552 S. KEDHARNATH 



reported on obtaining useful resistant mutants. Resistance to Victoria 

 blight, stem rust, and crown rust has been reported in oats; to stem 

 rust and stripe rust in wheat; to mildew in barley; to rust in flax; to 

 leaf spot and stem rot in peanuts, and to anthracnose in bean (Konzak, 

 1954; Frey, 1954; Shebeski and Lawrence, 1954; Frey and Browning, 1955; 

 Flor, 1955; Konzak et al., 1956; Myers et al . , 1956; Down and Anderson, 

 1956; Hansel and Zakovsky, 1956; Gregory, 1956; Konzak, 1959; Cooper and 

 Gregory, 1960; Favret , 1960a, b; Bhatia, Swaminathan, and Gupta, 1961). 

 These examples illustrate that it is possible to obtain disease-resistant 

 mutants through induced mutagenesis. 



It may be argued that the frequency of these mutations is still too 

 low for effective use. It should be possible, as Konzak (1956) and Gaul 

 (1964) have predicted, that improved efficiency in the use of mutagens 

 and screening procedures may increase not only frequency of mutations but 

 also their identification and recovery. Various chemical mutagens may 

 substantially increase the mutation frequency. For example, ethyl-methane- 

 sulphonate, diethyl -sulphate and ethylene imine are potent mutagens in 

 higher plants (Heslot, et al. , 1959; Ehrenberg, Gustafsson, and Lundquist , 

 1961; Swaminathan, Chopra and Bhaskaran, 1962; Konzak et al. , 1965). 

 Then too, screening procedure for disease-resistant genes are most effi- 

 cient when plants can be screened at the seedling stage under artificially 

 created epiphytotics . The technique developed by Wheeler and Luke (1955) 

 also merits attention. Using the toxin produced by the pathogen, 

 Helminthosporium viotoriae Meehan 5 Murphy, as a screening agent, they 

 tested two varieties of oats for resistant variants. In 100 bushels of 

 oats screened (approximately 45 million grains) , 973 blight-resistant 

 seedlings were obtained. 



RADIATION EXPERIMENTS WITH FOREST TREE SPECIES 



Radiations from physical sources, i.e., X-rays, gamma-rays and 

 neutrons, have been widely used with forest tree material with two objec- 

 tives in view. One objective was to study the immediate effects of acute 

 irradiation or radio-sensitivity and cumulative effects of chronic 

 irradiation on the developmental process of the treated material (Gustafsson 

 and Simak, 1958; Simak, Ohba, and Suszka, 1961; Snyder, Grigsby, and 

 Hidalgo, 1961; Mergen and Stairs, 1962; Okunewick, Herri ck, and Carlsen, 

 1964; Yim, 1964; Privalov, 1964, 1965; LaCroix, 1964; Stairs, 1964; 

 Sparrow et al. , 1965; Bevilacqua, 1965; Mergen and Cummings , 1965; 

 McMahan and Gerhold, 1965; Osborne and Lunden, 1965; Murai and Ohba, 

 1966; Kedharnath, 1966, 1967, 1968, 1969; Rudolph, 1967; Miksche and 

 Rudolph, 1968). The second objective was to induce genetic changes or to 

 overcome incompatibility barriers (Rudolph, 1965, 1966). Pollen 

 irradiation appears to be of particular usefulness in tree species (Stairs 

 and Mergen, 1964). The major advantages of this approach, compared with 

 using seed, seedling or other plant parts for irradiation, would be (1) 

 a reduction in time from irradiation to scoring the progeny and (2) 

 absence of possible diplontic selection against affected cells. 



