634 ri.A\ 1 c;ko\\ 111 wn i'lam communities 



growth ina\ also bo n-latotl to scopolotin. This compound, which is a 

 constituent of health) plant tissue, increases in amount in tomato 

 plants and tobacco plants as a result oH \irus infections ( Pavillard and 

 Beanchamp, 1957). It is inhibitor) to growth, and it has been sug- 

 gested that its increase in tobacco, as a result of \ irus infection, occurs 

 at the expense of indoleacetic acid (Pavillard and Beauchamp, 1957). 



Enzymes. A. F. Woods (1902) noted that mosaic-infected tobacco 

 contained a higher content of oxidizing enzymes than did healthy tis- 

 sue. This led him to postulate an enzyme theory of virus origin. \\'e 

 know that Woods" enzyme theor\- of the nature of \irus was wrong, but 

 his observation about diseased tissue ha\"ing higher levels of oxidases 

 was right. There are man\- obserxations recorded in the literature 

 which indicate that \ iruses effect an increase in oxidase and peroxidase 

 s\stems of the host. 



Reports have been made on \ irus effects on other enz\"me systems, 

 including, among others, chlorophyllase (Peterson and McKinney, 

 1938), dehydrogenase (Gerola and Testa. 1957). catalase (WVnd. 

 1942), and amylase (Balls and Martin, 1938). and these generally indi- 

 cate a stimulated enz\me acti\"it\- in infected tissue. It should be 

 pointed out, howexer, that the literature is not in agreement at all 

 points, and that this area of research has received disproportionately 

 little attention in recent vears. 



Host environment in telatioti to s^rowth and virus synthesis 



The manner and extent of plant growth are greatly dependent 

 upon enxironment, particular!) on mineral nutrition, temperature, and 

 light. Cellular growth, as enlargement or dixision, cannot occur without 

 a source of carbohydrate and nitrogenous foods, and it requires that 

 new proteins and cell-wall materials be produced and readily available. 

 These materials are s\ nthesized in green cells when the cells are pro- 

 \ided the proper enxironment. Amino-acid sxnthesis is dependent upon 

 carboh\drate metabolism for energ>' and building materials, and the 

 production of these in turn depends upon the mineral nutrition of the 

 plant. The major chemical elements of most critical importance in host 

 growth are nitrogen, phosphorus, and potassiiun— the elements with 

 w^hich commercial fertilizers are formulated. Each of these elements in- 

 creases plant growth up to a point where the concentration of the ele- 

 ment becomes toxic and growth is reduced (Figure 6). Nitrogen con- 

 stitutes a considerable portion of the mass of plant \iruses, and phos- 

 phorus is an important constituent of nucleotides, which have a central 

 role in nucleic-acid metabolism and in energy transfers in metabolic 

 reactions (Arnon, 1953). It is reasonable to assume, therefore, that 



