plants · priate quantities of CUSO4, ZnSO4, MnCl2 and ferric tartrate or ferric iron fromthe...

HEAVY-METAL NUTRITION AND IRON CHLOROSISOF CITRUS SEEDLINGS

PAUL F. SMITH AND ALSTON W. SPECHT

BUREAU OF PLANT INDUSTRY, ORLANDO, FLORIDA AND BELTSVILLE, MARYLAND

Received August 25, 1952

The recent widespread increase in iron chlorosis on acid, sandy soils inFlorida has led to studies of the underlying causes. The leaves on affectedtrees are deficient in iron (10). Field observations show that tree conditioncan be substantially improved by liming. Additions of iron sulphate to thesoil has not improved tree condition noticeably. Recently foliage conditionimproved when small quantities of chelated iron were used (12). It isnot known yet whether soil applications of this material will promote therehabilitation of the topsoil fibrous root systems, which have been severelydamaged in all cases investigated. The heavy metals Cu, Zn, and Mnaccumulate in the topsoil of citrus orchards despite its very sandy and acidicnature (8). This accumulation is independent of that of phosphate, whichtakes place simultaneously (8). These metals, as well as others, may inter-fere with Fe metabolism of plants (3, 5).

Little is known about the fundamentals of heavy-metal nutrition ofcitrus. There is actually little information on the relative toxicities, antago-nistic reactions, or the nature and extent of the accumulation of heavymetals in roots of any plant. Much additional research is needed to eluci-date the processes of their absorption and accumulation. The present studieswere made with the hope that some information on these aspects of nutri-tion could be gained and it is felt that the results presented will contributetoward this end.

Materials and methods

The plants tested were young citrus seedlings grown in seedbeds of ver-miculite as described elsewhere (9). Valencia orange seedlings were usedmostly; however, some Sunki mandarin seedlings were included in one ex-periment. The seedlings were one to two months old when used and theexperiments were continued two or three months, which gave sufficient timefor a widely differential growth response.

Two systems of culture were used. The first involved use of clay potsfilled with vermiculite and placed in glazed pottery vessels which permittedsub-irrigation with the desired nutrient solution. The solution was flushedthrough the pot twice weekly and adjusted frequently to keep the pH near 4and to maintain the liquid level. Fresh solutions were applied about everytwo weeks. The second system was a solution culture in one-quart glassjars that had been painted successively on the outside with asphalt and

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aluminum paints. The solutions were adjusted to pH 4 and brought to vol-ume with distilled water weekly and new solutions were supplied every fourweeks. In addition, the solutions were tested every few days to see that theapplied levels of Cu, Zn, Mn, and Fe remained nearly constant. In bothsystems of culture two seedlings were grown in each container.

The first vermiculite experiment had two sources of nitrogen (all NO3and half NH4), two levels of Cu (0.02 and 10 p.p.m.), two levels of Zn (0.05and 15 p.p.m.), and two levels of Mn (0.5 and 25 p.p.m.). All possible com-binations of these variables were used, making a total of 16 solution treat-ments. Iron from ferric tartrate was added thrice weekly at the rate of 0.5p.p.m. Fe. Nitrogen and the other unmentioned essential elements weresupplied here and elsewhere in levels equivalent to those in one fourthstrength Hoagland's culture solution. After 90 days these plants were har-vested, divided into roots, stems, and leaves, rinsed several times in distilledwater, dried and analyzed spectrographically for 10 elements. Only theheavy metal analyses are reported. Zinc was determined by standard chemi-cal procedures. The second vermiculite experiment and the solution cultureshad 75%o of the N supplied as NO3. The ammonium ion was used to retardshifts in pH. Four levels of Cu (0.02, 2.5, 5, and 10 p.p.m.), three Zn levels(0.05, 25, and 75 p.p.m.), and three Mn levels (0.5, 25, and 75 p.p.m.) wereused in combination with two Ca levels (40 and 160 p.p.m.) and two Felevels (1 and 5 p.p.m. from ferric tartrate). This made a total of 40 cul-tures, supplemented by a few additional 10 p.p.m. Cu cultures which con-tained 10 p.p.m. Fe from the chelated complex, KFe-EDTA (6). This ex-periment was also continued for 90 days but the only tissue analysis madewas for Fe in chlorotic leaves.

