y small q u a n t i t i e s of s o d i u m to their stems a n d leaves. T h e m e c h a n i s m w h e r e b y these p l a n t s d i s c r i m i n a t e a g a i n s t s o d i u m is little u n d e r s t o o d a n d has v a r i o u s l y been a t t r i b u t e d to a) t h e a b i l i t y of r o o t s to r e t a i n large a m o u n t of sodium, so t h a t r e l a t i v e l y small q u a n t i t i e s pass into t h e t r a n s p i r a t i o n s t r e a m (BEn~STEI~, B n o w ~ a n d HAYWAm), 1956; PEAUSO~, 1967), b) a m e m b r a n e or o t h e r s t r u c t u r e which i n h i b i t s transl o c a t i o n of s o d i u m into the v a s c u l a r tissue of t h e r o o t (GAucI~ a n d WADLEIG~I, 1954; BERNSTEIN a n d H A Y W ~ D , 1958), or c) a w i t h d r a w a l of s o d i u m from t h e t r a n s p i r a t i o n s t r e a m i n t o living cells a d j a c e n t to t h e v a s c u l a r tissue (JAcoBY, 1965). On t h e basis of electrochemical m e a s u r e m e n t s , i t has also been suggested t h a t s o d i u m is a c t i v e l y e x c r e t e d b y b a r l e y roots (PIT~A~ a n d SADnL~U, 1967) a n d b y o t h e r p l a n t ceils ( J E t ? c i t e s , 1968). I n order to i n v e s t i g a t e these h y p o t h e s e s further, e x p e r i m e n t s h a v e been carried o u t on y o u n g seedlings of maize, a species which t r a n s l o c a t e s o n l y small a m o u n t s of s o d i u m to t h e leaves (tIuFFAKEU a n d WALLACe, 21 Planta (Berl.), Bd. 86
302
M . G . T . SHONE, D. T. CLAR~:SO~ and J. SANDERSON."
1959). I n some e x p e r i m e n t s th e u p t a k e of sodium was c o m p a r e d w i t h t h a t of t h e c h e m i c a l l y similar b u t physiologically co n t r ast i n g ion potassium, using sodium-22 a n d potassium-42 in s h o r t - t e r m studies in w a t e r culture. This p r o c e d u r e necessarily measures t h e influxes a n d n o t t h e n et q u a n t i t i e s of ions t a k e n u p in a g iv e n time. Th e r e l a t i v e q u a n t i t i e s of t h e t w o ions a b s o r b e d b y t h e roots an d t r a n s l o e a t e d to t h e shoots were m e a s u r e d in i n t a c t seedlings. Studies were also m a d e on t h e x y l e m e x u d a t e of d e t a c h e d roots of v a r y i n g l e n g t h in order to i n v e s t i g a t e t h e t r a n s p o r t of t h e t w o ions w i t h i n t h e r o o t to t h e t r a n s p i r a t i o n stream. M i c r o - a u t o r a d i o g r a p h s p r e p a r e d f r o m t r a n s v e r s e sections of roots of seedlings labelled w i t h sodinm-22 were e x a m i n e d to d e t e r m i n e if t h er e was some tissue in t h e r o o t which r e d u c e d t h e r a t e of m o v e m e n t of s o d i u m i n t o t h e stele, or a l t e r n a t i v e l y w h e t h e r t h er e was a n y evidence of a c o n c e n t r a t i o n of s o d i u m in t h e cells bordering t h e x y l e m vessels.
Methods In all experiments seeds of Zea Mays L. (John Innes F 1 hybrid) were germinated in the dark on moist filter paper and transferred to perforated black polythene discs suspended above a dilute culture solution 1. The seedlings were then grown for four days in a constant environment cabinet (20~ 1,200 tic. for 16 h daily). The seminal roots had then extended to about 20 cm in length and were either used in experiments on the absorption of labelled sodium and potassium or in autoradiographie studies. The uptake solution used in all the experiments contained 0.6 m. equiv./1 potassium and 0.2 m. equiv./1 sodium and calcium as chlorides. Solid plant material and root exudate were assayed for sodium-22 and potassium-42 by counting twice at an interval of 7 days, in a Beckman Lowbeta II end window counter or in a well-crystal y scintillator. Similar counts made on Miquots of the uptake solution enabled the results to be expressed as the quantities of sodium or potassium in ~ equivalents which had entered the plant material from the uptake solution. For the autoradiographie studies maize seedlings were grown for 24 h at 23~ under continuous illumination with their roots either totally or partially immersed in the uptake solution labelled with 1 ~4 ~Ci sodium-22 per ml. At the end of the treatment the roots of the intact plants were rinsed with distilled water then blotted with tissue. Segments 1 cm in length were cut from roots and held with fine forceps in the centre of a small well recessed into a block of frozen liver which was maintained at --25 ~C. The well was then flooded with a liquid hydrocarbon mixture at --8~ which solidified shortly after contact with the liver. The whole block was chilled further by contact with liquid nitrogen. The essential feature of this technique, which will be described in detail elsewhere, is that the root is supported by, but isolated from the sodium-rich liver tissue by a collar of inert material immiscible 1. Composition (m.equiv/1) : K +, 0.6; Ca++, 0.3; Mg++, 0.3; Na +, 0.2; N0'a, 1.0; SOt", 0.3; H2PO"t, 0.1; d- minor nutrients (Fe as EDTA complex, Mn, Zn, Cu, Mo, C1, B).
Absorption and Translocation of Sodium
303
with water. Failure to provide such isolation permits contact between the root and the cut surface of the liver with consequent exchange of sodium between the tissues prior to freezing. The methods used for cutting and picking up sections and for exposing Kodak AR 10 stripping film differed in only minor details from the procedures described by APPLETO~r (1966), CR0SSETT (1967), and ROGEP,S (1967). All these operations were performed at --20 to --25~ Details of other methods used in individual experiments are given in the relevant sections.
