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CHAPTER 2 

icarf.* Understanding the Minimal Toxicity of 

Lead in Plants 

DAVID E. KOEPPE 

Department of Agronomy, University of Illinois, USA 

1. INTRODUCTION 

The presence of indigenous lead in the earth's crust at concentrations 

averaging 16 ^g/g soil (De Treville, 1964), its recent deposition near 

highways and other pollutant sources (Koeppe, 1977), and the knowledge 

that lead is highly toxic to various physiological processes have brought 

into focus over the last 15-20 years the question of the effects of lead on 

higher plants. This has seemed an especially relevant question when 

consideration has been given to the fact that certain higher plants probably 

suffer a lead insult that is greater than that imposed on any other living 

organism. Recent man-effected lead deposits have been near highways 

through the burning of leaded gasolines by automobiles (Cannon and 

Bowles, 1962), in soils near buildings painted with leaded paints (Getze/ al., 

1977), on agricultural lands fertilized with sewage sludge (Hinesly et al., 

1972) and in various localized situations near metal ore deposits, mining, 

smelting and other industrial operations (Antonovics et al., 1971). 

Pesticides containing lead have, dependent on usage, left substantial lead 

residues in orchards and other agricultural areas where they have been 

sprayed (Schuck and Locke, 1970). 

2. UPTAKE OF LEAD BY PLANTS 

Lead is available to plants from soil and aerosol sources. The actual uptake 

and association oflead with plants is affected (to varying degrees) by almost 

all environmental factors (Zimdahl and Koeppe, 1977). In a very general 

way it has been observed that the lead content (in and/or on) of plants 

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56 Effect of Heavy Metal Pollution on Plants 

~ % 

generally reflects the extent of lead insult, or what can be termed 

biologically-available lead. The chemical form oflead as it impacts plants is 

of critical importance, however,' as this is a factor in movements into plants, 

in translocation and in the toxic effectiveness of lead within the plantPlants growing near highways are usually exposed to more leadthan 

mosToflTer locations (z.imdahl and Hassett, l*)J/7). Ihe source of this leacGs 

primarily from aerosol automobile exhaust particulates as lead halogens 

wfiich are not water soluble!(Rolfe and Reinbold. 1977,. Even lead 

incorporated into soils is nearly always tightly bound to organic or colloidal 

materials, or in a precipitated form; all of which serve to reduce the uptake -

of' lead info plant roots by conventional processes involving soluble ionic 

movement (Zimdahl and Koeppe, 1977). These observations are supported 

by experiments which show that increased cation-exchange capacity 

(Baumhardt and Welch^JSI^Miller et al., 1975a; Miller et al., 19756; 

Motto et al., 1970),4owere3j)H (Cox and Rains, 1972; Lagerwerff et al., 

1973; Miller et al., 1975a; Miller et al., 19756; Zimdahl and Foster, 1976; 

John and Van Laerhoven, 1972), organic matter (Liebhardt and Koske, 

1974), phosphorus (Hassett etal., 1976; Miller and Koeppe, 1971; Rolfe, 

1973; Zimdahl and Foster, 1976; Sung and Jeong, 1977) and other leadprecipitating anions (Rolfe and Reinbold, 1977; Karamanos et al., 1976; 

Sung, 1976) reduced the uptake of lead into plant roots. 

Aside from the general low availability to plants of soil lead, the 

movement or translocation of lead from absorbing roots, or root hairs, is 

apparently impeded by a number of biochemical and/or physical processes 

involving lead binding, inactivation and/or precipitation. Early work by 

Hammett (1S»28) showed lead to be localized in the cell walls and nuclei of 

absorbing roots. Tandler and Solari (1969) found that lead was bound to 

orthophosphate ions within the nucleolus of onion root tips fixed in a lead 

solution. Other studies have shown that lead is often found in the cytoplasm 

of cells associated with electron-dense precipitates localized in membranous inclusions, vesicles or organelles. Malone et al. (1974) reported 

that the roots of corn plants exposed to lead in hvdroponic solution 

accumulated a surface lead precipitate and lead crystals in the cell wallsT 

They hypothesized that dictyo'some vesicles were responsible for an active 

extrusion of apparently soluble lead from root cells. In corn, an encased 

deposit of lead surrounded by the vesicular membrane was observed to 

migrate toward the outside of the cell where the membrane surrounding the 

deposit fused with the plasmalemma. The material surrounding the deposit 

then fused with the cell wall, resulting in a concentration oflead deposits in 

the cell wall outside the plasmalemma. Similar deposits were also observed 

1 
Lead: Understanding the Minimal Toxicity of Lead in Plants 57 

in stems and leaves which would support the hypothesis that once 

translocated, lead could be 'extruded' from cells throughout the plant. 

The impaction on plant leaves of aerosol lead particles from automobile 

exhaust has been well documented (see Zimdahl and Koeppe, 1977 for 

review). The uptake into plant cells of such lead has been a source of much 

controversy. Zimdahl and Koeppe (1977) concluded from a literature 

review that little, if any, aerosol-deposited lead in nature is accumulated 

through the cuticular barrier of higher plant leaves. The bryophyte 

Rhytidiadelphus squarrosus did take up lead from traffic exhaust, as 

electron-dense precipitates were recognized within cytoplasmic vesicles or 

vacuoles, chloroplasts, mitochondria, microbodies and plasmodesmata 

(Ophus and Gullvag, 1974). 

