Plant and Soil 48, 599-612 (1977) Ms. 3249

E F F E C T OF C H E L A T I N G A G E N T S

ON T H E K I N E T I C S OF D I F F U S I O N OF ZINC

TO A S I M U L A T E D R O O T SYSTEM AND

ITS U P T A K E BY W H E A T

by M. K. SINHA and B. PRASAD

Department of Soils Punjab Agricultural University

Ludhiana, Punjab, India

SUMMARY

The kinetics of Zn diffusion to a simulated root system was investigated at a controlled rate of exudation of mobile chelating agents through porous ceramic tubes into a soil tagged with 65Zn tracer. The chelating agents en- hanced the rate of Zn diffusion from the soil into the simulated root to varying extents depending upon their reIative efficiency in increasing the concentra- t ion gradient of diffusible Zn. The rate of Zn diffusion from the soil into the simulated root conformed to pseudo-first order kinetics. The uptake of Zn by wheat plants was significantly increased when a constant flux of DTPA, EDTA and FA (fulvic acid) into the 65Zn labelled soil was maintained during a 60 days growth period. A significant linear regression between the concen- trat ion of soluble Zn in soil and its uptake by wheat was observed. The cal- culated diffusive flux of Zn based on the assumption of a constant concentra- tion of Zn at the root surface bore a curvilinear relationship with Zn uptake by wheat.

INTRODUCTION

Chelation of metal ions occurring in the vicinity of plant roots plays an important role in overcoming the rate-limiting steps of dis- solution and diffusion which limit the movement of micronutrient cations from the soil to plant roots. Chelating agents may originate as plant root exudates, as substances released from decomposing organic matter, as metabolites of rhizosphere microorganisms or as chelated fertilizers added to soils. In a porous medium like the soil, the removal of metal ions from a chelating ligand by plant roots

600 M. K. SINHA AND B. PRASAD

gives rise to a dynamic system in which the meta l ions develop a concentra t ion gradient towards the root, the chelating agent de- velops a concent ra t ion gradient away from the root, and the chelat- ed meta l ions establish a concentra t ion gradient towards the root so tha t the diffusive movemen t of the chelated meta l ions to the plant roots is considerably augmented. The chelat ing l igand released at the root surface moves back from the root surface to the soil ma t r ix where it recombines with meta l cations resulting in a subsequent movemen t of the newly formed metal chelate towards the root. Thus, exudat ion of mobile chelating agents, chelat ion and back-diffusion of meta l chelates marked ly increase the t ranspor t and availabi l i ty of micronut r ient cations to plant roots 3 5 ]2. Es t imat ion of the con- t r ibu t ion of control led exudat ion of mobile chelating agents into the soil to the diffusion of Zn has been a t t e m p t e d using s imulated root systems. However , the kinetics of desorption of labile Zn and the subsequent diffusion of Zn from soils into such s imulated root sys- tems has not been invest igated in studies repor ted earlier. Such arti- ficial roots do not s imulate the active and selective physiological functions of a plant root, bu t do simulate the physical bounda ry conditions which regulate the diffusion of a nu t r ien t in the soil zone close to the plant root.

The objectives of this invest igat ion were: (i) to s tudy the kinetics of Zn diffusion to a s imula ted root at control led rate of exudat ion of soluble chelating agents to a 65Zn-labelled soil and (ii) to deter- mine the effect of a constant flux of chelat ing agents to the soil on the up take of Zn by wheat plants.

GENERAL CONSIDERATIONS

The rate of diffusion of Zn to a simulated root system when chelating agents exude into the soil at a controlled rate would be regulated by several constraints such as (i) the simultaneous equilibria of the chelating ligands with metal ions as defined by their chemical activity in the ambient soil solution, the formation constant of the metal chelates and the pFI of the soil suspension, (ii) diffusion of the chelating ligands away from the root in re- sponse to their concentration gradients, (iii) equilibrium of the chelating ligands with the solid phases in the soil system and (iv) the diffusion of chelated Zn towards the simulated root system in response to its own con- centration gradient.

