measurement of water fluxes and potentials in a single root-soil system

P l a n t a n d Soil 45, 577-594 (1976) Ms. 2717

M E A S U R E M E N T O F W A T E R F L U X E S A N D

P O T E N T I A L S I N A S I N G L E R O O T - S O I L S Y S T E M

I. THE TENSIOMETER--POTOMETER SYSTEM

by H. B. SO, L. A. G. AYLMORE and J. P. QUIRK*

SUMMARY

T he c o n s t r u c t i o n a n d o p e r a t i o n of a nove l t e n s i o m e t e r - p o t o m e t e r s y s t e m capab l e of m e a s u r i n g t h e x y l e m w a t e r p o t e n t i a l a n d f lux of w a t e r i n to t he roo t is descr ibed. T he v a l i d i t y of i ts m e a s u r e m e n t s has been i l l u s t r a t ed a n d i t was s h o w n t h a t a u n i q u e l inea r r e l a t i onsh ip exis ts b e t w e e n t h e r e s i s t ance to w a t e r flow a n d t h e w a t e r s t a t u s of t h e roo t t issues.

INTRODUCTION

The flow of water to plant roots is part of a catenary process of water transport from the soil through the plant and into the atmo- sphere s. A considerable amount of investigation has been carried out on the various sections of the water flow pathway. However, the region that has received the least attention is that of the root and the soil immediately around it. Progress in this area has been limited largely because of the difficulties associated with direct experimental measurements in this region. The root--soil interface is very complicated in its geometry and furthermore changes with time.

I t has long been recognised that the water potential at the root- soil interface is an important parameter as it determines to a large extent the availability of the soil water 16 and at the same time it has a large influence on the distribution of water potentials throughout the plant 14.

* Lecturer, Department of Agronomy and Soil Science, University of New England, Armidale, N.S.W.; Senior Lecturer, and Professor, Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, W.A.

578 H. B. SO, L. A. G. AYLMORE AND J. P. QUIRK

Recently increasing attention has been directed towards measurements of water potential gradients in the root-soil system and the associated root and soil resistances a 4 5, the relative magnitudes of which have become a matter of dispute 12 la. Except for the root psychrometer all others are destructive methods. However, it is not clear to which part of the root the potential, measured by the root psychrometer, relates.

This paper describes the construction and operation of a tensiometer--potometer capable of measuring in situ and simultaneously the xylem water potential, the root-soil interface water potential and the flux of water into a single root growing in soil. The use and applications of this tensiometer will be described in a second paper of this series.

METHODS AND MATERIALS

It was considered t h a t a t ens iome te r could be used for measur ing the root wa te r po ten t i a l if a mate r ia l could be found t h a t provides essent ia l ly im- med ia t e con tac t be tween the t ens iomete r and root surface. Tile r equ i r emen t for such a mater ia l is t h a t it should have as small a specific mois tu re capac i ty (d0/d~) as possible while p rov id ing a r easonab ly h igh and re la t ive ly co n s t an t conduc t iv i ty , suctl t h a t any d i s tu rbance b y the root is t r a n s m i t t e d a lmos t i n s t an t aneous ly to tile te l ls iometer . Hence i t should ac t essent ia l ly as an ex tens ion of the ceramic wall of the tens iometer . I t was felt t h a t good co n t ac t be tween the root and th is mater ia l would be ob ta ined by growing the root in to t h a t mater ia l . Wit t l these requ i rements in mind the t ens iome te r collar was cons t ruc t ed as out l ined below.

Construction o/the tensiometer and/low collars

The cons t ruc t ion Of the collar t ens iomete r s of 0.6 cm in te rna l d i ame te r is shown in the d iag ram of Fig. 1. Ceramic cyl inders * 0.6 cm I.D., 1.2 or 2 cm long and a wall th ickness of 0.2 cm were cemen t ed wi th pane l -me ta l epoxy c e m e n t * * at ti le ends inside pe r spex cyl inders leaving a gap be tween the ceramic and pe r spex to ac t as a wa te r reservoir . This reservoir was connec ted to the pressure t r ansduce r sy s t em b y means of ~" O.D. copper t ub ing and 'Gra - t ec ' *** vacuum connectors . The pressure t r ansduce r used is a Dynis - co**** model PT 25-30, 0 to :t: 15 psig, exc i ted a t 4 V giving all o u t p u t of 1.12 # v / m bar. This was amplif ied wi th a J o h n Fluke (Model 845 AB) micro- vo l tme te r and recorded on a Smi th ' s Servoscr ibe recorder (Fig. 2).