Cannon and Bowles (1962) reported that lead was associated with the 

above-ground portions of grass grown close to roads in concentrations as 

high as 3000 ug/g dry weight of plant matter. Schuck and Locke (1970) and 

Wedding et al. (1975, 1976) determined that substantial lead particulate 

matter was deposited on plant surfaces. Such deposition was dependent 

upon the characteristics of the leaf surface as well as the wind speed, and, to 

a lesser extent, other environmental conditions. Seven times more lead was 

deposited on pubescent sunflower leaves than on glabrous tulip poplar 

leaves (Wedding et al., 1975). These, and additional experiments (Wedding 

et al., 1975; Carlson et al., 1976) found no re-entrainment of monodispersed lead particulates from a leaf surface at wind speeds up to 

6-7 m/s, even though these winds caused a rapid fluttering of the test leaves. 

Wjtile topically applied lead was minimally removed by wind, it was readily 

removed by simulated rain conditions. A falling mist was more effective 

than large raindrops in lead removal (Carlson er al., 1976). The smaller 

drops of mist coalesced into larger drops, ran toward the leaf edge and fell 

off; while the larger, less effective drops splashed off at impact. 

Itjs almost certain that plants growing near highways possess elevated 

Igad levels, probably in a particulate form, associated with leaves and stems. 

Even if this lead is not absorbed into the plant cells, its presence as a topical 

coating on leaf surfaces must be considered. Does it have effects on plant 

processes or those of the microorganisms co-inhabiting these plant 

surfaces? 

3. EFFECTS • 

The ubiquitous presence oflead in soils, complicated and accentuated now 

by man-effected additions, makes questions of lead effect on plants of 
58 Effect of'Heavy Metal Pollution on Plants 

critical importance in formulating both short- and long-term environmental policy. The importance of these questions is amplified by two 

possibly conflicting observations that have been in the literature for many 

years. The first is that lead is highly toxic to many organisms under certain 

conditions. Secondly, even though very large concentrations of lead are 

present in localized plant environments, and even associated in or on 

plants, there are few reports of lead-induced toxic effects on plants grown in 

natural ecosystems that have been severely impacted with lead. > 

This section will consider the effects of lead on plants when it is available 

from rooting media or atmospheric sources. It will also discuss some of the 

studies which specifically ask jquestions of the effects of lead on in vitro 

physiological processes. Finally, the association oflead with plants will be 

summarized from the larger ecosystem-food chain vantage point. 

3.1. Distribution 

When an analysis is made of how lead may influence plant growth, 

questions relating to uptake! patterns and mechanisms of deposition 

(localization) within the plant are often the first to be considered. All studies 

show that most lead impacted at the root or leaf surface remains at that site 

without movement. When roots are treated, very little lead is translocated 

into edible fruits (Baumhardt and Welch, 1972; Ter Haar, 1970; Zimdahl 

and Koeppe, 1977). Early studies, including those by MacLean et al. 

(1969), Page and Ganje (1970), Aarkrog and Lippert (1971), and many 

others, have found the lead content of fruits, vegetables and grain to be less 

than in other vegetative plant parts. In plants exposed to lead-contaminated 

rooting media, the roots always contain lead at concentrations considerably greater than other above-ground tissues. Hevesv (1923.. in one of 

the earliest studies of this problem, concluded that lead is bound in the roots 

subsequent to root treatment, and that such binding serves to protect the 

remaining plant parts from injury. Recent studies utilizing various types of 

microscopical and hi.stochem.cal techniques support the concept that large 

amounts of lead are bound to the outside of roots, the 'free space', as well as 

cell walls throughout the roots (Malone et al., 1974). Studies where root 

metabolism has been controlled, suggest that the outside and free-space 

binding is passive (Goren and Wanner, 1971). Less is known about the 

energetics of the intracellular lead deposits associated with organelles and 

vesicles. It seems unlikely that the dictyosome vesicle lead deposits in corn 

reported by Malone et al. (1974) are passive, however, their studies did not 

seek to answer this question directly. Sharpe and Denny (1976) observed 

that the plasmalemma acted as a barrier to lead in Potamogetonpectinatas. 
Ik^imfe 

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^^S^/^^yLead: Understanding the Minimal Toxicity of Lead in Plants 59 

The movement of lead once it is clearly within the functional metabolic 

regions of the roots has been hard to evaluate. Whether lead moves as an 

ion, or as a chelated complex, is a reasonable question but not easily 

answered. Chelation of lead with EDTA does facilitate lead movement. 

Wallace et al. (1976) and Patel et al. (1977) observed that chelating agents 

modified the uptake of heavy metals from the' soil, and Marten and 

Hammond (1966) found that the presence of EDTA increased lead uptake 

by plants. 

Tanton and Crowdy (1971) reported that lead-EDTA entered roots only 

in the short root-hair zone, and that the root endoderms appears to limit the 

movement of the lead chelate into the transpiration stream. Once in the 

transpiration stream, however, they suggested that lead moved passively, 

and that it accumulated at the evaporative surfaces of the leaf. The 

possibility that lead moves into and within plants in a naturally chelated 

form is suggested from work at the University of Illinois where naturally 

exuded compounds were found which chelated lead (Gadde and Laitinen, 

.1972). 