The rate-limiting step during the back-diffusion of Zn to the simulated

KINETICS OF DIFFUSION OF Z n 601

root is the t r ans fo rmat ion of the labile solid phase Zn into soluble Zn com- plex. I t is general ly bel ieved t h a t the desorpt ion of solid-phase Zn is diffusion control led. There is also the possibi l i ty t h a t t he chela t ing agents form a surface comple x wi th adsorbed Zn which t h e n recedes into solution, In e i ther case, t he assumpt ion t h a t the ra te of diffusion to the s imula ted root sys tem is p ropor t iona l to t he a m o u n t of po ten t i a l ly diffusible Zn in soil remains valid. The overal l desorpt ion react ion which increases the concent ra t ion gra- dient of t o t a l diffusible Zn due to chelat ion m a y be indicated as follows :

Soil-Zn (labile) + L ~- ZnL + Soil (1)

A simple ra te expression for the above react ion and the resu l tan t diffusion of Zn to the s imula ted root would be second-order wi th respect to the amounts of labile Zn and the chela t ing ligands. Bu t in the present exper iment a con- s t an t f lux of the chela t ing agents to the soil was main ta ined and thus a pseudo-f i rs t order t r e a t m e n t of the da t a on diffusion of Zn was a t t empted . The appl icable in tegra ted ra te equa t ion is:

Log (N0-Rr,) = log No -- K/2.303(t) (2)

where K is the ra te constant , No is the to t a l diffusible Zn in the soil at zero t ime (expressed in counts of 65Zn), Nt is the a m o u n t of Zn diffused at any t ime, t (expressed in counts of 6~Zn in solut ion in receiving flask). I f a p lot of log (N0-N,) vs t ime is linear, the react ion is pseudo-first order wi th respect to the q u a n t i t y of diffusible Zn. The slope of the line will be - -K/2 .303 f rom which the diffusion ra te cons tan t can be computed . The in te rcep t of the line will be the log of the to t a l diffusible Zn of the soil at zero t ime.

The ra te of t r anspor t of Zn f rom the soil to a p lan t root depends on the ra te of r emova l of Zn ions f rom the root soil- interface by ion absorpt ion process in the p lan t 1. The concent ra t ion of a nu t r i en t at the ac t ive ly ab- sorbing root surface cannot be measured by present ly known methods 9. All a rb i t ra ry va lue of the concent ra t ion of the nu t r i en t at the root surface (Cr) m a y be chosen, or a bounda ry condi t ion assumed when Cr remains fa i r ly cons tan t t h roughou t the diffusion period or growth of plants. In the present s tudy, the concent ra t ion of chela ted Zn at the root surface was assumed to approach a cons tan t va lue of zero and was supposed to remain so dur ing the ent ire growth period of plants. O ' C o n n o r et al. s used such a boundary con- di t ion for calcula t ing the concent ra t ion of Fe at the root surface of corn when Fe chelates were appl ied to a soil.

The diffusive f lux of Zn to t he root surface of whea t plants was calculated using the E q u a t i o n :

~ t = 2 (Co-Cr) ] / D p b t (3) r

Mt = diffusive f lux (btg/cm2 of the root surface) ; Co ~ equi l ibr ium concen- t r a t ion of Zn in. soil solut ion (t*g/mt) ; Cr : concent ra t ion of Zn at the root surface ; Dp : self diffusion coefficient of Zn in soil (cm~ sec -1) ; b : capac i ty factor of soil for Zn and t = t ime of growth

602 M. K. SINHA AND B. PRASAD

MATERIALS AND METHODS

The soil used in this study was a loamy sand member of the Jodhan series in the group Ustochrepts. Bulk surface (0-28 cm) sample of the soil was col- lected. The chemical properties of the soil were as follows: p i t 8.4; C.E.C. 6.2 (meq/100 g); Organic carbon 0.18 per cent; I)TPA-extractable Fe 5.6 ppm; Cu 0.28 ppm; Zn 0.48 ppm and Mn 8.0 ppm.

I4inetics el Zn diNusion to the simulated root system

Ten kilograms of the soil were treated with deionized water containing 2 mCi of carrier-flee 65Zn isotope. After air-drying, the soil was wetted a second t ime with deionized water and air-dried again. The soil was then crushed and mixed thoroughly. A similar t reatment was given to another 10 kilograms of the soil with 5 ppm of Zn in the form of ZnSO4.

