The Essentiality of Certain Elements in Minute Quantity for Plants with Special Reference to Copper.

The recent discoveries of the importance of small amounts of boron, manganese, copper, and zinc (1, 2, 4, 7, 13) in the physiology of higher plants, and in a number of soil-plant problems of considerable agricultural importance, have added interest to studies on the role of these elements in plant nutrition. The mostobvious and important question is whether or not these elements are indispensable to the growth of plants. Recent investigations in this laboratory (12) on the importance of cer am metals in minute quantity in the economy of higher plants, have resulted in the development of an experimental technique which makes possible a consistent and reproducible demonstration of the essentiality of copper, manganese, and zinc for the growth of higher plants. Although the details of the procedures will be described elsewhere (12) it is desired to present at this time some considerations and conclusions which may be of general interest. It was deemed desirable to test experimentally the essentiality of each element according to the following criteria: an element is not considered essential unless (a) a deficiency of it makes it impossible for the plant to complete the vegetative or reproductive stage of its life cycle; (b) such deficiency is specific to the element in question, and can be prevented or corrected only by supplying this element; and (c) the element is directly involved in the nutrition of the plant quite apart from its possible effects in correctinug some unfavorable microbiological or chemical condition of the soil or other culture medium. From that standpoint a favorable response from adding a given element to the culture medium does not constitute conclusive evidence of its indispensability in plant nutrition. Conclusive proof of the indispensability of the so-called microelements, that is, elements required in minute amounts by plants, can be obtained by the use of artificial culture media (the water culture technique was used in this investigation), provided special procedures are employed to remove incidental impurities. The physiological importance of these contaminants is illustrated by the finding that young tomato plants grown in Pyrex glass containers with nutrient solutions deficient in zinc gave a measurable response to the addition of 1 gamma (0.001 mg.) of zinc to a plant (this gave a zinc concentration in the culture solution of 5 parts per billion). This response to a minute amount of zinc as well as similar responses to copper and manganese were consistently obtained only when redistilled water and purified chemicals were used. The metal content of ordinary distilled water was reduced by redistillation, using 371