AB + CD = AC + BD.
[Haselhoff (1892)] extracted several lots of 25 kgm. soil, each with 25 litres of water in which quantities of mixed copper salts varying from 0–200 mg. had been dissolved, the mixture consisting of three parts copper sulphate and one part copper nitrate. This operation was repeated 15 times, the soils being allowed to drain thoroughly after each treatment, so that altogether each 25 kgm. soil was extracted with 375 litres water. The drainage waters were analysed, so that the amount of copper absorbed by the soils could be estimated. It was found that by extracting with water containing such soluble copper salts as sulphate and nitrate, the food salts of the soil, especially those of calcium and potassium, were dissolved and washed out, copper oxide being retained by the soil. In this way a double action was manifest, whereby the fertility of the soil was reduced by the loss of plant food, while its toxicity was increased by the accumulation of copper oxide. So long as the soil contained a good supply of undissolved calcium carbonate the harmful action of the copper-containing water was diminished, but as soon as the store was exhausted by solution and leaching, the toxic influence became far more evident.
(e) Mode of action of copper on plants.
Quite early in the investigations on the effect of copper on plants the question arose as to its mode of activity—whether the toxicity was merely due to some mechanical action on the root from outside, whereby the absorptive power of the root was impaired, or whether the poisonous substance was absorbed into the plant, so acting directly on the internal tissues. [Gorup-Besanez] made definite experiments towards ascertaining the truth of these theories as far back as 1863, endeavouring first of all to see whether the plants take up any appreciable quantity of poisons which exist in the soil as mixtures or combinations and which are capable of solution by the cell-sap. Salts of arsenic, copper, lead, zinc and mercury were intimately mixed with soil, 30 grams of the poison being added to 30·7 cubic decimetres of soil, two plants separated by a partition being grown on this quantity. The test plants were Polygonum Fagopyrum, Pisum sativum, Secale cereale and Panicum italicum, and all the plants developed strongly and normally except the last named. The Panicum developed very badly coloured leaves in an arsenic-containing soil, and the plants were killed soon after they started in soils containing copper. After harvesting, the crops were analysed and no trace of copper was found in any one of the experimental plants by the methods adopted. Also the absorption capacity of different soils for different poisons was shown to vary, for basic salts are absorbed, while acids may pass completely through the soil into the drainage water.
These results obtained by Gorup-Besanez are possibly not altogether above criticism, for later workers showed that copper was absorbed to some extent by plants grown in water cultures, and if that is so it seems unlikely that no absorption should take place from soil. Nevertheless, the absorption is very slight, for apparently living protoplasm is very resistant to copper osmotically. [Otto] showed that beans, maize and peas can have their roots for a long time in a relatively concentrated solution of copper sulphate, and yet take up very little copper indeed, but analyses do reveal slight traces after a sufficient interval of time of contact has elapsed. [Berlese and Sostegni] indicate that the roots of plants grown in water culture in the presence of bicarbonate of copper showed traces of copper.
[Verschaffelt (1905)] devised an ingenious method of estimating the toxic limits of plant poisons, though it is rather difficult to see how the method can be put to practical use with water culture and soil experiments. Living tissues increase in weight when put into water on account of the absorption of water. Dead tissues do not, as they have lost their semi-permeable characteristics, so a decrease in weight takes place owing to part of the water passing out. This principle is applied by Verschaffelt to determine the “mortal limit” of external agents in their action on plant tissues. Root of beetroot, potato tuber, aloe leaves, and parts of other plants rich in sugar all came under review. The parts were cut into small pieces weighing about 3–5 grams, dried with filter paper, weighed, and plunged into solutions of copper sulphate of varying strengths from ·001–·004 gm. mol. per litre, and left for 24 hours. After drying and again weighing all were heavier owing to the absorption of water. The pieces were then immersed in pure water for another period of 24 hours, when after drying and weighing, those from the weaker strengths of copper sulphate (·001–·002) had absorbed yet more water, while those from higher concentrations (·003–·004) had lost weight. So the author assumes that for such pieces of potato the limit of toxicity lies between ·002 and ·003 gm. mol. copper sulphate per litre.
These experiments may possibly give some indication as to the action of copper salts on plant roots. So long as the solution of copper salt is dilute enough, the absorption layer of the root, acting as a semi-permeable membrane and upheld by the resistant protoplasm, is able to keep the copper out of the plant and to check its toxicity. As soon as a certain limit is reached the copper exercises a corrosive influence upon the outer layer of the root whereby its functions are impaired, so that it is no longer able efficiently to resist the entry of the poison. As the concentration increases it is easy to conceive that the harmful action should extend to the protoplasm itself, so that the vital activities of the plants are seriously interfered with and growth is entirely or partially checked, death ensuing in the presence of sufficiently high concentrations.
2. Effect of copper on germination.
The action of copper on the germination of seeds, spores and pollen grains has attracted a certain amount of attention, and although the results are apparently contradictory this is probably due to the different plant organs with which the observers have worked.