Only a few reports are available which assess the question of the chemical 

identity of lead in plants. One compound, lead pyrophosphate, has been 

identified in the roots of pinto bean where the lead concentration was 

32 400 ug/g dry weight (Zimdahl, 1972). There has also been a report that 

lead orthophosphate (Pb3(P04)2) was present in soybean roots (see 

Zimdahl and Koeppe, 1977). Other studies by Brown and Slingsby (1972) 

with the lichen Cladonia rangiformis detected lead bound to insoluble 

anionic sites in exchangeable form, but external to the cell membranes. 

The potentially tight and nearly irreversible binding of lead to most 

exchangeable surfaces reported by soil scientists, and those reports referred 

lo here with plant roots, adequately set the stage and predict, a priori, why 

plants show so little response to potentially toxic concentrations of lead in 

the plant environment. Numerous experiments have been conducted, 

however, which have quantitated the effects of lead on both general yield 

parameters (including gross vegetative yield and grain yield) and more 

specific or integrated whole-plant physiological processes. These studies 

will be considered in the next section of this chapter. 

3.2. Aerosol Effects 

Lead particles deposited on leaf surfaces could be hypothesized to affect 

plant processes in two general modes. First, intracellular physiological 

processes could be affected if lead is solubilized and moves into the cells of 

the leaf. In Section 2 the possibility of this seems to have been 
60 Effect of Heavy Metal Pollution on Plants 

experimentally discounted. Carlson et al. (1976) made measurements of 

lead movement from leaf surfaces (fumigated with 1-3-wm diameter PbCl2 

aerosol), to non-fumigated plant tissues. In two experiments at least, 

several fully developed leaves of soybean plants were selectively fumigated. 

Following a post-fumigation period, the lead associated with the fumigated 

leaves was determined, as was the lead concentration of several nonfumigated sink tissues, including newly developed leaves and developing 

bean pods. In both the above experiments and a third in which the lead 

concentration of fumigation was very high and conditions were optimized 

for the movement of lead from the source leaf, negligible movement into 

and/or within the plant was observed. 

In corollary experiments Carlson et al. (1977) measured the rate of 

photosynthesis of leaves after topical application of PbCl2 particles up to 

concentrations of 700/ig/g fresh leaf weight. Photosynthesis was not 

reduced significantly with this iextent of leaf contamination over a three 

week post-fumigation period. . : 

The question next arises as to whether lead particles may accumulate in 

the stomata of leaves, thus affecting the ability of the stomata to open and 

close diurnally and with environmental stress, such as that imposed by 

water deficits. Fliickiger et al. (1979) reported that the diffusive resistance of 

aspen leaves was reduced when the surface was impacted with chemically 

inert 5-jim diameter silica gel particles. Their interpretation of this situation 

was that particles of a proper size will affect plant-water relations by 

blocking stomata, thus preventing their closure during hot and dry periods 

that would be required to prevent drought injury. Carlson et al. (1976, 

1977), however, argue that 1-3-um diameter PbCl2 particles on soybean 

leaf surfaces are not likely to physically block stomata. They point out that 

to physically block only 10% of the stomatal pore area of a soybean leaf 

with 1-ym diameter PbCl2 particles would require about 7000 wg PbCl2/g 

of leaf. In using this reasoning, and their photosynthesis rate studies, they 

conclude that stomatal blockage will not effect decreased rates of 

photosynthesis unless particulate deposition reaches extremely hip,h values. 

"Other studies of physiological processes of plants exposed to air 

pollution (including lead) have been conducted with plants grown at 

varying distances from highways. A recent experiment by Fliickiger et al. 

(1978) has correlated Pb collected in filters at varying distances from a 

highway with growth, peroxidase activity and ATP formation in 

chloroplasts isolated from the exposed plants. Concentrations of lead 

decreased rapidly from the highway, being reduced to 33 % at a distance of 

10 m from the highway border. Peroxidase activity was increased in plants 
7

~^ymWVLead: Understanding the Minimal Toxicity of Lead in Plants 61 

at 2 m and 12 m distance when compared to activity in plants at 42 m and. 

186 m distance. The formation of ATP in isolated chloroplasts and plant 

growth exhibited an increasing inhibition with decreasing distance from the 

highway; plants at 2 m distance were the most inhibited. Unfortunately, 

these authors were not able to specifically isolate the effects of lead from 

other pollution sources arising in the vicinity of highways. And while it was 

inferred that dust and exhaust gases were responsible for the observed 

effects, the authors suggested that the exhaust gases exerted a predominant 

influence. 

Steenken (1973) directly fumigated, with automobile exhaust, a number 

of cultivated plants in greenhouses, seeing in certain situations lethal effects 

on older leaves, but usually only slight effects on younger leaves. 

Interestingly, in complementary experiments where fumigation was with 

Pb-free automobile exhaust, the effects were even more severe at most 

exposure levels. Although Steenken did find test plants with Pb 

concentrations of 350 mg Pb/kg dry matter, he was unable to show any 

statistically significant depressions of yield and growth due solely to lead. 

Smith (1976) has found that the lead burden of plants due largely to 

surface deposition was as high as 200 x the baseline levels for unwashed 

plants. He was unable to find, however, any acute and direct effects of this 

lead on the roadside biota. His work does point in another direction that 

we will consider later, that being the potential of a topical lead coating to 

exert a subtle impact on the microflora on the plant surface and to have foodchain effects ampnneq through foraging insects and animals. 

