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Barley stripe mosaic virus became systemic 3 days alter inoculation of barley plants in the two-leaf stage. The first leaf to show mosaic was leaf 3. which was 4.5 cm long at 3 days after inoculation when it was invaded by virus. Since all of Ieaf 3 showed mosaic, the events leading to mosaic occurred during cell expansion and after cell division. which is limited to the basal centimeter of young cereal leaves. Molecular changes associated with mosaic formation included a 509, reduction in chloroplast ribosomal RNA and chlorophyll, expressed as amount per unit DNA, in young leaves with an acute stage of infection. Cytoplasmic ribosomal RNA was not reduced. The amount of DNA per barley leaf and leaf fresh weight were reduced as a result of systemic virus infection. Chloroplast ribosomal RNA was not detectable by density gradient centrifugation in infected areas of maize leaves. An RN A-dependent RN A polymerase associated with virus infection reached maximum concentrations in young systemically infected barley leaves coincident with high rates of virus replication and first detectable decreases in chlorophyll and chloroplast ribosomal RNA.