PROCESS OF INFECTION AND VIRUS SYNTHESIS 



13 



(1956), it would appear that, under certain conditions, it is possible to cause 

 a mixed lesion to arise from the infection of a single hair ceU with two strains 

 of TMV. 



H. Changes in Virus Behavior during Early Stages of Infection 



When a delay is introduced between the time of rubbing the leaf with 

 virus and irradiating the host-virus system, a significant change in both the 

 slope and shape of the inactivation curve becomes apparent. Figure 3 shows 



4 6 8 



Dose (min) 



Fig. 3. UV survival curves of TMV strain U2 infective centers at different times after 

 inoculation of a local lesion host, N. glutinosa. The points indicate the number of lesions 

 on irradiated leaves plus virus compared with the number on unirradiated leaves. From 

 Siegel and Wildman (1956). 



the behaAaor of strain U2 in this respect. When irradiation is delayed 4-5 

 hours after inoculation, a marked change in the slope of the inactivation 

 curve is observed, although the exponential character of the curve is still 

 retained. Further extension of the time between inoculation and radiation to 

 7 and 9 hours produces a multitarget type of curve, indicative of the necessity 

 of inactivating more than one infectious unit at each focus of infection before 

 a lesion can be prevented from developing. Also noticeable is the fact that 

 the final slopes of the multitarget curves do not return to the slope encoun- 

 tered immediately following inoculation. 



Strain Ul, being much more resistant to uiactivation by radiation, has 

 not sho^^'n as much detail m the changes m its behavior toward inactivating 

 radiation as strain U2. The irradiation doses required to inactivate the Ul 

 strain are too large to exclude the possibility that lesions fail to appear 

 because of extensive damage to the host as well as the virus. However, we 



