Inorganic Plant Poisons and Stimulants

by Winifred Elsie Brenchley

(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.