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II. Physiology of Aluminum and Manganese Tolerance in Wheat

II. Physiology of Aluminum and Manganese Tolerance in Wheat

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ACID SOIL TOLERANCE IN WHEAT



125



grossly distorted walls; rupture of the cells of the epidermis and outer cortex was

noted at high Al concentrations. The ability of Al-damaged roots to synthesize

and translocate growth-promoting hormones such as cytokinins does not appear

to have been investigated.

An understanding of the dynamics of root growth inhibition by Al is important

in designing experiments to elucidate Al tolerance mechanisms. Recent studies

have shown that reduction in elongation of Al-challenged wheat roots commences

after a lag period that may be as short as 2 hr (Ownby and Popham, 1989; Ryan

ef al., 1992). Parker (1 994) has concluded from analysis of wheat root growth that

there are two responses to Al: an initial “acute” inhibition of growth that is followed by a later “chronic” A1 effect on root growth. In his study, some, but not

all, wheat cultivars became acclimated to low levels of Al and resumed growth

after the initial shock. Surprisingly, the acclimation phenomenon appeared in Alsensitive cultivar ‘Scout 66’ as well as in Al-tolerant ‘Atlas 66,’ and no correlation

between acclimation and Al tolerance was observed. The concept of acute vs

chronic growth inhibition by Al is useful in evaluating various approaches to the

study of Al toxicity. As Parker ( 1994) noted, short-term experiments by physiologists may reveal mainly acute effects of Al on growth, while long-term field work

and breeding studies are more likely to observe chronic phytotoxicity of Al, which

could have a different physiological basis. Likewise, short-term tests for Al tolerance such as root growth (Kerridge et al., 1971) and hematoxylin staining (Polle

et af.,1978) may not invariably predict long-term performance of crops in acid

soils. Indeed, examples of this have been observed (Scott and Fisher, 1989).

This concept may also be useful in resolving what appears to be contradictions

in the plant response to Al. Among the effects of Al that often appear after its

initial effect on growth are inhibition of DNA synthesis (Wallace and Anderson,

1984), alteration of cell membrane potential (Kinraide, I988), and reduction of

root apex H + efflux (Ryan ef al., 1992). These effects eventually contribute to

cessation of root growth (i.e., the chronic effect) but they appear to succeed an

initial response with which they may not be primarily involved.

Two major unresolved questions about Al in plants concern its chemical form

and cellular distribution. The general assumption that Al 3 + , the major form of

Al,,,,,,,,at pH <4.75, is phytotoxic has been confirmed in wheat (Parker et al.,

1988). However, as Taylor (1991) pointed out, the level of A13+that may exist in

cell cytoplasm can be no greater than 10 l o M , and is probably much lower due

to inorganic phosphate and other ligands. The results of studies in virro that assume reaction with Al 3 + thus may not apply to cytoplasmic conditions. Mononuclear hydroxy-Al does not appear to be very toxic in wheat but was suggested to

be more toxic than A13+ in dicots (Kinraide and Parker, 1989; but see Kinraide,

199 I). The polynuclear hydroxy-Al (A1 species may be a major toxic Al species

in the root apoplast. Parker et al. (1989) reported that Al,, was as toxic to AItolerant wheat cultivar ‘Seneca’ as to Al-sensitive cultivar ‘Tyler’; however, Kin~



126



B. F. CARVER AND J. D. OWNBY



raide (1991) later suggested that Al,, was less likely to form in wheat than in

dicots. In a study using purified cell walls, Bertsch et al. (1994) concluded that

Al,, is more mobile and is bound with less affinity to carboxyl groups of the cell

walls of Al-sensitive wheat cultivar ‘Caldwell’ than Al-tolerant cultivar ‘Yecora

Rojo.’ Clearly, the task of characterizing and developing Al-tolerant plant lines is

complicated by the diverse chemical species of Al that can form in plants and the

possibility that not all may act at the same cellular sites.

Zhang and Taylor (1989) examined Al uptake kinetics in wheat and observed

an initial 30-min period of binding of Al to cell walls, followed by an extended

period of linear uptake, the magnitude of which was about the same in both Alsensitive and Al-tolerant cultivars. However, the authors later concluded that this

linear phase consisted of two separate processes: fixation of Al in the apoplast that

was dependent on metabolism and permeation of the cell membrane (Zhang and

Taylor, 1990).