A simulated root system similar to that used by E l g a w h a r y et al. ~ was used to study the kinetics of Zn diffusion. After testing the ceramic tubes for air leakage, 750 grams of 85Zn-labelled soil were packed into each con- tainer. Before covering the container, water was added to bring the soil to saturation. Soil in different containers was allowed to equilibrate for 96 hours with water in a 1 litre-flask, 67 cm below the ceramic tube, establishing a suction of 67 cm of water in the soil solution. The solution remaining in equilibration flask contained no significant counts of 65Zn tracer. Convective movement of solution into the simulated root was eliminated by maintaining the same suction (67 cm) of water in the soil and in the ceramic tube. During operation, solution flowed slowly from the supply flask through the ceramic tube embedded in the soil to the receiving flask. The rate of flow was regu- lated at 100 ii11/24 hours by adjusting the hydraulic head (usually 2 to 3 cm) between the supply and receiving flasks.

The diffusion cells were placed in a constant temperature room at 25 ± 1 °C. Eight diffusion cells were constructed, each with different solution in the supply flask. The following solutions were used : 1. ttyO, 2. Citric acid, 10-3M, 3. Glycine, 0.1 g/1.4. HC1, 10-YM, 5-6. EI)TA, 10-~M, 10-4M. 7. Fulvic acid (FA), 10-3M, 8. DTPA, 10-SM.

Similarly six diffusion cells were prepared for the soil treated with 5 ppm Zn containing the above solutions except 3 and 6 in the supply flask. The solution collected in the receiving flask was transferred to a beaker and treated with 2 ml of 6 N I-IC1 containing 5 ppm of Zn. The solution was then concen- trated to a volume of 2 to 3 ml on a steam bath. The act ivi ty of 65Zn in the concentrated solution was determined on a single channel gamma-ray spectrometer. The activity of 65Zn ill the solution collected ill the receiving flask was considered as the amount of ~sZn diffusing into the simulated root System from the ambient soil matrix.

EHect el a constant flux o/chelating agents on the uptake o[ Zn by wheat

After the laboratory experiment, the simulated root system was shifted to a glasshouse and eight healthy wheat plants were grown in each container.

KINETICS OF DIFFUSION OF Zn 603

A constant flux of tile complexing agents and acid was maintained for a 60 day growth period of plants. The solutions were renewed at intervals of five days to prevent microbial growth. After 60 days, the plants were harvested. The root and shoot of plants were separated. These were then acid washed, dried, weighed, and ground. One gram of the ground materials was digested in a mixture of H2SO4, HNOa and I-IC104. The volume of the digested ma- terial was made to 50 ml and a 2-ml aliquot was used for counting GSZn in a well-type scintillation head on a gamma-ray spectrometer. The uptake of 65Zn by shoot and root was calculated using the specific act ivi ty of 65Zn for each t reatment . This was determined by shaking a 10 g sample fo 65Zn- labelled soil separately with 20 ml of the solution of each complexing agent.

RESULTS AND DISCUSSION

E//ect o/ chelating agents on cumulative di/]usion o/ Zn to simulated root The results depicted in Fig. 1 represent the cumulative net counts

per 100 seconds of 65Zn (corrected for background) collected in the receiving flasks. Measurements were made at 24-hour intervals over a 7-day period when the chelate solutions were passed through the simulated root system. Distilled water served as a standard reference solution.

A constant flux of DTPA, FA, and EDTA solutions into the soil signficantly increased the movement of Zn into the simulated root. When l O-3M solutions of DTPA, FA, and EDTA were passed through the ceramic tube into the soil, the diffusion of 65Zn into the simulated root was increased 18.7, 16.7 and 15.1-fold over that when water only was flushed into the soil through the ceramic tube. The EDTA at a concentration of 10 -4 gave a 9.2-fold increase in Zn dif- fusion into the simulated root over the amount of Zn diffused when water alone was used. Citric acid, glycine and HC1 exerted similar effect on the diffusion of Zn into the simulated root but their effect was not as pronounced as that of DTPA, EDTA and FA. The relative effect of the chelating agents and acid on the cumulative diffusion of Zn into the simulated root system varied in the follow- ing order:

DTPA > FA > EDTA > HC1 > Citric acid = Glycine > H20.