*1 

3.3. Whole Plant Effects (Lead in the Rooting Medium) 

Reports on the effects of lead on selected aspects of plant growth have 

appeared in the literature for over 50 years. Both growth stimulations and 

reductions have been reported to result from varying lead concentrations in 

the rooting media. Growth stimulations, mostly at low lead concentrations, 

have been reported for several different plants by Keaton (1937), Berry 

(1923), and others (see Tornabene et al., 1977 for review). Since most of 

these reports came before the definition of optimal conditions for 

hydroponic growth of plants, it might be suspected that the growth 

stimulation itself was not due to lead, but possibly to other ion(s) made 

available as a result oflead salt treatments to the rooting media. Numerous 

other studies utilizing lead-amended hydroponic culture have shown a 

significant reduction of growth with increasing lead concentrations. 

Evidence has also been presented that the magnitude of the lead effect 

various inversely with the phosphate concentration of the nutrient solution 
62 Effect of Heavy Metal Pollution on Plants 

and with the phosphate status of the plant itself (Miller and Koeppe, 1971; 

Rolfe, 1973). ;.s 

Other examples in which general decreases in growth parameters have 

been determined after lead treatment in hydroponic culture include studies 

with red maple (Davis and Barnes, 1973), oats (Fiussello and Molinari, 

1973), soybeans (Huang et ai, 1974), corn (Carlson et ai, 1975), sunflower 

(Carlson et al., 1975), rice (Fiussello and Molinari, 1973; Mukherji and 

Maitra, 1976,1977), loblolly pine (Davis and Barnes, 1973), and groundsel 

(Briggs, 1976). In most of these situations the time of treatment was a 

significant variable in the extent of the lead-induced toxicity. 

In a comparative hydroponic study to determine the relative toxicity of 

lead to a number of different plants, Fiussello and Molinari (1973) 

determined the following sequence of decreasing sensitivity to lead: „ 

Capsicum annuum > Beta vulgaris > Phalaris canariensis > Vicia sativa > 

Helianthus annum >Oryza sativa > Triticum vulgare > Avena sativa > 

Pisum sativum 

An example of the extent of the lead toxicity is that A. sativa, one of the 

least sensitive species, showed a M% decrease in fresh weight, a 23% 

decrease in dry weight and a 26 % decrease in chlorophyll content when 

grown for 21.days in nutrient solution amended with 10"* M Pb(N03)2-

Merakchiiska et al. (1976)j reported that Phaseolus vulgaris (bean) 

exhibited reductions in whole plant, leaf and root fresh weight after 10 days' 

treatment with 10"5 M PbCl2 in hydroponic solution. The reductions were 

about twice as great when the PbCl2 concentration was increased to 10"3M. 

With the 10 " 5 M PbCl2 treatment, the length of the whole plant was reduced 

by 34 % and the length of the root by 70 %. Leaf surface area was reduced 

26 %, but measurements of the length of the aerial part of the plant and the 

hypocotyl node each showed increases of 16%. 

With rice {Oryza sativa) root growth was completely inhibited by 

2 x 10~2 M lead acetate and germination by 10"' M lead acetate (Mukherji 

and Maitra, 1976). At 10"3 M lead acetate, shoot growth was inhibited by 

9%, whereas the inhibition of, root growth was 46%. 

Broyer et al. (1972) in experiments with Phaseolus vulgaris (bean, var. 

bush blue lake), Hordeum vulgare (barley) and Lycopersicon esculentuin 

(tomato) observed only slight reduction in plant top and root dry weight 

following treatments of up to 50/ig atom/litre initial lead supply. Possible 

reasons for the lack of observable lead effect may have resulted from.shorter 

treatment times and much lower lead concentrations over the entire 

fi, 
Lead: Understanding the Minimal Toxicity of Lead in Plants 63 

duration of the treatment period. The authors did find most of the lead 

^csnciatedwith the roots and little with the plant tops. This lead sorption to 

the roots of barley was very rapid and not influenced by temperature. It is 

often very difficult to compare the total amount of lead treatment with the 

roots of plants, and herein often resides the probable quantitative 

differences in lead-effected plant response. 

'In other experiments, lead salts have been incorporated into a number of 

different soils. Neither alfalfa nor corn were affected by lead concentrations 

:

JipTo"212/ig/gin Chester silt loam soil (Lagerwerff et al., 1973). Soybean 

(Glycine max. L.) and corn (Zea mays L.) were sown in a sandy-clay soil 

andwatered with fixed concentrations oflead and various anions (Sung and 

Jeong, 1977). When the plants were harvested before flowering they showed 

no^signs of jgad^toxicity in spite of 3773 /ig Pb/g soih_ Total lead 

concentrations in the plants did not exceed 3-64 /ig/g dry weight. In another 

study with 500/ig/g Pb in the soil, radish, carrot and spinach showed 

considerable uptake of lead but without reducing yield or toxicity 

symptoms (Judel and Stelte, 1977). Other work has revealed no yield 

reduction in alfalfa and bromegrass grown on soils amended with up to 

100/ig Pb/g soil (Karamanos et al., 1976). In many of the more recent 

«Midies the defined presence of anions such aTphosphate and sulfate has 

Tourer] lead uptake and/or effect (see Section 2). 

