462 MUTATION AND PLANT BREEDING 



Useful mutant occurrences in many kinds of plants have been 

 reported and a number of reviews have been published concerning 

 their usefulness in plant breeding (2, 17, 31, 38, 40, 46, 48, 51, 55). 

 Numerous characteristics, such as grain yield, straw stiffness, chloro- 

 phyll type, maturity date, grain weight, plant height, disease resist- 

 ance, alkaloid content, etc., have been observed to vary significantly 

 under the influence of mutagens. MacKey (31) concluded his paper 

 on mutation breeding in Europe by saying that the evidence attests 

 that any agronomic characteristic can be improved by induced 

 mutations. The more recent reviews of Borg, et al. (2), Gaul (17), 

 Prakken (40), Scholz (46), and Stubbe (51) have given generally 

 optimistic accounts of the possibilities of mutation breeding. 



In interpreting the results obtained with useful individual 

 mutants it should be remembered that the statistical characteristics 

 and therefore interpretations are considerably different in situa- 

 tions where useful deviates are recognized among a great number 

 of variants in an artificially mutated population and in situations 

 where a character is pre-chosen for study. Few bona fide cases of the 

 latter can be identified with certainty in the literature on mutation 

 plant breeding. Among these cases, Brock and Latter (6) pre-chose 

 flowering date in their studies on subterranean clover, Oka, et al. 

 (39) pre-chose heading date and plant height in studies with rice, 

 and Gregory and Gregory (unpublished) pre-chose flowering response 

 to day length in their study of Hibiscus. 



Some other pre-chosen characteristics have not proved so suc- 

 cessful as breeding ventures. For example, Gregory (unpublished) 

 pre-chose the reversal of geotropism in the young peanut fruit. 

 Following an individual observation of hundreds of thousands of 

 X 2 and X 3 plants he has not, to date, observed what could be estab- 

 lished as a diminution of the geotropic response in peanut fruits. 

 Apple (unpublished) found in three different flue-cured varieties 

 of tobacco, that two of the varieties were naturally very highly 

 susceptible to "black shank", Phytophthora parasitica nicotianae; 

 the third had a measurable tolerance to the disease. By making use 

 of artificial inoculation in a combination of seedling flat and field 

 techniques, Apple was able to screen extremely large numbers of 

 individual X 2 plants. After several years of work he has been unable 

 to discover any increase in tolerance to black shank in the two 



