III. Effect of Zinc on Certain of the Lower Plants.
Among the fungi, one species stands out in special prominence on account of the great amount of work that has been done on it with regard to its reactions to zinc salts. Aspergillus niger = Sterigmatocystis nigra van Tgh was used as a test plant by [Raulin (1869)], who evidently considered that zinc was an essential primary constituent of the food solutions of the fungi, ·07 parts zinc sulphate being added to each 1500 parts of water. In his experiments he tested (1) ordinary nutritive solution, (2) nutritive solution with various salts added, as zinc sulphate, (3) nutritive solution and salts (as 2) and also powdered porcelain. (2) gave a crop of Aspergillus about 3·1–3·5 times better than (1), while (3) was even better still. Sulphate of iron also proved stimulating in its action, but Raulin stated that zinc cannot replace iron, as both are essential.
[Ono (1900)] determined the relation between the weight of the mould crop in grams and the quantity of sugar used up in the presence of varying amounts of zinc sulphate. The amount of sugar used was always greater in the crops with ·0037–·0297% zinc sulphate by weight than in the control crops, indicating a stimulation caused by zinc.
[Richter (1901)] carried out rather similar experiments. When grown in solutions without and with 1/700,000 gram molecule zinc sulphate the dry weights of the mould were practically the same for the first two days, then the dry weight of the zinc crop shot ahead for a day or two, a depression setting in on the fifth day. Without zinc a less increase took place, and a similar drop was noticeable about the sixth day. The conclusion drawn is that the stimulation due to the zinc occurs chiefly in the first few days and also that the rise in the sugar consumed is more rapid at first with the moulds treated with zinc. Concentrations above 1/600 are harmful, but in weaker solutions zinc is a definite stimulant.
[Coupin (1903)] re-investigated some of Raulin’s work under more antiseptic conditions in order to see what substances were really needed by the mould and whether certain elements declared essential were really so. He concluded that iron and zinc are of no use in the nutrition of Sterigmatocystis nigra, but that the zinc retards the development of mycelium when food is abundant, killing it if it is badly nourished. This denial of stimulation was controverted by [Javillier (1907)] who re-tested Raulin’s solution with extreme care, growing Sterigmatocystis in
- (a) normal Raulin’s solution with zinc,
- (b) Raulin’s solution without zinc.
The ratio of crops a/b varied from 2·3–3·1 in four experiments, vindicating the favourable action of zinc. With regard to the optimum value for zinc the mould seemed to be perfectly indifferent to the presence of medium quantities but very sensitive to extremes, the maximum weights being reached in dilutions between 1/10,000,000 and 1/250,000, while quantities above 1/25,000 were toxic in their action. At a dilution of 1/50,000,000 stimulation was still evident, though in a less degree than with the optimal concentrations.
Javillier maintains that zinc is fixed by the fungus, the whole of the zinc present in dilute solutions being taken up, only part being utilised in stronger solutions. The value of accordance between the quantity of zinc fixed and the quantity supplied decreases rapidly with increase of concentration. Sterigmatocystis is able to fix without harm a quantity of zinc equal to more than 1/1100 of its weight. Zinc is regarded as a catalytic element, as essential to the well-being of the plant as are the more obvious nutrients, carbon, sulphur, phosphorus, &c., in spite of the minute traces in which it occurs.
A few tests on yeasts made by Javillier showed that with vegetative yeasts zinc has a specific action, a consistent increase occurring in the amount of yeast formed and in the amount of sugar consumed as the quantity of zinc increased from 0–1/10,000,000–1/10,000. With ferment yeast, however, zinc exerted no appreciable action. These results lend force to the conclusion of [Richards (1897)] who carried out experiments on fungi with various nutritive media with the addition of certain salts of zinc, nickel, manganese, iron, &c. He considered that his general results showed that the fact of a chemical stimulation of certain metallic salts upon the growth of fungi is established, although it must not be considered without further investigations that all fungi react in the same degree to the same reagent.
Conclusion.
As matters stand at the present day, it appears that it is still uncertain whether higher plants grown in water cultures are susceptible to stimulation by zinc salts. If a stimulus does exist, it must be at exceedingly great dilutions, but further evidence is needed. In soil cultures, however, the fact of increased growth seems to be more firmly established, certain species responding to zinc salts when used as manure, though no increase has been obtained with other species. It must always be remembered that the action may be an indirect one. The soil is very complex in its constitution, and it is impossible to determine the exact action of the added poison upon it, so that a stimulating effect need not necessarily be due to a direct action of a substance upon the plant, but it may be the result of more favourable conditions for life induced by the action of the substance upon the soil.
Among the fungi the stimulation of Aspergillus niger by minute traces of zinc compounds seems to be well proved, though again it does not necessarily follow that all fungi will react in the same way to zinc.