Results
1. Absorption o/ Sodium and Potassium by Intact Seedlings Seedlings with roots 20 cm long were sealed w i t h lanolin a t t h e b a s a l end of t h e r o o t i n t o p e r f o r a t e d discs of paraffin w a x a n d s u s p e n d e d a b o v e t h e labelled solution so t h a t t h e roots, a p a r t from t h e basal 1 cm, were in c o n t a c t w i t h t h e labelled u p t a k e solution. A t t h e end of t h e 18 h r u p t a k e period, t h e seedlings were r e m o v e d f r o m the solution, t h e r o o t s b l o t t e d a n d t h e seedlings d i v i d e d into shoots, seed a n d first i n t e r n o d e , five r o o t s e g m e n t s of equal length a n d 1 cm r o o t tip. Table 1. Absorption and translocation o~ labelled sodium and potassium in maize
seedlings After the uptake period of 18 hours, the seedlings were subdivided into shoots, seeds+ first internode, five equal portions of the 20 cm long roots and 1 cm root tip. Means of 6 replicates are shown with their standard errors. Tissue
Shoot Seed+ 1st internode Root seetion 1 2 3 4 5 Roottip
Concentration of labelled ions (gequiv. per g fresh weight) Na
K
0.003 J= 0.001 0.07 =h 0.01 1,24 J=0.19 0.62 /=0.08 0,45 =J=0,06 0.41 ~0.05 0.46 ~0.02 0.67 ~=0.13
7.01 ! 0.89 3.46 ~= 0.36 28.6 • 30.8 • 32,2 • 39.7 1 3 . 7 55.5 =k7.3 91.7 =t=19.6
Ratio Na/K
0.0005 ~ 0,0002 0.022 ~_ 0.005 0.041 /-0.005 0.019 ~0.003 0,015 ~0.007 0.010 ~0.002 0,010 ~0.003 0.006 ~0.001
T a b l e 1 shows m e a n values of t h e q u a n t i t i e s p e r g r a m fresh weight of labelled s o d i u m a n d p o t a s s i u m , a n d t h e r a t i o s of s o d i u m to p o t a s s i u m , in various p o r t i o n s of six m a i z e seedlings after a b s o r p t i o n for 18 hr. A l t h o u g h t h e q u a n t i t y of p o t a s s i u m a b s o r b e d b y t h e roots exceeds t h a t of s o d i u m b y one to t w o orders of m a g n i t u d e a n d t h e r a t i o of s o d i u m to p o t a s s i u m increases t o w a r d s the basal end of t h e roots, t h e d i s t r i b u t i o n of t h e two ions in t h e roots is b r o a d l y similar. T h e two ions c o n t r a s t 21"
304
M.G.T. S~O~E, D. T. CLARKSON and J. SANDERSON:
Fig. 1. Apparatus designed to study uptake by segments of intact plants in which the remainder of the plant is kept in an atmosphere of moist air. The "Perspex" holder A serves to support 6 seedlings B and allows 1.5 em lengths to be exposed to the uptake solution C. The aerator D maintains a moist atmosphere in the lower jar E. Air escapes through the capillary fitted into a hypodermic syringe barrel F which contains water. This ensures that the air in the upper vessel G is saturated with water, and at the same time maintains a small positive pressure within the lower jar E. This pressure helps to prevent breakdown and leakage from the paraffin wax - - rosin seals H. Air escapes through the hole I in the upper jar m a i n l y in t h e q u a n t i t i e s t r a n s l o e a t e d to t h e shoots; whereas t h e c o n t e n t of p o t a s s i u m in t h e shoots r e l a t i v e to t h a t in t h e b a s a l r o o t s e g m e n t is a b o u t 0.25, t h e corresponding r e l a t i v e c o n t e n t for s o d i u m is a b o u t 0.003. T h e r e is also a m a r k e d decrease in t h e r a t i o of s o d i u m to p o t a s s i u m in t h e p a r t s of t h e p l a n t s a b o v e t h e b a s a l s e g m e n t of t h e roots. I t seemed possible t h a t t h e e n t r y of s o d i u m m i g h t be localised to t h e region a r o u n d t h e r o o t tip, a n d t h a t this m i g h t lead to a r e d u c e d u p t a k e of this ion. A c c o r d i n g l y to s t u d y t h e a b s o r p t i o n of s o d i u m b y different p a r t s of t h e r o o t system, seedlings were sealed w i t h paraffin w a x / r o s i n m i x t u r e into small p e r s p e x cells containing 1 ml of labelled u p t a k e solution, t h e r e m a i n d e r of t h e roots a n d t h e shoots being k e p t in an a t m o s p h e r e of m o i s t air. B y this m e a n s a b o u t 1.5 em of t h e r o o t s were e x p o s e d to t h e labelled solution, which was a p p l i e d a t v a r y i n g
Absorption and Transloeation of Sodium
305
Table 2. Quantities o/labelled sodium absorbed and translocated ]rom 1.5 cm regions
of the roots of intact maize seedlings The labelled solution was applied for 18 hours at varying distances from the root tips; the remainder of the plants were maintained in moist air. Zone where sodium was applied
l~oot tip 2 . 5 4 . 0 em behind tip 10.0
11.5 cm behind tip
Total sodium absorbed (nano-equiv.)
14 21
Quantities of sodium in various sections (nano-equiv.) Treated segment
Below treated segment
Above treated segment
5.5 4.3
-0.7
8.5 16.0
51.5
24.6
~
86.9
Groups of means which do not differ significantly (P=0.05) are indicated by vertical lines.
distances from the root tip. The apparatus is illustrated and described in Fig. 1. T a b l e 2 shows t h a t t h e t o t a l q u a n t i t y of labelled s o d i u m a b s o r b e d b y t h e seedlings a n d t h e r e s p e c t i v e q u a n t i t i e s f o u n d in t h e t r e a t e d segment, a n d below a n d a b o v e this region increase with increasing d i s t a n c e f r o m t h e r o o t t i p ; t r a n s l o c a t i o n is m a i n l y basipetal. I n one e x p e r i m e n t , t h e r o o t s were sectioned i n t o 1 cm lengths a b o v e a n d below t h e t r e a t e d segment, which was 7 - - 8 . 5 cm f r o m t h e r o o t tip. T h e s o d i u m c o n t e n t 6 cm a b o v e a n d 1 cm below was o n e - t e n t h of t h a t in t h e t r e a t e d s e g m e n t a n d negligible a m o u n t s were f o u n d in t h e first i n t e r n o d e s a n d shoots. As e x p e c t e d f r o m t h e s h a r p decline in t h e s o d i u m c o n c e n t r a t i o n n e a r t h e base of t h e root, h i g h l y v a r i a b l e results were o b t a i n e d when t h e solution was a p p l i e d only to t h a t region.
2. Absorption o/ Sodium and Potassium and Transport o/ these Ions to the X y l e m Exudate in Detached Roots o/ Di//erent Length R o o t s were cut below w a t e r a t v a r y i n g distances from t h e r o o t t i p ; t h e i r b a s a l ends were i n s e r t e d in capillaries a n d t h e y were t h e n i m m e r s e d in a e r a t e d solutions labelled w i t h sodium-22 a n d p o t a s s i u m - 4 2 a n d m a i n t a i n e d a t 25~ The roots a n d t h e e x u d a t e collecting in t h e capillaries were s a m p l e d a n d weighed after 18 hr. T h e results are shown in T a b l e 3 ; t h e r e was a significant decrease in t h e r e l a t i v e q u a n t i t i e s of s o d i u m a n d p o t a s s i u m in t h e x y l e m e x n d a t e w i t h increasing r o o t length. This was l a r g e l y due to a decrease in t h e s o d i u m c o n c e n t r a t i o n of t h e e x u d a t e w i t h increasing length, a n d suggests
306
M . G . T . SHONe,, D. T. CLARKSOI~and J. SAI~DEI~SON:
a slower r a t e of m o v e m e n t of s o d i u m in t h e older p o r t i o n s of t h e x y l e m vessels. There was no large or consistent change w i t h l e n g t h in t h e r e l a t i v e q u a n t i t i e s of t h e t w o ions in t h e roots. Table 3. Absorption o/ labelled sodium and potassium by detached maize roots o/ different length: volume o/ exudate and concentration o/ these ions alter 18 hour8 in exudate and roots Means of seven replicates: values transformed to logarithmic basis for statistical analysis are in parentheses. Length of root (era)
Exudate Volume (m]/root)
4
0.026
8
0.071
16
0.092
21 a
0.179
Significant difference (P=0.05)
0,021
Roots Concentration (tzequiv./ml)
Ions absorbed (tzequiv./g fresh weight)
Na
K
Na
K
Ratio Na/K
0.618 (2.648) 0.261 (2.312) 0.088 (i.882) 0.006 (0.373)
2.07 0.264 (1.27)(3.377) 2.74 0.100 (1.40) (2.909) 3.14 0.028 (1.49)(2.400) 6.79 0.001 (1.74) (0.664)
0.195
10.9
0.023
0.144
11.0
0.014
0.173
11.1
0.017
0.288
19.0
0.016
(0.387)
(0.22)
0.084
5.7
0.009
Ratio Na/K
(0.321)
a Roots were excised at the basal region adjacent to the seed.
