The method of water cultures has been largely applied to determine the relation of copper compounds to plants. Twenty years ago (1893) [Otto] discovered the extreme sensitiveness of plants to this poison when grown under such conditions, as he found that growth was very soon checked in ordinary distilled water which on analysis proved to contain minute traces of copper. Controls grown in tap water gave far better plants, but this superiority was attributed partly to the minute traces of mineral salts in the tap water, and not only to the absence of the copper which occurred in the distilled water.
Tests made at Rothamsted have carried this point still further. Pisum sativum, Phaseolus vulgaris, Triticum vulgare, Zea japonica, Tropeolum Lobbianum, sweet pea (American Queen), nasturtium, and cow pea—the first three of these being the species used by Otto—were grown in (1) ordinary distilled water, which was found to contain traces of copper, (2) glass distilled water, for about a month, till no more growth was possible owing to the lack of nutriment. In every single case the root growth was checked in some degree in the ordinary distilled water, the roots seeming to the eye to be less healthy and less well developed. In Pisum, Tropeolum and Zea, the shoot growth of the coppered plants appeared stronger than that of the controls, and this was borne out when the dry weights of the plants were obtained. In every other case the coppered plants were inferior, root and shoot, to those grown in the pure water. With the first three plants it appears that while the toxic water has a bad effect on the roots, yet the growth of the shoots is increased. The idea suggests itself that this apparent stimulation is in reality the result of a desperate struggle against adverse circumstances. The roots are the first to respond to the action of the poison, as they are in actual contact; their growth is checked, and hence the water absorption is decreased. No food is available in the water supply from the roots, so the plant is entirely dependent on the stores laid up in the seed and on the carbon it can derive from the air by photo-synthesis carried on by the green leaves. The result of the root checking in these particular cases seems to be so to stimulate the shoots by some physiological action or other, that this process of photo-synthesis is hastened, more carbon being converted into carbo-hydrates, so that the shoot development is increased, yielding a greater weight of dry matter. In each of the other cases observed the shoot was obviously not stimulated to increased energy by the poison, and so the whole plant fell below the normal.
Other experiments showed that barley roots are peculiarly sensitive to the presence of minute traces of copper, as very little root growth took place in the copper distilled water, and root growth was also entirely checked by the presence of one part per million copper sulphate in the pure glass distilled water. Yet again, one litre of pure distilled water was allowed to stand on a small piece of pure metallic copper foil (about 11⁄2″ × 1⁄2″) for an hour, and even such water exercised a very considerable retarding influence upon the root-growth, checking it entirely in some instances.
Some years before True and Gies published their results, [Coupin (1898)] had grown wheat seedlings in culture solutions with the addition of copper salts for several days in order to find the fatal concentrations of the different compounds. Taking toxic equivalent as meaning “the minimum weight of salt, which, dissolved in 100 parts of water, kills the seedling,” the results were as follows:
| Toxic equivalent | Containing copper | |
| Copper bromide (CuBr2) | ·004875 | ·001387 |
| Copper chloride (CuCl2 . 2aq.) | ·005000 | ·001865 |
| Copper sulphate (CuSO4 . 5aq.) | ·005555 | ·001415 |
| Copper acetate (Cu{C2H3O2}2 . aq.) | ·005714 | ·001820 |
| Copper nitrate (Cu{NO3}2 . 6aq.) | ·006102 | ·001312 |
These numbers appear to be very close, so Coupin considered that it might be permissible to regard the differences as due to the impurities in the salts, and to the water of crystallisation which may falsify the weights, so that under these conditions one may believe that all these salts have the same toxicity. This is considerable, and is evidently due to the copper ion, the electro-negative ion not intervening with such a feeble dose. A recalculation of these toxic equivalents to determine the actual amount of copper present in each, gives results that are fairly approximate, but it is difficult to accept this hypothesis in view of other work in which different salts of the same poison are proved to differ greatly in their action on plant growth.
[Kahlenberg and True (1896)], working with Lupinus albus, found that the various copper salts, as sulphate, chloride and acetate, were similar in their action upon the roots. Plants placed in solutions of these salts of varying strengths for 15–24 hours showed that in each case 1/25,600 gram molecule killed the root, while with 1/51,200 gram molecule the root was just alive. These workers discuss their results from the standpoint of electrolytic dissociation, and concur in the opinion that the positive ions of the toxic salt are exceedingly poisonous.
The toxicity of the positive ion was again set forth by [Copeland and Kahlenberg (1900)]. Their water culture experiments were carried on in glass vessels coated internally with paraffin to avoid solution of glass, and in tests with seedlings of maize, lupins, oats and soy beans it was found that such metals as copper, iron, zinc and arsenic were almost always fatal to the growth of plants. As a general rule those metals whose salts are toxic, themselves poison plants when they are present in water. The assumption made was that the injury to plants when cultivated in the presence of pure metals depends on the tendency of the metal to go into solution as a component of chemical compounds and on the specific toxicity of the metallic ion when in solution.
(b) Masking effect caused by addition of soluble substances to solutions of copper salts.
Experiments were carried on with barley, in which the plants were grown in the various grades of distilled water indicated above, both with and without the addition of nutrient salts. It was found that the presence of the nutrients exercises a very definite masking effect upon the action of the poisonous substance, so that the deleterious properties of the toxic substance are materially reduced. Later work, in which known quantities of such toxic salts as copper sulphate were added to pure distilled water showed that in the presence of nutrient salts a plant is able to withstand the action of a much greater concentration of poison. For instance, a concentration of 1:1,000,000 copper sulphate alone stops all growth in barley, but, if nutrient salts are present, a strength of 1:250,000 (at least four times as great) does not prevent growth, though the retarding action is very considerable (Figs. [2] and [3]).