The solution culture experiment had variable quantities of Cu, Zn, andMn each at two levels of chelated Fe. These treatments are shown in detailin table IV. Each treatment was made in triplicate. The plants were grownunder differential treatment for 70 days. Only visual observations of thegrowth responses were made.

Differential quantities of the heavy metals were supplied by using appro-priate quantities of CUSO4, ZnSO4, MnCl2 and ferric tartrate or ferric ironfrom the chelated complex mentioned previously.

Results and discussionTOXICITY OF HEAVY METALS AND THEIR EFFECTS ON GROWTH

In the vermiculite cultures, growth was very good except in the supple-mentary Cu and 75 p.p.m. Zn treatments, which all caused chlorosis orstunting (fig. 1, C to G). Copper was the most toxic of the three heavymetals, causing severe suppression of root growth and inducing chlorosis atmuch lower levels of supply than either Zn or Mn. Neither the source of Nnor the Ca level had any visible effect on the behavior of the heavy metals.In the first vermiculite experiment where various combinations of the metalswere added, there did not appear to be any additive effect on growth sup-pression by the three elements. Solutions containing 10 p.p.m. Cu caused

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SMITH AND SPECHT: IRON CHLOROSIS OF CITRUS

severe stunting and chlorosis of the very few leaves that developed regard-less of the amiaounts of Zn and AIn. Zinc at the rates of 15 and 25 p.p.m. andMn at all rates including the 75 p.p.m. permitted normal growth of bothroots and tops. Examples of plants grown at certain levels of the metals areshown in figure 1, A to F.

The frequent additions of ferric tartrate did not prevent the developmentof chlorotic leaves in the supplemental Cu cultures of the first test. In the

FIG. 1. Effect of certain heavy-metal treatments on growth and chlorosis of citrusseedlings in vermiculite and solution cultures. All are Valencia orange except the planton the right in each clay pot which is a Sunki mandarin seedling. The full-nutrientsolution contained the following supplemental metals in p.p.m.: A, none; B, 75 Mn;C, 75 Zn; D, 2.5 Cu; E, 5 Cu; F, 10 Cu; G, 10 Cu; H, none; I, 0.25 Cu; J, 3 Zn; andK, 5 Mn. Solutions for D, E, and F contained 5 Fe from ferric tartrate. Solution forG contained 10 Fe from KFe-EDTA. Ruler is six inches long.

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PLANT PHYSIOLOGY

second vermiiculite experimiient, where Fe was a variable, no difference ingrowth, intensity of chlorosis, or reduction in metal toxicity to roots wasassociated with the rate of Fe-tartrate supplied. The use of chelated Fe,however, prevented the development of chlorosis but did not overcolmle thesuppressive effect of Cu on growth. (Comipare A, F, and G, fig. 1.)

Living roots that contained 500 or more p.p.m. Cu by dry weight werefound to be sharply altered in appearance. Elongation was interfered withso that numerous laterals were arrested in growth and looked like pegs alongtheir parent root. These roots were usually swollen and had a cavity wherethe apical meristeiim should normally be. Their color varied from very lightto dark brown. This apparently depended on the amount of Fe suppliedand the ratio Mn: Fe. A high iIn: Fe ratio in the solution gave very light-colored roots. The toxic effect of Cu on the roots is much like that describedfor Zn (3).

Vermiculite has a relatively high exchange capacity (1). It was foundnear the end of these experiments that nearly all of these heavy metals wereadsorbed by the vermiculite even though the pH was maintained near 4.This apparently reduced greatly the effectiveness of these elements as muchlower concentrations in the solution cultures showed marked effects ongrowth. It seems probable that the results obtained in vermiculite wouldbe somewhat comparable to those that would be found under acid soil con-ditions. Chlorotic seedlings with stunted roots are of rather common occur-rence under citrus trees in Florida orchards when Cu is high and pH low.