One of the more significant applications of lead to agricultural soils has 

been with numerous other heavy metals and other major and minor 

micronutrients in sewage sludge. With the. ever-present possibility that a 

number of these metals besides lead might be toxic to plants, it has not been 

possible to isolate any distinct lead effects from such sludge applications. In 

actuality, the presence of so many other competing ions probably serves to 

reduce the uptake and any effect of lead. Several studies have questioned, 

however, whether combinations of heavy metals that are known plant 

toxins might "at certain concentrations act synergistically (or antagonistically). Two reports from a trace element research project at the University 

of Illinois serve to illustrate this point. In a study by Miller et al. (1977) 

varying concentrations of lead and cadmium were incorporated into a 

sandy loam soil with a relatively low cation-exchange capacity. In 

measurements of 10-31 days old corn shoots there was a tendency for soil 

lead to increase the plant cadmium concentration and the total cadmium 

uptake. Conversely, soil cadmium reduced the total lead uptake and in 

some cases lead concentration of the shoots, Statistical analyses show that 

both lead and cadmium individually reduced corn shoot growth, and that 

there was a positive interaction of the two metals on growth. In another 
64 Effect of Heavy Metal Pollution on Plants 

study by Carlson and Bazzaz (1977), seedlings of American sycamore 

(Plantanus occidentalis L.) were grown in Drummer silty clay loam soil 

treated with various concentrations of lead, cadmium or lead plus 

cadmium. Plant growth and heavy metal content were measured after 

90 days of treatment. Concentrations of both cadmium and lead were 

increased in the plant tissue, and root growth, woody stem growth, new 

stem growth and foliage growth Were synergistically affected by the leadcadmium treatment. That is, the effect of the two heavy metals together 

was greater than the sum of th~y pfT^cts nf thrTin^jrhij^ljigwy meta i 

treatmeats. . 

Research with several plants including Lactuca sativa L., Raphanus' 

sativus, Oleifera sp., Setaria itdlica L., Raphanus sativus radicula L. and 

Tagetes sp. by Krause and Kaiser (1977) assessed the interaction oflead and 

sulfur dioxide fumigation. In their studies, neither the uptake and 

translocation of lead, nor its effects on yield were influenced by the sulfur 

dioxide treatment. ! 

As a result of observations of the uptake and effect oflead on the general 

growth of plants in the field and the literature cited here, along with many 

other studies too numerous to mention, the following concepts have 

emerged. First, that lead is taken up by plants with the magnitude of uptake 

being correlated inversely to soil or other artificial rooting-media 

conditions that bind lead. Such binding of lead reduces its activity (in~a 

physical chemistry sense) in the soil or rooting-medium solution. In other 

words^ good agricultural practice in monocultural or natural systems 

should virtually negate the uptake and effect oflead under field conditions. 

In the absence of agricultural disturbance, it seems that Pb uptake from the 

soil would be found only under conditions where the soils were light and 

had low fertility levels, low organic matter levels and/or a low cationexchange capacity. Even here, where some lead uptake is likely, studies have 

shown that 90 % or more of the lead taken into roots is tightly bound there, 

riot available tor translocation to other parts of the plant, and not an 

effective toxin, 'the cnances that enough lead can be taken up by any higher 

plant under natural conditions to effect a toxic response seem remote. 

Under controlled lead-treatment conditions, toxic responses influencing 

physiological parameters other than general growth have been quantitated, 

and are the subject of the next section of this chapter. 

3.4. Specific In Vivo Lead Effects (Lead in the Rooting Medium) 

This section will review studiesiin which plants were exposed to high lead 

concentrations in the rooting medium, and in which the response to lead of 

<*' 
Lead: Understanding the Minimal Toxicity of Lead in Plants 65 

processes other than general growth were measured. These effects might be 

considered in vivo as compared to in vitro studies where Pb was applied 

directly in the 'test tube' to enzymes, organelles or other subfractionated 

cellular components. 

Gas exchange, particularly the uptake of C02 in photosynthesis, has 

been determined for several different plant species treated with different 

concentrations of lead for increasing periods of time, Among the plants 

tested in these experiments, soybean (Glycine max. L.) (Bazzaz et al., 

19746), sunflower (Helianthus annuus) (Bazzaz et ai, 1975; Carlson et al., 

1975; Bazzaz et ai, 1974a), American sycamore (Plantanus occidentalis 

L.) (Carlson and Bazzaz, 1977), corn (Zea mays L.) (Bazzaz et al., 1975; 

Carlson et al., 1975), loblolly pine (Rolfe and Bazzaz, 1975), autumn olive 

(Rolfe and Bazzaz, 1975) and spruce (Keller and Zuber, 1970) exhibited 

rprinced rates of photosynthesis with increased soil lead concenTfattons. 

