GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 631 



by an increase in total N and a decrease in total carbohydrates of in- 

 fected leaves. There seems to be no question but that a reduced carbo- 

 hydrate content is characteristic of many mosaic diseases. In many 

 cases this is accompanied by an increase in total N, thus resulting in a 

 marked decrease in the C/N ratio. In other cases the C/N ratio is de- 

 creased by a reduction in leaf carbohydrates without a significant 

 change in nitrogen content. Yellows diseases were found by Dunlap to 

 effect a reduction in total nitrogen and often an increase in carbo- 

 hydrates. The literature has shown this to occur in a number of cases. 

 Although these different effects occur, the number of cases examined is 

 too low for this to be of use in virus classification, as has been 

 suggested. 



Starch accumulation in leaves is quite marked in some virus dis- 

 eases (e.g., potato leafroll) and much thought has been given to ex- 

 plain it. It was early postulated that starch accumulation in potatoes 

 affected with leafroll was due to deranged phloem. However, starch 

 accumulation begins before phloem necrosis is evident ( Thung, 1928 ) . 

 Data do not indicate that enzvmatic disturbances are the answer, al- 

 though this has not been adequately investigated. As Wynd (1943) 

 suggests, some change may occur in the permeability of cell mem- 

 branes, so that starch becomes locked in the cells. It has been generally 

 thought that the yellows viruses effect an increase in carbohydrates, 

 due to the fact that they commonly cause conductive-tissue derange- 

 ments which impede the movement of carbohydrates. The mosaic 

 viruses are commonly parenchyma invaders and do not cause marked 

 derangements of conductive tissues. However, mosaic viruses also 

 reduce the movement of starch from leaves, as can be shown by the 

 Holmes ( 1931 ) starch retention test. As Bawden and Pirie ( 1952 ) have 

 suggested, the answer to the different effects on the C/N ratio may lie 

 in differential virus effects on photosynthesis rather than on synthesis 

 or translocation of carbohydrates. 



Respiration. It was Bunzel (1913) who perhaps made the first sug- 

 gestion of increased respiration of virus-infected tissue and referred to 

 the phenomenon as a "fever," and Thung ( 1928 ) who performed the 

 first really significant experiments on the effect of virus infection on 

 respiration. Thung showed that potato tissue infected with leafroll 

 virus eliminated over twice as much carbon dioxide per gram of dry 

 weight as did healthy tissue. Subsequent reports have created much 

 confusion as to virus effects on respiration. Whitehead (1934) reported 

 an increase in respiration due to virus infection, but Lemmon ( 1935 ) 

 reported that mosaic-infected tobacco tissue had a lower respiratory 

 rate than healthy tissue. Takahashi ( 1947 ) , in carefully controlled ex- 

 periments, showed that in discs of tobacco leaves infected with 

 tobacco-mosaic virus the respiratory rate in both darkness and light 



