Abstract
The modifications that the vascular bundles undergo in the “resistant zone” surrounding TMV-induced necrotic lesions were studied by electron microscopy. It was found that, at a distance of about 1 mm from the lesions, the sieve plate pores and the plasmodesmata connecting the sieve elements with their companion cells were occluded by a callose-like substance. Nearer to the lesions, the tracheids were partially or totally occluded by plugs of electron-dense material, and the xylem parenchyma cells produced thickenings in their walls. The plugs in the tracheid lumen were not digested by pectinase or pronase, and were not preferentially stained by ruthenium red. They were markedly labelled after phenylalanine 3H uptake, and it is suggested that one of their main constituents might be phenolic compounds.
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