II. Effect of Zinc on the Growth of Higher Plants.

1. Toxic effect.

(a) Toxic action of zinc salts alone in water cultures.

In comparison with copper little work has been done with regard to the action of soluble zinc salts alone on higher plants when grown in water cultures. [Freytag (1868)] stated that zinc salts must be very dilute if the plants are not to be harmed, and that for zinc sulphate the concentrations must not be more than 200 mg. per litre (= 1/5000). [Baumann (1885)] carried out further experiments and concluded that zinc salts are far more toxic than Freytag suspected, 44 mg. zinc sulphate per litre[5] killing plants of 13 species belonging to 7 families (Coniferae excepted). The various plants withstand the action of the zinc salts in different degrees, the same concentration killing off the species in different times. With the 44 mg. zinc sulphate the following results were obtained:—

Trifolium pratensekilled in 16 days
Spergula arvensis „ 21 „
Hordeum vulgare„30„
Vicia sativa„31„
Polygonum Fagopyrum„60„
Beta vulgaris„76„
Onobrychis sativa„194„

With still less poison, 22 mg. zinc sulphate per litre, all the species mentioned were eventually killed with the exception of Onobrychis sativa, while 4·4 mg. zinc sulphate seemed to be harmless for all the plants tested except Raphanus sativus, which is evidently exceptionally sensitive to this toxic substance.

[Jensen (1907)] again indicated the poisonous action of zinc salts and also found that a relatively small reduction of toxicity was obtained by the addition of finely divided quartz to the solutions.

(b) Effect of soluble zinc salts in the presence of nutrients.

[Krauch (1882)] grew various plants in the presence of nutrient solutions and quantities of zinc sulphate varying from ·1 to ·8 gm. per litre (= 1/10,000 to 8/10,000). Barley proved to be very sensitive, even to the weakest strength of the poison, as the plants soon showed reddish flecks, while all were dead within six weeks, the control plants without zinc remaining quite healthy. Certain grasses took longer to kill than barley, those with ·4 gm. zinc sulphate per litre dying in about seven weeks, while 13 weeks elapsed before the others were killed. Even after this length of time the plants with ·1 gm. zinc sulphate per litre still survived, although in a very sickly condition. With willow, again, even ·1 gm. zinc sulphate per litre made the plants very sickly after four weeks, growth being weak, the leaves yellow, and the roots brownish. In this case the solutions were renewed, but the plants treated with zinc compounds were dead within eight weeks from the start, the controls being very healthy.

The next year (1883) [Storp] repeated these experiments made by Krauch and corroborated his results fully. Barley and grasses (timothy and others) grown in solutions of zinc sulphate, both with and without nutrients, soon lost their green colour and became covered with rusty brown flecks, the barley dying within 14 days, and the grasses soon after. With willow, too, the toxic action was again manifested.

[True and Gies (1903)] showed that the addition of calcium salts in appropriate concentrations reduced the toxicity of zinc salts considerably, a result similar to that which they obtained for copper.

Recent experiments at [Rothamsted] have shown that zinc sulphate is very toxic to barley, though the plant is able to make some slight amount of growth even in the presence of a solution of the anhydrous salt ZnSO4 as strong as 1/5000, rapid improvement occurring as the concentration decreases to 1/2,500,000 or less ([Fig. 6]). On the whole the higher strengths of zinc sulphate are less poisonous to peas than they are to barley. At a concentration of 1 in 1⁄4 or 1 in 1⁄2 million in different experiments the growth was nearly as good as with the control plants, though it consistently lagged a little way behind until a dilution of 1/10,000,000 was reached (Figs. [7] and [8]). Incidentally it is very striking to see the desperate efforts that badly poisoned pea plants make to reproduce themselves. Growth of the roots is nearly always checked in advance of that of the shoots, probably on account of the contact of the roots with the poison. In the greater strengths of such poisons as zinc and copper sulphate root growth is checked from the outset, but usually a very little shoot growth is made, and one frequently obtains ridiculous little plants about an inch high bearing unhappy and diminutive flowers, which are occasionally replaced by equally unhappy and miniature fruits. The same thing has also been noticed when unsuccessful attempts have been made to introduce spinach as a test plant for water cultures.

Fig. 6. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of anhydrous zinc sulphate and nutrient salts. (March 2nd–May 8th, 1911.)

Fig. 7. Photograph showing the action of anhydrous zinc sulphate on pea plants in the presence of nutrient salts. (Sept. 30th–Dec. 20th, 1912.)

1. Control.
2.1/5,000zincsulphate.
3.1/10,000„„
4.1/50,000„„
5.1/100,000„„
6.1/250,000„„
7.1/500,000„„
8.1/1,000,000„„
9.1/2,500,000„„

Fig. 8. Curve showing the mean values of the dry weights of nine series of pea plants grown in the presence of anhydrous zinc sulphate and nutrient salts. (May 18th–June 28th, 1910.)

