Toxic Effect of Boron on Fruit Trees

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Abstract
Boron injury of citrus and walnut trees consisted of crinkling, chlorosis, and mottling of leaves. Lemons were more sensitive than oranges and oranges more than grape fruit. Use of Fe2(SO4)3 reduces the toxicity of B, but so-called insoluble borates, e.g., MnB4O7, caused injury when present in concentrations of 10-40 p. p. m. of B. The effect of B is dependent upon the concentration and to some extent upon the presence of other ions. When Ca is deficient, injury consists of leaf spotting due to burning of small patches and of premature abscission. When injury is not too severe recovery follows washing of B from culture. Severity of injury induced by given amounts of B decreased as the trees put on new cycles of leaves thus indicating very little migration of B from mature to young leaves. Dropping of mature leaves and leaching of soil both tend to reduce B content of soil. Ash of walnut, orange, and lemon trees injured by B showed reduced Ca and increased K content, indicating that excess B interferes with absorption of Ca. Leaves injured by B showed no anatomical changes when compared with normal ones.