Role of Mineral Elements with Emphasis on the Univalent Cations

Abstract

Of the many inorganic elements that higher plants absorb in ionic form from the soil or aquatic environment, only a relatively few are known to be indispensible for metabolic processes. The essential macronutrient elements other than C, H, and 0 include N, P, K, S, Ca, and Mg. The concentration of these elements in higher plants is illustrated by analyses of Zea mays showing percentages of 1.45 N, 0.20 P, 0.92 K, 0.17 5, 0.23 Ca, and 0.18 Mg in dry material (159). Another group of mineral elements generally required by plants in small quantities and referred to as micronutrient elements include Fe, Mn, B, Cu, Zn, Cl, and Mo. Several elements are required by some spe­ cies of higher plants and microorganisms but not by others. Cobalt is a micronutrient for some microorganisms and symbionts (71), V is essential for Scenedesmus obliquus (6), Na is needed for certain blue-green algae and halophytes (4, 37, 270), and Si is reported (131) to be indispensable for the growth of diatoms. Although B is generally required for higher plants, it has not been shown to be essential for microorganisms. Even though the micronutrient elements occur in plant tissues at con­ centrations ranging from less than one to a few hundred ppm of dry matter, it seems that more effort has been devoted to the elucidation of the mode of action of these than to some of the macronutrient elements, such as K, that commonly make up 1 to 2 per cent of the dry material of plants. Authors of modern textbooks often state that the specific role of K in plants is unknown. They indicate that K in some way is involved in the unique organization of protoplasm, that it functions in cellular osmotic processes, and is necessary for cell membrane permeability. That univalent cations function as acti­ vators for certain enzymes usually is mentioned, but the impression is left that the importance of these cations in this capacity is questionable. A de­ cision was made by the writers, therefore, to first survey briefly the status of our knowledge of the roles of the essential mineral elements other than uni­ valent cations, referring for the most part to the pertinent recent reviews. The remainder and major portion of this review will deal with the role of the