(c) Effect of zinc compounds on plant growth when they are present in soils.

As soon as the presence of zinc in members of the vegetable kingdom was established the question arose as to its effect upon both the plant and the soil.

[Gorup-Besanez (1863)] grew plants in soil with which 30 grams of metallic poisons such as CuSO4, ZnSO4, HgO, were intimately mixed with 30·7 litres (“cubik Decimeter”) of soil[6]. On analysing the ash of Secale cereale, Polygonum Fagopyrum, and Pisum sativum after six months growth he failed to detect the presence of zinc in any one of the three. As the results varied with different poisons on different plants he concluded that the absorption capacity of the various kinds of soils for different poisons varies, that basic salts are absorbed, while the acid salts may pass completely through the soil in the drainage water.

[Freytag (1868)] stated that zinc is retained by the soil in the form of oxide, which is derived from dilute zinc compounds as they filter through the soil, by decomposition by the salts of the soil. For field earth the limit of absorption of zinc oxide from zinc sulphate is between ·21%–·24% of the earth.

[F. C. Phillips (1882)] corroborated Freytag’s statement as to the absorption of small quantities of zinc by the roots of plants, but he states as a fact that both lead and zinc may enter plant tissues without causing any disturbance in the growth, nutrition or functions of the plants, a conclusion that is obviously incorrect or at least incomplete in view of later work on the subject. His choice of plants was certainly unusual, including geraniums, coleas, ageratums and pansies, the poison used being zinc carbonate.

[Holdefleiss (1883)] stated that in spite of a soil content of 2% zinc the vegetation was not in any way harmed, clover fields and meadow lands on zinc soil presenting a normal appearance. This observation was quite inconclusive, as the author proceeds to say that of the plants that were able to absorb zinc salts without disadvantage the most luxuriant were the so-called zinc plants—the exceptions that prove the rule. Two years later [Baumann] showed that such insoluble zinc salts as the carbonate and sulphide in the soil cannot hurt plants. These salts are certainly dissolved to some extent by water containing CO2 but solution is hindered by the constitution of the soil. He also found that the various kinds of soil act differently upon zinc solutions, the absorptive power of pure humus soils (“reinem Humusboden”) for zinc solutions being the strongest. Clay and chalk soils also decompose such solutions energetically, while poor sandy soils have only a weak power of absorption. This selectivity of absorption may account for the difference in the toxicity of zinc salts to plants in the various soils.

[Storp (1883)] experimented to determine the changes in the various characters of the soil by the action of zinc salts on it, and he makes the remarkable statement that in some soils the presence of zinc generates free sulphuric acid, which is particularly injurious to plant life. Grasses, young oaks and figs showed a decrease in dry weight, nitrogen and fat, as the quantity of zinc compounds increased in the water added to the soil. Both the quality and the quantity of the crop were adversely affected. This decrease in the dry weight due to the presence of zinc was confirmed by [Jensch] later on, and also by [Nobbe, Baessler and Will (1884)], who state that both lead and zinc compounds work disadvantageously to vegetation even when they are present in such small quantities that the plants are outwardly sound, the harmful action appearing in the decrease of dry weight. Contrary to Baumann’s opinion, zinc carbonate is said to be one of the salts that exercises this insidious poisonous action. [Storp (1883)] noticed that the direct poisonous action of zinc compounds is largely destroyed by their admixture with soil, but he suggests that a secondary cause of harm is introduced by the accumulation of insoluble zinc salts, so that the fertility of the soil is impaired to the detriment of the vegetation.

[Ehrenberg (1908)] throws out a suggestion that zinc is specially harmful to plant life when it occurs in conjunction with ammonia, but no further evidence has come to light.

(d) Mode of action of zinc on plants.

The reason for the toxicity of zinc salts when present in soil forced itself upon the attention of some of the early investigators in this field. [Freytag (1868)] put forward the hypothesis that the zinc oxide is partly or exclusively absorbed by the roots on account of the cell walls of the root being corroded by the very thin layer of zinc salts lying in contact with it—the same theory as has been held with regard to copper. He stated also that the quantity of zinc oxide taken up by the plant through its roots is strictly limited, not being proportional to the quantity occurring in the soil, but varying between narrow limits. [Krauch (1882)] found himself unable to accept another hypothesis which at one time found favour, i.e. that the zinc salts kill the plants by coagulating the protoplasm. If this were so, he argued, no plants at all could grow upon soils containing zinc, and he was content to leave the cause as one yet to be explained. Even at the present time, thirty years after, we know very little more about the physiological cause of the toxicity of zinc.

