Plant scientists usually classify plant mineral nutrients based on the concept of ''essentiality" defined by Arnon and Stout as those elements necessary to complete the life cycle of a plant. Certain other elements such as Na have a ubiquitous presence in soils and waters and are widely taken up and utilized by plants, but are not considered as plant nutrients because they do not meet the strict definition of "essentiality". Sodium has a very specific function in the concentration of carbon dioxide in a limited number of C-4 plants and thus is essential to these plants, but this in itself is insufficient to generalize that Na is essential for higher plants. The unique set of roles that Na can play in plant metabolism suggests that the basic concept of what comprises a plant nutrient should be reexamined. We contend that the class of plant mineral nutrients should be comprised not only of those elements necessary for completing the life cycle, but also those elements which promote maximal biomass yield and/or which reduce the requirement (critical level) of an essential element. We suggest that nutrients functioning in this latter manner should be termed "functional nutrients". Thus plant mineral nutrients would be comprised of two major groups, "essential nutrients" and "functional nutrients". We present an array of evidence and arguments to support the classification of Na as a "functional nutrient", including its requirement for maximal biomass growth for many plants and its demonstrated ability to replace K in a number of ways, such as being an osmoticium for cell enlargement and as an accompanying cation for long-distance transport. Although in this paper we have only attempted to make the case for Na being a "functional nutrient", other elements such as Si and Se may also confirm to the proposed category of "functional nutrients".
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