Effects of darkness on the constitution of tobacco leaves and susceptibility to virus infection

An attempt was made to find the causes of increased susceptibility to virus infection when tobacco plants are kept in the dark before inoculation. The changes in certain nitrogen fractions, viz. insoluble-X, amino-X, amide-N, ammonia-N and nitrate-N, and in dry matter and water content were followed in tobacco plants subjected to a period of darkness before inoculation with tobacco aucuba mosaic virus. Only nitrate-N vas strongly correlated with the susceptibility to infection, but the evidence suggests that the correlation is indirect and not causal. Dry matter and water content, determined either as dry matter percentage of fresh weight or measured separately on a leaf-disk basis were found to vary directly with variation in susceptibility.