Comparative Mineral Nutrition of Plants

A uniformity in the mineral requirements of plants is frequently im­ plied in discussions of plant nutrition. Classroom reviews of this subject routinely begin with a listing of essential mineral elements and almost invariably include the suggestion that these elements are required by all plants. This perhaps is to be expected, for although the usually accepted criteria of essentiality (1, 2) are explicit regarding the establishment of a requirement by a specific species, there has been little agreement on the extent to which the results with one species, or even with several species, should be generalized to all plants. There obviously has been a tendency to assume that demonstration of essentiality of an element for one or a few species could be interpreted as a requirement for all plants. Further­ more, past experience seemed to justify such generalizations. This is not surprising in view of the emphasis on higher plants, and crop plants in particular, in nutrition studies, for these are organisms selected for growth under a relatively narrow range of nutritional conditions. The point to be emphasized most strongly in the present discussion is that the mineral nutrient requirements of all plants are not the same. This is true both qualitatively and quantitatively. Furthermore, although gen­ eralizations on nutrient requirements are often justified, nutritionists and plant physiologists have much to gain from recognizing and emphasizing variations in the mineral requirements of organisms throughout the plant kingdom. Some aspects of comparative mineral nutrition have been discussed in previous articles in the Annual Review of Plant Physiology. This would include certain phases of selective uptake of elements, particularly as re­ lated to mechanisms of salt accumulation [Epstein (3) J, and mineral nutrition in relation to plant ecology as reviewed by Billings (4). However, comparative mineral nutrition has not been considered as a separate topic. This discussion, therefore, will not emphasize current literature as much as does the average review article. Instead, emphasis will be placed on investigations which illustrate the various aspects of comparative nutrition and the possibilities for further contributions in this area. Because of ex­ tensive previous coverage, all references to selective uptake in relation to mechanisms of salt accumulation have been omitted.