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II. Nutrient Ion Accumulation in Roots

II. Nutrient Ion Accumulation in Roots

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76



CECIL H. WADLEIGH



types: simple and that involving Donnan equilibrium. The magnitude

of either type of diffusional process will be conditional upon the concentrations of ions a t the surface of the absorbing roots as well as the concentrations of the ions in the absorbing cells in addition to the permeability of the root cell membranes to the diffusate. Exchange adsorption

is involved in ionic accumulation by roots, but there is some question

regarding the extent to which this process is limiting in ion accumulation. Ion accumulation through metabolic processes is the most intriguing of the three categories mentioned. It is the mechanism by

which chemical energy released through catabolism effects the accumulation of ions in the root cells against concentration gradients. To the

extent to which one can precisely differentiate between the roles of the

three types of processes of ion entry into roots, metabolism probably has

the predominant influence under most conditions.

Let us consider the factors that are known to affect the metabolic

accumulation of ions by cells capable of absorption. These may be listed

as:

1 . Supply of metabolite in roots.

Hoagland and Broyer (1936) provided evidence that absorption

of K+ and NO, by barley roots was dependent on the supply of

respiratory substrate such as sugar.

2. Oxygen supply.

Numerous investigations in addition to Steward (1937), Hoagland and Broyer (1936), and Lundegardh (1940) reported that

a supply of oxygen to plant roots is essential for absorption of inorganic ions. It is of interest that the level of soil oxygen may

modify selectivity of roots with respect to ions absorbed. Hopkins

et al. (1950) found that decreasing the partial pressure of oxygen in air supply to the root zone was associated with an increase in the concentration of sodium in the tops of tomato,

soybean, and tobacco plants; whereas the opposite effect was

noted for the concentration of potassium, calcium, and phosphorus.

3. Carbon dioxide accumulation.

The inhibitory action of carbon dioxide in ion absorption has

been studied by Chang and Loomis (1945). They concluded

that increasing carbon dioxide content of the air supplied to

nutrient cultures may be toxic per se to plants over and above

any effect of inadequacy in oxygen supply. Under alkaline soil

conditions, increasing partial pressure of carbon dioxide brings

about an increase in concentration of the bicarbonate ion in the

soil solution. The presence of the bicarbonate ion i n the sub-



MINERAL NUTRITION O F PLANTS



77



strate was associated with an abnormally high level of potassium

accumulation and a low level of calcium content in the leaves

of pear trees (Lindner and Harley, 1944), and bean plants

(Wadleigh and Brown, 1952).

4. Supply of nutrient ions.

Numerous studies in addition to those of Hoagland and Broyer

(1936) and Robertson and Wilkins (1948) have shown that

metabolically controlled salt accumulation is conditioned by the

concentration of ions in the external medium.

5 . Level of internal accumulation.

One would deduce that cells with a relatively low concentration

of salt are more effective in displaying metabolically induced

salt accumulation than those having a relatively high salt level.

Hoagland and Broyer (1936) presented clear-cut evidence that

this is so.

6. Competition between ions.

Antagonism among ions diffusing through a membrane has

long been a field of investigation by physiologists. Studies of

this type have now evolved into ascertaining degrees of competition among ions for protoplasmic binding sites that effect

ion transport across the cytoplasm. Since this is a relatively active field of inquiry at the present time, it will be discussed at

some length.

7. Stimulative effects.

Viets (1944) found that increasing levels of Cat+ in the culture

solution up to 200 meq. per liter were associated with increasing

accumulation of potassium uniformly supplied at 5 meq. per

liter. This effect was not observed, however, under conditions

that inhibited oxidative metabolism. Caplin and Steward ( 1948)

observed that there was an active principle in coconut milk that

stimulated the mature secondary phloem tissue of carrot to

active and very rapid growth-conditions favorable to enhanced

absorption of nutrient ions.