Although Taylor and co-workers did not observe marked differences in A1 accumulation between Al-tolerant and Al-sensitive wheat cultivars, other studies indicate that there is less total uptake of Al into roots of Al-tolerant wheat cultivars

compared to sensitive cultivars, particularly in the zones of cell division and elongation. Rinc6n and Gonzales ( 1992) found that A1 accumulation in the terminal

2 mm of Al-sensitive wheat cultivar TAM 105 roots was about seven- to eightfold

greater than in tolerant Atlas 66 during the critical first 6 hr of exposure. Likewise,

Delhaize et al. (1993a) observed, within 4 hr of exposure, a markedly increased

uptake of Al in an Al-sensitive wheat genotype compared to a sibling tolerant

genotype.

Attempts to localize Al in root tissue, however, have not produced consistent

results, apparently varying according to the methods used. Early work with X-ray

microanalysis suggested that Al was detectable in the cell, especially in the nucleus (Naidoo et al., 1978). In more recent studies using better fixation procedures, X-ray microanalysis has revealed Al only in cell walls. Aluminum was

below the limit of detection by X-ray microanalysis in root cells of wheat (Ownby,

1993) and oat (Avena sativa L.) (Marienfeld and Stelzer, 1993) that were treated

for 24 hr with growth-inhibiting levels of Al.

In contrast to X-ray microanalysis, other methods indicate that considerable Al

crosses the plasmalemma of Al-intoxicated root cells. Aluminum was readily detected in the nuclei of Al-treated wheat root cells by the putative Al-specific dye

hematoxylin (Rinc6n and Gonzales, 1992) and the fluorochrome morin (Tice

et al., 1992). The latter study also used procedures to elute Al from the apoplast

(extracellular space) and concluded that, when Al-tolerant cultivar Yecora Rojo

and Al-sensitive cultivar Tyler were treated with sufficient Al to inhibit growth by

50%, about 55 to 70% of total tissue Al was in the symplast (inside the plasmalemma) after 48 hr. Partitioning of Al between the apoplast and symplast was the

same for both cultivars. Considering that initial root growth inhibition occurs



ACID SOIL TOLERANCE IN WHEAT



127



within the first 2 to 6 hr of treatment (the “acute” response), it is possible that

Tice ef a/. ( 1992) were observing the accumulation of A1 in the cytoplasm of Aldamaged cells. It cannot be concluded from this work that entry of A1 into the

cytoplasm was the sole cause of growth inhibition.

The ability to exclude A1 from shoots as well as roots also appears to be a trait

of Al-tolerant wheat cultivars. Foy and Peterson (1994) noted that when 10 wheat

lines differing in A1 tolerance were grown in Al-toxic Tatum soil (pH 4 . 3 3 , there

was a strong positive correlation between accumulation of A1 in shoots and growth

inhibition. Al-tolerant cultivars also contained two- to fourfold more shoot potassium than sensitive lines; however, there was no correlation between tolerance and

the level of shoot Ca, Mg, or P.

In summary, evidence now suggests that Al-tolerant wheat cultivars exclude A1

from root and shoot tissue better than Al-sensitive cultivars. This presumably results from the ability of tolerant cultivars to better exclude A1 from the root symplast; however, mechanisms of A1 exclusion remain unclear. Several physiological

processes by which plants could exclude A1 from the tissue as a whole or the

symplast in particular are described next.



B. ALUMINUMTOLERANCE

MECHANISMS

During the past two decades there has been no shortage of hypotheses to explain differential tolerance to A1 among plants. The reader is referred to reviews

by Roy et al. (1988), Haug and Shi (1991), and Taylor (1991) for a more complete

description of these hypotheses as well as earlier work on which they are based.

Rao et al. (1 993) have reviewed physiological and genetic aspects of breeding for

A1 tolerance in crops of the American tropics.

In general, strategies that various plants use to tolerate A1 fall into two categories: (1) external tolerance mechanisms, by which A1 is excluded from plant tissue, especially the symplastic portion of root meristems; and (2) internal tolerance

mechanisms, where A1 that has permeated the plasmalemma is sequestered or

converted into an innocuous form. Work published during the last 3 to 5 years

indicates that exclusion of A1 from plant tissue and cells is probably more important than internal mechanisms for A1 tolerance in wheat and most other crop

plants. Specific responses in Al-tolerant plants that have generated enthusiasm

among workers in this area include the following.

