Section 13 — Plant Genetics and Breeding 



parent, has been neglected. Yet many horticultural 

 crops are reproduced vegetatively, like apples 

 which are artificially budded on rootstocks; these 

 might in turn influence the free variability. 

 Strawberries are more naturally reproduced 

 from runners. These rot ultimately, so leaving 

 self-supporting ramets at the rooted nodes form- 

 ing an asexual population. Such clones provide 

 opportunity for assessing the variability of a 

 vegetative reproductive system not previously 

 analysed intensely. 



Plant clones can be of all ages, and of all 

 vegetative stages removed from the original 

 parent. A clone may be diploid or polyploid, 

 homozygous or heterozygous, healthy or virus 

 infected. Some clones may even transmit plas- 

 magenes vegetatively through successive gener- 

 ations. The clone is subject to the rejection or 

 accumulation of somatic mutations of oligogenic 

 or polygenic origins; hence they may be homo- 

 somatic or hetero-somatic. Not surprisingly a 

 clone may show vegetative segregation. 



The variability in ramet size is analysed for 

 octoploid virus-free cultivated strawberry clones 

 and those carrying plasmagene infection. The 

 internal variation is examined within grafted 

 clones of very old and new apples for the bio- 

 metrical character "stamen number". These 

 data are then assessed in the light of contempora- 

 ry views on genetic homeostasis, the "c" effect, 

 and the individual in the plant kingdom. 



13.43. Genetic Variability in Isogenic Lines of Barley. 

 C. O. Qualset and C. W. Schaller (Davis, 

 U.S.A.). 



Backcross-derived isogenic lines of Atlas 

 barley with two segments of chromosome 

 marked by independent genes governing the 

 length of the lemma awn are being used to study 

 types of quantitative gene action and the effect 

 of the awn on yield and various other characteris- 

 tics. The theoretical length of chromosome 

 associated with each of these loci retained from 

 the nonrecurrent awnless parent through the 

 backcrossing procedure has been calculated to be 

 about 5 crossover units. Several quantitative 

 characters are being studied to assess the 

 amount of genetic activity in these short segments 

 of chromosome. Many workers have found that 

 photosynthetic activity in the awn contributes 

 significantly to final kernel weight and hence 

 total yield. In this material the four homozygous 

 genotypes are phenotypically distinguishable by 

 the length and distribution of the awn. They are 

 designated full-awned (AABB), half-awned 



(AAbb), quarter-awned (aaBB), and awnless 

 (aabb). 



A study has been completed in which the four 

 genotypes were grown at five plant densities 

 (1/6 to 6 ft 2 /plant) in two years. Over both years 

 genetic variation was found for 5 of 8 characters 

 measured. For the quantitative characters 

 measured the genotypes were stable over the 

 various densities but interacted with years in 

 5 out of 8 cases. Significant additive genetic 

 variability was found at the A locus for kernel 

 weight, plant height, and spike density. At the B 

 locus only kernel weight and percent sterility 

 indicated additive effects. All of the genetic 

 variance for kernel number/spike was due to 

 interaction of the A and B loci. This interaction 

 resulted in a significant increase in total yield of 

 half-awned over full-awned in the first year and 

 equal yield in the second year in spite of a signifi- 

 cant decrease in kernel weight due to the reduced 

 length of awn in half-awned. 



13.44. The Effect of Different Treatments on the 

 Variability of Polygenic Characters in Wheat. 



S. Borojevic (Novi Sad, Yugoslavia). 



A study has been made of the effect of mineral 

 nutrients, rate, mode and date of planting on the 

 variability of ear and kernel characters as well as 

 on the yield of wheat. 



The absence of fertilizers, as well as the use of 

 nitrogen in the absence of phosphorus produced 

 the highest variability in all the characters studied. 

 Phosphorus in the absence of nitrogen did not 

 affect the variability of the length of ear and 

 number of spikelets but increased considerably 

 the variability of the number and weight of 

 kernels per ear. Potassium does not seem to affect 

 the variability. The lowest variability of the 

 characters studied and the highest yield have been 

 obtained when high doses of nitrogen, medium 

 ones of phosphorus and low ones of potassium 

 were applied. 



Heavier stands lead to the increase in vari- 

 ability of the number and weight of kernels but 

 not of other characters. The date of planting 

 affected the variability in a similar manner. The 

 mode of planting did not exert any great influ- 

 ence on the length of ear and number of spikelets. 

 However, larger distances between rows resulted 

 in lower variability and higher mean values. 



These findings have interesting bearing on the 

 exploration of the genetic capactity for yield and 

 indicate that from the increase of heritability 

 level in the number of kernels the most may be 

 expected. 



224 



