12 S. G. WILDMAN 



whether the virus was irradiated before or after making contact w!th the 

 host.i 



Altering the concentration of virus does not change the exponential char- 

 acter of the inactivation curves. This indicates that an exclusion phenomenon 

 operates to prevent more than one infectious virus unit from starting an 

 infection at a susceptible site. Increasing the concentration of virus so as to 

 present each susceptible site, on the average, with as many as 7 imits of 

 virus infectivity failed to change the exponential character of the inactiva- 

 tion curve, whereas it would have been predicted that if any of the 6 other 

 units present at any given site could have substituted for the mactivated 

 particle, the inactivation curve should have displayed a multitarget char- 

 acter. Apparently, only one unit of infectivity can institute the infection, and 

 if, after making the proper contact with the susceptible site, its activity is 

 destroyed by irradiation, other units at the site which may have escaped 

 inactivation cannot replace the unit that was inactivated. We have no 

 further information that would help to explam the nature of the phenomenon. 



At first sight, the exclusion phenomenon would appear to conflict vnth. the 

 observations of Kunkel (1934), Lauffer and Price (1945), and Friedrich- 

 Freksa et al. (1946), who find a small percentage of mixed lesions when two 

 different strains of TMV are inoculated simultaneously on N. glutinosa 

 leaves. We have also observed between 1 and 5 % mixed lesions when strains 

 Ul and U2 are used as a mixed inoculum. However, I am far from convmced 

 that mixed lesions necessarily arise from the entrance of both strains into the 

 same cell. On the contrary, I have often observed that a single lesion can 

 have its origin from two or more foci of infection. For example, when de- 

 tached A^. glutinosa leaves are inoculated with TMV and the leaves main- 

 tained under constant illumination at 20°C., tiny, dark specks (0.1 mm. or 

 less in diameter), as viewed by transmitted light, will appear in about 18 

 hours after inoculation. Lesions develop from these specks. Occasionally, two 

 such specks will be in such close proximity that the resulting lesion, when 

 observed later, cannot be distinguished from lesions knowTi to have their 

 origin from a single speck. Indeed, with this consideration in mind, I would 

 interpret the results of mixed inocula experiments as strong evidence in 

 favor of an exclusion phenomenon. However, from Benda's experiments 



^ As far as TMV is concerned, photoreactivation by visible light of virus inactivated 

 by irradiation has not been demonstrated, although Bawden and Kleczkowski (1953) 

 have shown that tobacco necrosis virus is subject to photoreactivation. UV has a 

 profound effect on the susceptibility of plant cells to virus infection. Irradiation of leaves 

 prior to rubbing thena with virus will greatly reduce the amount of infection. Exposure 

 of UV-irradiated leaves to visible light results in photoreactivation of the susceptibility, 

 for with a sufficient length of visible light treatment, infection will be as great as if the 

 leaves had not been treated with UV (Bawden and Kleczkowski, 1952; Benda 1955; 

 Siegel and Wildman, 1956). 



