mac key: induced mutation in crop improvement 343 



In principle, this can be achieved in two different ways. One is to 

 hinder or eliminate competition between primordia or shoots already 

 differentiated before the mutagenic treatment. In radiation experi- 

 ments with seed of cereals, Freisleben and Lein (24, 25) suggested and 

 Gaul (28, 29) more definitely proved that the highest relative yield 

 of mutation will be recorded if the Mi plants are permitted to develop 

 a maximum of one tiller. In this way only one of the three to five 

 differentiated ear primordia of the treated embryo can develop. The 

 simplest method to suppress tillering is dense and late sowing, prefer- 

 ably with an ordinary drill and with proper calculation with respect 

 to lowered seed vitality. High temperature and light deficiency, as in 

 ordinary greenhouse conditions, are also efficient. In vegetatively 

 propagated plants, Bauer (5) showed that the detection of mutations 

 greatly improved if bud competition was reduced. This can be 

 done by cutting back the treated shoot successively or dividing it into 

 small cuttings which are allowed to root and develop separately. 



The other method of overcoming the disadvantages of diplontic 

 selection is to adjust the dose and its time of application. If only one 

 initial is allowed to develop, a stronger dose will increase the fre- 

 quency of mutated over normal meristematic cells. The relative rate 

 of mutation will thus increase much longer with dose than if com- 

 petition between affected and unaffected initials is allowed to counter- 

 select (29). A treatment in a late ontogenetic phase of plant develop- 

 ment will decrease both inter- and intrameristematic competition, 

 since the floral shoots gradually lose their interdependence and the 

 time for the diplontic selection to work will be shortened. The most 

 radical solution along this line will be to treat meiotic stages or even 

 gametes and then, preferably, pollen (13, 21, 29). In connection with 

 ionizing radiation and treatment with radiomimetic chemicals, such 

 extreme efficiency may, however, at least in certain species, be coun- 

 teracted by the exceptional response of very condensed chromosomes 

 resulting nearly entirely in chromosome aberrations (98, 101, 102). 

 The delay in treatment should, under such circumstances, be 

 restricted preferably to the somatic stages of the plant ontogeny. 



As Mericle (unpublished) has shown, treatment in early embry- 

 onal stages offers similar advantages without the undesirable effects 

 of treating gametes. A higher mutation frequency and a higher ratio 

 of mutant to normal offsprings can thus, in this way, be combined 

 with a much lower demand on dose. 



