THE PHYSIOLOGY OF HOST–PARASITE RELATIONS: I. THE ACCUMULATION OF RADIOACTIVE SUBSTANCES AT INFECTIONS OF FACULTATIVE AND OBLIGATE PARASITES INCLUDING TOBACCO MOSAIC VIRUS
Excised, parasitized leaves were placed with their bases in radioactive solutions or exposed to radioactive carbon dioxide and the uptake and distribution of the tracers followed by autoradiography. Carbon dioxide and 20 other carbon compounds, including sugars, amino and organic acids, phenols, and indoleacetic acid, and/or their metabolic products, as well as phosphate and calcium, accumulated strongly at uredial and conidial colonies of the obligate parasites Puccinia and Erysiphe, respectively. Accumulation of glucose occurred in the host tissue at the sites of mildew colonies on leaves from which the ectoparasitic fungal mycelium had been removed, paralleled the respiration, rate, increased progressively with the development of the parasites, and was most marked at type 4 infections. It was inhibited by sulphur dust (with mildew), sodium azide, 2,4-dinitrophenol and, reversibly, by anaerobiosis and was, therefore, dependent on aerobic respiration. On the other hand the tracers did not accumulate within senescent rust and mildew colonies or within the necrotic lesions produced by bacterial and fungal parasites which kill the tissues of their hosts. Radioactive glucose did not, but calcium and phosphate did, accumulate within young local lesions of tobacco mosaic virus on Nicotiana. The tracers did not enter dead areas on mechanically-wounded leaves and only accumulated at fresh wounds under conditions conducive to a high rate of water loss from the damaged surface. In conjunction with more conventional methods, tracer techniques provide a powerful method of attack on the problems of host–parasite relationships.