* A1-4, 1300°C ceramic tubes supplied by Gallard and Robinson, Sydney, N.S.W. ** Shelley's product.

*** Gra-tee Inc., Los Altos, Calif. U.S.A. **** Dynisco Ine.~ Mass. U.S.A.

MEASUREMENT OF WATER FLUXES AND POTENTIALS. I 5 7 9

BD E

Fig. 1. D i a g r a m of a t e n s i o m e t e r or f low collar. A. P e r s p e x cy l inder ; B. Ceramic cy l inder ; C. W a t e r reservoi r ; D. Ceramic pa r t i c les ; E. P a n e l m e t a l c e m e n t ; F. Hole for bo l t ; G. Copper t u b e ;

H. Gra- tec v a c u u m connec tor .

T he t e n s i o m e t e r ' s w a t e r reservoi r is a t t h e same t i m e connec t ed to a w a t e r s u p p l y b o t t l e u n d e r con t ro l l ed suc t ion t h r o u g h a 3 -way h i g h v a c u u m glass t a p a n d a c a l i b r a t e d glass cap i l l a ry tube . W h e n t h e 3-way t a p is closed, t h e s y s t e m opera tes as a t ens iome te r . However , w h e n t he 3-way t a p is open to t h e cap i l l a ry t u b e t h e s y s t e m opera tes as a f low cell where t h e f lux of w a t e r i n to t i le roo t is m e a s u r e d b y t i le m o v e m e n t of a n air b u b b l e in t i le cap i l l a ry tube . I n such a s i t u a t i o n t h e p ressure t r a n s d u c e r measures t h e p o t e n t i a l of t h e w a t e r in t h e reservoir .

Fig. 2.

RECORD

Circuit diagram of the pressure transducer system.

580 H . B . SO, L. A. G. AYL3/IORE AND J. P. QUIRK

TABLE 1

Flow parameters of the ceramic particles ~-F ~ average water potential; R ~ resistance to water flow of ceramic particle column 1.2 em long, 0.60.D. and 0.16 cm I.D.; k = conductivity for water

~1 ~ Alundum particles Al-4 particles (G. ~ R.) (era H20)

R k R k (HR em -2) (cm HR -1) (HR em-~) (eln HR -1)

- 5 1.7 1.10 × 10 -1 50 3.68 × 10 -a - 1 0 0 131 1.4 × 10 -a 73 2.54 × 10 -8 -200 602 3.03 x 10 -4 76 2.42 x 10 -8 -305 1913 9.6 x 10 -5 88 2.08 x 10 -a -405 3348 5.5 × 10 -5 120 1.52 x 10 -3 -505 6648 2.8 x 10 -5 - -600 152 1.21 x 10 -3

700 264 6.95 x 10 -4

The Flow Collars are cons t ruc ted in exac t ly the same w a y as the tensio- m e t e r collars bu t are connec ted d i rec t ly to t he wa te r supp ly bo t t l e unde r cont ro l led suct ion t h rough a ca l ibra ted glass capi l lary tubing. These collars measure t he f lux of wa te r in to t he root unde r control led suct ions.

The t ens iome te r collar was packed wi th ground ceramic par t ic les (0.25- 0.5 mm). The mos t sa t i s fac to ry mater ia l s were A l u n d u m (Coors Porcelain, Colorado, U.S.A.) which were used for t he init ial expe r imen t s and A1-4, 1300°C (Gallard and Robinson, Sydney, N.S.W.) which were used for t he la ter exper iments . These mater ia ls fulfil the requ i rements of a ve ry small specific mois ture capac i ty (d0/d~0) as shown in Figure 3 and have a re la t ive ly h igh and c o n s t a n t conduc t iv i t y as shown in Table 1. The pore size dis t r i - bu t ion of A1-4 is m u c h more uni form t h a n t h a t of t he Alundum, hence a more c o n s t a n t conduc t iv i t y is observed wi th changes in mois ture po ten t i a l (Fig. 4). The ceramic mater ia l was ground wi th a rubbe r pes t le and the par t ic le size 0.25-0.5 m m was found the mos t sa t i s fac tory .

The flow collars were packed wi th t he same ceramic par t ic les or soil as desired.

