The physiological action of arsenic compounds on plant life early attracted the attention of investigators. [Chatin (1845)] put forward some rather curious and unexpected considerations with regard to this action. He stated that the effect of arsenic on plant growth is determined more by the constitution and temperament of individual plants than by their age, and that apparently difference in the sex of plants is of no significance. The chief determining agent, however, is the species, and Chatin found that as a general rule Cryptogams are more sensitive than Phanerogams, and Monocotyledons than Dicotyledons, as is shown by the fact that under treatment the former perish first. Some extreme exceptions exist, though, as Mucor mucedo and Penicillium glaucum will grow on moist arsenious acid, whereas leguminous plants are killed by an arsenical solution in a few hours. Chatin held the view that elimination of the poison succeeded its absorption, and that this elimination is complete if the plant lives long enough. Here again the species exerts a great influence on the excretory functions of the plants. Lupins and Phaseolus are presumably able to eliminate in six weeks all the arsenious acid they can absorb without dying. Most Dicotyledons need 3–5 months, while Monocotyledons retain traces of poison for six months after its absorption. Lichens are said to eliminate it more slowly still. Again, woody species are longer in freeing themselves than herbaceous, and young plants carry out the elimination more easily than old plants. The excretory function is influenced by other physiological factors such as dryness and season. The toxic effects and elimination are supposed to act inversely and parallel, the absorbed arsenious acid combining with alkaline bases, making a very soluble salt which is excreted by the roots. Calcium chloride is given as the antidote to arsenious acid, all soluble acid being “neutralised” by it. This view of the elimination of arsenic apparently did not gain much support, as no further references to the matter have so far come to light. In view of the work of some modern investigators (Wilfarth, Römer and Wimmer) on the excretion of salts by plant roots, the idea may prove of fresh interest. Chatin also found that moving or still air influenced the working of the poison, indicating that the external physical conditions affect the toxic action considerably. Nearly forty years later [Nobbe, Baessler and Will] found that, if transpiration were hindered by placing plants in a dark or moist room, it was possible to keep the plants turgescent in arsenic solutions for a long time without thereby increasing the toxic effect later on. The poisonous action proceeds from the roots, of which the protoplasm is disorganised and the osmotic action hindered. Finally, in the presence of sufficient of the poison, the root dies without growth.
Stoklasa ([1896], [1898]) again found that phanerogamic plants can withstand arsenic poisoning for some time in the dark or in CO2-free air, provided that glucose is given in the food solution. The arsenic poisoning is at its maximum during carbon assimilation by means of chlorophyll. The toxic action of arsenious and arsenic acids, especially in phanerogams, is due to injury to the chlorophyll activity. The destruction of the living molecule is far more rapid in the chlorophyll apparatus than in the protoplasm of the plant cell.
Thus it seems that the physiological cause of the toxicity of arsenic is partly a direct action on the root protoplasm, whereby its osmotic action is hindered, and partly a detrimental action upon those functions which are directly concerned with the elaboration processes of nutrition.
2. Effect of arsenic compounds on germination.
In view of the great toxicity of arsenic to plants in their various stages of development, one would naturally expect to find a similar action with regard to the germination of the seeds. [Davy (1859)] casually mentioned cases in which watering with arsenical solutions or dipping seeds in arseniated water prevented germination. [Heckel (1875)] found that arsenious acid checks germination and kills the embryo at relatively feeble doses, ·25 gm. to 90 gm. water.[10] Guthrie and Helms ([1903]–[4]–[5]) carried out a systematic series of experiments to test the effect of arsenic compounds upon different farm crops. Various amounts of arsenious acid were added to soil in pot experiments, and the seeds of the several crops were then sown. With barley, wheat and rye 0·10% arsenious acid had little or no effect on germination, while an increase in the poison exercised a retarding action. Maize could withstand 0·40% arsenious acid without retardation being perceptible. The aftergrowth with the different crops varied considerably. The wheat plants with 0·10% arsenious acid grew all right at first, but later on they developed weakly. The toxic action increased rapidly as the strength of the poison rose in the different pots. Barley proved even more sensitive than wheat, for even 0·05% arsenious acid affected the growth adversely. After a time the plants with 0·05–0·06% recovered and grew strongly, though not so well as the controls, but those with 0·10% practically died off. Rye behaved in the reverse way from wheat. The plants with 0·10% were slightly checked at first but later recovered and made growth quite equal to the check plants. Growth was stunted with 0·20% arsenious acid, and the plants were killed with 0·30%, so that rye is far less sensitive than barley. With maize the growth was slightly affected with 0·05% As2O3, and increasingly so with greater quantities. It was also found that the action of 0·8% As2O3 was strongly adverse to the germination of all plants, and that above this strength germination was altogether prevented.
The results show very clearly how impossible it is to draw any general conclusions with regard to the action of arsenic compounds on plants, as they emphasise the strong individuality of the species in their reaction.
3. Do arsenic compounds stimulate higher plants?
The question of stimulation due to arsenic does not seem to have engaged the attention of investigators to any extent. Water culture experiments at Rothamsted have so far yielded negative results, and no stimulation has yet been obtained with any plant, with the possible exception of white lupin with sodium arsenite. In a single series a stimulus was suggested, beginning to make itself felt at 1/500,000, rising to an optimum at 1/10,000,000. No stress can be laid on this result, as it is never safe to draw any certain conclusions without several repetitions of the same experiment. With arsenic acid on barley a possible stimulus is sometimes indicated to the eye, the plants being fine and of a particularly healthy dark colour, but this is not corroborated by the dry weights. Additional tests were made with peas and barley, treated with sodium arsenite and arsenate, the dilutions being carried down to 1/250,000,000, but no evidence of stimulus was obtained, so that it hardly seems possible that arsenic can act as a stimulative agent for these two plants when grown in water cultures. It had been thought that the failure to find a stimulation point hitherto might be due to the too great concentration of the toxic substance rather than to the actual inability of the poison to stimulate, but this hypothesis must now be dismissed so far as these plants are concerned.