Like most of the lead-effects experiments, the effective rooting-media lead \ 

concentrations were usually very high as compared with the concentrations \ 

of lead encountered by almost all plants growing naturally or in J 

monoculture. Because of the techniques and exj)gjirignt^l prnrfflj"irs ^ 

involved, few corrections have been madeTJetween the lead content of the 

and tKeir_ rates of photosynthesis. One study making these 

(1974a) who reported for 

leaves 

measurements was done by bazzaz et 

sunflower that at leaf lead concentrations of 193 /ig Pb/g dry weight the rate 

of net photosynthesis was reduced by 50 %. Rolfe and Bazzaz (1975) found 

no effect of Pb on either photosynthesis or transpiration of loblolly pine 

(Pinus laedd) or autumn olive (Elaeagnus umbellata) at tissue concentrations below 60 and 72/ig/g dry weight respectively. At these 

concentrations photosynthesis was reduced in loblolly pine by 11 % and in 

autumn olive by 17%. In other studies carried out by Bazzaz and coworkers (Bazzaz et al., 1974a; Bazzaz et al., 19746; Carlson et ai, 1975; 

Rolfe and Bazzaz, 1975) there was a strong correlation between leadeffected decreases in photosynthesis and concomitant decreases in rates of 

transpiration. The suggestion from this is that decreases in the rates of 

whole-plant photosynthesis may be due to induced closure of stomata 

rather than a direct effect on the process of photosynthesis residing directly 

within the chloroplasts. This point may be questioned, however, as reports 

are available of in vitro effects of lead on isolated chloroplasts (Miles et al., 

1972; Bazzaz and Govindjee, 1974; Wong and Govindjee, 1976). 

The possibility of in vivo effects of lead on chloroplasts that may be 

responsible for cell death has been reported after studies with the 

hydrophyte Ceratophyllum demersum (Rebechini and Hanzely, 1974). In 
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Effect of Heavy Metal Pollution on Plants 

this study, striking changes in chloroplast fine structure resulted from 

growth in an aquatic medium containing Pb(NO,)j chelated with EDTA. 

Lead-exposed leaf cells exhibited a reduction in grana stacks, a reduction in 

the amount of stroma in relation to the lamellar system, and showed an 

absence of starch grains. • 

In bean, however, Yordanovand Merakchiiska(1976) found that 10"3 M 

PbCl2 stimulated photosynthesis on a leaf-area basis, but effected a 

15-20 % reduction on a mg-chlorophyll basis. These authors also reported 

no significant effect on chloroplast lamellar protein composition or spectral 

properties of the pigment system. The membranes of proplastids in the 

meristematic cells of lettuce root tips did show destruction of the marginal 

membranes after treatment with tetramethyl lead (Herich and Bobak, 

1974). 

The respiration of corn root tips decreased bv 10-17% after a 1-h 

treatment with 20mM lead, and by 28-40 %"after a 3-h treatment (Koeppe, 

1977). These respiratory decreases were accompanied by a concomitant 

decrease of the energy charge of the treated tissue. 

Lead added to a soybean rooting media of sand-vermiculite, reduced 

pod fresh weight by 35 % with daily waterings of 300 /iM lead (Huang et ai, 

1974). This reduction was correlated with decreases in shoot, root, leaf and 

nodule dry weight; and witfi nodule ammonia, protein and carbohydrate 

content. Decreases in the rate of photosynthesis and nodule acetylene 

concentration were also observed. 

Pigment concentration changes in leaves are often a visible sign of 

elemental toxicity or deficiency. In PbCl2-treated Acer rubrum (red maple) 

the relative anthocyanine content increased as much as 270% (Davis and 

Barnes, 1973). In another quantitative study, Pb(N03 ) 2 in solution culture 

for 21 days effected a decrease in chlorophyll content oi Avena sativa (oat) 

by approximately 26 % (Fiussello and Molinari, 1973). Other researchers 

have referred to the chlorotic, appearance of leaves following lead treatment, 

but have usually not quantitated the loss of chlorophyll. In these studies 

there have been few attempts to correlate a chlorotic plant appearance only 

with lead accumulation. 

In a study by Walker et al. (1977) with Zea mays L. (corn), PbCl2 was 

added to a sandy loam soil in concentrations of 125-250/ig/g dry soil 

weight. After 20-30 days treatment the plant concentrations of boron, 

manganese, copper and zinc >vere reduced from 20-39 %. The mode oflead 

action in this study was not questioned. Kannan and Keppel (1976) did 

observe in short-term experiments with pea (Pisum sativum) seedlings that 

lead in a Hoagland's nutrient medium was highly inhibitory to the 

<W 

'!•£<• 

ft" 
P.14 

Lead: Understanding the Minimal Toxicity of Lead in Plants 

and Zn2+ 

67 

absorption of Fe , Mn2 + , and Zn2+ ions. They reasoned that the 

inhibition was of a physical nature, like blocking the entry or binding of the 

ions to the ion-carrier. 

Several researchers have measured the activities of enzymes in generally 

crude homogenates of lead-treated plants. Lead acetate (6 x 1_0"2 M^injthe— 

liqiiifj media of germinating rice effected about a 50%"decrease in the 

activities of protease and a-amylase in endosperm after four days (Mukherji 

and Maitra, 1976). The activity of RNase was reduced by 17%, but that of 

DNase was stimulated by 12%. 

A comprehensive study of enzymes, selected nutrients and nitrogenous 

compounds of soybean leaves following 10 days of Pb(NOf), treatment in 

-Qlulinn r-nitiirp led Lee et al. (1976) to conclude that lead caused an 

advance in senescence of the seedlings. At lead concentrations of 100 /ig/ml, ' 

leaf respiration was increased 22% while several enzymes" showedlme 

following effects when compared to zero-lead treated controls: acid 

phosphatase, +23%; peroxidase, + 154%; malic dehydrogenase, +20%; 

j-amylase, +280%; and glutamine synthetase, -89%. Under the same 

lead treatment phosphorus, calcium and nitrate decreased, but total free 

amino acids, soluble protein, ammonia and lead increased. 

