Demonstration of Phloem Necrosis in Plants Affected by Potato Leafroll

Abstract

In potato plants infected with potato leafroll Luteovirus, sieve tubes and accompanying cells in the primary phloem become necrotic (phloem necrosis). Even before this phenomenon can be observed, the infection manifests itself by accumulation of starch in the leaves. The phloem necrosis starts with a swelling of the sieve tubes in which lignification may take place. The phloem shrinks, and the walls of the lumina of individual sieve tubes and accompanying cells are no longer visible. The phloem parenchyma cells, however, are not affected. The necrotic cells can be made visible with a stain, e.g. Diamant fuchsin or phloroglucinol, which are reagents for pentoses or condensed pentoses, such as lignin. In healthy plants, only the xylem and the phloem fibres are stained red, whereas all other elements remain unstained. In potato leafroll-diseased plants, however, necrosis-showing groups of sieve tubes and accompanying cells are stained bright- or dark-red with Diamant fuchsin, and red or yellowish-red with phloroglucinol.