The experiment was repeated after adding 5 ppm of Zn to the soil in the form of 65ZNSO4. Results indicated that this application of Zn further increased the cumulative diffusion of Zn into the sim- ulated root.

604 M. K. SINHA AND B. PRASAD

"t / tn ,DTPA 1(3~M

8 < 12OO U') ,/e FULV,C ACID 163M

m . / . . O EDTA 103M O , A A ~ t S

p,,.

0 0 et o w I- J

t n

- 4 I O M ,, ~" ...~A EDTA

6 0 0 - . 1

j .

0 4 0 0 - , , " z ,#,. Q ,"

2 0 0 - ~, ~ - " .• CITRIC ACID 163M s" ~ . l t ~ - ~

u. - - - - - " GLYCINE O'19/,~ -4 ~, - ~ ' x ~ - " - - ~ " ~ : - . ~ CITRIC ACID I0 M

~ 0 T ,o I 2 3 4 5 6 7

TIME IN DAYS

Fig. I. C u m u l a t i v e n e t c o u n t s of 65Zn v s t ime.

The observed findings may be explained on the premise that in response to a concentration gradient, a part of tile mobile chelating agents diffused outward through the wall of the ceramic tube into the surrounding soil and increased the concentration of Zn in soil solution by forming soluble Zn complexes. The newly formed Zn complex established a concentration gradient towards the simulated root cylinder. A fraction of the newly complexed Zn diffused back into the ceramic tube and was carried to the receiving flask. Con- vective flow of Zn into the simulated root was negligible. The 2.6 fold increase in diffusion of Zn caused by citric acid was considerably less than the 100-fold increase reported by H o d g s o n et al. 5 but was very similar to the 3 fold increase in Zn diffusion in a soil noted by E l g a w h a r y et al. 3.

The differences in tile results of the investigations mentioned above are those of closed and open systems in which the relative

14(

KINETICS OF DIFFUSION OF Z n 605

/ /

1 2 0 0

I000

8 0 0

600

4 0 0

200

[ I I I I I I 01-0 I -4 1.8 2 . 0 2.2 2 -4 2-6

Fig. 2. Cumulative diffusion of Zn from soiI to root v s square root of time.

efficiency of the sinks for accumulating the diffusing Zn ions differed widely. In the present experiment, only a small fraction of the total complexed Zn actually diffused back toward the simulated root. In real soil-plant systems, however, the convective flow of water to the root may bring the chelated metal closer into the root zone. The interaction of convective movement and diffusion of Zn is expected to further augment the flux of Zn to the plant roots4.

606 M. K. SINHA AND B. PRASAD

E//ect o/chelation on the kinetics o I Z n di//usion to simulated root

The rate-limiting step affecting the movement of Zn is the con- version of solid phase labile Zn into soluble Zn complex and its dif- fusion back to the simulated root. The relative efficiency of different chelating agents in enhancing diffusion of Zn from the soil to the simulated root would depend on the kinetics of the desorption pro- cess. The cumulative diffusion of Zn from the soil to the simulated root was linearly related to the square root of time, (t½), throughout the 7-days of intermittant diffusion. This relationship is illustrated in Fig. 2. It has been reported that diffusion controlled reactions display similar characteristics 6 10 11

The data on cumulative diffusion of Zn into the simulated root were fitted into a diffusion equation given by C r a n k 2. This equation describes the radial diffusion of a substance into a cylinder, in which the concentration of the diffusing substance is initially uniform. This model may be applied to diffusion of zinc into the simulated root from the soil matrix in which the amount of diffusible zinc is assumed to be uniformly distributed. Q u i r k and C h u t e 10 used such an Equation:

~ - " ~ = r : ½ \ a 2 / t~ a 2 (4)

where Mt = quanti ty of zinc differing into the simulated root cy- linder in time t; Me = corresponding quanti ty of Zn that may dif- fuse after infinite time; D = diffusion coefficient (cm 2 sec -1) and a = internal radius of the simulated root cylinder (cm).

