On Phloem Necrosis in Plants Infected with Potato Leaf Roll Virus

Abstract

Physalis floridana and Datura stramonium plants were inoculated with potato leaf roll virus by using viruliferous green peach aphids (Myzus persicae), and at various intervals after the inoculation, virus recovery tests were made by using non-viruliferous aphids. The earliest recovery time from upper leaves, inoculated leaves and lower leaves was 2-4, 0-2 and 6-8 days, respectively, after inoculation. Percentage of virus recovery was highest in upper leaves after 2 days following inoculation.Symptoms appeared on 6-7th day after inoculation in upper leaves and on 10-12th day in other leaves.Ultrathin sections of leaflet midrib tissues of these plants were examined by an electron microscope. Necroses were observed in phloem cells of upper and inoculated leaves 5 days after inoculation. In samples of 10-15 days after inoculation, the number of necrotic cells was approximately three times of that in samples of 5 days after inoculation.There were two types in necroses observed in the phloem: coagulation or disintegration of cytoplasm (DC type) and reticulate or fibrous structures in cytoplasm (NC type). DC type cells were observed more frequently in upper leaves than in lower ones in earlier period after inoculation. NC type cells were seen more frequently in lower leaves than in upper ones in later period after inoculation. The fibrous structures were recognized also in necrotic cells of healthy plants.Virus particles were observed within DC and NC type cells with reticulate structres, but not in NC type cells with fibrous structures.The following time sequence is suggested: virus multiplication→phloem necrosis→external symptoms.