No abstract

With Plate 15)When plants were kept at 36" C. for some time before inoculation, their susceptibility to infection by five mechanically transmissible viruses was greatly increased. When kept at 36" after inoculation, fewer local lesions were produced than at lower temperatures, hut the effects of the post-inoculation treatment differed with different viruses. Tomato spotted wilt and tobacco mosaic viruses multiply in plants at 36", and the post-inoculation treatment reduced the local lesions they caused to numbers that varied between 10 and 90% of the control; these two viruses also have large thermal coefficients of heat inactivation. By contrast, tobacco necrosis, tomato bushy stunt and cucumber mosaic viruses, were much affected by post-inoculation treatment, lesion formation being completely prevented by exposure to 36" for a day or more. These three viruses appear not to multiply in plants at 36", and although they have high thermal inactivation points, they have small temperature coefficients of thermal inactivation.The extent to which lesion formation was affected by pre-or post-inoculation exposure of plants to 36" depended not only on the length of the treatment, but also on the physiological condition of the plants.The symptoms of infected plants changed considerably if kept at 36". At 36" Nicotiana gfutinosa, inoculated with tobacco mosaic virus, gave chlorotic local lesions instead of necrotic ones, and became systemically infected. When systemically infected plants were brought to ordinary glasshouse temperature, the infected tissues all collapsed and died in a day. 157

SUMMARYThe Rothamsted culture of tobacco necrosis virus contains two serologically unrelated viruscs one of which, called the 'satellite virus' (SV), causes no lcsions and multiplics detectably only when the other (TNV) is present.It decreases the size of necrotic local lesions formed by TNV. Inocula containing both viruses gave the same, fewer or more lesions than inocula containing only TNV ; the results depended on the ratio of the two viruses in the mixed inoculum, on the species and age of the tcst plants and, particularly, on the tcmpcrature a t which the plants were kept after the inoculation. The conccntration of TNV decreased when SV was also present; the extent of the decrease depended on the ratio of the two viruses in the inoculum and othcr factors. Inoculation with SV 5 days before inoculation with TNV still decreased lesion size and led to the multiplication of SV. Phenol-disrupted SV affected lesion size only when inoculation with TNV followed within a few hours, but when inoculation with TNV was delayed by up to 2 days, SV somctimcs multiplied deteetably. SV has the smallest particle reported for any plant virus; diameter c. 17 mp and weight equivalent to a molecular weight of about 1.9 x lo8, of which 20% is nucleie acid. It is exceptionally stable, retaining infectivity after 17 years a t 3" and after heating for 10 min. at 90'. TNV was much more susceptible to inactivation by heat and by ultraviolct light than SV. The two direrered greatly in electrophoretic mobility and were readily scparated from mixed preparations by electrophoresis. IhTRODUCTIONThe Rothamsted culture of tobacco necrosis virus was long known to contain spherical particlcs of two sizes (Bawden & Pirie, 1942, 1950 but the significance of this was not understood until they were found to be serologically unrelated viruscs, the smaller of which multiplies dctcctably only in the prcsence of the largcr. The simultancous multiplication of the two produces smaller necrotic local lesions in French bean than when the large particles are multiplying alone; and the ratio of large to small lesions falls as the proportion of small particlcs in the inoculum increases; 8 great excess of small particles can decrease the number of lesions (Kassank & Nixon, 1960, 1961. The experiments now to be described show othcr effects of interaction between the two viruses and how their interactions depend on the species of host plant and changes in the environment. Also, by using the

No abstract

With Plate 10)By growing the excised apical meristems of sprouts from the potato varieties King Edward and Xrran Victory, infected respectively with potato paracrinkle virus and potato virus S , virus-free plants were obtained. Although the method failed to produce virus-free plants from varieties infected with potato virus X , this virus also seems not to be present in apical meristems, for no virus could be demonstrated in callus tissue that developed from excised meristems less than 200 /L across. The concentration of tobacco mosaic virus in tomato roots and tobacco stems is also much less near the growing point than in older cells, but there is no evidence that the meristematic region is virus-free.