A plot of

( M r ) , 1 . ~ - versus t~

should yield a straight line of slope

\ a ~ /

from which the diffusion coefficient may be calculated. The calculated diffusion coefficients are given in Table 1. A peru-

sal of the relative values of the diffusion coefficient for water and different complexing agents and HC1 indicates considerable varia-

KINETICS OF DIFFUSION OF Zn 607

TABLE 1

Effect of chelating agents on kinetic parameters of zinc diffusion in an alkaline soil (pH -- 8.4)

Complexing agents Cumulative Zinc diffusion Zinc diffusion Zinc diffusible potential rate constant diffusion Zinc after coefficient

seven days (cp 100 see.) (ep 100 see.) (day -I) (clnSsec -I)

Nt No K × 10 s D

a b a b a b a

l. H 2 0 70 140 130 240 0.094 0.11 125.4

2. Citric acid (10-~M) 180 270 247 669 0.068 0.16 144.0

3. Glyeine (0.1 g/litre) 144 -- 365 -- 0.062 -- 39.6

4. HC1 (10-2M) 260 400 515 1002 0.068 0.069 58.2

5. t~DTA (10-SM) 1056 1600 5102 7625 0.095 0.037 4.5

6. F A (10-SM) 1170 1720 7062 8799 0.024 0.032 2.3

7. DTPA (10-3M) 1310 1960 8506 11282 0.023 0.029 2.3

a - No Zn added. b - 5 p p m Zn added.

tion. The values for H~O, citric acid, glycine and HC1 are of higher magnitude than for EDTA, FA and DTPA. But the influence of EDTA, FA and DTPA on the cumulative diffusive flux of Zn into the simulated root cylinder was relatively higher than that induced by HC1, glycine, citric acid and water. This observation is easily reconciled when the potential capacity of different chelating agents, HC1 and water in increasing the concentration gradient of diffusible Zn is compared. The predicted concentrations (moles/litre) of soluble zinc in the ambient soil solution with water and different chelating agents were as follows:

H20 -- 10-SAM, Citric acid -- 10-7-8M, Glycine -- 10-7"7M, EDTA -- 10-5.5M, FA -- 10-4.5M, DTPA --3.6M.

The observed increase in cumulative diffusion of zinc induced by the complexing agents relative to water into the simulated root cylinder were not of the same order as that predicted from the po- tential increase in the solubility of zinc because of the mutually compensating the effect of the opposite trend in the variations of the

6O8

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3"~

3'8

3"7

3"6

*q z i o 3,S z

O .J

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M. K. S INI tA AND B. PRASAD

3.3

3%

- ~ ' ~ " ~ D TPA IO3M

FA d3M

~ ED TA 1(~3M

D TA f_4M6 I 2 3 4 S 6 7 8

DAYS

Fig. 3. See t ex t .

diffusion coefficient and the concentration gradient of diffusible zinc.

The lines drawn in Fig. 2 depict the best fitting regression between the amount of Zn diffused and t~. Thus, the values of slopes given in the last column of Table 1 apply to straight-line regressions shown in Fig. 3. The rate of Zn diffusion (Cp 100/sec/t~) for the eight so- lutions differed considerably, and varied in the order: DTPA, 10-aM(731) > FA, 10-3M(684) > EDTA, 10-aM(568) > EDTA, 10-4M(381) > HC1, 10-2M(125) > Citric acid, 10-aM(83) > Glyci- he, 0.1g/1 (68) > H20 (34).

This observations suggests that DTPA exerted a higher cumula- tive effect on Zn diffusion than other complexing agents. This is because DTPA is more efficient than other chelating agents and

KINETICS OF DIFFUSION OF Zu 609

water in enhancing the concentration gradient of diffusible Zn in the soil solution. The diffusive flux of Zn into root cylinder increased with increasing concentration of the complexing agents, for example, the change in concentration of EDTA from 10-4M to 10-aM in- creased the flux of Zn diffusion from 381 to 568.