1. Accumulation and/or Secretion of Organic Acids



The ability of various di- and tricarboxylic acids to form strong complexes with

A1 has led to various studies attempting to show that plants use this as a defense

mechanism against A1 toxicity. Galvez e t a / . (1991) observed that Al-tolerant sor-



128



B. F. CARVER AND J. D. OWNBY



ghum [Sorghum bicolor (L.) Moench] cultivar ‘SC283’ increased root organic

acid content more than Al-sensitive ‘ICA-Nataima’ in response to Al. In the latter

study, however, the total tissue level of potential Al-complexing organic acids

such as citrate and malate was ca. 400 and 1600 pM, respectively (authors’ estimate), even in the Al-sensitive cultivar. This is probably much higher than the

concentration of tissue Al; thus, even Al-sensitive plants would seem to have sufficient organic acids to complex Al.

Perhaps more convincing are studies on the ability of Al-tolerant plants to release organic acids into the root environment when challenged by Al. Miyasaka

et al. (1991) observed that Al-tolerant snapbean (Phaseolus vulgaris L.) cultivar

‘Dade,’ when grown under sterile conditions for relatively long periods (8 days),

exuded citric acid to a level that reached 26% of initial A1 (mol/mol). This response was not seen in Al-sensitive cultivar ‘Romano.’ The authors concluded that

exudation of citric acid into the medium provided Al tolerance in snapbean, either

by chelating external AI and thus preventing its entry into the root (see Bartlett

and Riego, 1972) or by mobilizing phosphate that had been precipitated with Al

in the root apoplast.

The idea that exudation of organic acids may function as an Al tolerance

mechanism in wheat is supported by Delhaize and co-workers. When two nearisogenic lines of wheat were challenged with Al, the Al-tolerant but not the AIsensitive line released malate into the medium (Delhaize et al., 1993b). Exudation

of malate was not associated with Al-induced phosphate deficiency; the Alsensitive line did not release significant amounts of malate when grown in the

absence of phosphorus. The time course of release of malate, shown in Fig. 1,

demonstrates that this phenomenon is rapid enough to account for resistance to an

initial acute phase of growth inhibition by Al. The amount of secreted malate

corresponded to about 35% of the initial All,,,,,, (mol/mol) in the medium. Although malate release was quite dramatic in response to high levels of Al, the

difference in malate exudation between Al-sensitive and Al-tolerant lines was

much less pronounced at lower Al levels that inhibited root growth only in the

sensitive line (Delhaize et al., 1993a).

Recent work has reinforced the concept that Al tolerance in wheat may be based

on exudation of malate and its chelation of Al. In a survey of 36 wheat cultivars,

there was a strong correlation between long-term (7 day) tolerance to Al and shortterm (80 min) efflux of malate (Ryan et al., 1994b). In the Al-sensitive genotype

ES3, root growth inhibition by 3 pM Al was partially reversed by 10 pM malate

and completely reversed by 20 pM malate. The activity of two enzymes presumably involved in malic acid synthesis, PEP carboxylase and NAD-malate dehydrogenase, was the same in Al-tolerant wheat genotype ET3 and Al-sensitive ES3,

and was not altered when roots were grown in Al (Ryan et al., 1994a). However,

they observed that a variety of anion channel blockers inhibited malate efflux,

suggesting that Al 3 + -induced malate efflux involved activation of these channels



ACID SOIL TOLERANCE IN WHEAT



-



129



T



0.41



d

I



d



0. 0



5



Al-sensitive



5



Time (h)



Figure 1. Time course of malic acid secretion by two near-isogenic lines of wheat, one Al tolerant and the other AI sensitive. Five 6-day-old seedlings were incubated in flasks containing 20 ml of

sterile nutrient solution, pH 4.1, to which 50 pA4 A1 as AIK(S0,)2 was added at zero time. This

treatment reduced root growth of the Al-tolerant line to 70% of control and the Al-sensitive line to

less than 10% of the control during a 5-day exposure to Al. The two lines of wheat were derived from

a cross between Al-tolerant cultivar Carazinho and Al-sensitive cultivar Egret. Adapted from Delhaize

et al. (l993b) by permission.



in Al-tolerant wheat lines. K appears to serve as a counter-ion to Al-stimulated

malate efflux. Malate most likely functions in the apoplast and the unstirred

boundary layer surrounding roots by chelating A1 to form Al-malate and thus

shielding potential sites of injury such as the plasmalemma (M. Delhaize, personal

communication).