The uniformity of response in the nutrient solution experiment was good.The plants in favorable solutions showed growth that was equal or superiorto that in good soil or in vermiculite with applied nutrient solutions. Rootdevelopment was surprisingly good (fig. 2) despite the lack of mechanicalaeration. Under these conditions similar stunting and chlorosis were causedby all three heavy metals (fig. 1, H to K). This picture and those in fig-ure 2 were taken 70 days after treatments were begun.

The observations reported in table I were made on the sixty-first day.The suppression of growth as Cu was increased is evident. The greatestintensity of chlorosis occurred at the intermediate Cu levels rather than atthe highest because some growth must occur before chlorosis can appear.One p.p.m. Cu was very toxic and lethal to some roots, with the consequencethat new leaf development was essentially curtailed. When the chelated Fewas increased from 0.5 to 5 p.p.m. the Fe chlorosis pattern in the new leavesnever developed but after several weeks they showed the distinctive Mn de-ficiency pattern (2). The increased Fe was slightly toxic at the lowestlevels of the other metals, showing some leaf scorch and some reduction innew leaf production (table I). The scorch was identical with that producedby ferrous sulphate (2). No beneficial effect of the increased Fe on rootgrowth was noted at any Cu level used. The absence of Fe chlorosis in theleaves made the high-Fe plants appear better for the first few weeks, buttheir general appearance after two months showed only doubtful improve-ment. About the only difference was the replacement of Fe chlorosis with

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AIn chlorosis in the leaves as a symptomatic expression of an extremelyunhealthy root condition.

The word "stunted" as used in table I indicates severe suppression of rootgrowth and many pegged laterals. None of the roots were dead. The term

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FIG. 2. Valencia seedlings grown in solution cultures with two levels each of Cu(0.1 and 0.5 p.p.m.), Zn (3 and 15 p.p.m.), Mn (5 and 50 p.p.m.), and Fe (0.5 and 5p.p.m.). The plants with 0.5 Cu and 5 Fe do not show chlorosis but the roots areseverely stunted (compare with check in center row).

very stunted is used to indicate extreme metal toxicity and presence of someswollen (bulbous) root tips. The root systems are capable of new growthwhen placed in a favorable environment. The roots indicated as dead wereflaccid, discolored, and in various stages of decomposition. In appearancethe plants at 3 p.p.m. Zn were about like those at 0.25 p.p.m. Cu. Actually

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the root condition was solmiewlhat better. The 15 p.p.m. Zn plants were like-wise similar to the 1 p.p.m. Cu plants. Thus, under these conditions Cuseems to be about 12 to 15 times as toxic as Zn. Botlh elements block theupward translocation of Fe and thus induce Fe chlorosis in tlle leav-es. Theincreased level of clhelated Fe elimiiinated this symptom witlh Zn also and this

TABLE IGROWTH RESPONSES OF VALENCIA ORANGE SEEDLINGS AFTER TWO MONTHS INCOMPLETF NUTRIENT SOLUTION WITH HEAVY METALS VARIED AS INDICATED.

New Chlorosis General ap- ConditionCu Zn Nmn Fe leaves* type of tops of roots-~~~~~~~ftp

p.p.m. p.p.m. p.p.m. p.p.m.

0.004 O.01 0.1 0.50.1 0.01 0.1 0.50.25 0.01 0.1 0.50.5 0.01 0.1 0.5

0.75 0.01 0.1 0.5

1 0.01 0.1 0.5

0.004 0.01 0.1 5

0.1 0.01 0.1 5

3219 Fe13 Fe8 2 Mn

3 Fe**6 Fe

Very goodFairPoorPoor

Poor

4 Fe Poor

25 Mn Good; slightFe toxicity

22 Mn Fair0.5 0.01 0.1 5 12 Mn Poor

1 0.01 0.1 5 9 Mn Poor

0.004 3 0.1 0.5 12 Fe Poor0.004 15 0.1 0.5 4 Fe Very poor

0.004 30 0.1 0.5 0 .... Five dead0.004 3 0.1 5 19 Mln Fair0.004 15 0.1 5 2 MIn Very poor