In lead-treated young corn plants Maier (1978) found that the activity of 

acid phosphatase decreased in the roots but increased in the leaves. In 

experiments with other enzymes Maier (1977a) observed that NAD+-

dependent malate dehydrogenase increased in leaves of Medicago sativa 

and Zebrina pendula, but decreased in the Medicago roots. Esterase 

analyzed after in vivo lead treatment by Maier (19776) increased, or 

decreased, as did its number of multiple forms, with the length of lead 

treatment and the temperature. 

Several investigations have found that various forms of lead produce 

chromosomal lesions or other disturbances. Included in these are 

tctramethyl lead effects on Lactuca sativa (Sekerka and Bobak, 1974), lead 

acetate on onion root tip cells (Mukherji and Maitra, 1976). and di- and 

trialkyl lead on onion root tip cells (Allium cepa) in which spindle 

disturbances were noted (Ahlberg et ai, 1972). 

Yet another interactive effect of lead is suggested in several studies, in 

which the hypothesis is advanced that the real toxic effect of lead is not 

direct on any cellular component, but rather indirect by killing cells 

through deprivation of phosphate. This was recently argued by Schulze and 

Brand (1978) following experiments with Chlamydomonas. and had been 

earlier argued with less data by Merakchiiska et al. (1976) in studies with 

lead effects on Phaseolus vulgaris. 

"Z^'-J1 
68 Effect of Heavy Metal Pollution on Plants 

3.5. Lead Effects on In Vitro Plant Systems 

Numerous reports from the animal literature have shown that lead forms 

mercaptideswith the—SH group of cysteine and less-stable complexes with 

other amino acid side chains (see Vallee and Ulmer, 1972 for review). Lead 

has also been shown to bind and alter the activity of many membranous 

subcellular particles includingmitochondria,chloroplasts, nucleotides and 

nucleic acids and chromosomes (see Vallee and Ulmer, .1972 for review). 

Using mitochondria isolated from etiolated corn seedlings Koeppe and 

Miller (1970) reported that PbCl, stimulated the oxidation of exogenous 

NADH, but inhibited the rate:of succinate oxidation by more than 80%. In 

a reaction medium containing sucrose or KC1 as an osmoticant the effect of 

lead on the oxidation rates was immediate as was the reversal of the leadinduced succinate inhibition ;with the addition of inorganic phosphate. 

Inorganic phosphate did not reverse the stimulation of the rate of NADH 

oxidation by lead, possibly suggesting an irreversible binding oflead or that 

lead effected an irreversible change in the mitochondrial complex 

responsible for NADH oxidation. Other data strongly suggested leadinduced decreases in oxidative1

 phosphorylation. In a subsequent study with 

isolated plant mitochondria (Bittell et ai, 1974) the concentration of lead 

effecting a 50 % inhibition ofj succinate oxidation was not different from 

similarly effective concentrations of cadmium or zinc. Lead (0-1 mM) caused 

an extensive increase in swelling of mitochondria in a reaction medium 

containing KC1. In binding studies it was observed that the maximum 

capacity of a mitochondrial suspension to bind lead was nearly 10 times 

greater than the 58 nM of Cd2;

+

, Zn2 +, Co2 +, Ni2 + and Mn2 + bound/mg 

mitochondrial protein. The affinity was also observed to be very high for 

lead. Studies by Goyer and other workers (Goyer, 1968; Goyer and Kroll, 

1969; Goyer et ai, 1968 and Scott et ai, 1971) detailing the effects of lead on 

animal mitochondria indicate that lead has very similar effects on both 

plant and animal mitochondria. 

Effects oflead on isolated chloroplasts were first reported by Miles et ai 

(1972). These researchers observed that lead inhibited photosynthetic 

electron transport associated! with photosystem II, but had no effect on 

electron transport through photosystem I. Fluorescence induction curves 

indicated that the primary site of lead inhibition was on the oxidizing side of 

photosystem II, between the primary electron donor of photosystem II and 

the site of water oxidation. Bazzaz and Govindjee (1974) later observed that 

PbCl2 either stimulated or inhibited photosystem II activity depending on 

the pH of the reaction medium. Further studies with isolated maize 

chloroplasts by Wong and Govendjee (1976) reported the direct inhibition 

m 
?spt^: "c%i**&°; 

Lead: Understanding ihe Minimal Toxicity of Lead in Plants 69 

by lead of the reaction centre P700 of photosystem I. They hypothesized 

through observations of the dark decay rates of P700+, that lead altered the 

rate of electron transfer to P700+. Thus, lead was seen to have two 

simultaneous effects: an inhibition of P700 photooxidation and an 

alteration of the kinetics of re-reduction of P700+. 

The photosynthetic fixation of CO, by isolated spinach chloroplasts was 

observed 5y~Hampp et ai (1973a) to be non-competitively inhibited by 

lead. These inhibitions were most pronounced at concentrations up to 

-y ij I1 ~ 20 /tM, and likely were correlated with a drastic reduction in ATP formation 

ff

 , ,-. —""By irradiated chloroplasts. Interestingly, even at^QOj^jconcentrations of 

..•••

:

] j ! "~ lead, 15% of the C0 2 fixation capacity was not inhibited. ~" 

&•' "' "• • , r y r-y As predicted from numerous animal studies, lead also affects the activity 

,! >'• ' (_' of numerous isolated plant enzymes. In extracts of roots or leaves of 

spinach, Hampp et al. (19736) observed the inhibition of ribulose-1,5-

diphosphate carboxylase and ribulose-5-phosphate kinase activity at 5/iM 

Pb(N03)2. Lead stimulated the activity of pyruvate kinase up to 

concentrations of 100 /iM. Both a stimulation and inhibition, depending on 

lead concentration, was observed in an NAD+ or NADP + glyceraldehyde3-phosphate dehydrogenase phosphoglycerate kinase system. 