At present, exudation of organic acids is the most promising mechanism of Al

tolerance yet studied. Preliminary evidence showing that organic acid release correlates with Al tolerance in crosses between tolerant and sensitive cultivars further

supports this model (Delhaize et al., 1993b). The demonstration that mutant lines

deficient in organic acid exudation also become more A1 sensitive would further

bolster this mechanism of Al tolerance.

+



2. Binding or Fixation of Aluminum in the Cell Wall Region

The interaction of A1 with cell wall constituents remains a relatively unexplored

aspect of Al phytotoxicity. It is generally thought that binding of A1 to charge sites

on the cell surface is a prerequisite for uptake and toxicity. Plants with a high root

cation exchange capacity (CEC) are generally more sensitive to A1 than similar

lines with low CEC (Blarney er al., 1990). In terms of a specific interaction with

Al, Blarney and co-workers have provided evidence that Al displaces Ca from cell



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B. F. CARVER AND J. D. OWNBY



wall pectic acids, which reduces the movement of water and mineral nutrients

through the cell wall interstices (Blarney er al., 1993). This rapid response to Al,

at least in model systems, is consistent with the time course of “acute” effects of

A1 on root elongation. Hunter and Bertsch (1994), using cell wall fractions isolated from wheat cultivars differing in A1 tolerance, showed that Al may disrupt

the hydrogen bonds between cellulose molecules; the extent of this response correlated well with the degree of sensitivity to Al among wheat cultivars.

In contrast to these reports, Kinraide et al. (1992) have concluded that cell

surface negative charges, derived from cell wall pectins as well as charge sites on

membrane lipids and proteins, do not play a significant role in differential Al tolerance in wheat. They based this conclusion on the observation that cultivars

Atlas 66 and Scout 66, the latter much more sensitive to Al than the former, exhibited about the same level of root growth inhibition when treated with La3+.The

two cultivars were expected to show the same relative response to La3+as to A13+

if surface charge was the basis of differential sensitivity.

Many early studies found an association between A1 toxicity and accumulation

of A1 phosphate precipitates in the apoplasm (Clarkson, 1967). It is still not clear

if Al-tolerant plants actively release phosphate to immobilize A1 in the apoplast.

Evidence for active efflux of cell phosphate in Al-tolerant sugarbeet (Beta vulgaris L.) cultivars was provided by Lindberg (1990). However, it should also be

noted that cellular phosphate often leaks into the cell wall region as part of the

Al stress effect, seen for example in the Al-sensitive wheat cultivar ‘Victory’

(Ownby, 1993). In fact, the higher levels of total Al observed in roots of Alsensitive wheat cultivars (see earlier discussion) could be due in part to the accumulation of relatively innocuous Al phosphate complexes in the apoplasm of Aldamaged roots. Such precipitation would reflect cell damage by Al, not an Al

tolerance mechanism per se. Various studies have suggested, moreover, that tolerance of low phosphate, and high efficiency in uptake and distribution of phosphate, may be characteristics of Al-tolerant wheat cultivars (Foy el ul., 1978). De

Miranda and Rowell ( 1990), for example, have provided evidence that Al-tolerant

wheat cultivars are better able to absorb and translocate phosphate to shoots in the

presence of Al.



3. Production of Root Mucilage

Horst et al. (1982) demonstrated that in cowpea (Vigna unguiculuta),removal

of root cap mucilage caused an increase in Al uptake and phytotoxicity. Among

10 cultivars of winter wheat, Henderson and Ownby (1991) noted a strong correlation ( r = 0.82) between root mucilage volume and A1 tolerance as determined

by root growth assays. The mechanism of protection by mucilage is not clear.

Although it is generally assumed that mucilages contain Al-binding pectic acids,



ACID SOIL ‘TOLERANCE IN WHEAT



131



mucilage droplets of Al-treated wheat and cowpea did not stain with hematoxylin

even under conditions where the root surface was readily stained (Henderson and

Ownby, 199 I ). Because Al-organic acids complexes do not react with hematoxylin (Ownby, 1993), it was suggested that A1 in the mucilage was not associated

with pectic acids, but rather with organic acids released by the root. The mucilage

droplet would thus create a “boundary layer” in which diffusion of Al to the root

surface is slowed and where the organic acid/Al ratio would likely be much more

favorable than in the rhizosphere as a whole (Henderson and Ownby, 1991).



