92 elements have been identified on earth and 17 of these are known to be essential to all plants. The essential elements required in relatively large amounts (>0.1% of dry mass) are called macronutrients and include C, H, O, N, S, P, Ca, K, Mg. Those required in much smaller amounts (<0.01% of dry mass) are referred to as micronutrients or trace elements and include Ni, Mo, Cu, Zn, Mn, B, Fe, and Cl. Plant growth and development depends on a balanced supply of these essential elements and thus the plant has a range of homeostatic mechanisms operating to ensure that this is maintained. Beneficial elements which promote growth and may be essential to some taxa, include Na, Co, Al, Se and Si. Elements such as the heavy metal Cd and the metalloid As have no demonstrated biological function in plants, but are nevertheless taken up and cause severe toxicity in most plant species. The concept for this special issue is the plant ionome, a word coined to encompass all these elements and allow focussed discussion and investigations on the mechanisms that co-ordinately regulate these elements in response to genetic and environmental factors reviewed in Salt et al., 2008).
macronutrients, micronutrients, homeostatic mechanisms, promote plant growth, plant ionome
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