628 PLANT GROWTH AND PLANT COMMUNITIES 



in cells is not surprising, but the literature shows disagreement as to 

 the nature and extent of such effects. Stanley ( 1937 ) reported that 

 some viruses increased the total nitrogen of leaves and the concentra- 

 tion of protein in saps, whereas other viruses decreased them. Some 

 reports (Martin et al., 1938; Holden and Tracy, 1948) indicate that 

 tobacco-mosaic virus increases the soluble protein concentration but 

 does not affect the total nitrogen concentration of plant saps, whereas 

 others (Wildman et al., 1949) report no increase in protein but rather a 

 decrease in normal nucleoproteins proportionate to the increase of 

 virus. Bawden and Kleczkowski (1957) demonstrated differential virus 

 effects on the soluble-protein content of infected tobacco tissue and 

 wide fluctuations in the soluble-protein content of infected tissue in re- 

 lation to healthy tissue— fluctuations which were affected by host en- 

 vironment, tissue sampled, etc. Thus the conditions under which plants 

 were grown and sampled may account for some of the discrepancies in 

 the literature. 



From what is virus produced in the cell? From elaborated host 

 protein? From the same pool of building blocks from which host pro- 

 tein is produced? These questions are only partly answerable. Wild- 

 man et al. (1949) and Wildman (1959) concluded that virus synthesis 

 occurred at the direct expense of elaborated host proteins, on the basis 

 of their findings that as virus nucleoprotein increased in inoculated 

 leaves, a corresponding decrease occurred in soluble host proteins. 

 This idea has not been generally accepted, and there is considerable 

 evidence that virus synthesis does not result from hydrolysis of host 

 protein. Meneghihi and Delwiche (1951), using labeled nitrogen 

 (N^^H4C1), concluded that tobacco-mosaic virus is formed in tobacco 

 from nitrogenous compounds, such as amino acids or polypeptides, 

 which have a more rapid exchange of nitrogen with ammonium ion 

 than does the cytoplasmic extractable protein of the cell. Labeled nitro- 

 gen appeared in virus fractions at a rate two to three times as great as 

 in cellular proteins. These authors believed that the process of virus 

 synthesis is irreversible, and that the virus behaves as a foreign protein 

 which is not in dynamic equilibrium with the rest of the cell constitu- 

 ents. Commoner and Dietz (1952) reached a similar conclusion. They 

 showed that the largest difference between infected and healthy tissue 

 was in the ammonia content. Virus synthesis was correlated with a 

 drain of nitrogen from the soluble nitrogen pool. They concluded that 

 most of the nitrogen of the virus protein came from the leaf's pool of 

 soluble nitrogen (ammonia, amino acids, and amides), and that virus 

 is formed from ammonia nitrogen and non-nitrogenous carbon sources. 

 This is further indicated by the fact that during the period of rapid 

 virus synthesis there is a deficiency of a number of amino acids that 

 occur in the virus molecule ( Commoner and Nehari, 1953 ) . Commoner 



