GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 



641 



allowed to multiply in Nicotiana glutinosa L. ( Figure 8 ) , a concentra- 

 tion gradient identical with that in horseradish and the reverse of that 

 in cabbage occurs (Pound and Weathers, 1953b). In N. multivalvis (also 

 in the solanaceae) air temperature has little effect on concentration of 

 these viruses in systematically-infected plants. In all of these hosts 

 ( horseradish not tested ) virus synthesis in inoculated leaves is a direct 

 function of temperature, increasing as temperature increases. This in- 

 dicates that virus synthesis is actually enhanced by the 28° temperature 

 but movement of the virus out of inoculated leaves of some hosts into 

 other portions of the plants is impeded at this temperature. The growth 

 of cabbage increases with increase in temperature up to 24° C, above 

 which it levels off or drops, depending upon the variety. The growth 



200 



24"C. 



28C. 



I- 



If) 



X 



I- 

 $ 



o 



a. 

 o 



en 

 o 



X 



Figure 8. The relation of air temperature to host growth and concentration 

 of turnip mosaic vims in systemically-infected plants of Nicotiana glutinosa 

 L. and cabbage at two weeks after inoculation. 