A series of t ens iome te r and flow collars make up the t e n s i o m e t e r - p o t o m e t e r sys tem. A long t h i n hypode rmic needle smal ler t h a n the selected root b y a p p r o x i m a t e l y 0.1 ram, was pos i t ioned in t he cent re of t he collars dur ing packing and its r emova l p rov ided an access hole for t he root to grow th rough . The collars were sepa ra ted by a brass washer filled wi th a soft mix tu re of paraf f in oil and w a x (melt ing po in t 45°C) sandwiched be tween two lens t issue pape r smeared wi th t he same mixture . This was necessary to p r e v e n t leakage of wa te r be tween collars. A schemat ic d iagram of t he t ens iomete r - p o t o m e t e r sys t em is shown in Fig. 5.

, 60

MEASUREMENT OF WATER FLUXES AND POTENTIALS. I 581

g

3 <

.GO fill o (.3

, j .20

pc i - #

.10 o

0 0

Fig. 3.

ILl

A L - 4 , 1 3 0 0 ° C .50 o o. . o . o

~0 _~ o I

ALUNDUNI 30

20 > i;o 2~o 3;0 4;0 s;o Goo

TENSION - ~ - iVIBARS

Mois ture cha rac t e r i s t i c cu rve of t he ceramic par t ic les .

Lfl

t~

8

oJ e~

Fig. 4.

-20

.1B

.16

.14

.12

.10

.08

.06

.04

.02

0 ~o.ooo 3.000 looo ~o ~ 30

PORE ,RADIUS

Pore-s ize d i s t r i b u t i o n of t h e ce ramic ma te r i a l s used.

582 H. 13. SO, L. A. G. AYLMORE AND J. P. QUIRK

TPRE.SSU#E TENSIOMETER ~.. ,~.-.ri~:~,uu=l~ COLLAR_ AIR no

TI°. .i-==~ ........ 0 SUCTIONH20 . . . ~ I

/ VOLTMETER RECIRDER

Fig. 5. Schemat ic d iagram of the tensiometer-pressure t ransducer and flow collar systems in the exper imenta l set-up.

Environmental control o/ the plant

A schemat ic d iagram of the env i ronmenta l control of the exper imenta l p lan t is shown in Fig. 6. The aerial pa r t of the p lant is grown in a perspex cabinet (20 × 20 × 30 cma). Light was supplied by a bank of eight 40 W fluorescent tubes and two 200 W incandescent bulbs, a t all in tens i ty of 3800 ft. candles ha l fway up the cabinet for 14 hours/day. A fan dr iven by a t o y electric motor keeps the air cons tan t ly s t i r red to ma in ta in uniform conditions. Dry air was supplied at a ra te of ½ to 2 l /rain and t ranspi ra t ion rates were measured by the increase in weight of the Si-gel t rap.

Fig. 6.

8 L,0WATT FLUORESCENT

FLOW REGULATOR

200 WATT BULB

Si-GEL

R~.SEX 8R - CABINET

TENSIOMETER " '

POTOMETER IR

AIR

Schemat ic d iagram of the env i ronmenta l control sys tem.

MEASUREMENT OF WATER FLUXES AND POTENTIALS. I

TABLE 2

Composition of standard nutrient solution used

Compound Conc. of nutrients/Iitre

CaCI2.2H20 2000 ~tM KNOa 200 ~xM MgSO4.7HsO 100 [xM NH4NO3 100 ~zM NaH2PO4.7H20 50 [xM Fe-sequestrian 9 ~M NaSiO8 100 [zM

ZnSO4.7H~O .475 [xM (NH4) aMoT024.4H20 .02 lxM HaBOa .3 [zM CuSO4.5H20 .1 ~M CoSO4.7H~O .045 [xM MuSO4.H20 .50 [xM

583

The day t empera tu re in this growth cabinet was 32.5 ± 0.5°C whereas the n ight t empe ra tu r e was equal to the room t empera tu re of 22 ° ± 0.5°C.

W i t h the except ion of a preselected root, the root sys tem was grown into a 600-ml suct ion flask conta in ing aera ted nu t r i en t solut ion at s tandard or 2½ t imes t h a t of s t andard s t rength provid ing osmotic pressures of -- 106 and --266 m bars (measured on a commercia l osmometer) . Composi t ion of the s tandard nu t r i en t solut ion is given in Table 2. This solut ion was renewed every morning and evening to p reven t s ignif icant changes in concentrat ion. The selected root was directed ver t ica l ly into the access hole by means of a curved th in po ly thene tube. The top of the root sys tem and the selected root were kep t mois t by wrapping wet Meenex tissues around i t w i thou t ac tua l ly touching the roots.

Thus s teady s ta te flow of wa te r th rough the p lan t was app rox ima ted by main ta in ing cons tan t condit ions th roughou t the day.

