MEASUREMENT OF CONCENTRATION OF 

 PLANT VIRUS SUSPENSIONS 



J. G. Bald 



University of California 



Los Angeles 



Rational methods for measuring the concentration of plant viruses 

 were initiated in 1927 by McKinney. McKinney used tobacco mosaic 

 virus, and single tobacco plants, as units for infection. Two years later 

 Holmes suggested the use of local lesions for measuring virus concen- 

 tration. Local lesions are visible injuries caused by virus particles enter- 

 ing an inoculated leaf, infecting it, and multiplying around the points 

 of entry. They have become the principal unit for measuring virus 

 concentration. The use of local lesions is limited to virus-host combina- 

 tions, which easily produce countable lesions after mechanical in- 

 oculation. 



A sample of virus, suspended in plant juice, water or some other 

 watery medium, is rubbed lightly over the upper surfaces of a series 

 of leaves. The virus enters through minute wounds into leaf hairs or 

 other surface cells. Entry is almost instantaneous. Where infections 

 occur, viruses multiply locally, spreading to a group of adjacent cells, 

 killing or injuring them. The boundaries of such lesions remain well 

 defined, and the lesions are generally separated by healthy cells from 

 other lesions. Within limits, the more infectious the virus particles in 

 the suspension used for inoculation, the more lesions are likely to appear 

 on the host plant. Different samples of virus may be compared on 

 similar series of leaves, and rated for relative concentration according 

 to the greater or smaller numbers of lesions produced. 



The nature of the inoculated surface sharply separates the local 

 lesion method for measuring the concentration of plant viruses from 

 the egg-membrane method for animal viruses. Epidermal cells of the 

 leaf surface are mature, differentiated, and variable. They are pro- 

 tected by a cuticle and a cell wall of cellulose pectin and suberin. No 

 virus placed on this surface can penetrate it directly. Wounding is 

 therefore a prerequisite for entry by the virus and for the initiation of 

 each single lesion. During inoculation, it seems that only a fraction 

 of a percent of the cells on an inoculated leaf surface are wounded in 

 the right way and to the right degree for the entry and establishment 

 of virus in living leaf cells. Making suitable wounds for entry of the 

 virus probably implies either (1) rupturing the cuticle and rigid wall 

 of epidermal cells without destroying the underlying cell protoplasm, 

 or (2) getting virus into injured cells, the contents of which are then 



