Lanthanides as Beneficial Elements for Plants

The lanthanide or lanthanoid (Ln) series of chemical elements encompasses 15 elements in the f-block of period six in the periodic table, with atomic numbers ( Z ) 57–71, from lanthanum (La) through lutetium (Lu), thus including the following elements listed in alphabetical order: cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), terbium (Tb) thulium (Tm), and ytterbium (Yb). While this series of metallic elements can be categorized as transition metals, they exhibit unique properties and characteristics that let them being set apart from the rest of the elements in the periodic table. Together with scandium (Sc; Z =21) and yttrium (Y; Z =39), they are all collectively called rare earth elements (REE). The terms ‘lanthanide’ or ‘lanthanoid’ were originally coined taking into consideration the first element of the series, La. The lanthanides share many analogous physical and chemical hallmarks, including common oxidation states 3+, molecular compounds formed with coordination numbers >6 (often 8–9), tendency to form chemical bonds with more electronegative elements (i.e. O or F), and unique ground and excited state properties that confer these elements special optical and magnetic behaviour. Of the 15 lanthanides, La and Ce have received most of the attention from plant biologists. Herewith, we describe the most salient findings regarding La and Ce as beneficial elements in plants. In agriculture, plants treated with low concentrations of either La or Ce display improved performance, with evident hormetic dose-response effects. Other lanthanides such as Pr, Nd, Eu, and Gd have also demonstrated positive effects, but they have been less investigated in plants.