Tests with white lupins gave no conclusive results, as for some reason it proved very difficult to get satisfactory plants in water cultures. When they are grown under such conditions the roots always tend to get more or less diseased and covered with slime, probably fungal in nature. In the presence of much boric acid the roots remain in a much healthier condition, which suggests that the acid has in this case a strong antiseptic action, and protects the roots. With high concentrations the lower leaves of the plant are badly affected, just as with peas and barley, turning brown and withering at an early date. Various experiments have been made with yellow lupins, but these again are very difficult to grow well in water cultures, as they are apt to drop their leaves for no apparent reason. Generally speaking, the evidence goes to prove that boric acid is toxic down to a concentration of about 500 parts in 25 million. It is difficult to get a true control with which to make comparisons as the plants without boric acid are encumbered with the slime on their roots, which naturally interferes with normal growth, while the plants in the presence of boric acid have the unfair advantage due to the probable antiseptic action of the boron. The effect of the boron poisoning is again evident in the dying off of the lower leaves, which become flaccid and drooping and finally drop off. The lupins grown with boron are very active in the putting forth of lateral roots, so much so that the cortex of the roots is split along the line of emergence of the laterals, which are very numerous and crowded.
(b) Toxic action of boron compounds in sand cultures.
[Agulhon (1910 a)] moistened 2 kgm. pure sand with 500 c.c. nutritive solution for each pot, and boron was added at the rate of 0, 0·1, 1, 10, and 50 mg. boric acid per litre of nutritive solution. Twenty wheat seeds were sown in each pot, and after twelve days the healthy plants in the first four pots were 6–8 cm. high, but those with the maximum amount of boron showed yellowish leaves only 3 cm. long. After three months’ growth the plants were harvested, when those with most boron were found to have died after making about 10 cm. growth. The toxic doses in sand proved to be weaker than those in water cultures, probably because evaporation from the surface of the sand caused concentration of the poisonous liquid.
(c) Toxic action of boron compounds in soil experiments.
Long before any experimental work was done with boron in water cultures, the poisonous properties of the substance were recognised with regard to plants growing in soil. [Peligot (1876)] grew haricots in porous earthenware pots, the plants being watered by rain and by solutions, each containing about 2 grams per litre of such substances as borax, borate of potassium, and boric acid, other pots receiving various fertilisers, as potassium nitrate, sodium nitrate, &c. This quantity of boron completely killed off the plants receiving it, whether it was applied as free or combined boric acid, while the fertilised plants completed their development well. On this account the deleterious action was attributed to the boric acid and not to the sodium or potassium base supplied. Peligot hinted at the improbability of a substance like boron, which is so poisonous to plants, being really innocuous to human beings when it is used as a preservative for foods.
[Nakamura (1903)] also found that borax is harmful in pot cultures if present in large quantities, 50 mg. borax per kgm. of soil exerting a very injurious influence, while even 10 mg. per kgm. did some damage. [Agulhon (1910 c)] found that the toxic doses of boric acid in soil cultures approached those in nutritive solutions rather than in sand cultures, a phenomenon that he attributed to the fact that the boric acid was fixed by the soil, probably as insoluble borate of calcium, so that the surface concentration obtained with sand cultures was avoided. He found that the ash of plants grown with excess of boron contained more than the normal amount of boron, while the weight of ash per 100 dry matter was also increased. He concluded that the plant thus suffers an over-mineralisation and in consequence an augmentation of its hold on water, so that the fresh weight of the plant may indicate a more favourable action of the boric acid than does the dry weight. Other investigators ([Fliche and Grandeau 1874]) had found the same increase in the proportion of ash in chestnut trees grown on too calcareous soil, so Agulhon concluded that one is here dealing with a general reaction of plants to an excess of a useful element.
Other experiments were carried on in the open field, maize being grown on control plots and on plots receiving 2 gm. boron per square metre. At first the latter plants were behind, the dose being too strong. Eventually, however, they pulled up level and the dry weights from the two plots proved to be nearly the same, the fresh weights being identical. Maize is evidently far less sensitive to boron poisoning than are peas and oats, for with these one-half the original amount of boron (= 1 gm. per sq. metre) proved toxic.
Interesting results were obtained ([Agulhon 1910 a]) by repeated experiments with the same soil containing boron. It was found that sand or soil containing a proportion of boron which is lethal or toxic to a first culture will allow much better growth with a second and subsequent crops. Repeated experiments on the same soil may show the change from a lethal dose to a toxic one, thence to an indifferent and finally to an optimum concentration. Furthermore ([Agulhon 1910 b]) the very plants may accustom themselves to greater quantities of boron, the increased power of resistance being transmitted. He concluded from his experiments that the progeny of the second generation of maize were able to withstand quantities of boron that were toxic to control plants[13]. Agulhon once again emphasised the fact that for toxic doses of boron the first symptom is the more or less marked disappearance of chlorophyll, though the aerial parts are not affected so soon as the roots.
2. Effect of boron compounds on germination.
One of the first indications that boron compounds affect the germination of seeds was given by [Heckel (1875)] who found that germination was retarded for 1–3 days by weak solutions of borates (·25 gm. to 20 gm. water), and was stopped altogether by stronger solutions (·60 gm. to 20 gm. water). [Archangeli (1885)] tested the germination of a variety of seeds of Leguminosae, Gramineae, and of Cannabis, Iberis, Raphanus, Collinsia, and Linum in the presence of boric acid. The seeds were placed in bowls with solutions of ·25, ·5, and 1% boric acid at temperatures ranging from 16°–23° C. The bowls were covered with glass plates to prevent evaporation and consequent increase of concentration, controls in spring water being dealt with under similar conditions. 1% boric acid was found to check germination altogether, and the weaker the concentration the less was the process hindered. [Morel] soaked seeds of haricots and wheat in various solutions of boric acid, and found that germination was generally hindered or inhibited. The deleterious action diminishes as the strength of the solution or the time of contact diminishes, but solutions of the same concentration do not act equally on all seeds. Boric acid and borax proved to be similar in their action qualitatively.