P l a n t material

The p lan t used was hybr id maize (supplied by Wesfarmers T u t t B r y a n t P ty . Ltd.) and the same ba tch of seeds was used for all experiments . The seeds were soaked in aera ted water for 4 hours and germinated on a plast ic t r a y conta in ing a th ick layer of k leenex wet ted wi th 2000 # M CaC12. The t r a y was then covered wi th 'g ladwrap ' and kep t ill the dark at room t empera - ture for 4 days af ter germinat ion. This produced seedlings wi th 1½-2 inches of Hypocoty l . 6 un i form seedlings wi th well developed root systems were selected and t ransferred into a gallon plast ic bucke t conta ining s tandard nu t r i en t solution. The seedlings were suppor ted b y plast ic foam pieces inside 1" holes made in the top of the buckets , hence the need for long

5 8 4 H. B. SO, L. A. G. AYLMORE AND J. p. QUIRK

hypoco ty l s . These were t h e n g rown u n d e r iden t i ca l cond i t ions of l ight , t e m p e r a t u r e a n d d a y l e n g t h as those g rown in t he g r o w t h c a b i n e t d u r i n g t h e a c t u a l expe r imen t s . T he roots were k e p t a t 21°C. A su i t ab le p l a n t was selected a t t he age of 20-21 days w h e n t he second c rown of noda l roots were 3 - 5 cm long. A su i t ab le roo t to use is one t h a t ha s a c o n s t a n t d i a m e t e r ove r i t s l e n g t h (in c o n t r a s t to some t h a t are tapered) . A p l a n t would be selected w i t h such a roo t growing in t h e oppos i te d i r ec t ion f rom t h e h y p o c o t y l (which is now t h e sec t ion be tween f i rs t node a n d seed) since th i s f ac i l i t a t ed i ts a s s e m b l y in to t h e e x p e r i m e n t a l sys tem.

Experiment 1 : T he t e n s i o m e t e r response t ime. The response t i m e of t h e t ens iome te r -p r e s su re t r a n s d u c e r s y s t e m w i t h o u t

t he ce ramic par t ic les was m e a s u r e d b y enclos ing t h e col lar in a n a i r t i g h t w a t e r s y s t e m on to wh ich cont ro l led suc t ions can be appl ied. The response t i m e w i t h ce ramic par t ic les packed in to t h e col lar was m e a s u r e d b y us ing a n A l u n d u m t u b e (1.6 m m O.D.) as an ar t i f ic ia l root . A s u p p l y b o t t l e was c o n n e c t e d to th i s t u b e a n d con t ro l l ed suc t ions can be appl ied to t h e w a t e r in t h i s bo t t le .

Experiment 2: T he response of t h e t e n s i o m e t e r to roo t w a t e r p o t e n t i a l changes .

T he t e n s i o m e t e r col lar was e m b e d d e d 3 cm down in a c o l u m n of soil a n d a se lected single roo t was g rown in to a n access hole in t h e soil such t h a t i t grew in to t h e cen te r of t h e t ens iomete r . The t e n s i o m e t e r was d r a ined to a 60 m b a r suc t ion to ass is t t he roo t in r e m o v i n g t h e w a t e r in t h e i n t e r p a r t i c l e pores of t he ce ramic bed a n d i ts r esponse was t h e n m o n i t o r e d ove r a pe r iod of 6 days. T he m a i n roo t s y s t e m was g rown in n u t r i e n t so lut ion. The a i r supp ly used for th i s e x p e r i m e n t i n v o l v e d h u m i d i t y towers * w i t h re la t ive humid i t i e s of 33% a n d 76%.

Experiment 3 : 2 series of 3 e x p e r i m e n t s were car r ied ou t w i t h t h e p o t o m e t e r ( t ens iome te r a n d 1 flow col lar in series) p a c k e d w i t h ce ramic par t ic les to check t he v a l i d i t y of t h e t e n s i o m e t e r m e a s u r e m e n t s . The f i rs t series was car r ied ou t w i t h t h e p l a n t in da rkness to reduce t r a n s p i r a t i o n to a m i n i m u m . T he g r o w t h c a b i n e t was comple t e ly covered w i t h Al-foil for th i s purpose . T he second series was car r ied ou t w i t h t h e p l a n t s t r ansp i r ing .

The t e n s i o m e t e r was nsed to m o n i t o r t he x y l e m tens ions a n d t h e f low col lar or t e n s i o m e t e r col lar was used to measu re t h e f lux of w a t e r in to l he roo t u n d e r d i f fe ren t suc t ions appl ied to t h e s u p p l y bot t le .