3. Variation with T i m e in the Quantity o/ Sodium and Potassium Absorbed by the Roots and Released to the X y l e m Exudate 8 cm sections of r o o t s were i n s e r t e d in capillaries a n d p l a c e d in labelled solution as in t h e p r e v i o u s e x p e r i m e n t , a n d e x u d a t e a n d roots were s a m p l e d a f t e r 4, 8 ~nd 24 hr. To collect sufficient m a t e r i ~ l for counting a t t h e 4 a n d 8 h r s a m p l i n g times, t h e e x u d a t e s from t h r e e a n d t w o r o o t s r e s p e c t i v e l y were bulked. A t b o t h these times, samples of six r o o t s were t a k e n for e s t i m a t i n g t h e influx of sodium-22 a n d potassium-42, t h e r e m a i n i n g r o o t s being r e m o u n t e d in capillaries. T a b l e 4 shows t h a t t h e r a t i o of s o d i u m to p o t a s s i u m in t h e e x u d a t e decreased m a r k e d l y w i t h time. This was due to a significant decrease in t h e r a t e of t r a n s f e r of s o d i u m to t h e x y l e m sap. There was no significant change w i t h t i m e in t h e r a t i o of s o d i u m to p o t a s s i u m in t h e roots.
Absorption and Translocation of Sodium
307
Table 4. Variation with time o/the rates o~ exudation and trans/er o/labelled sodium and potassium to the xylem sap o/detached roots 8 om long and o/the concentrations o/these ions in the root tissue. Means o] six replicates Period over which exudate was sampled, hours
Mean rate of exudation (ml/hr per root)
Mean rate of transfer of ions to exudate (nano equiv./hr per root)
Quantities of ions in roots at end of sampling period (~equiv./g fresh weight)
Na
K
Ratio Na/K
Na
K
Ratio Na/K
0~4 4--8 8--24
0.0038 0.0035 0.0016
0.235 0.161 0.059
2.56 5.80 3.73
0.098 0.031 0.016
0.119 0.147 0.171
6.52 9.90 11.9
0.019 0.016 0.015
Significant difference (P=0.05)
0.0012
0.081
2.78
0.033
0.066
7.9
0.014
4. Autoradiographic Studies
A f t e r 24 hr u p t a k e of sodinm-22 from a labelled culture solution t h e d i s t r i b u t i o n of s o d i u m in t h e tissues of t h e r o o t was e x a m i n e d b y microa u t o r a d i o g r a p h y of t r a n s v e r s e sections. Fig. 2 shows t h a t in t h e zone 5 cm from t h e r o o t t i p t h e t r a c e r s o d i u m was more h i g h l y c o n c e n t r a t e d w i t h i n t h e stele t h a n in t h e cortex. This t y p e of d i s t r i b u t i o n was also seen in sections 1 a n d 12 cm d i s t a n t from t h e a p e x a l t h o u g h the s o d i u m c o n t e n t of t h e sections v a r i e d a p p r e c i a b l y along t h e axis (Table 1). L a t e r a l r o o t i n i t i a t i o n was in progress 12 em from t h e t i p of t h e r o o t a n d i t is i n t e r e s t i n g to n o t e (Fig. 3) t h a t l a t e r a l roots which h a d y e t to emerge f r o m t h e c o r t e x were r e a d i l y d i s t i n g u i s h a b l e b y their higher s o d i u m content. The r o o t from which this section (Fig. 3) was c u t was i m m e r s e d o n l y to a d e p t h of 7 em in t h e labelled solution, t h e r e m a i n d e r , including t h e zone f r o m which sections were cut, was k e p t in m o i s t air. Thus t h e t r a c e r s o d i u m l o c a t e d in t h e stele (Fig. 3) is likely to h a v e been t r a n s l o e a t e d t h r o u g h t h e v a s c u l a r tissue. I t w o u l d seem t h a t lateral roots are able to w i t h d r a w t r a c e r s o d i u m f r o m t h e t r a n s l o e a t i o n s t r e a m . A l t h o u g h t h e e x p e r i m e n t s were n o t carried o u t u n d e r sterile conditions t h e r e is no evidence in t h e a u t o r a d i o g r a p h s of h e a v y a c c u m u l a t i o n s of labelled s o d i u m in b a c t e r i a l colonies a t t h e p e r i p h e r y of t h e sections. F a i l u r e to m a i n t a i n s t e r i l i t y in e x p e r i m e n t s w i t h phosphorus-32 (BA~BE~, SA~DE~SO~ a n d RUSSELL, 1968) a n d scandium-46 (CLARKSON a n d SANDERSON) results in a h i g h l y d i s t i n c t i v e loealisation of radioa c t i v i t y in a d i s c o n t i n u o u s b e l t a t t h e surface of t h e r o o t epidermis. T h e a u t o r a d i o g r a p h s of sections f r o m t h e e x t r e m e base of a r o o t which h a d been wholly i m m e r s e d in a solution labelled w i t h sodium-22 differ s o m e w h a t from those o b t a i n e d in t h e first 12 em of t h e axis.
308
M.G.T. S~oNE, D. T. CLAI~KSONand J. SA~rD~RSO~: o
N
9
Figs. 4 and 5 show autoradiographs from the basal region with and without an underlying section of root. In both plates there is a strong impression that the labelling of the main xylem vessels is appreciably lower than the surrounding stelar parenehyma. This was confirmed by grain counting (Fig. 6) which shows that peaks of grain density are
Absorption and Transloeation of Sodium
309
0 o
o o
a~
~.~ ~ 9
..
~|
o,..~
~.'~ 9
found over the stelar parenchyma and the cells of the pericycle, while the troughs in the stele are associated with the main vascular elements. The average grain density over the stele is still several times that over the cortex and the labelling of the epidermis and the outer ranks of cortical cells is barely above the background of the film.
~
310
M.G.T. S~ONE, D. T. CbARKSON and J. SANDERSON:
cq
O
r
~b
Discussion In the experiments described above, the concentrations of sodium ~nd potassium in the external solution were not varied. Moreover, because the entry of tracer sodium and potassium was studied, the quantities of these ions which were absorbed by the roots ~nd tr~ns-
Absorption and Translocation of Sodium
311
~
. .'~
ported to the xylem sap represent influxes and not net quantities of ions moved 9 These limitations of the experimental conditions must be borne in mind in interpreting the results (CAN~Y and ASXI~AM, 1967). The salient feature of the comparative distribution of sodium and potassium shown in Table 1 is the sharp decline in the sodium content
m
N
~-. ~
312
M.G.T. SHO~E, D. T. CLA~:SOX and J. SAnDeRSOn.' a
b
c d
II 120
e
I
d
I
c II
Tissue plan
b
a
/ /
eq
l ._c m 80
U3
0
0
z~O0
800
1 200
Distance, p.
