THE EFFECT OF DARKENING ON THE SUSCEPTIBILITY OF PLANTS TO INFECTION WITH VIRUSES I. RELATION TO CHANGES IN SOME ORGANIC ACIDS IN THE FRENCH BEAN

The susceptibility of French bean plants to infection by the Rothamsted strain of tobacco necrosis virus as measured by the local-lesion method is increased by a rise in temperature and usually by darkening the plant before inoculation. If part only of a leaf is darkened, that part becomes more susceptible. Plants in full light also become more susceptible if carbon dioxide is removed from the air, whereas the susceptibility of plants in the dark is not altered. Darkening leaves decreases their content of malic, fumaric, succinic and glycolic acids and increases the content of citric acid; the content of oxalic and malonic acids remains constant. These changes occurred in winter and summer and whether or not darkening increased susceptibility. The effect on susceptibility of individual acids infiltrated into the leaf was measured in leaves kept in the light or in the dark before inoculation. None of the acids used produced any large change in susceptibility.