WHILE the trace elements are necessary for the normal growth and development of plants, excess of them may be as injurious as their deficiency. Trace elements in excess are, in fact, poisons, and toxic symptoms characteristic of one or other of the trace elements in their action on particular species have been recorded in a number of instances. Such injurious effects are, of course, not limited to micro-nutrients and may be produced by a large number of elements, particularly the so-called heavy metals. The amount of any element which has to be absorbed in order to produce injury varies not only with the element and the plant but also probably with other factors such as the age of the plant or organ and the amount of other elements present in the medium and the plant.

MANGANESE

The visual effect of excess of manganese on the cereals wheat and barley is the browning of the roots and the subsequent formation of brown spots in the leaves. This was shown as long ago as 1902 by Aso, while more recently this symptom of manganese excess in barley leaves was recorded by Williams and Vlamis (1957). More commonly excess of manganese has been found to produce chlorosis. This was reported in pineapple plants by Kelley (1909, 1912), in oats by Rippel (1923), in Citrus by Haas (1933) and in dwarf bean (Phaseolus vulgaris) and common vetch (Vida sativa) by Lohnis (1950). In the dwarf bean excess of manganese induces chlorosis which at first affects the leaf margins and then in the younger leaves extends to the areas between the veins. As the leaves age they become crinkled while in the intervenal regions there appear whitish blotches and finally necrotic spots. The petioles and older parts of the stem are also affected, brownish purple spots developing on them. In the vetch the intervenal chlorosis is accompanied by a purplish discoloration of the margins of the leaflets.

The development of purplish spots on the primary leaves as a symptom of manganese toxicity was observed in cowpeas by Fujimoto and Sherman (1948).