8. Toxic effects.

Since salt accumulation by roots is usually dependent on concomitant aerobic respiration, Machlis ( 1944) proceeded to

study the effects of the oxidase inhibitors, cyanide and azide,

and of the dehydrogenase inhibitors, iodoacetate and malonate,

on the respiration and bromide accumulation by excised barley

roots. Cyanide and azide were found to inhibit approximately

two-thirds of the respiration and, at the same time, prevent any

accumulation of bromide. Machlis suggested that cytochrome



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CECIL H. W A D L E I G H



oxidase was the enzyme affected. Iodoacetate and malonate were

found to inhibit both respiration and salt accumulation. Malate,

succinate, fumarate, and citrate reversed iodoacetate inhibition

of both respiration and accumulation of salt and neutralized

malonate inhibition of accumulation. This suggested that the

citric acid phase of Krebs' cycle was an integral part of the

respiratory system essential for salt accumulation.

9. Membrane specificity.

Specificity in ion absorption is especially evident in the wide

range in accumulation of sodium by different species when

grown at a given level of sodium supply. Collander (1941) determined sodium content of the herbage of 21 species supplied

with 2 meq. of sodium per liter in the culture solution. Very

little sodium was found in buckwheat, with the other species

ranging up to the very high accumulation of this ion found in

Atriplex hortense.

10. Temperature.

Ulrich (1941) showed the concurrence among trends of 0, consumption, CO, evolution, and salt absorption by excised barley

roots as affected by temperature. At 5O C., 100 g. of fresh roots

consumed 41 ml. of oxygen, produced 36 ml. of CO,, and accumulated K+ to a level of 13 meq. per liter of sap during 8

hours. These three effects increased concurrently with increasing temperature and at 35O C. 100 g. of fresh barley roots consumed 246 ml. of oxygen, evolved 247 ml. of CO,, and accumulated K+ to 41 meq. per liter of sap for the 8-hour period.

11. Water.

The role of soil moisture in the mineral nutrition of plants has

been reviewed by Wadleigh and Richards (1951) . Water in the

soil affects ionic accumulation in plants in three ways: (a) by

conditioning the availability of nutrient ions; (b) by regulating

plant growth processes dependent on soil moisture availability;

and (c) by the effects of anaerobiosis resulting from excess

water in the soil.

The foregoing factors affecting metabolic ion absorption are interrelated in their effects on the conflux or processes involved. Lundegardh

(1940) has developed an intriguing concept pertaining to the mechanism by which chemical energy released through the respiratory

process is used in ion accumulation by plant cells. Since this theory has

received considerable attention in recent reviews (Broyer, 1951 ; Robertson, 1951; Burstrom, 1951; Overstreet and Jacobson, 1952) on the min-



MINERAL NLI'I'RITION



O F PLANTS



79



era1 nutrition of plants, it is well to consider it briefly, even though

recent investigations have emphasized its inadequacies.

The Lundegardh hypothesis assumes that the cytochrome systems

are an integral part of ionic transfer from the outer cell surface across

the cytoplasm into the vacuole. The chain of events from energy release by respiration to inward transfer of cations is set forth briefly as

follows: When the hydrogen atom liberated by the dehydrogenase

phase of respiration reaches the cytochrome system, the cytochrome

picks up the electron and the hydrogen ion is freed to exchange for

external cations. The resulting ferrocytochrome effects movement of

the electron to the outer cell surface, where the electron is lost via

cytochrome oxidase to externally supplied oxygen; and the oxidized

ferricytochrome acts as an inward carrier for anions. Robertson and

Wilkins (1948) pointed out that if the Lundegardh theory is valid, the

maximum rate of anion accumulation in the cell should take place

when each electron leaving via the cytochrome system is exchanged for

an anion from the external medium. On the assumption that respiration

is proceeding by the cytochrome system, all molecular oxygen involved

in the process becomes combined as water, and each molecule of oxygen

requires four electrons and four hydrogen atoms. Hence, the maximum

rate of salt accumulation should be 4 gram moles of monovalent salt

accumulated for each gram mole of oxygen used, or salt accumulation/salt respiration = 4. Robertson and Wilkins (1948) studied chloride intake by carrot tissue in relation to respiration and found that the

value of the aforementioned ratio tended to approach a value of four if

neither the rate of respiration nor the rate of chloride accumulation was

limited by the external concentration. These results were confirmatory of the Lundegardh theory.

