646 PLANT GROWTH AND PLANT COMMUNITIES 



diverts the metabolic machinery of the cell from the synthesis of nor- 

 mal cellular constituents to that of virus nucleic acid and virus protein, 

 which are subsequently assembled into the virus nucleoprotein. This 

 invariably results in growth abnormalities of the host, which are ex- 

 pressed as anatomical or physiological changes. 



Virus eflFects on host anatomy include suppressed or stimulated 

 cell division and differentiation, necroses, vascular occlusions, plastid 

 degeneration, and intracellular inclusion bodies. Teratological effects, 

 such as tumorous overgrowths, giant flower buds, adventitious roots 

 and shoots, and alamiiiate leaves, are not common to all virus diseases 

 but are conspicuous in some. 



Physiological effects are numerous but still poorly defined. Carbo- 

 hydrate, protein, and nucleic-acid metabolism are all affected. Respira- 

 tion is often increased and photosynthesis decreased by virus infection. 

 Reactions are not the same for all host-virus combinations, and it is not 

 possible to draw general conclusions which would encompass all virus 

 infections. Many conclusions that have been drawn have been based 

 upon experiments dealing with sap extracts of plants, excised leaf discs, 

 or otherwise altered living systems; these experiments, at the most, may 

 be only indicative of what happens within the cell. 



Studies of host environment have revealed definite relationships 

 between host growth and virus synthesis. In many instances the condi- 

 tions of environment that promote the best host growth (increase in 

 weight) are also the best conditions for virus synthesis. Notable excep- 

 tions occur, however. In some cases, virus synthesis is directly related 

 to the amount of nitrogen supplied the host, even though the nitrogen 

 concentration may be great enough to cause marked stunting. In other 

 cases, the effect of nitrogen on virus synthesis directly parallels its ef- 

 fect on host growth. Similar results have been obtained with phos- 

 phorus. These elements are major constituents of the virus nucleopro- 

 tein, and it is reasonable to assume that deficiencies of them limit virus 

 synthesis directly by limiting the necessary materials for synthesis and 

 indirectly by upsetting the growth processes of the plant. Other ele- 

 ments which are not constituents of the virus particles probably affect 

 virus synthesis indirectly through their effects on host metabolism, and 

 in such instances virus synthesis generally parallels host growth. How- 

 ever, the synthesis of tobacco-mosaic virus is greater in manganese- 

 deficient tobacco than in plants receiving adequate manganese. While 

 growth differences in tobacco due to iron nutrition are easily estab- 

 lished, only under extreme deficiency is synthesis of tobacco-mosaic 

 virus affected. Boron also has only limited effects on virus synthesis, 

 while host growth effects are very marked. 



The effects of temperature and light on virus synthesis are very 

 complex. In inoculated leaves, virus synthesis is generally a direct func- 