4. Exclusion of Aluminum at the Plasmalemma

Considering its essential role in cell metabolism and growth, it is not surprising

that the plasmalemma has been postulated to be the site of selective Al toxicity.

The effects of Al on membrane integrity and function include binding of A1 to

membrane lipids (see Haug and Shi, 1991), as well as inhibition of ATPase activity (Matsumoto and Yamaya, 1986), NADH-linked electron transfer (Loper

et al., 1993). and ion channel functions (Rengel and Elliott, 1992). In sugarbeet,

Al toxicity was associated with an increase in the ratio of phosphatidylcholine to

phosphatidylethanolaminewhich could increase membrane permeability (Lindberg and Griffiths, 1993). Caldwell (1989), using the luminescent cation terbium

[Tb(lll)], observed that wheat root membranes isolated from Al-sensitive cultivar

‘Anza’ appeared to bind more Al than did tolerant cultivar ‘BH 1136.’ He also

inferred that Al could displace Ca from membrane protein-binding sites.

However, it has been suggested that the plasmalemma continues many of its

functions well after initial Al toxicity effects are noted. These functions include

maintenance of membrane potential and H + efflux (Kinraide, 1988) and K + absorption (Petterson and Strid, 1989; but see Nichol et al., 1991). Huang et al.

(1993) observed that A1 could inhibit the uptake of Ca in wheat seedlings, although it was demonstrated in a later part of this study that levels of A1 sufficient

to inhibit growth did not affect Ca uptake (Ryan et al., 1994~).

Although Pifieros

and Tester (1993) found that 70 puM A1 completely blocked the Ca channels of

plasmalemma-enriched fractions from wheat roots, the Ca channels of membranes isolated from Al-tolerant wheat cultivar Atlas 66 and Al-sensitive cultivar

Scout 66 were equally sensitive to Al, indicating that differences in channel proteins did not account for differential Ca absorption (Huang et d , 1994).

The idea that the plasmalemma is the primary target for Al toxicity remains

attractive; however, the evidence is certainly not unequivocal. It remains to be

shown if fundamental differences in membrane organization contribute to differential A1 tolerance in plants. Long-term effects of A1 toxicity, however, certainly

do involve disruption of plasmalemma integrity and function as part of the overall

disruption of cell metabolism (Meharg, 1993). Many of the cellular stress re-



I32



B. F. CARVER AND J. D. OWNBY



sponses observed in Al-intoxicated roots, for example, are elicited not only by Al,

but also by other factors such as heavy metal toxicity and pathogen invasion in

which damage to the plasmalemma occurs.



5. Synthesis of Aluminum Tolerance Proteins

Aniol ( 1984a) suggested that plants could develop A1 tolerance through the

synthesis of proteins that bind or sequester A1 and render it innocuous within the

symplast. Since then there has been an intense effort by a number of laboratories

to identify proteins synthesized in tolerant but not sensitive cultivars during AI

challenge. As A1 tolerance is determined by potentially many genes, the task of

identifying specific proteins that might confer Al tolerance is difficult.

What is clear from recent work, however, is that Al-challenged roots synthesize

a considerable number of proteins as part of the cellular stress response itself.

Table I1 lists proteins whose synthesis or activity is either induced or upregulated

in roots experiencing A1 stress. In addition to those proteins listed in the table,

Snowden and Gardner (1993) also identified, by screening a wheat cDNA library

from Al-treated roots, three other gene products whose deduced amino acid sequence was not homologous to any known protein. The pattern of expression of

these genes, however, was not consistent with a role in Al tolerance. Table I1 also

suggests some possible functions of these “A1 stress” proteins in Al-intoxicated

roots, although in none of the examples given have their roles been unequivocally

established. Slaski (1990) observed that Al-tolerant cereal species had more total

NAD kinase activity than Al-sensitive cereal species. Among ditelosomic lines of



Table I1

Proteins Whose Synthesis or Activity Is Increased in Response to A1 Challenge in Plant Roots

Protein

( I ,3)-/3-glucan

synthase”

NAD kinase



TAIL18

Phenylalanine ammonia lyase-like

protein

Metallothionein-like

protein



Possible function

Synthesis of callose as part of wounding

response

Upregulation of pathway of synthesis of

secondary compounds (?)

Pathogenesis-related protein; defense

against fungal pathogens (?)

Synthesis of Al-binding flavonoids (?)



Unknown



“Evidence from cowpea, not wheat.



