THE REACTION OF THE VIRUSES OF TOMATO SPOTTED WILT AND TOBACCO MOSAIC TO THE pH VALUE OF MEDIA CONTAINING THEM1
Authors: BEST EUPERT J. , SAMUEL GEOFFREY 
Published: February 2008

The interpretation of lesion counts in terms of the relative concentration of active virus units is discussed in its bearing on our results.

The effect of pH. value on the activity of the virus of tomato spotted wilt has been examined with the following results:

Activity‐time curves at constant pH. value show that in some instances in the absence of oxygen and at 0o C. suspensions of the virus buffered at pH 7 maintain their activity without loss for at least 11 hours. In other instances a fall in activity sets in after about 6 hours and in others even sooner, but in general suspensions remain at constant activity under these conditions for 6 hours or more. Time‐activity curves at pF values of 5‐2, 6‐0, 8‐7 and 9‐2 are also recorded.

An activity‐pH curve for the virus (in the absence of free oxygen and at 0o C.) has been constructed. It has been found that the virus is rapidly inactivated at and below a pH value of 5 and above a pH. value of about 10. There is no significant difference in activity over the pH range 6–8‐9 for a half‐hour or 5‐hour contact.

A study of the effect of pH. value on the activity of ordinary tobacco mosaic virus has revealed the following relationships:

Activity‐time curves at constant pH value show that:

Suspensions of this virus buffered at pH. 7 do not change in activity over a period of at least 24 hours at room temperature (ca. 20o C).

Suspensions at pH. about 9 undergo a rapid fall in activity, but eventually reach a steady state and then remain constant. If the suspensions (at pH 9) are adjusted to pH 7 shortly after preparation a marked reactivation takes place, the difference in activity of the adjusted (to pH 7) and unadjusted suspensions getting progressively smaller with time. It is concluded that portion of the virus was completely and irreversibly inactivated, that the amount so inactivated increased with time, that a small portion was reversibly inactivated (i.e. was reactivated when suspensions were adjusted to pH. 7), and that the portion capable of reactivation decreased progressively with time.

Suspensions at pH 2 when inoculated at this pH. value cause significantly fewer lesions than the corresponding control inocula, whereas the activity of suspensions held at pH 2 for various times and adjusted to pH 7 before inoculation is not significantly different from that of the controls at pH. 7. The differences are discussed and in part at least ascribed to an effect on the host plants.

Two activity‐pH curves have been constructed for tobacco mosaic virus. One, when the suspensions were adjusted to pH 7 after 12‐hour contact at the test pH values and another when the suspensions were inoculated at the test pH. value. It is concluded from these tests that the virus undergoes inactivation above pH 8‐2 and below pH 2. The extent of this inactivation varies with the pH. value and is complete at pH. 11 and 0–5. There is no significant difference in the activity of the virus over the pH. range 2–8 in the adjusted suspensions and over the pH. range 6–8 for the unadjusted suspensions. Differences between the apparent activity of adjusted and unadjusted suspensions at low pH. values are recorded and their probable causes discussed. Between pH. values of 2 and 5 these differences are relatively small and are probably due in the main to an effect of the acidity of the inoculum on the host plant.

The bearing of these results on other problems connected with plant viruses is briefly discussed, and it is pointed out that in the case of tobacco mosaic virus at any rate they are strong evidence for the non‐organismal nature of this virus.