0.004 30 0.1 5 0 .... All dead0.004 0.01 5 0.5 16 Fe Fair0.004 0.01 25 0.5 6 Fe Poor0.004 0.01 50 0.5 3 Fe Poor; Mn

toxicity0.004 0.01 5 5 37 .... Very good0.004 0.01 25 5 26 .... Good0.004 0.01 50 5 22 .... Poor; Mn

toxicity0.004 0.01 0.1 25 0 .... Dead

Very goodStuntedVery stuntedVery stunted;

partly deadVery stunted;


mostly deadGood

StuntedVery stunted;


mostly deadSlightly stuntedVery stunted;

partly deadAll deadSlightly stuntedVery stunted;

partly deadAll deadSlightly stuntedStuntedStunted;

partly deadVery goodSlightly stuntedStunted

Died after tops

*Total on six plants.**Of six plants, 2 showed Min chlorosis, 3 Fe chlorosis.

again resulted in MAn-deficient leaves. The increased iron level in this casesubstantially improved the top growth but did not entirely overcome thetoxic effect of Zn on the roots (fig. 2).

AManganese was much less toxic than Cu or Zn but readily caused Fechlorosis. It would appear from these comparisons that Cu is about 50times as toxic as Mln. At the low Fe rate, 5 p.p.m. Mn caused very severe

376

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SMIITH AN-D SPECHT: IRON CHLOROSIS OF CITRUS

chlorosis and stunting of the top and the roots showed ev-idence of toxicity.Substantial imiiprovement in both root and top condition Nas achieved bvincreasing the clhelated Fe to 5 p.p.m. (fig. 2). Plants with 5 p.p.m. Mn and5 p.p.m. Fe were aml-ong the best in the experiment. At 50 p.p.m. MIn causedsevere chlorosis and stunting of the tops and was toxic to both leaves androots. AVith Fe increased to 5 p.p.m. chlorosis of the leaves was preventedbut yellow orange spots and curling caused by 'Mn toxicity (4) developed.The roots in these cultures also showed typical heavy metal toxicity. Even25 p.p.m. Fe appeared to be non-toxic to roots. Excessive absorption ofchelated Fe, however, killed the leaves in a few days. The initial symptomof Fe toxicity was a speckled scorch, followed by death of the leaf. Theroots were still alive and light-colored several days after the leaves andstems were dry and brown. Cu, Zn, and MAIn at the supplementary levelsslhown in table I, each prev-ented the leaf scorch that 5 p.p.m. Fe caused at thelow levels of these elemiients. A few supplementary jars were given 25 p.p.m.chelated Fe along with the h-iigh rates of Cu, Zn, and 'Mn; i.e., 1, 30, and 50p.p.m. respectively. No Fe leaf scorch appeared in any of these combina-tions but the roots still showed evidence of heavy metal toxicity. The topswere not chlorotic in any of the treatments but they were severely stunted.

The relative effects of Cu, Zn, and 'Mn on citrus seedlings are of the sameorder as described by HEWITT (5) for sugar beets. His observations wereapparently limited to the upper parts of the plant since root behavior wasnot mentioned. He states that it is possible to induce chlorosis with theseand other metals at levels that are non-toxic to plants. All the presentobservations on Cu and Zn on citrus have shown that toxicity to roots eitheraccomiipanied or preceded leaf chlorosis. The effect of MIn was not clearlyestablished. It may be possible to find conditions under wlhich IMn wouldinduce Fe chlorosis without being toxic to roots. It appeared in the presenttests, howzever, that MIn reduced root growth before chlorosis developed inthe leaves. The responses of different plant species may vary somewhat,of course.