In a later contribution, Robertson ef al. (1951) presented evidence

that the Lundegardh concept is not adequate to explain the complexities

of the mechanism of ion absorption. They found that 2,4-dinitrophenol

inhibited the accumulation of KC1 by carrot cells while enhancing the

rate of respiration. Furthermore, 2,4-dinitrophenol increases the leakage of ions from cells transferred from salt to water. The respiration

stimulated by dinitrophenol is sensitive to inhibition by cyanide. Although cyanide inhibits salt accumulation, it apparently does not induce leakage of ions from the cells. On the evidence (Bonner, 1949)

that dinitrophenol blocks the transfer of energy-rich phosphate to

growth processes, Robertson ef al. ( 1951) suggest that the Lundegardh

theory requires modification to allow for the participation of energyrich phosphate.

In their review of the mechanisms on ion absorption by roots, Over-



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CECIL H . WADLEIGH



street and Jacobson (1952) point out that probably the greatest service

of the Lundegardh concept has been in showing the very close relationship between salt accumulation and the cytochrome system. Because of

the quantitative and qualitative linking of ionic accumulation with

cytochrome-mediated respiration, it seems probable that any theory of

salt absorption will involve the cytochrome system. They regard the

actual identification of the cytochrome-cytochrome oxidase system with

the actual ion carrier as premature.

These reviewers emphasize Rosenberg’s ( 1948) thermodynamic

treatment of ionic accumulation involving the postulation of ionic

donators and acceptors in diverse parts of the system: external substrate-membrane-internal

phase. The mechanisms of transport so

analyzed are involved in the Lundegardh theory. Nevertheless, Overstreet and Jacobson (1952) point out several objections to this theory,

viz.: (1) different rates of absorption of diverse ions with the same

charge, (2) unequal rates of absorption of cations and associated anions,

and (3) the mutual reciprocal effect of ion pairs. Furthermore, as

Epstein (1953) points out, there is now valid evidence for specific binding sites on protoplasm for cations in contrast to the concept that cations

move passively through the protoplasm as mere electrovalent companions to the anions that are transferred metabolically. Epstein, by supplying cations on exchange resins, has also accrued unpublished evidence that “salt respiration” is just as much associated with cation accumulation as with anion accumulation.

It is pertinent to consider the current evidence on the importance

of cytoplasmic binding sites in ion accumulation. Jacobson and Overstreet ( 1 947) have set forth the properties that must characterize hypothetical compounds or reactive groups that are capable of fixing in plant

cells inorganic ions taken from the culture medium in exchange for

equivalent ions released by the cells:

“1. The ion fixing compounds must be related to the oxidative

metabolism of the plant since there is a parallelism between ion absorption and oxygen tension. Under conditions of arrested metabolic activity

such as at low temperature or in solutions in equilibrium with low oxygen tensions, very little or no accumulation takes place in root systems

or in a variety of other cells.

“2. The ion fixing compounds in the protoplasm must form compounds with ions such as K+, Rb+, Ca++,Br-, NO,-, SO,=, HPO,=, and

others, in which the ions are held by relatively strong bonds since plants

are able to accumulate these mineral nutrients offered at extremely low

.levels of concentration. Furthermore, plant roots can compete with soil

colloids which bind nutrient cations very firmly.



MINERAL NUTRITION O F PLANTS



81



“3. The ion fixing compounds should account not only for the

absorption of anions and cations, but also for the very wide range in

the absorption of different ions of the same sign. This would presumably

require two or more classes of compounds or groups.

“4. Although the ions may be bound quite strongly to the ion fixing

complex, the combinations so formed must nevertheless possess a high

degree of instability since the evidence is that the ions pass into free

solution in the vacuoles. Moreover, it is a familiar fact that in response

to injury and death, ions absorbed in cells freely diffuse into the surrounding medium. For example, radioactive K could be completely

leached from ether killed barley roots.”