* Madden , J. J. and Nordon , P. Air supply of constant humidity. Rev. Sci. Instr. 58, 561 (1967).

M E A S U R E M E N T O F W A T E R F L U X E S A N D P O T E N T I A L S . I

T A B L E 3

R e s p o n s e t imes of t he t e n s i o m e t e r - p r e s s u r e t r a n s d u c e r s y s t e m

585

C h a n g e in app l i ed R e s p o n s e t i m e for

s u c t i o n s (S) (era H~O) .63 AS .95 AS .99 AS

0 to 10 1.9 see 2.9 see 4 sec

500 to 525* 1.6 sec 2 .2 sec 4.5 sec

525 to 500* 1.4 sec 3.0 see 5.8 see

E x p o n e n t i a l r e sponse TR 3 T ~ 4 . 6 T ~

* Resu l t s a r e a v e r a g e s of t h ree r ep l i ca te s .

R E S U L T S A N D D I S C U S S I O N

Response o/the tensiometer-pressure transducer system The tensiometer--pressure transducer system without the cera-

mic particles showed a very fast response to suction changes at the ceramic surface as shown in Table 3.

When packed with ceramic particles (Alundum) these particles formed essentially an extension of the tensiometer cup itself. How- ever the response time is slower (Table 4) and a typical response curve is shown in Figure 7. These response curves are not expo- nential and for that reason the response time for 0.99 AS was used. These were less than 5 rain. for the tensiometer with ceramic

T A B L E 4

R e s p o n s e t i m e s o f t e n s i o m e t e r a n d A l u n d u m pa r t i c l e s

S in i t i a l S f ina l R e s p o n s e t i m e for

(cm H20) (era H20) .99 AS (minutes )

20 30 4.0 30 40 2.0

0 40 3.6 40 0 3 .4

0 100 1.75

100 0 4 .50

100 110 0.95

100 200 2 .50 200 300 1.75

400 425 1.33 500 525 4.50

586 H. B. SO, L. A. G. AYLMORE AND J. P. Q U I R K

b

T INITIAL

~120 MBARS

"r F1NAL

200 MBARS

Fig. 7.

1

TIME (MIN)

Response curve ot tensiometer.

particles packed into them, which was considered small enough compared to the expected slow rate of change of the root water potentials.

However, the artificial root used in this experiment has an infinite capacity to absorb or supply water to the tensiometer system and hence the tensiometer response time was determined only by the flow properties of its components as defined by the equation 7 15.

TI~ ~ 1/KS where TI~ = tensiometer response time

K = conductance of the tensiometer cup S = sensitivity of the pressure gauge

A real plant root, however, does not have an infinite capacity to absorb water and therefore the rate of water uptake will tend to determine the overall response of the tensiometer-root system.

Response o~ the tensiometer to root water potential changes

The result of Experiment 2, presented in Figure 8, is plotted as root water tensions versus time. On tile third day the tension suddenly increased sharply and continued to do so at a rate of approximately 9 mbars hour. This was due to the active portion of the root growing through the tensiometer. Since the tensiometer was approximately 3 cm away from the root tip initially, this would correspond to a root growth rate of about 1/cm day. Previously measured rates ranged from 1½ to 2½/cm day under similar conditions. However, the abrasive ceramic particles of the tensiorneter


Z

Z

h t

~201 B o

Fig. 8.

, . . . . . : , - . . . . . ,, , . . . . . , . . . . : L I G H T . . . . . . : . . . . . i : ,

D A R K ! l , l ', ',

! ', z i , ' : 0 ',

i i i

I ,2 y ,2 y 12 y 12 2 r, ,2 2t, SEPt . 17.19'71 l g 19 2 0 21 2 2

Response of t he t ens iome te r to root tens ion changes induced by var ious t r e a t m e n t s .

offer a high resistance to deformation and would have reduced the growth rate of the root. When the lights and fan were switched on that morning the tension increased even more rapidly at approximately 23 mbars/hour, and this rate was maintained throughout the day. When the lights and fan were switched off at night the tension dropped rapidly. This did not happen on the previous night as the tensiometer was in the process of coming to equilibrium with the root water potential. A time lag of approximately ½ hour exists between the switching off of the lights and the rapid decrease in tension. The largest effect on the root tension was that of light which was expected since light controls the sto- mates, a major factor in transpiration. Relatively minor effects were noticed from changing the humidity of the air. The tension dropped rapidly on the sixth day when the soil column was wetted from the bot tom and it appears that free water leaked into the tensiometer, hence the rapid decrease to almost zero tension.

