Effects of light and temperature on symptom development and virus content of tobacco leaves inoculated with potato mop-top virus

Necrotic spots or small rings develop after 3–4 days in leaves of Nicotiana tabacum cv. Xanthi-nc inoculated with potato mop-top virus and kept at 14 °C in continuous light (4320 lux); a series of concentric necrotic rings of increasing diameter then form at 2- to 3-day intervals around each initial lesion. Successive rings take longer to appear when either the light intensity or the photoperiod is decreased. Virus accumulation is much decreased and lesions rarely develop either at 14· in darkness or at 22° in light. Virus accumulates rapidly when plants are transferred from these conditions to 14° in light (4320 lux), and necrotic spots or rings develop whose size depends on the interval between inoculation and transfer, and on the conditions during this period. In such plants, necrosis seems to occur only when conditions become favourable for virus synthesis, it is confined to recently infected cells and it does not prevent virus spread to further healthy cells. From the sizes of the necrotic rings, the virus is estimated to invade tissue in light (4320 lux) at c. 38 μm/h at 22° and c. 16 μm/h at 14°. Invasion in darkness at either temperature is very slow. Necrotic rings develop, and the rate of virus accumulation increases when inoculated plants are transferred from 22° in light (4320 lux) to 14° in darkness, but no lesions appear when the order of the treatments is reversed. The process of lesion formation thus includes an early phase requiring light and a later phase requiring low temperature. The light-requiring phase takes about a day at 14° but less at 22°. The later phase takes about 2 days in light (4320 lux) or 3 days in darkness.