In a recent study by Maier (19776) in vitro lead inhibitions of esterase 

activity were observed in enzyme preparations from Zea mays, Medicago 

saliva and Zebrina pendula. 

Lead (2 x 10"6-2 x 10"3 M) was found in experiments by Zegers et al. 

(1976) to inhibit the IAA-induced elongation of Avena sativa coleoptile 

segments. These authors hypothesized that lead was interfering with a 

factor(s) directly involved in cell elongation, suggesting the specific 

involvement of enzymes in the wall and the ATPase associated with the 

plasmalemma. 

i It is suspected that many other enzyme systems would be inhibited by 

lead if the studies were conducted as they have been with in vitro animal 

systems. 

• 

4. SUMMARY 

As a result of localized pollution, highly elevated levels of lead are becoming 

associated with plant roots and foliage. Movement of lead into higher 

plants has been convincingly demonstrated through roots, but not from 

lead particles deposited on leaf surfaces. Association of lead with foliage is 

as a topical coating, with more lead deposited on pubescent leaves than 

. ^..^^^.^^mi^Sme*-. ^.^iJSSCW-.--^>'^teWBt*.- •- s.wjifci^-\..A.JaiUi«!»SJBL.«-.-_ 
Effect of Heavy* Metal Pollution on Plants f§ijl 

-is 

leaves with smooth surfaces. Moderate winds effecting a fluttering leaf £ 

motion cause little lead removal. Rainfall does remove large quantities of ^ 

leadTwith slow mist-type treatme^Us being the most effective. Lead""d"eposits '~~ 

on leaves have mtje effect on stnnhptal aperture and gas exchange, but are of .$ 

considerable importance as potential toxins to the microflora associated ;: 

witlTleai surfaces and to grazing; herorvores ranging trom insects to horses". '; 

Quite possibly the most important effect of lead associated with plant • 

foliage is in food chains where plants in essence act as passive lead carriers. '& 

Under certain soil conditions (including low pH, low cation-exchange f 

capacity, low organic matter levels and low phosphorus levels) large 

alnouiiis uf lead can be-taken up by higher plant roots. Lead taken up by 

roots generally has noftoxiCyeffect on plants, except at extremely high rootmedia concentrations that have little relevance to natural conditions. This 

general lack of lead effect from high lead concentrations in roots has led to 

the hypothesis that most lead |taken up by roots is quickly inactivated 

through deposition in^he roots. Lead phosphate deposits are formed on 

root surfaces, in peripheral extracellular spaces (including cell walls) and 

within dictyosome vesicles of root cells outside the endodermis. 

Environmental factors, plant age and plant speciation are important 

variables in lead uptake by plant roots. Generally, an alteration of any soil 

vanable.wtiich makes lead more available in soil solution increases root lead 

uptake. Most studies with lead in hvdroponic solution show a direct 

correlation with time of lead treatment, uptake into and translocation 

within the plant, and toxic effect. 

Lead additions to both in vitro subfractionated cellular systems and to 

roots have produced effects on many different enzymes or other integrated 

subcellular systems. One is ledjto suspect that the choice of the enzyme, 

organelle or whole-plant process under study has been in many instances a 

result of the expertise and interest of the investigators rather than searches 

for truly critical lead-effected rate-limiting reactions. Effects of high lead 

concentration have been reported on photosynthesis, respiration, energy 

relations and an array ot selected enzymes as well as yield (either tresh or 

dry weight, or grain). Lead has been shown to cause chromosomal 

aberrations and to become nAtimately associated with many different 

cellular and subcellular membranes. This association_f>ft<m results in-the 

manifestation of toxic lead effects~"such as reductions in the activity of 

phosphatase and enzyme-bound ATPase. 

Lead accumulations in localized areas near pollution sources probably 

have little direct effect on plants grown in these areas. This lack of effect 

most often is due to nearly irreversible binding of lead to soil exchange 

•4 

<* 
.mmr^zgm sr^"-:r^ 

Lead: Understanding Ihe Minimal Toxicity of Lead in Plants 71 

surfaces and to extracellular root surfaces. Lead aerosol particulates do not 

enter plants and have no demonstrable physical effect on the foliar surface 

of plants. At this time, lead can be hypothesized to pose little, if any. 

problem to plants themselves, with the possible exception of areas where 

joil fertility, organic matter and colloid content are very low. Here only 

modest lead binding will occur in the plant's p'ivi'-""T"Mn, TMTMP lead will be 

riirectlv available lor uptake, and some toxic effects of lead might be 

prearcrjdT^ 

The more significant role of plants with lead in the ecosystem resides in 

food chain situations where a topical lead coating may be ingested by 

"hTfoTvores grazing on these plants. The potential relevance of this problem 

is supported by documented deaths of horses (Rabinowitz and Wetherill, 

1972) grazing in lead-contaminated roadside areas. 

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*& 
Lead: Understanding the Minimal Toxicity of Lead in Plants 75 

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