24 



S. G. WILDMAN 



C. Course of Virus Synthesis in Directly Inoculated Leaves 



Goodchild ei al. (1958) made siirndtaiieoiis measurements of the 

 relative amoimt of infectivity, the number of particles detectable by electron 

 microscopy, and the amount of specific virus nucleoprotein that could be 

 extracted from directly inoculated leaves of N. tahacum var. Turkish Samsun 

 as a function of time, after rubbing the surface of the leaves with a solution 

 of TMV. The results of the infectivity determinations are presented in Fig. 9. 



) 5 10 15 20 

 Time (days) 

 Fig. 9. The relative infectivity that can be extracted from directly inoculated leaves 

 of N. tabacum var. Turkish Samsun as a function of time of infection Avith TMV strain 

 Ul. Different kinds of points represent repetitions of bioassay for infectivity. From 

 Goodchild et al. (1958). 



(The three methods showed a close correspondence throughout the period of 

 infection, leading to the conclusion that the virus does not change in specific 

 infectivity.) The inoculum contained 10"^ mg. of strain Ul TMV/ml. An 

 increase in relative infectivity could be detected in these greenhouse-grown 

 plants two days after rubbing. As time of infection continued, a steady rise 

 in relative infectivity ensued until a marked flattening of the curve set in 

 about 8-10 days after the start of infection. Steere (1951) has found virus 

 increment curves of similar shape for both TMV and bushy stunt virus by 

 counting newly synthesized virus particles by means of electron microscopy. 

 The increment curves flatten in lesser time than that shown in Fig. 9. 



The great probability that only a miimte percentage of the total number 

 of cells comprising the surface of a leaf becomes infected at the mstant of 

 inoculation almost automatically leads to the conclusion that a curve such 

 as Fig. 9 is a composite of at least two processes, (1) the multiphcation of 

 virus within individual cells, and (2) the spread of virus into other ceUs, 

 together with multiphcation in these cells. 



