Mutagenesis—A Potential Approach for Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 156

Initial studies on induced mutations were mainly directed to finding optimum
combination of mutagen and dose to elicit the best response. Both physical and
chemical mutagens were tested in various crop species such as wheat, barley, rice,
tobacco, corn, Brassica, fruit crops and vegetables. These studies helped to initiate
large-scale mutation breeding experiments for various practical applications (Cho-
pra 2005 ). Since various physical and chemical mutagens are known to act in dif-
ferent ways to cause DNA lesions, combined effects of mutagens were investigated.
In wheat, combined treatment with UV and X-rays showed dose-dependent ef-
fects. UV pretreatment of seeds reduced the frequency of mutations at low doses of
X-rays (11-16 kr) but increased it at high doses of 22-30 kr (Swaminathan and Na-
tarajan 1959 ; Bansal et al. 1962 ). In barley, treatment with S-2 aminoethylisothiou-
ronium bromide hydrobromide (AET) was tested as both pre- and post-treatment
with X-rays. Frequency of chromosome aberrations and chlorophyll mutations reg-
istered a significant drop when AET treatment was followed by X-ray irradiation.
On the other hand, post x-ray treatment of AET caused a slight drop in chromosome
aberrations (Chopra et al. 1965 ). Similarly, combined treatment of two chemical
mutagens, ethyl methane sulfonate (EMS) and hydroxyl amine (HA), was inves-
tigated in wheat. Data of chlorophyll and viable mutations indicated that EMS is
a potent mutagen in Triticum dicoccum but HA is a weak mutagen. But when HA
was administered after EMS treatment, there was a significant drop in mutation
frequency indicating that HA may be involved in mutational repair process (Cho-
pra and Swaminathan 1966 ). Studies with Drosophila showed that formaldehyde,
which is not mutagenic in female flies, could enhance mutation frequency when
administered following X-ray treatment. This suggested that formaldehyde might
be blocking some DNA repair process (Mahajani and Chopra 1973 ).
MutagensandTheirDoses
One of most crucial requirements for a successful breeding programme is the selec-
tion of an effective and efficient doze of a mutagen for mutagenizing the starting
material. Historically, the effectiveness of a mutagen has been measured in terms
of biological effect that it produces. It is, however, desirable to establish a relation-
ship between the observed biological effect to a well-defined and easily measurable
physical quantity characterizing the amount of radiation or chemical mutagen re-
sponsible for that effect (Roychowdhury 2011 ). Therefore, the mutagenic effect in
biological targets is commonly and conveniently described in terms of dose-effect
relationships. In quantitative radiation biology, the 'simple dose' (D) is the amount
of energy absorbed per mass of irradiated matter at the point of interest. The special
unit of D is rad (1 tad 100 erg/g = 10 −2 joule/kg), expressed in terms of time as rad/h,
rad/min and rad/s. Thus, among others, changes in radiation doses and duration of
exposure of biological material to the irradiation are important parameters of physi-
cal mutations. In case of chemical agents, the dose of treatment is determined based
on several parameters viz., (i) concentration, (ii) duration of treatment and (iii) tem-