Fig. 6. Distribution of silver grains across an autoradiograph of a transverse section cut from the basal end of a maize root after absorption of sodium 22 for 24 hr. Seetion thickness, 20 ~, radioactivity 34 counts min-1, exposure 5 days. a epidermis, b cortex, e endodermis and perieyele, d vascular tissue, e parenchyma of the portions of plant immediately above the basal end of the root. This suggests that movement of sodium in the xylem may be slow in this region. Although it is possible that the high sodium content of the basal zone of the root may have been due to local accumulation, Table 2 shows that a substantial proportion of the labelled sodium applied at varying distances from the root tip can move towards the base of the root. Further evidence of this is provided in Fig. 3, which illustrates a section cut 5 em above the level of the labelled solution. I t seems therefore that sodium translocated from distal portions of the root is aeeumulated at its base. Experiments on detached roots of different lengths (Table 3) show a progressive decrease in the concentration of sodium in the xylem exudate with increasing length of root. This suggests that discrimination against sodium is not restricted to the upper portion of the plant, but continues throughout the lower parts of the root. Discrimination is particularly effective in the basal region of the root; there is evidence from the autoradiographs of a lower concentration of sodium in the main xylem vessels (Figs. 4 and 5). BA~OE and v a n VLIET (1961) in studies using stable sodium and potassium found that whereas the rate of absorption of potassium by intact maize seedlings was constant or increased with time, that ol sodium decreased over a period of 24 hr; a comparable effect is evident in the decrease of the ratio of sodium to potassium in the xylem sap of detached roots (Table 4), although the ratio of the quantities of the two ions absorbed by the roots remains relatively constant. BA~G~ and
Absorption and Translocation of Sodium
313
VAN"VLIET proposed that the transport of sodium and potassium from absorption sites to the xylem might be mediated by different mechanisms, or that in the course of its movement to the sap, some of the sodium could be transferred back to a " p o o l " in the root tissues where it is accumulated.
PITMAN,COUI~TICEand LEE (1968) showed that during the accumulation of sodium by barley roots the decreasing net rate of uptake of sodium was brought about by a considerable reduction in sodium influx, as measured with sodium-22. Selectivity in the transport of potassium and sodium to the shoots was correlated more closely with the ratio of the influxes, rather than net uptake of these ions by the roots. These authors further suggest that the high ratio of potassium to sodium in barley roots is due to a comparably high ratio in the cytoplasm, which is maintained by inward transport of potassium and outward transport of sodium at the plasmalemma. The miero-autoradiographs indicate that the major fraction of the sodium absorbed by the roots is accumulated within the endodermis; this tissue evidently provides no barrier to lateral movement of sodium, which exchanges rapidly with sodium in the external solution (unpublished data, see also WALLACE, 1968). Furthermore, the sodium content of the stelar tissues increases with increasing distance from the root tip. I t may be concluded that the movement of sodium towards the shoot is progressively slowed down by factors associated with either the walls of the xylem or the surrounding stelar parenchyma; in many respects this explanation is similar to that advanced by JACOB~r (1965) to explain the slow rate of ascent of sodium entering the cut ends of bean stems. I t has already been suggested (BANGE and VAN VLIET, 1961 ; PITMAN, COURTICEand LEE, 1968) that a two-way movement of sodium takes place across certain tissues of the root and it would be of great interest to determine to what extent the low concentration of sodium in the transpiration stream is maintained by an outward transport of this ion to the surrounding tissues and the ambient medium. The authors thank Dr. 1%.ScoTTRUSSELLfor his interest in this work and for helpful comments. They are also grateful to Mr. B. O. BARTLETT for statistical analyses and Miss A. V.WooD for assistance in the experimental work. References
APPLETON,T. C. : 1%esolvingpower, sensitivity and latent image fading of solublecompound autoradiographs. J. Histochem. Cytochem. 14, 4 1 4 ~ 1 9 (1966). BA~GE, G. G. J., and E. VANVLIET: Translocation of potassium and sodium in intact maize seedlings. Plant and Soil 15, 312--328 (1961).
314
M . G . T . S~OZCE et al: Absorption and Translocation of Sodium
BARBER~D. A., J. SANDERSON,and R. S. RUSSELL: Influence of microorganisms on the distribution in roots of phosphate labelled with phosphorus-32. Nature (Lond.) 217, 644 (1968). BEI~NSTErN, L., J. W. BaOWN, and It. E. HAYWARD: The influence of rootstock on growth and salt accumulation in stone-fruit trees and almonds. Proc. Amer. Soc. Hort. Sci. 68, 86--95 (1956). - - , and H. E. HAYWARD: Physiology of salt toleran6e. Ann. Rev. Plant Physiol. 9, 25--46 (1958). CA~Y, M. J., and M. J. ASKHAI~'r: Physiological inferences from the evidence of translocation of a tracer: a caution. Ann. Bot., N.S. 31, 409--416 (1967). CLAt~XSOI% D. T., and J. SAI~DEI~SO~:In preparation. CROSSETT, R. N.: Autoradiography of 32p in maize roots. Nature (Lond.) 213, 312--313 (1967). GAUCH,H. G., and C. H.WADLEIGH: The effects of high concentrations of sodium, calcium, chloride and sulphate on ionic absorption by bean plants. Soil Sci. 59, 139--153 0954). HUFFAKnR, R. C., and A.WALLACE: Sodium absorption by different plant species at different potassium levels. Soil Sci. 87, 130--134 (1959). JACO~Y, ]3. : Sodium retention in excised bean stems. Physioiogia Pt. 18, 730--739 (I965). JE~);IN~S, D. H. : Halophytes, succulence and sodium in plants - - a unified theory. New Phytol. 67, 899--911 (1968). PEArSOn, G. A. : Absorption and translocation of sodium in beans and cotton. Plant Physiol. 42, 1171--1175 (1967). PIT~IA~, M. G., A. C. COURTIC~, and B. LEE: Comparison of potassium and sodium uptake by barley roots at high and low salt status. Aust. J. biol. Sci. 21, 871--881 (1968). - - , and I5. W. SADDLEn: Active sodium and potassium transport in cells of barley roots. Prec. nat. Acad. Sci. (Wash.) 57, 44--52 (1967). ROOERS, A. W.: Techniques of autoradiography. Amsterdam: Elsevier 1967. WALLACe, A. : The effect of temperature and p H on sodium translocation and sodium exchange reactions in bush beans. Soil Sci. 106, 144--148 (1968). Dr. M. G. T. SI~O~E Agricultural Research Council Letcombe Laboratory Wantage, Berkshire, U.K.