2. Effect of zinc compounds on germination.

In the course of his investigations on the influence of zinc on vegetation [Freytag] just touched upon the question of seed germination. According to his statement the presence of zinc oxide in the soil does not exercise much influence upon germination and the growth processes of plants. Little zinc is stored up in seeds and on this account seeds originating from plants containing zinc germinate quite normally and do not seem to be affected by the peculiar nutritive conditions of the parent plants.

In certain cases light seems to have something to do with the harm zinc compounds work on plants. [Storp] found that when clover seeds were germinated in the dark on filter paper moistened with water containing ·025 gm. ZnO per litre (added in the form of zinc sulphate) no deleterious action was observed. Barley seeds were soaked for four days in (a) distilled water, (b) water with ·9 gm. ZnO per litre, which was frequently changed. These seeds were then placed in the dark on filter papers soaked respectively with water and with the solution containing ZnO. So long as no light was admitted, for a period of eleven days, germination was uniform in both sets, but directly the covers were removed the growth of the seeds with zinc ceased almost entirely, and they did not assume the green colour taken on by the unpoisoned seedlings. With maize the germination was retarded by zinc even in the dark, but the harmful action of light on the plants with zinc was again established. These results seem to indicate that the formation and activity of chlorophyll is impaired by the toxic agent, and this hypothesis is borne out by the fact that in many fungi and non-assimilating higher plants the toxic action of zinc is not evident.

[Micheels (1906)] approached the matter from a totally different standpoint, seeking to discover what influence the valency of a metal has upon the toxicity of its salts. In each of a series of experiments 1000 c.c. of 5⁄8 decinormal solution of sodium chloride in pure distilled water were used, with the addition of varying strengths of calcium sulphate. Grains of wheat, which previously had been soaked in distilled water, were placed in the solutions, and it was found that the stronger the calcium sulphate solution (up to 1⁄64 normal—the limit of experiment), the better the growth. The calcium sulphate was then replaced by salts of other bivalent metals, as zinc, lead and barium, with analogous results, the quantity necessary to obtain the maximum development varying with one and another; with zinc, n/128 gave the maximum. In this case the toxic action of both sodium chloride and zinc sulphate on germination were considerably reduced by their mutual presence—a result which fits in perfectly with what is known as to the masking effect of soluble substances upon toxic action. The same fact obtains in the animal kingdom, where [Loeb] and others have found that the toxicity of solutions of sodium chloride for marine animals is reduced by the introduction of salts of the bivalent metals.

3. Stimulation induced by zinc compounds.

While the toxic action of zinc on the higher plants is so obvious that it forced itself upon the attention of investigators at an early date, the question of possible stimulus is so much more subtle that it has only come into prominence during the last twelve years, during which time an extraordinary amount of experimental work has been done with regard to it. One investigator, [Gustavson], was somewhat in advance of his time, for as long ago as 1881 he hinted at the possibility that zinc, aluminium and other substances might act as stimulants or rather as accelerators. He indicated that the rôle of certain mineral salts in the plant economy is to enter into combination with the existing organic compounds, the resulting product of the reaction aiding in the formation of yet other purely organic compounds which ordinarily require for their formation either a very high temperature or a long time—in other words, such a mineral salt acts as a kind of accelerator.

This work was apparently not followed up immediately, but it evidently contains the germ of the “catalytic” hypothesis of which so much has been made during recent years.

The work dealing with zinc as a stimulant to plant growth has yielded such various and apparently contradictory results that the question cannot yet be regarded as settled—it is even still more or less uncertain whether zinc compounds act as stimulants, or whether they are merely indifferent at concentrations below the toxic doses.

(a) Stimulation in water cultures.

[True and Gies (1903)] suspended seedlings of Lupinus albus for 24–48 hours with their roots in solutions of zinc sulphate and calcium sulphate (m/256)[7], and found that while zinc sulphate alone at m/8192 retarded growth, yet with m/2048 ZnSO4 and m/256 calcium sulphate growth was more than twice as rapid as in controls grown in water, indicating a marked stimulation. The presence of the calcium exercised a definite ameliorating influence, reducing the toxicity of zinc to one-sixteenth at most. The hypothesis put forward is that interior physiological modifications are responsible for the observed differences in growth rate, the cell processes being so affected as to bring about different results on cellular growth—i.e. that where mixtures of salts are concerned growth rate represents the physiological sum of oppositely acting stimuli or of antagonistic protoplasmic changes.

[Kanda (1904)] found that peas were stimulated in dilute solutions of zinc sulphate in the absence of nutrients, the optimum concentration being between ·00000287% and ·000001435% (about 1 in 34,840,000 and 1 in 69,700,000), higher concentrations being poisonous when the solutions were changed every four days. [Jensen (1907)] stated that he obtained no stimulation at all with water cultures, even in a solution as dilute as n/100,000 (about 1 in 1,239,000), but he suggested that it was quite possible that in proper concentration the zinc sulphate might prove to be a stimulant.

