2.3.1. Physical Mutagens
Man’s ability to deliberately induce mutations in plants derives directly from the discoveries of X-rays by Roentgen in 1895; radioactivity by Becquerel in 1896; and radioactive elements by Marie and Pierre Curie in 1898. For these achievements, the Nobel Prize for Physics was awarded to Roentgen in 1901 and to Becquerel, Marie and Pierre Curie in 1903 [15]. Deriving directly from these discoveries were the seminal demonstrations soon afterwards that radiation (X-rays) caused alterations to the genetic make-up of fruit flies [16] and in the crop plants—maize and barley [17–21]—respectively. These proofs of concept proved to be the watershed moments in induced mutagenesis as they provided the impetus for the subsequent widespread adoption of this mimicry of nature in crop improvement and more recently as a strategy to discover genes and elucidate their functions. Ionizing radiations constitute the most commonly used physical mutagens [22,23]. These are parts of the electromagnetic (EM) spectrum that, on account of their relatively high energy levels, are capable of dislodging electrons from the nuclear orbits of the atoms that they impact upon. The impacted atoms therefore become ions hence the term ionizing radiation. These ionizing components of the EM include cosmic, gamma (γ) and X-rays. Ultra violet (UV) light, though non-ionizing, is capable of some level of tissue penetrability and has been used in inducing mutations as well. The mutagenicity of UV derives from its ability to react with DNA and other biological molecules as its wavelengths are preferentially absorbed by bases in DNA molecules and by the aromatic amino acids of proteins.