The Absorption and Translocation of Sodium by Maize Seedlings M. G. T. StrOPhE, D. T. CLAUKSOX a n d J. SA~D~USO~ Agricultural Research Council Letcombe Laboratory (incorporating Radiobiological Laboratory) Wantage, Berkshire, U.K. Received March 3/April 14, 1969
Summary. The absorption and subsequent distribution of sodium and potassium has been examined in maize seedlings in short-term experiments using sodium-22 and potassium-42. The absorption and translocation of sodium by different segments of intact seedlings was also investigated. Although absorption of potassium exceeded that of sodium by a factor of about 50, there was no evidence that the entry of sodium was confined to a small region of the root. Determinations of the relative quantities of sodium and potassium in the xylem exudate of detached roots showed that the ratio of sodium to potassium decreased with increasing length of the root. These results suggested that upward movement of sodium in the xylem vessels was progressively reduced towards the basal part of the root. This conclusion was supported by microautoradiographs, which showed that although the concentration of sodium within the endodermis was greater than that in the cortex, there was an apparent decrease in the sodium content of the major xylem vessels at the basal end of the root. Introduction Most crop plants, a p a r t f r o m those of h a l o p h y t i c origin t r a n s l o c a t e o n l y small q u a n t i t i e s of s o d i u m to their stems a n d leaves. T h e m e c h a n i s m w h e r e b y these p l a n t s d i s c r i m i n a t e a g a i n s t s o d i u m is little u n d e r s t o o d a n d has v a r i o u s l y been a t t r i b u t e d to a) t h e a b i l i t y of r o o t s to r e t a i n large a m o u n t of sodium, so t h a t r e l a t i v e l y small q u a n t i t i e s pass into t h e t r a n s p i r a t i o n s t r e a m (BEn~STEI~, B n o w ~ a n d HAYWAm), 1956; PEAUSO~, 1967), b) a m e m b r a n e or o t h e r s t r u c t u r e which i n h i b i t s transl o c a t i o n of s o d i u m into the v a s c u l a r tissue of t h e r o o t (GAucI~ a n d WADLEIG~I, 1954; BERNSTEIN a n d H A Y W ~ D , 1958), or c) a w i t h d r a w a l of s o d i u m from t h e t r a n s p i r a t i o n s t r e a m i n t o living cells a d j a c e n t to t h e v a s c u l a r tissue (JAcoBY, 1965). On t h e basis of electrochemical m e a s u r e m e n t s , i t has also been suggested t h a t s o d i u m is a c t i v e l y e x c r e t e d b y b a r l e y roots (PIT~A~ a n d SADnL~U, 1967) a n d b y o t h e r p l a n t ceils ( J E t ? c i t e s , 1968). I n order to i n v e s t i g a t e these h y p o t h e s e s further, e x p e r i m e n t s h a v e been carried o u t on y o u n g seedlings of maize, a species which t r a n s l o c a t e s o n l y small a m o u n t s of s o d i u m to t h e leaves (tIuFFAKEU a n d WALLACe, 21 Planta (Berl.), Bd. 86
302
M . G . T . SHONE, D. T. CLAR~:SO~ and J. SANDERSON."
1959). I n some e x p e r i m e n t s th e u p t a k e of sodium was c o m p a r e d w i t h t h a t of t h e c h e m i c a l l y similar b u t physiologically co n t r ast i n g ion potassium, using sodium-22 a n d potassium-42 in s h o r t - t e r m studies in w a t e r culture. This p r o c e d u r e necessarily measures t h e influxes a n d n o t t h e n et q u a n t i t i e s of ions t a k e n u p in a g iv e n time. Th e r e l a t i v e q u a n t i t i e s of t h e t w o ions a b s o r b e d b y t h e roots an d t r a n s l o e a t e d to t h e shoots were m e a s u r e d in i n t a c t seedlings. Studies were also m a d e on t h e x y l e m e x u d a t e of d e t a c h e d roots of v a r y i n g l e n g t h in order to i n v e s t i g a t e t h e t r a n s p o r t of t h e t w o ions w i t h i n t h e r o o t to t h e t r a n s p i r a t i o n stream. M i c r o - a u t o r a d i o g r a p h s p r e p a r e d f r o m t r a n s v e r s e sections of roots of seedlings labelled w i t h sodinm-22 were e x a m i n e d to d e t e r m i n e if t h er e was some tissue in t h e r o o t which r e d u c e d t h e r a t e of m o v e m e n t of s o d i u m i n t o t h e stele, or a l t e r n a t i v e l y w h e t h e r t h er e was a n y evidence of a c o n c e n t r a t i o n of s o d i u m in t h e cells bordering t h e x y l e m vessels.
Methods In all experiments seeds of Zea Mays L. (John Innes F 1 hybrid) were germinated in the dark on moist filter paper and transferred to perforated black polythene discs suspended above a dilute culture solution 1. The seedlings were then grown for four days in a constant environment cabinet (20~ 1,200 tic. for 16 h daily). The seminal roots had then extended to about 20 cm in length and were either used in experiments on the absorption of labelled sodium and potassium or in autoradiographie studies. The uptake solution used in all the experiments contained 0.6 m. equiv./1 potassium and 0.2 m. equiv./1 sodium and calcium as chlorides. Solid plant material and root exudate were assayed for sodium-22 and potassium-42 by counting twice at an interval of 7 days, in a Beckman Lowbeta II end window counter or in a well-crystal y scintillator. Similar counts made on Miquots of the uptake solution enabled the results to be expressed as the quantities of sodium or potassium in ~ equivalents which had entered the plant material from the uptake solution. For the autoradiographie studies maize seedlings were grown for 24 h at 23~ under continuous illumination with their roots either totally or partially immersed in the uptake solution labelled with 1 ~4 ~Ci sodium-22 per ml. At the end of the treatment the roots of the intact plants were rinsed with distilled water then blotted with tissue. Segments 1 cm in length were cut from roots and held with fine forceps in the centre of a small well recessed into a block of frozen liver which was maintained at --25 ~C. The well was then flooded with a liquid hydrocarbon mixture at --8~ which solidified shortly after contact with the liver. The whole block was chilled further by contact with liquid nitrogen. The essential feature of this technique, which will be described in detail elsewhere, is that the root is supported by, but isolated from the sodium-rich liver tissue by a collar of inert material immiscible 1. Composition (m.equiv/1) : K +, 0.6; Ca++, 0.3; Mg++, 0.3; Na +, 0.2; N0'a, 1.0; SOt", 0.3; H2PO"t, 0.1; d- minor nutrients (Fe as EDTA complex, Mn, Zn, Cu, Mo, C1, B).
Absorption and Translocation of Sodium
303
with water. Failure to provide such isolation permits contact between the root and the cut surface of the liver with consequent exchange of sodium between the tissues prior to freezing. The methods used for cutting and picking up sections and for exposing Kodak AR 10 stripping film differed in only minor details from the procedures described by APPLETO~r (1966), CR0SSETT (1967), and ROGEP,S (1967). All these operations were performed at --20 to --25~ Details of other methods used in individual experiments are given in the relevant sections.
