Fig. 5. Curve showing the mean values of the dry weights of four series of pea plants grown in the presence of copper sulphate and nutrient salts. (Oct. 3rd–Dec. 20th, 1912.)
Yet, in spite of all the accumulated evidence as to the consistent toxicity of copper salts in great dilution, the possibility still remains that the limit of toxicity has not yet been reached, and that a stimulating concentration does exist, so that it is still uncertain whether beyond the limits of toxicity copper salts act as indifferent or stimulative agents.
4. Action of copper on organs other than roots.
The bulk of the work on the relations of copper with the life-processes of plants has dealt with those cases in which the metal has been supplied to the roots in some form or other, and many of the results may be said to apply more strictly to the theoretical, or rather to the purely scientific aspects of the matter, than to the practical everyday life of the community. This statement is hardly correct, in that the two lines of work are so inextricably interwoven that the one could not be satisfactorily followed up without a parallel march of progress along the other. In practice, copper has proved remarkably efficient as a fungicide when applied as sprays in the form of Bordeaux mixture to infested plants and trees. Observations on the action of the fungicide have shown that the physiological processes of the treated plants are also affected to some degree, and a number of interesting theories and results have been put forward.
(a) Effect of copper sprays on leaves.
[Frank and Krüger (1894)] treated potato plants with a 2% Bordeaux mixture, and obtained a definite improvement in growth, which they attributed to the direct action of the Bordeaux mixture upon the activities of the plant. The effect of the copper was most marked in the leaves, and was chiefly indicated by increase in physiological activity rather than by morphological changes. The structure of the sprayed leaves was not fundamentally changed but they were thicker and stronger in some degree, while their life was lengthened. Apparently, treatment increased the chlorophyll content, and, correlated with this, was a rise in the assimilatory capacity, more starch being produced. Rise in transpiration was also observed. While the leaves were the organs most affected, a subsidiary stimulation occurred in the tubers, since the greater quantity of starch produced required more accommodation for its storage. In different varieties the ratio of tuber formation on treated and untreated plants was 19:17 and 17:16. In discussing the meaning of this stimulation these writers, following the custom then in vogue, were inclined to hold that it was due to a catalytic rather than to a purely chemical action, an idea similar to one which later on came much into prominence in connection with the work of Bertrand’s school on manganese, boron and other substances.
The imputed increase in photo-synthesis seems to have met with approval and acceptance, but nevertheless it did not pass unchallenged. [Ewert (1905)] brought forward a detailed discussion and criticism of the assumption that green plants when treated with Bordeaux mixture attain a higher assimilation activity than untreated plants. His experiments were made to test the effects of differing conditions of life on plants treated in various ways, and his conclusions lead him to assert that “instead of the organic life of the plant being stimulated by treatment with Bordeaux mixture it is rather hindered.”
While Frank and Krüger indicated a rise in transpiration when copper compounds were applied to the leaves as sprays, [Hattori (1901)] attributed part of the toxic effect of copper salts, when applied to the roots, to a weakening action on the transpiration stream, and he maintained that the toxic effect of the copper salts is therefore connected with the humidity of the air. No further confirmation or refutation of this statement has so far come to light.
In certain plants the application of cupric solutions as sprays causes a slight increase in the quantity of sugar present in the matured fruits. Chuard and Porchet ([1902], [1903]) consider that such a modification in the ripe fruit during the process of maturation occurs in all plants which ripen their fruits before leaf-fall begins. Injection of solutions of copper salts into the tissues of such plants as the vine causes more vigorous growth, more intense colour and greater persistence of the leaves; in other words the copper acts as a stimulant to all the cells of the organism. A similar effect is produced by other metals such as iron or cadmium. By injecting small quantities of cupric salts into the branches of currants an acceleration of the maturation of the fruits was caused, identical with that obtained by the application of Bordeaux mixture to the leaves. If the quantity of copper introduced into the vegetable organism was augmented, the toxic action of the metal began to come into play. These investigators attributed the stimulus, as shown by the earlier maturation of the fruits, to a greater activity of all the cells of the organism and not to an excitation exercised only on the chlorophyll functions.