Jacobson and Overstreet (1947) studied intake and loss of radioactive strontium and iodide by barley roots to gain some insight into the

nature of ion fixation within the plant. Many of the determinations

were carried out at Oo C. in order to avoid complications due to longitudinal translocation. Observations on live roots at Oo C. and 2 5 O C. indicated the marked dependence of intake of radioiodide on metabolic

activity. There was a marked difference between behavior of iodide

and strontium in that the intake of iodide increased by a factor of 7.8

for the 25O C. rise in temperature, whereas intake of strontium increased by a factor of only 1.5. The difference between absorption of

iodide and strontium was further emphasized by observations on etherkilled roots. Dead roots showed relatively little intake of iodide at 2 5 O C.,

whereas they were found to take in more strontium at 25O C. during 15

minutes than live roots. Dead roots were found to release absorbed ions

to the external medium much more rapidly and completely than live

roots. The data suggest that strontium is held differently in live than

in dead roots; but even in living tissue, the exchange curves indicate that

no appreciable fraction of strontium or iodide is held in a nonexchangeable form. On the basis of these data, Jacobson and Overstreet (1947)

were able to make certain observations on the nature of ion carriers:

“(a) The ion carriers are intermediate metabolic products or closely

related substances; (b) the carriers are not stable in vitro; ( c ) they

undergo chemical alteration in the course of their carrier function; and

(d) they probably function as chelated complexes.”

Studies on the kinetics and characterization of binding sites for ions

in absorbing membranes now constitute a most promising field of inquiry in the mineral nutrition of plants. In order to evaluate the influence of microbial activity on the entry of nutrient ions into plant

roots, it would be helpful to consider briefly a few recent observations

on the nature of entities in protoplasm capable of binding ions. Roberts

and co-workers (Roberts et al., 1949; Roberts and Roberts, 1950; Cowie



a2



CECIL H. WADLEIGH



et al., 1949) have produced evidence that the binding sites for potassium

in Escherichia coli are closely associated with carbohydrate metabolism.

Jacobson et al. (1950) investigated the competition between K+

and Ca++and found that although the presence of K+ reduced the absorption rate of Ca++,the effect of Cat+ on K+ was more complicated. I n

certain concentration ranges, Ca++depressed absorption of K+, but at

other levels it enhanced entry of K+. It was concluded that probably a

single binding substance serves both Ca++and K+, but a n additional special role must be assigned to Ca++in the absorption process. This is in

line with the earlier observations of Viets (1944).

Epstein and Hagen (1952) devised a novel approach to the kinetics

of ion binding during absorption by considering the reaction of an ion

with the binding entity as analogous to the equilibria between enzyme

and substrate. They hypothesize that the absorption involves the formation and breakdown of an intermediate labile complex, MR, of the

metal ion, M, with a metabolically produced binding compound or carrier, R. The analysis is essentiaIly identical with the kinetic treatment

of enzyme reactions presented by Lineweaver and Burk (1934) for

analysis of the equilibria between the substrate, S, the enzyme, E, and

the labile complex, ES. In such an analysis, the interfering ions assume

the role of inhibitors or alternate substrates.

Epstein and Hagen (1952) concluded from their work as follows:

“It was found that K and Cs interfere competitively with Rb absorption, and it is concluded that these three ions are bound by the same

binding sites or reactive centers.

“Except at high Rb or Na concentrations, Na does not interfere competitively with Rb absorptions; that is, is not bound by the same sites.

At low Rb concentrations, Na over a wide range of concentrations entirely fails to interfere with Rb absorption. Li is not competitive with

regard to the K-Rb-Cs sites; i.e., it is not bound by them. At relatively

low concentrations of Rb and Li, Li increases the rate of absorption

of Rb.

“The findings are considered to be consistent with the hypothesis and

indicate the existence of several distinct binding sites of which one

group binds K, Rb, and Cs in preference to Na and Li.”

Epstein and Leggett ( 1954) have studied competition among Sr,

Ba, Ca, and Mg for binding sites in metabolic absorption by barley

roots. They found that Ca, Sr, and Ba compete for identical binding

sites, but that the Mg ion does not compete for these same sites. Furthermore, Epstein (1953) has presented evidence on the absorption of

anions indicating that Br and C1 compete for the same binding sites,

but the NO, ion is noncompetitive on the halide binding.



