|Which metals are essential for plant health?|
As a general rule a plant is healthy as long as the supply of a nutrient matches the plant's requirement for that nutrient. A shortage of a nutrient will result in symptoms of deficiency, and, at very low supply, in early mortality. An excess will cause injury, and at high levels, may cause toxicity and even death. The range between slight deficiency and slight toxicity often varies by no more than a factor of ten.
Only a few of the many metals present in the environment are essential to all plants. Normal performance of plants requires that nine chemical elements, the so-called macronutrients, be absorbed in relatively high amounts; these are carbon, hydrogen, nitrogen, oxygen, phosphorus, sulphur, and the metals calcium, magnesium and potassium. In addition, eight other chemical elements are necessary in small amounts, and are called micronutrients. These include boron and chlorine, and the metals copper, iron, manganese, molybdenum, nickel and zinc. A few plants living in symbiosis with nitrogen-fixing micro-organisms require cobalt as well. Two criteria are used to define a metal as essential for plant health: (1) it is required by the plant to complete its life cycle; and, (2) it is part of a molecule of an essential plant constituent or metabolite.
Plants differ from nearly all other organisms in that they photosynthesize, i.e., they use light energy to convert water and carbon dioxide into energy-rich carbohydrates and oxygen. Plant growth is therefore dependent on photosynthesis, which, in turn, is dependent on a sufficient supply of a number of chemical elements, including the metals copper, iron and manganese, which play an important role in metal-protein complexes necessary for regulating the primary processes of photosynthesis. With the exception of carbon dioxide, which is absorbed from the air through the leaves, all other elements are taken up from the soil and water by the roots.
To cope with a shortage of a metal, plants can increase the availability of metals in the root environment by lowering the pH through exudation of protons and organic acids, or by excretion of metal-complexing agents. As soon as a sufficient supply is achieved, a signal from the shoot to the root stops the exudation process. In addition, many plant species are associated with mycorrhizal fungi which help to facilitate the plant's uptake of water and nutrients, including metals. The amount of a required metal may depend on the chemical form of other nutrients in the environment. For example, in the presence of nitrates in the soil, healthy plant growth is impossible without molybdenum. By contrast, in the presence of ammonium, plants are able to synthesize amino acids almost without molybdenum.
During their evolution, plants did not develop uptake mechanisms that differentiate between essential and non-essential metals. Due to this lack of selectivity, the presence or concentration of a metal in the tissue of a plant does not tell us anything about the plant's requirement for that metal. In this respect, the accumulation of metals is related to the plant's age. This is one of the reasons for the observed high variability of metal concentrations in plants.
Plants may deal with a surplus of a metal by storing the metal in deciduous plant parts which can later be shed, or in special compartments of plant cells in such a manner as to render the metal metabolically inactive. This compartmentalization of excess metal may serve other functions as well. For example, in zinc-hyperaccumulators, where zinc levels may reach 10 g zinc per kg dry leaf mass, it serves to deter herbivorous animals. It can also provide a reserve of metals in times of low supply, when the plant is able to remobilize and transport the required metal from one tissue to another. Plants are therefore well adapted to the metal supply of their environments, having evolved mechanisms to deal with conditions of excess and deficiency.
About the author: Dr. Wilfried H.O. Ernst is Professor of Botany at the Free University of Amsterdam, The Netherlands. His research has been primarily concerned with the role of mineral nutrition in ecology and ecotoxicology of wild plants. He has authored about 200 research papers, chapters and reviews, and has written three books. He has served on numerous task groups and was an advisor to a number of international and national research committees and bodies.