Essential Elements for Plant Growth

Additional Ambiguities in Essential Elements


Even when specific physiological functions in particular enzymes are well-identified, questions can still arise as to whether plants can survive and reproduce without those enzyme activities under certain circumstances, or whether enzyme functionality is sufficiently maintained by substitution with a chemically similar element.

For example, the most well-established function of Mo (molybdenum) is in nitrate reductase, an important enzyme that converts nitrate, NO3-, to ammonium, NH4+. Many plants, if supplied with small amounts of ammonium, can survive without nitrate, and therefore have no need of nitrate reductase (or the Mo in it) to complete a life cycle. Indeed, many plants, among them wetland rice and cranberry, actually prefer ammonium nutrition over nitrate as a N source, thus stretching the limits of Arnon's first criterion for an essential element.

Even when Mo is absent, the chemically similar W (tungsten) can substitute for Mo in the synthesis of nitrate reductase with reduced effectiveness, but still maintaining some function, stretching the second criterion for Mo.

Yet other elements are essential for large groups of certain plants through associated organisms. For example, leguminous plants (ex. soybeans, alfalfa, peanuts, clover, etc.) "fix" atmospheric dinitrogen (N2) with the aid of symbiotic bacteria that require Co (cobalt) as well as Mo for their nitrogenase enzyme systems. In addition, leguminous plants that internally transport fixed nitrogen as ureides (natural urea-like compounds) require Ni (nickel) for the enzyme urease, which is otherwise not required by nonlegumes.


This page was last modified by Phillip Barak, Univ. of Wisconsin, on 4 Mar 95. All rights reserved.