[Javillier (1910)] grew wheat in nutritive solutions with quantities of zinc salts containing from 1/5,000,000–1/250,000 zinc, and found that the dry weight of the plant was increased in so far as the stems and leaves were concerned, though it remained uncertain whether a similar increase occurred in the grain.

A consideration of the [Rothamsted experiments] shows that up to the present time there is no conclusive evidence that zinc sulphate acts as a stimulant to barley grown in water cultures. As a general rule the growth of those plants with 1/5,000,000 ZnSO4 approximates closely to that of the controls. Beyond this the growth varies in different experiments. In some cases lower concentrations from 1/5,000,000 to 1/50,000,000 seem to cause some slight improvement in comparison with the normal, indicating a possible stimulus, but this improvement is not at all well marked. In other cases these great dilutions are apparently indifferent, neither a poisonous nor a stimulative action being exerted on the growth of the plant ([Fig. 6]). With peas some increase has been obtained with 1/20,000,000, and although the rise is only slight, yet it is possible that it may indicate the setting in of a stimulus which would make itself more strongly felt with still weaker concentrations ([Fig. 7]).

(b) Stimulation in sand cultures.

While [Jensen] denied stimulation in wheat grown in water cultures even when the solutions were as dilute as n/100,000 zinc sulphate, yet he found increase of growth with the same plant in artificial soil (quartz flour) to which much stronger solutions of zinc sulphate, from 5n/10,000–n/10,000, had been added.

(c) Increased growth in soil.

[Nakamura (1904)] dealt with a few plants of agricultural importance, adding ·01 gram anhydrous zinc sulphate to 2300 grams air-dried soil. The marked individuality in the response of the various plants to the poison is very striking. Allium showed signs of increased growth throughout; Pisum was apparently improved in the early stages of growth, but when the dry weights were taken at the end of the experiment no increase manifested itself in the weights of the plants treated with zinc; with Hordeum the same quantity of zinc exercised a consistently injurious action. These results with peas and barley corroborate those obtained in the Rothamsted experiments with water cultures in that zinc sulphate proved to be less toxic to peas than to barley.

[Kanda] found that both peas and beans when grown in soil as pot cultures were improved by larger quantities of zinc sulphate than when they were treated as water cultures—a result in full accordance with current knowledge.

Wheat is evidently peculiarly sensitive to the effects of zinc compounds under differing conditions. [Javillier (1908 c)] pointed out that while wheat is very susceptible to the toxic action of zinc, yet it can benefit by the presence of sufficiently small quantities of the compounds of the metal. Rice is another cereal that is said to respond to the action of zinc sulphate, as [Roxas], working in pot cultures with soil both with and without the addition of nutritive salts, obtained an acceleration of growth on the addition of m/1000 zinc sulphate, a quantity so remarkably great that it might be expected to act as a toxic rather than as a stimulant.

With phanerogams the zinc question is not only concerned with the effect of the metal upon germination, but also with its effect upon the later growth of the green plants, and on the physiological functions involving the construction of substances at the expense of mineral elements and the carbon dioxide of the air. Javillier holds that the indications are that zinc would prove to be profitable if applied to crops as a “complementary” manure.

4. Direct action of zinc salts on leaves.

[Dandeno (1900)] applied zinc sulphate in drops to the leaves of Ampelopsis, and found that the solution was not all absorbed by the leaf, but that a slight dark ring of a yellow colour was produced, and he was induced to think that some local stimulation was produced if the salt was presented in sufficient dilution.

[Klopsch (1908)] discussed the effect on plant growth of zinc derived from industries producing zinc fumes. Zinc oxide from the fumes is deposited on the leaves, and Klopsch stated that the rain and dew containing dissolved zinc compounds find entrance to the tissues by way of the stomates and work injury to the plants. Against this, however, it must be remembered that these same fumes also contain other substances which are admittedly harmful to plant life, and so the deleterious effect may be partly or even chiefly due to these substances rather than to the zinc. Yet it is probable that at least some of the depreciation is due to the zinc. [Treboux (1903)] tested the effect of zinc sulphate on shoots of Elodea canadensis. If the shoots were placed in n/100,000 (= ·000016%) zinc sulphate no reduction of assimilation (as observed by counting the number of oxygen bubbles emitted per minute) took place, and replacement in water apparently had no effect either way. When however the shoots were placed in (1) water, (2) ·00008% zinc sulphate, (3) fresh ·00008% zinc sulphate, (4) water again, it was found that while the first solution of zinc sulphate had apparently no effect on assimilation, yet during the second immersion a gradual reduction in assimilation set in, which reduction was continued after the return to pure water, so that the toxic action of the zinc sulphate upon the shoots was clearly demonstrated.