This preliminary experiment showed that the tensiometer did in fact respond to root water potential changes, the largest changes being associated with changes in transpiration due to opening and closing of the stomata stimulated by light. As water could only move into or out of the tensiometer through the root itself, water vapour transfer being negligible, the tensions measured must be related to those of the root: The question arises as to which part of the root was involved, the root-soil interlace, the cortex or the xylem? Also, how efficient is the tensiometer in reacting to root water potential changes ?


The root water potential (Experiment 3)

The tensiometer will only measure the water potential at the root surface if tile epidermis is a perfect semi-permeable membrane. Although the epidermis of maize is known to have a higher resistance to water flow than the cortex 17, the existence of the root free spacel 9 proces that it is not a semi-permeable membrane. Hence it eliminates the possibility of the tensiometer measuring the root- soil water potential.

At equilibrium the tensiometer will measure the xylem water potential only if no longitudinal movement of water occurs in the cortex of the root. There is no a priori reason why such movement will not occur and if it occurs to a significant magnitude, then the tensiometer would measure the water potential of a point in the cortex region.

The results of the first series of tensiometer-potometer experiments are presented in Figure 9 where the tension measured by the tensiometer was plotted against time. It is obvious that it varied with light and darkness although on curves B and C the changes were relatively small. When a change in suction was applied to the flow collar adjacent to the tensiometer, no systematic or significant

m E

~0

o

Fig. 9.

- - - 2 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . (A)

I . . . . . ' 600 / ~ Z,5 6 • ,7 8 9 1 0 11 ', 4OOl l i \ . i f i ~ * ' ~ ' ~

I i i i 1 / I " - ? :

200 ) ,1 , : : • i i = i

0 lz : :12 : 1,z 17 ~? 17 : ols~t2g/3o [c~1 ~ 2 ~ 3 ~ 4

1971 24 • (B)

D_A~K ITGHT DARK 500 : ' " ' ! :- "': ~ r . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11112

I ! : i [ ! ~ 4 5 67 891o i ~ "

Z°°l ' ! ' " ~ i n112 : 12 ! 12 12 12 12 12 J 12 - lOCI:. 22 I 23 I 2z. I 25 I 26 I 27 l 28 ,,I.

2419~1 24 24 24 24 24 7.4 (c)

6 ° ° I i - : ' , = . . . . . . . . . . . . . . . . . . . . i r-- ' , i i ' 8 i

4oo \', L ~ ~ 5 67;~

2001 : ' '~ ~ '

I : , : , ,

O, 12 ', ', ;12 12 1,2; : :1,2 NOV28 214 29 214 30 /DEC.1 / 2 I 1971

Tens iomete r readings wi th t ime for exper imenta l series 1.

M E A S U R E M E N T O F W A T E R F L U X E S A N D P O T E N T I A L S . I 589

changes occurred in the tensiometer readings. If longitudinal movement in the cortex was significant then a change in suctions on the flow collar should be reflected by the tensiometer. The amount of water necessary to bring about such a change is very small as the volume displacement of the pressure transducer is approximately 10 -3 cc for a full scale deflection of 1000 m bars. Therefore it was concluded that the tensiometer readings must be associated with the xylem water potential.

This conclusion can be supported by a simplified calculation of the ratio of the longitudinal and radial resistances involved. Assume a root radius r0 of 0.5 mm. Microscopic examinations revealed that the inner radius ri of the endodermal ring was approximately 0.2 mm. If K is the conductivity of the parenchyma cells in any direction, the longitudinal resistance RL can be obtained from the expression of Flux F = A+/RL = ~(r0 2 -- ri2)K • (A~/l) where 1 is the length of the root section involved (1.2 cm for tensiometer). Thus RL = 18.2/K. Similarly the radial resistance Rg can be obtained from the solution to the flow equation given by G a r d n e r a as A~ = (Q/4rcK)in (ro/ri) 2. Thus RI~ when calculated equals 0.145/K. Therefore the ratio RL/Rg = 126. Although admittedly

Legend to Fig. 9

C U R V E A. C U R V E B. C U R V E C.