MINERAL NITTRITION O F PLANTS



83



These recent investigations emphasize the diversity and specificity

of labile “ion carriers” or binding sites that undoubtedly occur in the

absorbing membranes of plant cells. There is sound evidence that these

“carriers” are metabolically energized in the protoplasm and they appear to be the key determinants in mineral nutrient absorption by plant

roots.

111. MICROBIAL

ACTIVITIES

A N D IONACCUMULATION

The soil microbiologists have accrued information that would aid

them in predicting or indicating the manner and extent to which microbial activity may affect or be involved in the eleven previously listed

factors conditioning metabolically induced ion absorption by roots. Norman ( 1951) and Clark ( 1949) have recently reviewed the influence of

soil microorganisms on the availability of mineral nutrients to crop

plants. Let us consider briefly, nevertheless, some possible and probable

effects that soil microbes may have on these eleven factors.

1. Supply of metabolite in roots.

Since the population of soil microorganisms is especially intense

at the surface of roots, presumably owing to the supply of energy

material sloughed off or emanating from the roots, it is conceivable that the supply and quality of metabolite in the root may

affect the density of population in the rhizosphere, which, in

turn, may affect availability of minerals in the soil. Furthermore, microbial activity under certain conditions, influences the

availability of minor elements in the soil (Clark, 1949), which,

in turn, may affect chlorophyll formation in the leaves (Brown,

1953) supplying metabolite to the roots.

2. Oxygen supply.

Microorganisms carry on respiration and may contribute to depleting the oxygen supply under conditions of poor aeration

(Norman, 1951). Low oxygen supply will not only affect the

metabolism of root cells but will also condition the availability

of nitrogen and such minor elements as iron and manganese

( Wadleigh and Richards, 1951) .

3. Carbon dioxide accumulation.

Norman (1951) points out that “under optimum conditions in

soils well supplied with organic matter, carbon dioxide evolution may attain rates as high as 100 pounds per acre per day,

though figures of 20-30 pounds per day are more general.” Under poor soil aeration, such activity could contribute to bringing

about a level of CO, at the root surface that is inhibitive to ion

absorption (Chang and Loomis, 1945).



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CECIL H. WADLEIGH



4. Supply of nutrient ions.

Clark (1949) has reviewed the evidence on this point. The activities of microorganisms can be a major consideration in the

supply of nitrogen, phosphorus, sulfur, and minor elements to

plants.

5. Level of internal accumulation.

Soil microbes would have an indirect effect on this point in that

they may have affected the supply of nutrients to the plant at

an earlier stage of growth, thereby influencing the facility with

which the plant effects ion accumulation at the later stage.

6. Competition between ions.

The production of CO, by microbes may effect an increase in

hydrogen ion concentration in the soil and thereby alter the

relative availability of cations and phosphate. Any effect microbial activity may have on depleting oxygen supply may also

have the further effect of modifying any inherent capacity the

plant root possesses with respect to selectively excluding sodium

(Hopkins et al., 1950).

7. Stimulative effects.

Schmidt ( 1951) reviewed the evidence relating the effects of

growth-stimulating substances produced by bacteria. Numerous

investigators have shown that various growth regulators for

higher plants are produced by bacterial synthesis. Substances

so produced have been shown to stimulate seed germination and

nodule formation on legumes. It is quite possible that certain of

these growth regulators could promote metabolically controlled

ion absorption.

8. Toxic effects.

Steinberg (1947) grew tobacco seedlings in aseptic culture in

the presence of diffusates of certain common soil bacteria and

found that various types of chlorosis developed on the leaves. In

a later work, Steinberg (1951) collected samples of soil adjacent

to the roots of tobacco plants which were either growing normally or were severely frenched. In six of seven paired samples

of rhizosphere soil, higher populations of 3acillus cereus were

found for the frenching soil than were found in normal soils.

Roots of frenched tobacco showed especially large populations

of B. cereus, with values several times those hitherto reported

for normal plant roots. The study indicated a good probability

that B. cereus has a causal relationship in frenching of tobacco.

As Norman (1951) points out, organic acids are a product of

bacterial activity under anaerobic conditions. Little is known as



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II. Nutrient Ion Accumulation in Roots

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