Breeding of Vegetable, Spice and Tuber Crops

Mutagens and their mode of action

• It refers to various physical or chemical agents which greatly enhance the frequency of mutations. Two types of mutagens
• Physical mutagens -X rays, Gamma rays, Alpha particles, Beta particles, Fast and thermal neutrons, UV rays

Gamma rays

• Gamma rays have generally a shorter wavelength and hence possess more energy per photon than X-rays. Gamma radiation in usually obtained from radioisotopes in contrast to X-rays. A gamma radiation facility can be used essentially in the same manner as an X-ray machine for acute or semi-acute exposures.
• The most distinct advantage of gamma radiation source for prolonged treatments is that it can be placed in a greenhouse or field so that plants can be exposed as they develop over long periods of time. Cobalt-60 and Cesium -137 are the main sources of gamma rays used in mutation breeding. They are stored in lead containers when not in use and operated by remote control mechanisms to irradiate plant material.

UV rays

• Ultraviolet light has limited penetrating ability, therefore its use is limited to treating spores, pollen grains cells and cultured tissue. Wavelengths in the range of 2,500 to 2,800 nm are biologically most effective because this is the region of maximal light absorption by nucleic acids.

Beta particles

• Beta particles such as those from Phosphorus-32 and Sulfur- 35 produce effects in tissues similar to those of X- or gamma rays. The penetrating ability of beta particles is lower than that of X- and gamma rays.
• The lower penetrating ability of beta particles can be overcome by putting the radioisotope in a solution and administering them to the plant material. P-32 and S-35 may then be incorporated directly into cell nucleus giving somewhat greater localization of the site of action. But, because of the variability from tissue to tissue and cell to cell, it is difficult to determine the exact dose given by an internal emitter in plant material.

Neutrons

• Neutrons have been shown to be highly effective for the induction of mutation in plants but, a certain degree of confusion exists concerning the results of early experiments and due to lack of adequate dosimetric techniques.
  

Ion beams

• Ion beams can give a large amount of energy with high LET (Linear Energy Transfer) to the localized position in tissues. Therefore, we can expect different biological effects will be given to plant material compared with low LET radiations, such as, gamma- and x-rays.
• Also ion beams can produce large structural changes in chromosomes and DNA. So we can expect to induce different kinds of mutations in plants with high frequency than gamma- and x- rays. But further basic and practical researches should be made to use ion beams efficiently in the future.