Potentially di//usible soil Zn

The first estimates of Zn diffusion potential were obtained using tile expression, I/N, = 1/No + b/t, where Nt is the cumulative dif- fusion of Zn during time t, b = slope and No = Zn diffusion poten- tial (counts/100 sec.). The estimates of No by this method given in column 3 of Table 1 were derived from regression analyses based on cumulative diffusion of Zn as a function of time.

The above relationship is based on the assumption that the rate of diffusion is proportional to the amount of potentially diffusible Zn and may be expressed by the equation, dN/dt = - -KN. Thus the integrated rate Equation (2) was employed to arrive at the values of No giving the best linear fit for the regression of log (No -- Nt) on t. As a first step (No -- Nt) was derived from values of No in column 3 and cumulative amounts of Zn-diffused, 'Nt' in column 2 of Table 1.

A perusal of the curves obtained by plotting (No -- Nt) on semilog graph paper (Fig. 3) indicated that the chosen No was approximate- ly equal to the No denoting the best linear fit. Finally, the No giving the best fit was found by an iterative process involving successive evaluation of (No -- Nt) vs t, by regression analyses, based on dif- ferent choices of No. These estimates were regarded as second approx- imation of No.

Comparison of the 'Chosen' and 'Calculated' values of No associat- ed with the best fitting regression of (N9 -- Nt) on t offer an even more direct basis for assessing relative goodness of fit attained with data from 0 to 7 days. For 0 to 7 days, the equation for the regression of No (calculated) on No (chosen) was:

No (Calc) = 4986 + 0.9976 No (chosen); (r 2 = 0.99). These esti- mates indicate that the kinetics of Zn diffusion to the simulated root can be described conveniently by a pseudo first-order equation.

E[/ect o/ a constant/lux o/ chelating agents through the simulated root on the uptake o/ Zn by wheat

The results of the experiment on the effect of a constant flux of

610 M. K. S INHA AND B. PRASAD

T A B L E 2

Effect of a constant flux of water and chelating agents on dry mat te r yield and uptake of zinc after 60 days of growth of wheat plant

Complexing agents Dry ma t t e r (g/pot) Shoot/

Total Shoot Root root

Zn-uptake (~g/pot) Shoot/ Calcu-

Total Shoot Root root lated diffusive

flux to the root

(~g/cm~)

1. H20 2.095 1.380 0.715 1.93 4.50 2.64 1.86 1.43 1.6 (1) (1) (1) (1) (1) (1)

2. Citric Acid 2.510 1.540 0.970 1.59 12.04 6.66 5,38 1.29 5.6 10-3M (1.2) (1.1) (1.4) (2.7) (2.5) (2.9)

3. Glycine 0.1 g/1 2.790 1.650 1.140 1.45 16.06 9.92 6.14 1.92 7.3 (1.3) (1.2) (1.6) (3.5) (3.7) (3.3)

4. EDTA 10-aM 3.125 2.070 1.056 1.96 53.01 33.30 19.71 1.68 18.9 (1.5) (1.5) (1.5) (11.8) (12.6) (10.6)

5. FA 10-aM 3.887 2.400 1.487 1.61 75.77 45.63 30.14 1.51 20.6 (1.9) (1.7) (2.1) (16.8) (17.3) (16.2)

6. DTPA 10-3M 3.960 2.470 1.490 1.66 104.93 64.30 40.62 1.58 21.5 (1.9) (I.8) (2.1) (23.3) (24.4) (21.8)

Bracket - Fold increase as compared to water.

chelating agents through the ceramic tubes into the soil on dry matter yield and uptake of Zn by wheat after 30 days growth are presented in Table 2. It is evident that a constant flux of the chelat- ing agents into the soil increased the dry matter yield of wheat in the following order of variation: DTPA, 10-3M(1.9) = FA, 10-SM (1.9) > EDTA, 10-3M(1.5) > Glycine 0.1 g/1 (1.3) > Citric acid, 10-3M (1.2) > Water (1). The figures in brackets indicate the rela- tive increase in yield at a constant flux of various complexing agents over the flux of water alone. The same sequence of variation in the yield of shoot and root was also noted.