Results
1. Absorption o/ Sodium and Potassium by Intact Seedlings Seedlings with roots 20 cm long were sealed w i t h lanolin a t t h e b a s a l end of t h e r o o t i n t o p e r f o r a t e d discs of paraffin w a x a n d s u s p e n d e d a b o v e t h e labelled solution so t h a t t h e roots, a p a r t from t h e basal 1 cm, were in c o n t a c t w i t h t h e labelled u p t a k e solution. A t t h e end of t h e 18 h r u p t a k e period, t h e seedlings were r e m o v e d f r o m the solution, t h e r o o t s b l o t t e d a n d t h e seedlings d i v i d e d into shoots, seed a n d first i n t e r n o d e , five r o o t s e g m e n t s of equal length a n d 1 cm r o o t tip. Table 1. Absorption and translocation o~ labelled sodium and potassium in maize
seedlings After the uptake period of 18 hours, the seedlings were subdivided into shoots, seeds+ first internode, five equal portions of the 20 cm long roots and 1 cm root tip. Means of 6 replicates are shown with their standard errors. Tissue
Shoot Seed+ 1st internode Root seetion 1 2 3 4 5 Roottip
Concentration of labelled ions (gequiv. per g fresh weight) Na
K
0.003 J= 0.001 0.07 =h 0.01 1,24 J=0.19 0.62 /=0.08 0,45 =J=0,06 0.41 ~0.05 0.46 ~0.02 0.67 ~=0.13
7.01 ! 0.89 3.46 ~= 0.36 28.6 • 30.8 • 32,2 • 39.7 1 3 . 7 55.5 =k7.3 91.7 =t=19.6
Ratio Na/K
0.0005 ~ 0,0002 0.022 ~_ 0.005 0.041 /-0.005 0.019 ~0.003 0,015 ~0.007 0.010 ~0.002 0,010 ~0.003 0.006 ~0.001
T a b l e 1 shows m e a n values of t h e q u a n t i t i e s p e r g r a m fresh weight of labelled s o d i u m a n d p o t a s s i u m , a n d t h e r a t i o s of s o d i u m to p o t a s s i u m , in various p o r t i o n s of six m a i z e seedlings after a b s o r p t i o n for 18 hr. A l t h o u g h t h e q u a n t i t y of p o t a s s i u m a b s o r b e d b y t h e roots exceeds t h a t of s o d i u m b y one to t w o orders of m a g n i t u d e a n d t h e r a t i o of s o d i u m to p o t a s s i u m increases t o w a r d s the basal end of t h e roots, t h e d i s t r i b u t i o n of t h e two ions in t h e roots is b r o a d l y similar. T h e two ions c o n t r a s t 21"
304
M.G.T. S~O~E, D. T. CLARKSON and J. SANDERSON:
Fig. 1. Apparatus designed to study uptake by segments of intact plants in which the remainder of the plant is kept in an atmosphere of moist air. The "Perspex" holder A serves to support 6 seedlings B and allows 1.5 em lengths to be exposed to the uptake solution C. The aerator D maintains a moist atmosphere in the lower jar E. Air escapes through the capillary fitted into a hypodermic syringe barrel F which contains water. This ensures that the air in the upper vessel G is saturated with water, and at the same time maintains a small positive pressure within the lower jar E. This pressure helps to prevent breakdown and leakage from the paraffin wax - - rosin seals H. Air escapes through the hole I in the upper jar m a i n l y in t h e q u a n t i t i e s t r a n s l o e a t e d to t h e shoots; whereas t h e c o n t e n t of p o t a s s i u m in t h e shoots r e l a t i v e to t h a t in t h e b a s a l r o o t s e g m e n t is a b o u t 0.25, t h e corresponding r e l a t i v e c o n t e n t for s o d i u m is a b o u t 0.003. T h e r e is also a m a r k e d decrease in t h e r a t i o of s o d i u m to p o t a s s i u m in t h e p a r t s of t h e p l a n t s a b o v e t h e b a s a l s e g m e n t of t h e roots. I t seemed possible t h a t t h e e n t r y of s o d i u m m i g h t be localised to t h e region a r o u n d t h e r o o t tip, a n d t h a t this m i g h t lead to a r e d u c e d u p t a k e of this ion. A c c o r d i n g l y to s t u d y t h e a b s o r p t i o n of s o d i u m b y different p a r t s of t h e r o o t system, seedlings were sealed w i t h paraffin w a x / r o s i n m i x t u r e into small p e r s p e x cells containing 1 ml of labelled u p t a k e solution, t h e r e m a i n d e r of t h e roots a n d t h e shoots being k e p t in an a t m o s p h e r e of m o i s t air. B y this m e a n s a b o u t 1.5 em of t h e r o o t s were e x p o s e d to t h e labelled solution, which was a p p l i e d a t v a r y i n g
Absorption and Transloeation of Sodium
305
Table 2. Quantities o/labelled sodium absorbed and translocated ]rom 1.5 cm regions
of the roots of intact maize seedlings The labelled solution was applied for 18 hours at varying distances from the root tips; the remainder of the plants were maintained in moist air. Zone where sodium was applied
l~oot tip 2 . 5 4 . 0 em behind tip 10.0
11.5 cm behind tip
Total sodium absorbed (nano-equiv.)
14 21
Quantities of sodium in various sections (nano-equiv.) Treated segment
Below treated segment
Above treated segment
5.5 4.3
-0.7
8.5 16.0
51.5
24.6
~
86.9
Groups of means which do not differ significantly (P=0.05) are indicated by vertical lines.
distances from the root tip. The apparatus is illustrated and described in Fig. 1. T a b l e 2 shows t h a t t h e t o t a l q u a n t i t y of labelled s o d i u m a b s o r b e d b y t h e seedlings a n d t h e r e s p e c t i v e q u a n t i t i e s f o u n d in t h e t r e a t e d segment, a n d below a n d a b o v e this region increase with increasing d i s t a n c e f r o m t h e r o o t t i p ; t r a n s l o c a t i o n is m a i n l y basipetal. I n one e x p e r i m e n t , t h e r o o t s were sectioned i n t o 1 cm lengths a b o v e a n d below t h e t r e a t e d segment, which was 7 - - 8 . 5 cm f r o m t h e r o o t tip. T h e s o d i u m c o n t e n t 6 cm a b o v e a n d 1 cm below was o n e - t e n t h of t h a t in t h e t r e a t e d s e g m e n t a n d negligible a m o u n t s were f o u n d in t h e first i n t e r n o d e s a n d shoots. As e x p e c t e d f r o m t h e s h a r p decline in t h e s o d i u m c o n c e n t r a t i o n n e a r t h e base of t h e root, h i g h l y v a r i a b l e results were o b t a i n e d when t h e solution was a p p l i e d only to t h a t region.