The development of Mn-deficiency symptoms after correction of Fechlorosis in plants grown with excess Cu and Zn agrees with the publisheddata (10) in which it was shown that both Fe and MF\n were at very lowlevels in the leaves of chlorotic orchard trees. It would seem logical thatthis same response might occur in orchard trees that were treated withchelated iron. The failure of iron to reduce the toxicity of Cu to roots inthese tests would seem to suggest that the application of chelated Fe tochlorotic citrus trees in the orchard may not promote the re-establishmentof roots in the soil zone containing relatively large amounts of accumulatedCu. The Fe may, however, be beneficial for other reasons. Thus, for exam-ple, a tree with restored photosynthetic capacity may develop additionalroots in a different soil lhorizon or it may recover much more rapidly whenFe is used in conjunction with lime. The fact that citrus is much moresensitive to Cu than to Zn and -Mn, along with the evidence (7, 8) that moreCu than either Zn or MIn has accunmulated in the soil of afflicted orchards,

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PLANT PHYSIOLOGY

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suggests strongly that excessive use of Cu is the chief factor involved inproducing the chlorotic condition in Florida citrus trees on acid soil.

EFFECTS OF N SOURCE AND RATE OF APPLICATION OF HEAVY METALS ON THE

DISTRIBUTION OF THE LATTER IN CITRUS SEEDLINGS

Although N source had no effect on growth, it did have some effect onthe behavior of each of the metallic ions (table II). Copper mobility wasslightly increased by NH4 so that more of it got to the tops of the plants.The reverse was true of MIn, while Zn and Fe were not affected. More Feaccumulated in the roots in the presence of NH4 than in its absence. Incontrast, less Zn and MIn accumulated in the roots in the NH4 series.

One striking response was the absence of any apparent relation of leafCu concentration to the supplied amount or that accumulated in the roots.Thus, while the difference in rate of supplied Cu was 500-fold, the corre-sponding difference in the root concentration was only about four-fold andthere was no difference in the leaf concentration.

Zinc was found to increase in all plant parts when the supply wasincreased. The root concentrations found at the two Zn rates differed evenless than with Cu. The leaves, however, show a substantial increase.

The MIn concentrations in all tissues at the two rates of supply differedmuch more than was the case with either Cu or Zn. The ratio of the leafconcentrations is even larger than that of the roots. It is perhaps note-worthy that with this element the supply differential is only a factor of 50instead of several hundred as with the other two metals.

The information just given in regard to heavy-metal distribution wouldsuggest that Cu can induce Fe chlorosis without itself being transported tothe foliage in increased amounts. If either Zn or Mln were the offendingmetal, however, one would expect to find an abnormally high foliage concen-tration of these elements. It has been reported (10) that chlorotic orangeleaves from Florida orchards on acidic soil have unusually low Fe and Mlncontents while Cu is present in normal amounts. Unpublished data havesince shown that Zn concentration in such leaves is in the low-to-normalrange. Thus, evidence from the pattern of metal distribution within theplant corroborates other evidence such as soil analysis and toxicity to rootsin showing that excessive use of Cu is the chief factor that has brought onthe chlorotic condition in many Florida citrus orchards.

Table II also shows how one metal may influence another. High Cucaused a reduction of Zn in all plant parts. It repressed 'Mn accumlulationin roots and Fe in leaves. The Fe value of 74 is a mean from samples whichcontained mixtures of chlorotic and green leaves. Chlorotic leaves analyzedby themselves contained 26 p.p.m. Fe. Similar low Fe values were found inchlorotic leaves from plants grown in solution cultures with excess Zn orexcess AIn. None of the metals influenced root accumulation of Fe however.Earlier Nork (3) showed this same relation between the Fe content of theroots and the leaves of lemion seedlings grown in solution cultures withexcess Zn.

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PLANT PHYSIOLOGY

TABLE IIIEFFECT OF NITROGEN SOURCE AND COPPER AND ZINC LEVELS ON THE

CONCENTRATION OF MANGANESE IN VALENCIA ORANGE ROOTS.

Concentration of MnSupplied metals

Nitrogen all N03 Nitrogen half NH4p.p.m. p.p.m. p.p.m.