Reference

Horst er rrl. ( 1991)

Slaski ( 1990)

Cruz-Ortega and

Ownby (1993)

Snowden and Gardner

(1993)

Snowden and Gardner

(1993)



ACID SOIL TOLERANCE IN WHEAT



133



‘Chinese Spring’ wheat, however, the elevation of NAD kinase activity in response to A1 treatment was about the same in both sensitive and tolerant lines.

The PAL homologue was suggested to function in synthesis of flavonoids that

could bind Al. None of the other proteins appear to have any direct role in conferring Al tolerance to plant roots. Cruz-Ortega and Ownby (1993) have suggested

that roots experiencing Al stress are more susceptible to other toxic elements and

to pathogens. As a defense response, the cells may thus synthesize PR proteins

such as TAI- 18 (Cruz-Ortega and Ownby, 1993) as well as callose (Horst et al.,

1991) and possibly the MLPs that could function in binding heavy metals (Snowden and Gardner, 1993).

To date there has been no unequivocal demonstration of a protein that is

the product of a gene conferring Al tolerance. Picton et al. (1991) used twodimensional PAGE to identify five proteins that were more abundant in Al-tolerant

wheat cultivar ‘Waalt’ than in Al-sensitive ‘Warigal’ in the absence of Al. These

proteins subsequently appeared in Warigal during Al stress, but none have yet

been characterized. Likewise, Delhaize et al. ( 1991) observed polypeptides specific to Al-tolerant wheat cultivar ‘Carazinho,’yet none cosegregated with the Altolerant phenotype when Carazinho was crossed with Al-sensitive cultivar ‘Egret’.

Basu et al. (1994) described two forms of a 51-kDa protein, called RMP51,

in the microsomal fraction extracted from the terminal 5 mm of wheat root

tips. RMP51 was rapidly induced in Al-tolerant cultivar ‘PT741’ but not in sensitive cultivar ‘Neepawa’ during challenge with Al. Although also present in Cdstressed plants, RMP5 1 was not induced by heat shock or toxic levels of Mn and

Cu, and turned over when Al was removed from the growth medium. From the

preliminary data, RMP51 seems to fit the criteria of an “A1 tolerance” protein.

Much more characterization of this protein is needed.

To summarize, various proteins that are elicited during Al toxicity have been

identified, but none have been identified whose presence can account for differential A1 tolerance among plants. It is assumed but not proven that proteins conferring Al tolerance are inducible. The possibility that products of genes providing

Al tolerance are constitutive or that they encode protein isoforms that are indistinguishable from those in Al-sensitive plants on one- or two-dimensional gels cannot be ruled out. Some of the properties of a novel, inducible “A1 tolerance”

protein would include: (1) consistently high concentrations in various tolerant

lines and reduced levels or absence in all sensitive lines; (2) cosegregation with

tolerant phenotypes when tolerant and sensitive cultivars are crossed; (3) relatively specific to Al toxicity; and (4) a physiological role that is consistent with

proposed mechanisms for metal tolerance (e.g., a protein involved in production

or secretion of chelating ligands or one that is part of an Al efflux pump or some

other exclusion process).

As this discussion illustrates, we are still a long way from completely understanding the physiological basis of Al tolerance. Current work suggests, however,



134



B. F. CARVER AND J. D. OWNBY



that if one could design an Al-tolerant wheat cultivar, it would secrete organic

acids for complexing Al, produce copious mucilage, and probably have a low

CEC. Future work should identify other traits that play a major role in the physiological tolerance to Al in plants.



c. UPTAKE AND DISTRIBUTION

OF MANGANESE IN PLANTS

Like Al, Mn is a potential source of phytotoxicity in acid soils. However, many

soils, including highly weathered acidic soils of the tropics, are generally low in

Mn (Marschner, 1986). Manganese toxicity has not been considered the threat to

crop growth and yield that Al toxicity is, especially in wheat-producing areas.

Only recently has Mn toxicity in wheat begun to be examined in physiological

terms. Two very thorough reviews that discuss general aspects of Mn toxicity in

crop plants are Foy er al. (1988) and Mukhopadhyay and Sharma (199 I).

