Some effects of host-plant nutrition on the multiplication of viruses

The amounts of tobacco mosaic virus present in systemically infected tobacco plants varied greatly with the mineral nutrition of the plants and were related to the effects on plant growth. With plants in soil, supplements of phosphorus produced the greatest increases in plant size, in virus concentration of expressed sap, and in total virus per plant; nitrogen increased plant size only when phosphorus was also added, and only then increased virus concentration and total virus per plant. Combined supplements of phosphorus and nitrogen doubled the virus concentration of sap and increased the total virus per plant by factors up to forty. Potassium slightly reduced the virus concentration of sap, though it usually increased plant size and total virus per plant. From all plants, only about one-third of the virus contained in leaves was present in sap. Virus production seemed to occur at the expense of normal plant proteins, and the ratio of virus to other nitrogenous materials was highest in plants receiving a supplement of phosphorus but not of nitrogen. The effects of host nutrition on the production of virus in inoculated leaves resembled those in systemically infected leaves, but were more variable. No evidence was obtained, with plants grown in soil or sand, that host nutrition had any consistent effect on the intrinsic infectivity of tobacco mosaic virus. The concentration of virus in sap from potato plants systemically infected with two strains of potato virus X was not consistently affected by fertilizers; the chief effect of host nutrition on virus production was indirect by altering plant size.