250 MUTATION AND PLANT BREEDING 



Gaul: I agree completely with you. For instance, it might be expected 

 that it will be easier to find high-yielding earliness mutants in a late- 

 ripening variety than in an early one. I think however, that a thorough 

 selection of the genotype of the starting material is an objective not only 

 valid for the use of mutations but is common for all breeding methods. 



Sarvella: Are the first five tillers referred to in the theory of diplontic 

 selection formed from the five buds present in the seed after irradia- 

 tion (primary axillary spikes), or from the first five tillers emerging 

 from the ground? Would you expect the mutation rate at a node to be 

 lower in the tillers which are formed after the primary axillary tiller 

 is formed? 



There is evidence that the mutation rate per spike and per seedling 

 in the plant appear to be the same for the whole plant, for the primary 

 axillaries, and for the whole plant excluding the apical spikes and the 

 primary axillary spikes (secondaries, etc.) at some nodes. The primary 

 axillary spikes were tagged when the X-l seedlings were 3 weeks old. 



Caul: In reference to your first question. The hypothesis of diplontic 

 selection considers the ontogenetically first five tillers, which do not 

 always correspond with the first five tillers emerged from the ground. 

 In reference to your second question. The first second-order tillers 

 derived from axillary bud 1, 2, and perhaps also 3 presumably contain 

 fewer mutations. With excessive tillering, the mutation rate of the later 

 second-order tillers depends, I suppose, on the number of surviving 

 L II-initial cells of the corresponding axillary bud. If there are several 

 surviving initial cells in these buds or primordias, the mutation rate 

 of the later second-order tillers will usually be smaller than that of the 

 tillers derived from primary axillary buds. This is a consequence of inter- 

 cellular competition. But if there is only one surviving L II-initial cell 

 and this carries a mutation, no drop of the mutation frequency is 

 expected because the generative tissue of the later secondary tillers is 

 supposed to originate from this initial cell. This question is fully dis- 

 cussed in the paper "Studies on diplontic selection" which is now in 

 press (Symposium on the Effects of Ionizing Radiation on Seeds and 

 their Significance for Crop Improvement, Karlsruhe, 1960). 



We have now evidence from four different experiments that the muta- 

 tion rate of the later formed tillers is smaller than that of the first by 

 about five. 



Nilan: From the literature and some discussions at this symposium, I 

 find that frequently the induced mutation technique and the cross 



