GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 



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NITROGEN (PPM) PHOSPHORUS (PPM) 



Figure 6. The relation of nitrogen and phosphorus nutrition of tobacco to host growth 

 and concentration of tobacco-mosaic virus. Plants were grown in a sand medium and 

 harvested 21 davs after inoculation. 



nitrogen and phosphorus deficiencies can affect \irus synthesis directly 

 by limiting the supply of necessary building materials, and indirectly 

 by upsetting vital functions of the cell related to protein and nucleic- 

 acid synthesis. 



The mineral trace elements, as will be shown below, also affect 

 virus synthesis, probably indirectly through their effects on cellular 

 metabolism. 



Growth processes of plants are also functions of temperature and 

 light. The most rapid respiration occurs at the points of rapid growth 

 {e.g., terminal growing points and other embryonic tissues). Respira- 

 tory substrates in the form of readily oxidizable carbohi\clrates must be 

 in plentiful supply. This is provided by translocation, a process pro- 

 ceeding most efficiently in darkness. Kinetic activity and, therefore, 

 diffusion rates increase with increase in temperature. \\'ithin limits, 

 enzyme reactions such as respiration are increased by increases in tem- 

 perature, and strictly chemical reactions generally have a Qio value of 

 2 to 3 (Went, 1953). Over a range of 10^ C. to 25° C, and under a 

 suitable light intensity and concentration of CO2, the Qio \d\ne of pho- 

 tosynthesis is approximately 2. In general, increased light intensity 

 will, up to a certain point, increase photosynthetic activity unless some 



