GROWTH ASPECTS OF PLANT VERUS INFECTIONS 



637 



spinach and the turnip-mosaic virus in Nicotiana species, the virus 

 concentration closely parallels the host growth (Figure 7). In cases 

 where the same virus has been studied in more than one host, identical 

 patterns of nitrogen effects have been found. 



Generally, increasing the phosphorus level increases the virus con- 

 centration in plants (see Figure 6), even though host growth is mark- 

 edly reduced at the higher phosphorus levels ( Cheo et al., 1952; Pound 

 and Weathers, 1953b; Weathers and Pound, 1954; Helms and Pound, 

 1955b). In occasional assays of the potato X virus and the tobacco- 

 ringspot virus Helms and Pound ( 1955a ) found that the highest virus 

 concentration was obtained at the phosphorus level giving the greatest 

 host growth. In these cases, however, phosphorus had much less 



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Figure 7. Nitrogen effects on host growth and virus concentration in plants 

 of Nicotiana glutinosa L. infected with tumip-mosaic virus. Plants were 

 grown in a sand medium and harvested 21 days after inoculation. 



