VIRUS INCREMENT CURVES OBTAINED FROM COUNTS OF PARTICLES IN CLARIFIED PLANT JUICE

Abstract

DURING THE COURSE of investigations into the nature of virus multiplication a need has arisen for quantitative assays of the number of virus particles in samples of gently clarified plant juice and for additional inform,ation concerning the rate of multiplication of virus particles in the inoculated leaves of infected plants. If ,a strict quantitative interpretation of any non-uniformity among virus particles, as observed by electron microscopy, is to be made, certain conditions must be imposed: (1) A method must be developed for accurately assaying the number of virus particles in any sample studied. This is necessary in order to equalize, among the samples, the chances of observing deformed individuals or accidentally formed groups of two or more virus particles. (2) Curves must be obtained which will show increments in the number of virus particles in inoculated leaves as a function of the age of infection. This is necessary in order that one may be sure of comp,aring a rapidly-growing infection with a static one, and in order that any observed signs of a multiplication process may be quantitatively correlated with the rate of increase in the number of virus particles. Since the anticipated investigations -are to involve the comparison of virus particles by electron microscopy rather than the infectivity of, or the total weight, or nitrogen content of the virus present in the various samples, the most suitable method for obtaining increment curves would be one in which the number or combined lengths of virus particles is obtained by direct measurements or counts. (3) Methods must be found for altering at will the rate of increase of virus particles in selected samples. This is desirable in order that one may compare rapidly-growing infections with static ones at the same age of infection, and hence eliminate the time factor that is implicit in the comparison mentioned under (2). A review of methods available for assaying virus concentration in preparations from infected plants has been published recently by Takahashi (1951). Of these procedures, that employing the precipitin reaction (Beale, 1934) is the most nearly suited to a study of the concentration of virus particles in gently clarified plant juice. This method is capable of showing the presence of at least some noninfective virus particles as well as the infective ones. It has not been shown, however, that all particles of a suitable size and shape to be judged virus par-