1. ~ of root sys tem cut off. 2. Suct ion increased: 80 to

2 2 0 m b a r s 3. Lights and fan turned

off. 4. All roots except tested

cut off. 5. S d e c r e a s e d : 220 to 120

6. S i n c r e a s e d : 120 to 320

7. S d e c r e a s e d : 320 to 220

8. S i n c r e a s e d : 220 to 320

9. S d e c r e a s e d : 320 to 220 10. S increased: 220 to 407 11. S increased: 407 to 520

l. ½ of root sys tem cut off. 2. Lights and fan turned

off. 3. S i n c r e a s e d : i00 to 200

4. S i n c r e a s e d : 200 to 300

5. S d e c r e a s e d : 300 to 100 6. S i n c r e a s e d : 100 to 200

7. S i n c r e a s e d : 200 to 500

8. S d e c r e a s e d : 500 to 300

9. S d e c r e a s e d : 300 to 200 10. S decreased: 200 to 100 11. Tens iometer f lushed and

readjusted 12. S i n c r e a s e d : 100 to 300

1. Lights and fan turned off. 2. S i n c r e a s e d : 80 to 120

3. All roots except tested cut off.

4. S i n c r e a s e d : 120 to 160

5. S i n c r e a s e d : 160 to 208

6. S i n c r e a s e d : 208 to 280

7. S d e c r e a s e d : 280 to 120

8. Tens iometer f lushed and readjusted.

9. S i n c r e a s e d : 120 to 368

590 H. B. SO, L. A. G. AYLMORE AND j . P. QUIRK

- .0005

APPLIED SUCTION

(rnbor s)

6°° l

30C

200'

100'

nsiometer ading~

o o

• 0005 .0010 .0015

FLUX OF WATER C C / H R / 1 . 2 C M ROOT

Fig. 10. Flux v s suction applied to the flow collar (Exp. I).

an oversimplification, this indicates that leakage of water from adjacent collars through the cortex amounts to less than 1%.

This conclusion is further supported when the flux of water measured by the flow collar is plotted against the suction applied on that collar (Fig. 10) producing a curvilinear relationship. The tensiometer readings agree well with the suction that produced zero flux. The other two experiments were not plotted in this fashion as the fluxes were too small. I t is interesting to note from Fig. 10 that when the suction applied was greater than the xylem tension, water moved out of tile root, a phenomena which has been previously observed 11. This indicates that the tensiometer should react to a decreasing as well as an increasing xylem tension.

The second series of experiments with the plants transpiring was more concerned with the flux-suction relationship. No continuous recording of the tensiometer was at tempted mainly because the flow collar was not operating satisfactorily during these experiments and flux measurements were carried out on the tensiometer collar.

The results of the first two experiments were presented in Fig. 11. The horizontal lines are estimates of the errors in measurements. Here again the flux-section relationships are curvilinear and the


L A (EXP.4)

o 100 5 \

<[

50

Fig. 11.

B(EXP .5 )

4

150

5 0

• 0005 .0010 .0005 .0010

FLUX - CCIHR/1.2 CM ROOT F L U X - CCIHR/1.2 CM ROOT

Flux-suction relationships for the roots of experimental series 2.

15x10' o

u 10

u u

z

cD

s

o

0 0

Fig. 12.

/

4 /o

o EXP. 4

a EXP. 5

CJ EXP. G

J

* i i i /

100 200 ~00 400 500 600

TENSION DIFFERENCE MBARS

Root resistance plotted against tension difference for experimental series 2.


tensiometer readings agree well with the suction producing zero flux 0f water.

It is obvious from Fig. 10 and 11 that within the conditions of these experiments the root tissues show a similar flux-suction relationship regardless of whether the driving force is predominantly osmotic (in the absence of transpiration) or transpirational in origin.

Curves A and B of Fig. 11 are almost identical and subsequent measurement indicates that this replication was undoubtedly for- tuitous. However, each curve was in itself reproducible and did not exhibit any significant hysteresis effect, indicating the uniqueness of the flux-suction relationship. Hence a unique relationship must exist between the resistance of the root to water flow and the suction gradients across the tissues. This is a direct proof that the analogy with Ohm's law is valid for the steady state flow of water through the plant root.

When this resistance was calculated and plotted against the suction difference across the root tissue, a linear relationship is obtained as shown in Fig. 12. The correlation coefficient for each line was highly significant. The root resistance is the total resistance of the tissues of 1.2 cm of root to the radial flow of water, hence the unusual dimensions used.

Since the xylem potential remained constant, the linear relationship could also be interpreted as the resistance decreasing propor- tionally with increasing average tension of the root tissues. This is in agreement with observations by earlier investigators 2 10. How- ever, the mechanism of this reduction is not known.