0.5 Mn:0.02 Cu plus 0.05 Zn 2,500 9600.02 Cu plus 15 Zn 1,000 97010 Cu plus 0.05 Zn 1,200 37010 Cu plus 15 Zn 1,200 480

25 Mn:0.02 Cu plus 0.05 Zn 16,000 13,0000.02 Cu plus 15 Zn 12,000 . 4,70010 Cu plus 0.05 Zn 8,600 4,00010 Cu plus 15 Zn 6,600 3,200

Zinc appeared intermediate between Cu and Mn in its effect on tissuecomposition. At the low rate of Cu supply, high Zn depressed the Cu con-centration in the roots. At the high-Cu level, Zn had no effect on Cuaccumulation. This interaction resulted in a non-significant overall effectof Zn on Cu uptake (table II). High Zn depressed Mn concentration in alltissues but had no effect on Fe distribution in this test where it had no effecton growth.

Manganese appeared to have little effect on the other metals. Theeffects of N source and Cu and Zn levels on accumulation of Mn by rootsis shown in detail in table III. The effect of N source on Mn accumulationis clearly shown, but the cause is not understood. The suppression of Mnby Zn is obvious at the high-Mn rate but it was not consistent at the low-Mnrate at which it appears that the Cu level may have had some influence.Copper interfered with Mn accumulation regardless of Mn or Zn level. Theeffects of Cu and Zn were supplementary in the high-Mn treatments.

TABLE IVCONCENTRATION OF OTHER HEAVY METALS IN VALENCIA ORANGE ROOTS

LISTED IN THE ORDER OF INCREASING COPPER CONCENTRATION.Rate of applied metal Concentration in dry matter

Cu Zn Mn Cu Zn Mn Fe Al

p.p.m. p.p.m. ppm. p.p.m. p.p.m.Low High Low 100 9,100 1,000 1,145 1,025Low High Low 140 6,500 970 950 1,100Low High High 170 3,950 8,350 1,250 1,220Low Low Low 195 3,200 1,730 785 830Low Low High 240 2,580 14,500 1,120 1,175High Low Low 510 2,100 785 1,150 930High. Low High 550 1,920 6,300 810 1,025High High Low 665 3,230 840 1,100 1,200High Low Low 740 2,100 750 910 760

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SMITH AN'D SPECHT: IRON CHLOROSIS OF CITRUS

That Cu and Zn may influence the adsorption of each other is seen intable IV. Root analyses of plants from individual treatments were selectedfor the ascending Cu-values shown. No distinction was made between theN sources. Some evidence of antagonism between different pairs of elementsmay be seen, the most obvious being the reciprocal relation between the con-centrations of absorbed Cu and Zn. The relation is not stoichiometric, nordoes it appear to be controlled directly by metabolic processes (9). It maybe possible to elucidate these adsorption responses by research on non-livingexchange systems.

The data in table IV suggest that one element can simultaneously sup-press the adsorption of two other elements. This adsorption can be seen bycomparing the Cu, Zn, and IMn values in lines 1, 4, and 6, which representtreatments at the low-_Mn level. When the rate of Zn was increased, bothCu and Mn were depressed. Likewise, when Cu was increased, both Zn andMn were depressed. Similar effects at the high-iMn level may be seen inlines 3, 5, and 7. Aluminum was not supplied in these solutions. The ver-miculite apparently supplied that which was adsorbed. The parallel be-havior of Al and Fe is perhaps noteworthy since it agrees with the closeparallel found in leaf accumulation of these two elements by orchard trees(11).

It seems apparent that the heavy metals influence the action of oneanother whichl points to the desirability of considering them as a group instudying heavy-metal nutrition. Single-element studies could lead to errone-ous conclusions regarding ion belhavior.

Summary

Studies were made on the responses to differential supplies of copper,zinc, manganese, and iron shown by sweet orange and mandarin seedlingsgrown in vermiculite and solution cultures with the pH maintained near 4.

