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PRODUCTION OF RUST RESISTANT TREES BY MUTAGENESIS 553 



EXPERIMENTS WITH CHEMICAL MUTAGENS ON FOREST TREE SPECIES 



Hanover (1964) treated seeds of Finns montioola Dougl. with ethyl - 

 methane-sulfonate (EMS). The objects were to determine the physiological 

 effects of the mutagen on the seeds and seedlings and to attempt to 

 induce genetic resistance against the blister rust fungus (Cronartium 

 ribieola J. C. Fisch. ex Rabenh.). Kedharnath (1968) treated seeds of 

 P. roxburghii Sarg. with EMS and found that germination, hypocotyl length 

 and survival were affected with increasing concentration. Yim (1963) 

 using seeds of P. rigida Mill, for treatment with EMS also found a 

 decrease in germination and seedling growth with increasing concentration 

 of EMS. Similar studies by Kedharnath (1969) on Dalbergia sissoo Roxb., 

 a broad-leaved species, comparing results obtained with those reported 

 earlier for P. roxburghii , showed that (1) at pH 9 the sensitivity of 

 both species to the mutagen was higher than at pH 5 and (2) D. sissoo was 

 more sensitive than P. roxburghii. 



Hanover and Hoff (1966) treated P. montioola pollen with EMS. They 

 obtained good seed set by using treated pollen in controlled crosses, but 

 the seedlings are yet to be scored. Kedharnath (unpublished) treated 

 pollen of Morus alba L. with EMS and used it in controlled crosses with 

 good seed set. Here again seedlings are yet to be scored. 



POSSIBLE APPROACHES TO MUTATION- INDUCED RESISTANCE 



The basidiospores of C. ribioola are formed on Ribes spp. Infection 

 of white pines with these spores occurs during cool moisture periods. 

 Since the basidiospores are the product of meiosis following nuclear 

 fusion in the telutospores, each basidiospore is potentially unique. 

 This uniqueness could also be true for the character of pathogenicity on 

 the pines and will naturally depend upon the number of genes conditioning 

 pathogenicity of the fungus in relation to the pine host. 



If pathogenicity is controlled by one or a few genes, the possible 

 number of unique types of pathogenicity would also be few. In such a 

 case, major gene-conditioned resistance in the host may be useful. On 

 the other hand, if a large number of genes control pathogenicity of the 

 fungus, the host plants will be exposed to a large number of unique types 

 of spores. In such a situation, it would be more profitable and even 

 essential to look for and exploit polygene-controlled field or horizontal 

 resistance. Unfortunately we have little information on this aspect. 



Flor (1955) working with linseed rust, has shown convincingly that 

 there exists a 1 to 1 relationship between the genes for pathogenicity in 

 the fungus and genes for resistance in the host. If we assume that a 

 similar relationship also holds in the present case then a consideration 

 of the available information on the nature of resistance in the host 

 plant, i.e., pines, would be of interest. 



We still do not have definite information about the number of genes 

 conditioning resistance to blister rust in the different species of 

 native and exotic white pines that have been studied in the United States 

 and Canada. There may be more than one source of resistance within the 

 same species and some of these sources may be conditioned b> a single 

 gene, some by two or more and some by polygenes. That such a situation 

 can exist is supported by evidence for several other vegetable and crop 

 plant diseases including rust of wheat. According to Heimburger (1962) 