The tensiometers response time

As previously discussed, the response time of the tensiometer- root system will depend largely on the flux of water into the root. This was measured at the end of the first experiment of the second series. The tensiometer was equilibrated with a suction of 100 m bars. The xylem tension was approximately 200 m bars. The capil- lary flow system was then cut off and the tensiometer allowed to equilibrate with the xylem tension. After 10 hours, the tensiometer finally equilibrated at 244 m bars. The amount of water drained from the ceramic particles between 100 and 224 m bars was approximately 0.007 cc estimated from its moisture characteristic curve. Thus the average rate of water uptake was roughly 0.0007 cc

M E A S U R E M E N T OF W A T E R F L U X E S AND POTENTIALS. I 593

h -1 agreeing favourably with the observed rates of uptake of curve A in Figure 11, which ranged from 0 at 206 m bars to 0.001 cc h -1 at 100 m bars suction.

The above calculations clearly show the dependence of response times on the ability of the root to absorb water. With higher uptake rates such as that in the last experiment of the second series (range: 0-0.01 cc h -1) the response time could be reduced considerably to the order of 1 to 2 hours for 0.99 AS.

It is clear that with response times of this order, the tensiometer is more suitable for steady state rather than transient conditions.

In conclusion, it has been shown that the root tensiometer described in this paper is capable of measuring the xylem water potential of a plant root subjected to steady state conditions with respect to water flow. It has also been shown, that under the con- dition of this experiment a unique linear relationship exists between the flux of water or the root resistance to water flow and the water status of the root tissues.

A C K N O W L E D G E M E N T

T h i s w o r k w a s f i n a n c e d b y g r a n t s f r o m t i l e W e s t e r n A u s t r a l i a n W h e a t I n d u s t r y R e s e a r c h C o m m i t t e e w h o s e s u p p o r t is g r a t e f u l l y a c k n o w l e d g e d .

Received 30 December 1974

R E F E R E N C E S

1 ]3r iggs , G. E. and R o b e r t s o n , R. N. Apparent free space. Ann. Rev. Plant Physiol. 8, 11-30 (1957).

2 B r o u w e r , R., Water absorption by the roots of Vicia/abe at various transpiration strengths. III. Changes in water conductivity artificially obtained. Proc. Kon. Ned. Akad. Wet. C57, 68-80 (1954).

3 C a r y , J. W. and F i s c h e r , H. D., Plant water potential gradients measured in the field by freezing point. Physiol. Plantarum 24, 397-402 (1971).

4 De Roo, H. C., Water stress gradients in plant and soil root systems. Agron. J. 61, 511-515 (1969).

5 F i s c u s , E. L., In situ measurements of root-water potentials. Plant. Physiol. SO, 191-193 (1972).

6 G a r d n e r , W. R., Dynamic aspects of water availability to plants. Soil Sci. 89, 63-73 (1960).

7 K l u t e , A. and G a r d n e r , W. R., Tensiometer response time. Soil Sci. 93, 204-206 (1962).

8 H o n e r t , T. H. v a n der , Water transport in plants as a ca tenary process. Dis. Faraday Soc. 3, 145-153 (1948).

594 MEASUREMENT OF WATER FLUXES AND POTENTIALS. I

9 K r a m e r , P. J., Outer space in plants. Science 125, 633-635 (1957). 10 K u i p e r , P. J. C., Water transport across membranes. Ann. Rev. Plant Physiol. 23,

157-172 (1972). 11 Mii l le r -Stohl , W. R., The problem of water outflow Iromroots, in B. S l a v i k (Ed.),

Water stress in plants, pp. 21-29. Czech. Acad. Sci. Prague (1965). 12 N e w m a n , E. I., Resistance to water flow in soil and plant. I. Soil resistance in

relation to amounts of root. Theoretical estimates. J. Appl. Ecol. 6, 1-12 (1969). 13 N e w m a n , E. I., Resistance to water flow in soil and plant. II . A review of experi-

mental evidence on the rhizosphere resistance. J. Appl. Ecol. 6, 261-272 (1969). 14 Ph i l ip , J. R., Plant water relation - some physical aspects. Ann. Rev. Plant

Physiol. 17, 245 268 (1966). 15 R i c h a r d s , L. A., Methods of measuring soil moisture tensions. Soil Sei. 68, 95-112

(1949). 16 S l a t y e r , R. O., Plant water relationships. Academic Press, London & N.Y. (1967). 17 Woo l l ey , J. T., Radical exchange of labelled water in intact maize roots. Plant

Physiol. 40, 711-717 (1965).

measurement of water fluxes and potentials in a single root-soil system

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