The first three metallic ions were all found to interfere with Fe metabo-lism in such a way as to induce chlorosis under certain conditions. Rela-tively low concentrations of these elements also were toxic to roots. Thesetwo effects appeared to be related in that it was not possible to inducechlorosis without simultaneously inducing toxicity to roots. They were pro-duced in solution cultures Nith 0.1 p.p.m. Cu, 3 p.p.m. Zn or 5 p.p.m. M\n,the lowest supplementary levels used. Copper appeared to be about 50times as toxic as -\'n and 12 to 15 times as toxic as Zn.

These metals appeared to cause Fe chlorosis by interfering with theupward movement of Fe since they did not reduce the accumulation of Fein roots. Strongly chelated Fe overcame this interference in some way andprevented the developmnent of chlorosis. It did not, however, prevent Cutoxicity in roots. It appeared to reduce slightly Zn toxicity in roots and toalleviate greatly toxicity due to -Mn.

Chelated Fe from ferric potassium ethylenediamiiine tetra-acetate wastoxic to leaves when used at 5 p.p.m. Fe in solution culture if all the othermetallic ions were relatively low in concentration. The foliage scorch pro-

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PLANT PHYSIOLOGY

duced was identical with that produced by FeSO4. This toxicity was pre-vented by increasing the amounts of any of the other three metals.

The analytical data presented show that Cu accumulates in roots, evento the point of toxicity, without being transported to the upper parts of theplants in increased amounts. Both Zn and Mn show marked accumulationin the roots as well as substantial distribution within the plant when theseelements are increased in the nutrient solution. The data also show severalcases of interference in accumulation of one element by another. The formof nitrogen also appreciably affected the behavior of several of these metals.

LITERATURE CITED1. BARSHAD, I. Vermiculite and its relation to biotite as revealed by base

exchange reactions, x-ray analyses, differential thermal curves, andwater content. Amer. Mineral. 33: 655-678. 1948.

2. CHAPMAN, H. D. and KELLY, W. P. The mineral nutrition of citrus.In: The Citrus Industry, Vol. I, University of California Press.1943.

3. CHAPMAN, H. D., LIEBIG, G. F., JR., and VANSELOW, A. P. Some nutri-tional relationships as revealed by a study of mineral deficiencyand excess symptoms on citrus. Proc. Soil Sci. Soc. Amer. 4: 196-200. 1939.

4. HAAS, A. R. C. Some nutritional aspects in mottle-leaf and otherphysiological diseases of citrus. Hilgardia 6: 483-599. 1932.

5. HEWITT, E. J. Relation of manganese and some other metals to theiron status of plants. Nature 161: 489-490. 1948.

6. JACOBSON, L. The maintenance of iron supply in nutrient solutions bya single addition of ferric potassium ethylenediamine tetra-acetate.Plant Physiol. 26: 411-413. 1951.

7. REUTHER, W. and SMITH, P. F. Effects of high copper content of sandysoil on growth of citrus seedlings. Soil Sci. 75: 219-224. 1953.

8. REUTHER, W., SMITH, P. F., and SPECHT, A. W. Accumulation of themajor bases and heavy metals in Florida citrus soils in relation tophosphate fertilization. Soil Sci. 73: 375-381. 1952.

9. SMITH, P. F. Heavy-metal accumulation by citrus roots. Bot. Gaz.(In press). M k

10. SMITH, P. F., REUTHER, W., and SPECHT, A. W. Mineral compositionof chlorotic orange leaves and some observations on the relation ofsample preparation technique to the interpretation of results. PlantPhysiol. 25: 496-506. 1950.

11. SMITH, P. F., REUTHER, W., and SPECHT, A. W. Seasonal changes inValencia orange trees. II. Changes in micro-elements, sodium, andcarbohydrates in leaves. Proc. Amer. Soc. Hort. Sci. 59: 31-35.1952.

12. STEWART, I. and LEONARD, C. D. Chelates as sources of iron for plantsgrowing in the field. Science 116: 564-566. 1952.

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