2. Absorption o/ Sodium and Potassium and Transport o/ these Ions to the X y l e m Exudate in Detached Roots o/ Di//erent Length R o o t s were cut below w a t e r a t v a r y i n g distances from t h e r o o t t i p ; t h e i r b a s a l ends were i n s e r t e d in capillaries a n d t h e y were t h e n i m m e r s e d in a e r a t e d solutions labelled w i t h sodium-22 a n d p o t a s s i u m - 4 2 a n d m a i n t a i n e d a t 25~ The roots a n d t h e e x u d a t e collecting in t h e capillaries were s a m p l e d a n d weighed after 18 hr. T h e results are shown in T a b l e 3 ; t h e r e was a significant decrease in t h e r e l a t i v e q u a n t i t i e s of s o d i u m a n d p o t a s s i u m in t h e x y l e m e x n d a t e w i t h increasing r o o t length. This was l a r g e l y due to a decrease in t h e s o d i u m c o n c e n t r a t i o n of t h e e x u d a t e w i t h increasing length, a n d suggests
306
M . G . T . SHONe,, D. T. CLARKSOI~and J. SAI~DEI~SON:
a slower r a t e of m o v e m e n t of s o d i u m in t h e older p o r t i o n s of t h e x y l e m vessels. There was no large or consistent change w i t h l e n g t h in t h e r e l a t i v e q u a n t i t i e s of t h e t w o ions in t h e roots. Table 3. Absorption o/ labelled sodium and potassium by detached maize roots o/ different length: volume o/ exudate and concentration o/ these ions alter 18 hour8 in exudate and roots Means of seven replicates: values transformed to logarithmic basis for statistical analysis are in parentheses. Length of root (era)
Exudate Volume (m]/root)
4
0.026
8
0.071
16
0.092
21 a
0.179
Significant difference (P=0.05)
0,021
Roots Concentration (tzequiv./ml)
Ions absorbed (tzequiv./g fresh weight)
Na
K
Na
K
Ratio Na/K
0.618 (2.648) 0.261 (2.312) 0.088 (i.882) 0.006 (0.373)
2.07 0.264 (1.27)(3.377) 2.74 0.100 (1.40) (2.909) 3.14 0.028 (1.49)(2.400) 6.79 0.001 (1.74) (0.664)
0.195
10.9
0.023
0.144
11.0
0.014
0.173
11.1
0.017
0.288
19.0
0.016
(0.387)
(0.22)
0.084
5.7
0.009
Ratio Na/K
(0.321)
a Roots were excised at the basal region adjacent to the seed.
3. Variation with T i m e in the Quantity o/ Sodium and Potassium Absorbed by the Roots and Released to the X y l e m Exudate 8 cm sections of r o o t s were i n s e r t e d in capillaries a n d p l a c e d in labelled solution as in t h e p r e v i o u s e x p e r i m e n t , a n d e x u d a t e a n d roots were s a m p l e d a f t e r 4, 8 ~nd 24 hr. To collect sufficient m a t e r i ~ l for counting a t t h e 4 a n d 8 h r s a m p l i n g times, t h e e x u d a t e s from t h r e e a n d t w o r o o t s r e s p e c t i v e l y were bulked. A t b o t h these times, samples of six r o o t s were t a k e n for e s t i m a t i n g t h e influx of sodium-22 a n d potassium-42, t h e r e m a i n i n g r o o t s being r e m o u n t e d in capillaries. T a b l e 4 shows t h a t t h e r a t i o of s o d i u m to p o t a s s i u m in t h e e x u d a t e decreased m a r k e d l y w i t h time. This was due to a significant decrease in t h e r a t e of t r a n s f e r of s o d i u m to t h e x y l e m sap. There was no significant change w i t h t i m e in t h e r a t i o of s o d i u m to p o t a s s i u m in t h e roots.
Absorption and Translocation of Sodium
307
Table 4. Variation with time o/the rates o~ exudation and trans/er o/labelled sodium and potassium to the xylem sap o/detached roots 8 om long and o/the concentrations o/these ions in the root tissue. Means o] six replicates Period over which exudate was sampled, hours
Mean rate of exudation (ml/hr per root)
Mean rate of transfer of ions to exudate (nano equiv./hr per root)
Quantities of ions in roots at end of sampling period (~equiv./g fresh weight)
Na
K
Ratio Na/K
Na
K
Ratio Na/K
0~4 4--8 8--24
0.0038 0.0035 0.0016
0.235 0.161 0.059
2.56 5.80 3.73
0.098 0.031 0.016
0.119 0.147 0.171
6.52 9.90 11.9
0.019 0.016 0.015
Significant difference (P=0.05)
0.0012
0.081
2.78
0.033
0.066
7.9
0.014
4. Autoradiographic Studies
A f t e r 24 hr u p t a k e of sodinm-22 from a labelled culture solution t h e d i s t r i b u t i o n of s o d i u m in t h e tissues of t h e r o o t was e x a m i n e d b y microa u t o r a d i o g r a p h y of t r a n s v e r s e sections. Fig. 2 shows t h a t in t h e zone 5 cm from t h e r o o t t i p t h e t r a c e r s o d i u m was more h i g h l y c o n c e n t r a t e d w i t h i n t h e stele t h a n in t h e cortex. This t y p e of d i s t r i b u t i o n was also seen in sections 1 a n d 12 cm d i s t a n t from t h e a p e x a l t h o u g h the s o d i u m c o n t e n t of t h e sections v a r i e d a p p r e c i a b l y along t h e axis (Table 1). L a t e r a l r o o t i n i t i a t i o n was in progress 12 em from t h e t i p of t h e r o o t a n d i t is i n t e r e s t i n g to n o t e (Fig. 3) t h a t l a t e r a l roots which h a d y e t to emerge f r o m t h e c o r t e x were r e a d i l y d i s t i n g u i s h a b l e b y their higher s o d i u m content. The r o o t from which this section (Fig. 3) was c u t was i m m e r s e d o n l y to a d e p t h of 7 em in t h e labelled solution, t h e r e m a i n d e r , including t h e zone f r o m which sections were cut, was k e p t in m o i s t air. Thus t h e t r a c e r s o d i u m l o c a t e d in t h e stele (Fig. 3) is likely to h a v e been t r a n s l o e a t e d t h r o u g h t h e v a s c u l a r tissue. I t w o u l d seem t h a t lateral roots are able to w i t h d r a w t r a c e r s o d i u m f r o m t h e t r a n s l o e a t i o n s t r e a m . A l t h o u g h t h e e x p e r i m e n t s were n o t carried o u t u n d e r sterile conditions t h e r e is no evidence in t h e a u t o r a d i o g r a p h s of h e a v y a c c u m u l a t i o n s of labelled s o d i u m in b a c t e r i a l colonies a t t h e p e r i p h e r y of t h e sections. F a i l u r e to m a i n t a i n s t e r i l i t y in e x p e r i m e n t s w i t h phosphorus-32 (BA~BE~, SA~DE~SO~ a n d RUSSELL, 1968) a n d scandium-46 (CLARKSON a n d SANDERSON) results in a h i g h l y d i s t i n c t i v e loealisation of radioa c t i v i t y in a d i s c o n t i n u o u s b e l t a t t h e surface of t h e r o o t epidermis. T h e a u t o r a d i o g r a p h s of sections f r o m t h e e x t r e m e base of a r o o t which h a d been wholly i m m e r s e d in a solution labelled w i t h sodium-22 differ s o m e w h a t from those o b t a i n e d in t h e first 12 em of t h e axis.
308
M.G.T. S~oNE, D. T. CLAI~KSONand J. SA~rD~RSO~: o
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9
Figs. 4 and 5 show autoradiographs from the basal region with and without an underlying section of root. In both plates there is a strong impression that the labelling of the main xylem vessels is appreciably lower than the surrounding stelar parenehyma. This was confirmed by grain counting (Fig. 6) which shows that peaks of grain density are
Absorption and Transloeation of Sodium
309
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o o
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~|
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found over the stelar parenchyma and the cells of the pericycle, while the troughs in the stele are associated with the main vascular elements. The average grain density over the stele is still several times that over the cortex and the labelling of the epidermis and the outer ranks of cortical cells is barely above the background of the film.
