GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 633 



benstein (1959) showed tliat sweet-potato plants infected with the 

 vein-clearing virus showed a much higher rate of respiration than 

 healthy plants, and that this elevated rate of respiration was main- 

 tained for as long as 70 days without decline. As with tobacco-mosaic 

 virus, the absolute values of respiration are greatly dependent on en- 

 vironment, leaf age, etc. 



Photosynthesis. It is to be expected that plant viruses should affect 

 photosynthesis, in view of the derangement they cause in chloro- 

 phyllous tissue. Little quantitative work has been done, however, to 

 establish these effects. Owen ( 1957, a, b; 1958) has shown that the rate 

 of photosynthesis in tobacco plants infected with tobacco-mosaic virus, 

 tobacco-etch virus, or potato X virus is lower than in healthy plants. In 

 the case of tobacco-mosaic virus, the reduction is measurable within 

 one hour after inoculation. Since epidermal cells are not active photo- 

 synthetically, this would indicate that the virus either moves into 

 chlorophyllous tissue much more rapidly than has been thought, or 

 that its presence in epidermal cells can markedly affect the photosyn- 

 thetic rate of uninfected cells. In the case of tobacco leaves infected 

 with the potato X virus or with tobacco-etch virus, respiration and pho- 

 tosynthesis remained unchanged until symptoms appeared, at which 

 time the respiration rate rose and the photosynthesis rate declined 

 (Owen, 1957a, 1958). Thus two different viruses may show a funda- 

 mental difference in regard to both respiration and photosynthesis in 

 the same host. It is obvious that generalizations cannot be made in 

 regard to virus effects on these important growth processes. 



Growth substances. Certain symptoms produced by viruses re- 

 semble effects produced by toxic levels of growth-promoting chemical 

 substances. Because of this and known effects of viruses on plant 

 growth, some interest has been shown in relating the virus effects to the 

 activity of these substances in the host. Thung ( 1951 ) suggested that 

 the presence of virus in plants more or less neutralized the influence of 

 growth substances on host development. The few quantitative studies 

 of the effects of virus infection on auxin content that have been made 

 indicate that infection results in reduced auxin activity (Grieve, 1943; 

 Pavillard, 1952, 1954; Hirata, 1954), These results would indicate that 

 the general stunting caused by viruses is perhaps the result of reduced 

 or inhibited auxin activity. It should be pointed out, however, that too 

 few critical studies have been made for this to be accepted categori- 

 cally. Recent studies ( Maramorosch, 1957; Chessin, 1958 ) have shown 

 that stunting produced by viruses may be counteracted with gibber- 

 ellic acid. Quantitative effects on virus multiplication have not been 

 made, but the fact that conspicuous leaf symptoms remain on the 

 gibberellin-treated plants would suggest that the reversal of the stunt- 

 ing effect does not necessarily reduce virus synthesis. Virus effects on 