The uptake of Zn by wheat plants in different treatments differed in the following order: DTPA, 10-~M(23.3) > FA, 10-~M(16.8) > EDTA, 10-3M(11.8) > Glycine, 0.1 g/1(2.7) = Citric acid, 10-3M (2.7) > H~O (1).

It is obvious that at a constant flux, DTPA is relatively more ef- fective than other chelating agents in solubilization and transport of Zn from the bulk of soil to plant roots. The flux of the chelating agents resulted into the conversion of the solid phase Zn into soluble Zn complex and thereby increased its concentration gradient to-

K I N E T I C S O F D I F F U S I O N O F Zn 611

wards the plant root and thus leading to enhanced uptake of Zn and dry matter yield. O ' C o n n o r et al. s also reported that increasing levels of added Fe-chelates increased the diffusion of Fe to plant roots.

The relative values of soluble Zn in different treatments; H20, Citric Acid, Glycine, EDTA, FA and DTPA were 0.02, 0.08, 0.10, 0.40, 0.50 and 0.62 ~g/ml in the equilibrium soil solution. A linear relationship between the total uptake of Zn by wheat plants and the concentration of soluble Zn in the soil solution was observed. (Fig. 4). The regression for this linear relationship was as follows: Y = --2.48 @ 160.99 X (r = 0.98) (5), where Y = Zn uptake.

The correlation between uptake of zinc and its calculated diffusive flux indicated the relationship to be curvilinear. A linear relation- ship of log Zn uptake with the calculated diffusive flux of Zn to root was derived: log Y = 0.0694 + 0.058 X (r = 0.98) where Y = up- take of Zn (ag/pot), and X = calculated diffusive flux of Zn to root in 60 days. This suggests that chelation reactions in the ambient soil matrix significantly increase the diffusive flux of Zn to plant roots.

ti0-

100. 90.

@

~ 7 0 .

. 6 0 .

5 0 .

40.

30' 20" t 0

Fig. 4.

/ L ~ F L U X OF ELA IONSH P

0:! 6"2 0"3 d.4 0"5 0:6 0.70~B 0'-9 1:0 1:1 1:2 CONCENTRATION OF SOLUBLE- ZlNC(/L~/11~)

i l

The relationship of Zn uptake to concentration and diffusive flux of Zn from soil to plant root.

612 KINE T ICS OF D I F F U S I O N OF Z N

A realistic appraisal of the contribution of chelating agents to the transport of micronutrients to plant roots requires a consideration of several parameters such as the metal chelates involved, the equili- brium relationship of the metal chelates, and the concentration gra- dient of the metal chelate and the chelating ligand in the soil matrix surrounding plant roots. It may be emphasized that relatively little is known about the amounts and concentration gradients of the chelating agents secreted by roots and how effectively a root lowers the ambient concentration of the micronutrient cations at its sur- face. The results of the present study indicate that diffusion is the main rate-limiting step in the movement of Zn in alkaline and cal- careous soils. Thus mobile chelating agents added to, secreted by the plant roots or generated in the soil during organic matter decompo- sition seem to be very effective in augmenting the rate of diffusion of Zn and similar micronutrients to plant roots.

Received 26 Ju ly 1976

R E F E R E N C E S

9

10

11

12

1 B o u l d i n , D. R., Mathematical description of diffusion processes in the soil-plant system. Soil Sei. Soe. Am. Proe. 25, 476-480 (1961).

2 C r a n k , J. , Mathematics of diffusion, p. 66, Clarendon Press, Oxford (1957). 3 E l g a w h a r y , S. M., L i n d s a y , W. L. and K e m p e r , W. D., Effect of complexing

agents and acids on the diffusion of zinc to a s imulated root. Soil Sci. Soc. Am. Proe. 34, 211-214 (1970).

4 H o d g s o n , J. F., Contribution of metal organic complexing agents to the t ransport of metals to roots. Soil Sei. Soe. Am. Proc. 33, 68-75 (1969).

5 H o d g s o n , J. F., L i n d s a y , W. L. and K e m p e r , W. D., Contributior~ of fixed charge and mobile eomplexing agents to the diffusion of zinc. Soil Sci. Soc. Am. Proe. 31, 410-413 (1967).

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