GROWTH ASPECTS OF PLANT VIRUS INFECTIONS 643 



concentration. That host growth does create a dilution eflFect was in- 

 dicated by the fact that no dechne in virus concentration over a 14-day 

 period could be detected in mature tobacco leaves in which growth 

 was negligible. 



At low temperatures virus synthesis parallels host growth for a 

 longer period, and maximum virus concentrations are reached much 

 later than at high temperatures. Since host growth is less rapid at low 

 temperatures, less of a dilution effect occurs, and less of a drop in virus 

 concentration follows. 



The concentrations of cucumber-mosaic virus and tobacco-mosaic 

 virus in tip leaves of tobacco show a cyclic pattern, in that maximal 

 concentrations occur at any given temperature at different time inter- 

 vals (Cheo and Pound, 1952; Bancroft and Pound, 1956). At any tem- 

 perature, the virus concentration follows a low-high-low-high pattern. 

 The frequency of the cycle depends upon temperature, and since these 

 reversals occur while growth is steadily increasing, some factor other 

 than the temperature effect on host growth must control this pattern. 

 Two possible suggestions occur to explain this phenomenon: (1) The 

 virus, while using the products of the host's enzymes for its own syn- 

 thesis, competes with these host enzymes for constituents necessary 

 for the synthesis of both. This successful competition by the virus 

 allows an initially rapid virus build-up. \Mien certain of these host en- 

 zymes become depleted, virus synthesis is retarded until the host re- 

 plenishes its enzymatic machinery, after which the products again 

 allow a new burst of virus synthesis. (2) Some by-product of virus syn- 

 thesis acts as an inhibitor of certain host enzymes by a feedback mech- 

 anism, such that the virus periodically checks its own synthesis while 

 allowing the host to recover. 



These experiments dealt only with \ irus concentrations in tissue 

 samples at stated intervals of time and gave no consideration to the 

 possibility of a dynamic equilibrium between the synthesis and inac- 

 tivation of virus. There is evidence against proteolysis of virus in es- 

 tablished systemic infections (Meneghini and Delwiche, 1951), but 

 some recent studies indicate that in some host-virus combinations, virus 

 synthesis and virus inactivation may occur simultaneously in the plant, 

 and that the measurable virus concentration at any one time may re- 

 flect the relative intensity of these opposing reactions. Harrison ( 1956), 

 working with tobacco-necrosis virus in the French bean, found that in- 

 activation greatly exceeded synthesis in plants growing at 30° C, and 

 he obtained some evidence that these two processes may occur simul- 

 taneously at lower temperatures as well. Kassanis (1957) reported 

 that tobacco-mosaic virus did not accumulate in tobacco plants kept at 

 36°, and that virus produced in plants at lower temperatures actually 

 disappeared from systemically-infected leaves when plants were shifted 



