Toxic Effect of Boron on Fruit Trees

Boron is toxic to plants when present in relatively small concentrations. 2. Lemon seedlings are more sensitive to boron than orange seedlings. The new growth of seedlings affected with boron may be chlorotic. 3. By the continuous flow method of supplying the culture solution, the relationship was studied between the concentration of boron in the culture solution, the effect on the tree growth, and the concentration of boron in the leaves. 4. The effect of boron on citrus depends on the concentration of boron present in the nutrient solution and the concentration of other nutrient ions. 5. Additions of boron greatly intensified the mottling tendency of the Valencia orange trees when grown in soil treated with nitrate of soda. 6. Citrus and walnut leaves may become thin, mottled, chlorotic, and crinkled as a result of a toxic agent such as boron. 7. The addition of various amounts of ferric sulphate to cultures of lemon seedlings tended to overcome the toxicity of boron. So-called insoluble borates in water cultures may furnish low concentrations of boron to the plant roots, but the continued absorption of these small amounts may be sufficient to produce toxicity. Several so-called insoluble borates were found to be distinctly toxic. 8. Unless badly defoliated repeatedly, trees injured by boron may recover if the toxic agent is leached out with water. 9. With toxic concentrations of boron, no changes in the vascular anatomy of the affected leaves were observed such as have been reported when boron was absent. 10. A single application of borax, ranging from 50 to 400 gm. per tree, to basined soil in which walnut trees were growing in the field produced a mottle and a leaf burn chiefly along the margins. 11. Citrus and walnut leaves affected with boron contain reduced amounts of calcium and increased amounts of potassium. The composition is typical of mottled and of immature leaves. The leaves, it is believed, are prevented from becoming mature in regard to ash composition as a consequence of the paralyzing action on the growth processes.