Some anatomical and chemical effects of "flat-limb" virus on apple var. Gravenstein

Abstract

Anatomical investigations were made of lateral branches of 3-year-old healthy and "flat-limb"-infected nursery trees, and of the main stem of 5-year-old healthy and 10-year-old flat-limb-infected orchard trees of apple var. Gravenstein.

Anatomical abnormality as a result of virus infection occurred in the late autumn of the first year of growth (7–8 months after infection in the nursery tree), spread laterally in spring of the second year, was overcome somewhat in summer, and spread more widely again in the autumn. In the nursery tree, anatomical studies indicated that there was a dilution of virus effects (including lessening of surface depressions in the stem) or lag in their appearance in passing up from the original point of infection to the top of the tree and to the tips of its lateral branches.

Both wood and phloem structure were abnormal in the virus-infected trees. Fibres, tracheids, and vessels in the wood, and fibre-sclereids and sieve-tube elements in the phloem, failed to differentiate. The wood consisted of parenchymatous cells, often much enlarged and distorted, with lignified and thickened walls and lumina packed with starch grains, and the phloem of a disordered array of loosely packed parenchymatous cells containing tannins and crystals. Schizogenous intercellular spaces developed in abnormal wood. The cambium was scarcely distinguishable from the phloem in the 10-year-old virus-infected orchard tree. Despite this there were indications that it had not lost its power to divide and provide derivatives differentiating as normal fibres when partial recovery from infection occurred. Deposition of starch and tannin in the pith was enhanced by virus infection.

The fact that sections of infected stems gave deep yellow solutions in water while those from healthy wood gave pale yellow solutions led to chemical analysis for quantitative and qualitative differences in methanol extractives between healthy and infected wood. Qualitative differences in the extractives from healthy and diseased 2- year-old wood were established.

Enhanced starch and extractives contents as a result of virus infection were interpreted as effects of disrupted translocation.