~
310
M.G.T. S~ONE, D. T. CbARKSON and J. SANDERSON:
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Discussion In the experiments described above, the concentrations of sodium ~nd potassium in the external solution were not varied. Moreover, because the entry of tracer sodium and potassium was studied, the quantities of these ions which were absorbed by the roots ~nd tr~ns-
Absorption and Translocation of Sodium
311
~
. .'~
ported to the xylem sap represent influxes and not net quantities of ions moved 9 These limitations of the experimental conditions must be borne in mind in interpreting the results (CAN~Y and ASXI~AM, 1967). The salient feature of the comparative distribution of sodium and potassium shown in Table 1 is the sharp decline in the sodium content
m
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312
M.G.T. SHO~E, D. T. CLA~:SOX and J. SAnDeRSOn.' a
b
c d
II 120
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I
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Tissue plan
b
a
/ /
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Fig. 6. Distribution of silver grains across an autoradiograph of a transverse section cut from the basal end of a maize root after absorption of sodium 22 for 24 hr. Seetion thickness, 20 ~, radioactivity 34 counts min-1, exposure 5 days. a epidermis, b cortex, e endodermis and perieyele, d vascular tissue, e parenchyma of the portions of plant immediately above the basal end of the root. This suggests that movement of sodium in the xylem may be slow in this region. Although it is possible that the high sodium content of the basal zone of the root may have been due to local accumulation, Table 2 shows that a substantial proportion of the labelled sodium applied at varying distances from the root tip can move towards the base of the root. Further evidence of this is provided in Fig. 3, which illustrates a section cut 5 em above the level of the labelled solution. I t seems therefore that sodium translocated from distal portions of the root is aeeumulated at its base. Experiments on detached roots of different lengths (Table 3) show a progressive decrease in the concentration of sodium in the xylem exudate with increasing length of root. This suggests that discrimination against sodium is not restricted to the upper portion of the plant, but continues throughout the lower parts of the root. Discrimination is particularly effective in the basal region of the root; there is evidence from the autoradiographs of a lower concentration of sodium in the main xylem vessels (Figs. 4 and 5). BA~OE and v a n VLIET (1961) in studies using stable sodium and potassium found that whereas the rate of absorption of potassium by intact maize seedlings was constant or increased with time, that ol sodium decreased over a period of 24 hr; a comparable effect is evident in the decrease of the ratio of sodium to potassium in the xylem sap of detached roots (Table 4), although the ratio of the quantities of the two ions absorbed by the roots remains relatively constant. BA~G~ and
Absorption and Translocation of Sodium
313
VAN"VLIET proposed that the transport of sodium and potassium from absorption sites to the xylem might be mediated by different mechanisms, or that in the course of its movement to the sap, some of the sodium could be transferred back to a " p o o l " in the root tissues where it is accumulated.
PITMAN,COUI~TICEand LEE (1968) showed that during the accumulation of sodium by barley roots the decreasing net rate of uptake of sodium was brought about by a considerable reduction in sodium influx, as measured with sodium-22. Selectivity in the transport of potassium and sodium to the shoots was correlated more closely with the ratio of the influxes, rather than net uptake of these ions by the roots. These authors further suggest that the high ratio of potassium to sodium in barley roots is due to a comparably high ratio in the cytoplasm, which is maintained by inward transport of potassium and outward transport of sodium at the plasmalemma. The miero-autoradiographs indicate that the major fraction of the sodium absorbed by the roots is accumulated within the endodermis; this tissue evidently provides no barrier to lateral movement of sodium, which exchanges rapidly with sodium in the external solution (unpublished data, see also WALLACE, 1968). Furthermore, the sodium content of the stelar tissues increases with increasing distance from the root tip. I t may be concluded that the movement of sodium towards the shoot is progressively slowed down by factors associated with either the walls of the xylem or the surrounding stelar parenchyma; in many respects this explanation is similar to that advanced by JACOB~r (1965) to explain the slow rate of ascent of sodium entering the cut ends of bean stems. I t has already been suggested (BANGE and VAN VLIET, 1961 ; PITMAN, COURTICEand LEE, 1968) that a two-way movement of sodium takes place across certain tissues of the root and it would be of great interest to determine to what extent the low concentration of sodium in the transpiration stream is maintained by an outward transport of this ion to the surrounding tissues and the ambient medium. The authors thank Dr. 1%.ScoTTRUSSELLfor his interest in this work and for helpful comments. They are also grateful to Mr. B. O. BARTLETT for statistical analyses and Miss A. V.WooD for assistance in the experimental work. References
APPLETON,T. C. : 1%esolvingpower, sensitivity and latent image fading of solublecompound autoradiographs. J. Histochem. Cytochem. 14, 4 1 4 ~ 1 9 (1966). BA~GE, G. G. J., and E. VANVLIET: Translocation of potassium and sodium in intact maize seedlings. Plant and Soil 15, 312--328 (1961).
314
M . G . T . S~OZCE et al: Absorption and Translocation of Sodium
BARBER~D. A., J. SANDERSON,and R. S. RUSSELL: Influence of microorganisms on the distribution in roots of phosphate labelled with phosphorus-32. Nature (Lond.) 217, 644 (1968). BEI~NSTErN, L., J. W. BaOWN, and It. E. HAYWARD: The influence of rootstock on growth and salt accumulation in stone-fruit trees and almonds. Proc. Amer. Soc. Hort. Sci. 68, 86--95 (1956). - - , and H. E. HAYWARD: Physiology of salt toleran6e. Ann. Rev. Plant Physiol. 9, 25--46 (1958). CA~Y, M. J., and M. J. ASKHAI~'r: Physiological inferences from the evidence of translocation of a tracer: a caution. Ann. Bot., N.S. 31, 409--416 (1967). CLAt~XSOI% D. T., and J. SAI~DEI~SO~:In preparation. CROSSETT, R. N.: Autoradiography of 32p in maize roots. Nature (Lond.) 213, 312--313 (1967). GAUCH,H. G., and C. H.WADLEIGH: The effects of high concentrations of sodium, calcium, chloride and sulphate on ionic absorption by bean plants. Soil Sci. 59, 139--153 0954). HUFFAKnR, R. C., and A.WALLACE: Sodium absorption by different plant species at different potassium levels. Soil Sci. 87, 130--134 (1959). JACO~Y, ]3. : Sodium retention in excised bean stems. Physioiogia Pt. 18, 730--739 (I965). JE~);IN~S, D. H. : Halophytes, succulence and sodium in plants - - a unified theory. New Phytol. 67, 899--911 (1968). PEArSOn, G. A. : Absorption and translocation of sodium in beans and cotton. Plant Physiol. 42, 1171--1175 (1967). PIT~IA~, M. G., A. C. COURTIC~, and B. LEE: Comparison of potassium and sodium uptake by barley roots at high and low salt status. Aust. J. biol. Sci. 21, 871--881 (1968). - - , and I5. W. SADDLEn: Active sodium and potassium transport in cells of barley roots. Prec. nat. Acad. Sci. (Wash.) 57, 44--52 (1967). ROOERS, A. W.: Techniques of autoradiography. Amsterdam: Elsevier 1967. WALLACe, A. : The effect of temperature and p H on sodium translocation and sodium exchange reactions in bush beans. Soil Sci. 106, 144--148 (1968). Dr. M. G. T. SI~O~E Agricultural Research Council Letcombe Laboratory Wantage, Berkshire, U.K.