Manganese toxicity usually occurs when low soil pH, low soil aeration, or both

prevent the oxidation of Mn2+ to MnO, by soil microorganisms (Mengel and

Kirkby, 1982). Manganese toxicity is usually aggravated by Fe deficiency. Recent

studies suggest that this may be less important in grasses than in dicots, presumably because of the different modes of Fe uptake by the two groups. Iron-chelating

phytosiderophores released by Fe-deficient cereals appear to have little affinity for

Mn (Zhang, 1993). However, in iron-deficient peas, the inducible iron-reductase

system reduced Mn at a rate 1 1-fold greater than in Fe-sufficient plants, which

correlated with a 2.7-fold increase in leaf Mn in Fe-deficient plants (Norvell ef al.,

1993).

Manganese as Mn2+in the soil solution is readily absorbed into root tissue and

is translocated to the shoot, where most toxicity symptoms are observed. Thus,

while exclusion mechanisms seem to play a major role in A1 tolerance, there is

general agreement that Mn tolerance is based almost entirely on internal mechanisms. Why do plants treat these two metals that are characteristic of acid soils so

differently? For one, Mn is an essential element. It participates in redox reactions

as manganoprotein on the water-splitting side of photosystem I1 (Hoganson and

Babcock, 1988) and also functions in mitochondria1 Mn-containing superoxide

dismutase (Sevilla er al., 1980). Second, the biological and chemical similarities

between manganese and magnesium may enable Mn to enter the plant by the same

absorption pathway used by Mg, so that any Mn exclusion process would run the

risk of likewise reducing uptake and transport of Mg. For most plants, the best

strategy under conditions of high soil Mn seems to be to allow Mn to accumulate

to supraoptimal levels in the shoot and then to use internal mechanisms to isolate

it from sites of cell metabolism. [For discussion of plants that do seem to tolerate

Mn through exclusion, see Mukhopadhyay and Sharma (1991)l.



ACID SOIL TOLERANCE IN WHEAT



135



Once Mn is absorbed into the root, its movement in the xylem stream seems to

vary among species studied. Mn was reported to move as free Mn2+ in tomato

(Lycopersicon esculentum Mill.) (Tiffin, 1967) and ryegrass (Lolium perenne)

(Bremmer and Knight, 1970). White et al. (198 1) calculated that 63 and 28% of

xylem sap Mn moved as complexes with citric and malic acid in soybean and

tomato, respectively.

Manganese is only slightly mobile in phloem and typically does not accumulate

in regions of growth. Visible toxicity symptoms generally develop in mature

leaves, including chlorosis in tobacco (Petolino and Collins, 1985) and spring

wheat (Macfie and Taylor, 1992). In the latter study, the rate of photosynthesis

decreased more than could be accounted for by the decrease in chlorophyll, especially in Mn-sensitive cultivar ‘Columbus.’ Cheniae and co-workers concluded

that the reduction in photosynthesis observed in Mn-stressed tobacco was most

likely due to interference by Mn in the Mg activation of rubisco (Houtz et a[..

1988). Although a three-fold increase in leaf polyphenol oxidase was observed

in the same study (Nable et al., 1988), nonspecific inactivation of chloroplast

proteins by polyphenols does not seem to play a major role in the reduction of

photosynthesis.

The most diagnostic Mn toxicity symptoms are dark necrotic spots, usually

<3 mm in diameter, that develop near margins and midveins (Bergmann, 1992).

These necrotic lesions are reported to be rich in MnO, (Marschner, 1986), but

Wissemeier and Horst (1992) observed that brown spots in cowpea were also

comprised of oxidized phenolic compounds, consistent with the idea that the spots

represent sites of immobilization of Mn 2+ through oxidation and precipitation as

Mn-phenolic complexes.

Early work by Morgan et al. (1976) showed that Mn-stressed cotton had elevated levels of IAA oxidases, which could lower the amount of the growth hormone IAA in the shoot. Reduction in shoot IAA probably affects growth more

than leaf necrotic lesions, which are sometimes seen in plants exhibiting little

reduction in growth or yield. The authors are not aware of any recent work corroborating this interesting hypothesis. With regard to other hormones, Wilkinson

and Ohki ( I 988) demonstrated that supraoptimal levels of Mn in wheat reduced leaf

gibberellic acid levels and also inhibited the in v i m synthesis of the GA precursor

ent- kaurene. These responses, however, were observed at nutrient Mn levels considerably higher than those correlating with the onset of growth inhibition.



D.



MECHANISMS OF MANGANESE

TOLERANCE



Progress in identifying Mn-tolerant wheat cultivars has been limited by lack of

rapid, reliable assays for predicting Mn toxicity. Goss er al. (1991) observed a



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