644 PLANT GROWTH AND PLANT COMMUNITIES 



to 36°. Observations with such Hmited numbers of host- virus combina- 

 tions and without measurements of absolute virus contents would not 

 warrant general conclusions that these opposing processes are the gen- 

 eral pattern. These experiments of holding plants at high temperature 

 (30° to 36° C.) do indicate that host metabolism can be upset to a 

 point where virus synthesis is no longer supported but where host 

 growth continues. 



Temperature not only aflFects the quantity of virus synthesized but 

 also the quality. A legume strain of tobacco-mosaic virus has a different 

 amino-acid composition if produced in tobacco plants held at 30° C. 

 than if produced at 20° C. (Bawden, 1958). Another indication that 

 temperature effects are qualitative in nature is that some strains of a 

 virus ( e.g., cucumber-mosaic virus ) will multiply in a host at a temper- 

 ature which will not permit synthesis of other strains ( Hitchborn, 1956, 

 1957). 



The relatively little work that has been done on soil temperature 

 indicates that soil-temperature effects usually parallel those of air 

 temperature in direction but not in magnitude. Soil-temperature effects 

 on virus synthesis are less marked than those of air temperature ( Cheo 

 and Pound, 1952; Pound and Weathers, 1953a; Pound and Helms, 1955; 

 Bancroft and Pound, 1956 ) . 



Light. Little experimentation has been done to determine the qual- 

 itative and quantitative effects of light on virus synthesis. It is a gen- 

 eral observation that keeping plants in reduced hght for short periods 

 prior to inoculation increases their susceptibility to virus infection. The 

 reason for this is unknown, but it is possibly related to carbohydrate 

 metabolism. Suificient data are at hand to indicate some relationships 

 between the effects of light on host growth and on virus synthesis. 

 With cucumber-mosaic virus ( Cheo and Pound, 1952 ) , tobacco-mosaic 

 virus (Pound and Bancroft, 1956), and squash-mosaic virus (Bancroft, 

 1958 ) , long photoperiods and high light intensities generally favor virus 

 synthesis during the early days of infection. Thus in inoculated leaves 

 the effects of light on the initial virus increase parallel the effects on 

 host growth. In systemically-infected plants, however, this parallelism 

 does not hold. In systemically-infected plants, tobacco-mosaic virus 

 synthesis in tobacco under different treatments of photoperiod or light 

 intensity is not along a straight-line gradient, as growth is, and a given 

 group of plants may show exactly opposite virus-concentration patterns 

 in relatively short periods. A reversal of virus-concentration gradients 

 while the growth gradient remains the same indicates that virus syn- 

 thesis, at least virus accumulation, is not necessarily related to growth. 

 Similar negative correlations between host growth and virus concen- 

 tration were reported by Bancroft ( 1958 ) for squash mosaic. 



A direct correlation does exist between host growth and virus con- 



